Post by: Peter Dow on 18/02/2020 09:16:34
(https://ichef.bbci.co.uk/news/660/cpsprodpb/15BB7/production/_110851098_bridge5.jpg)
BBC: “Falling ice causes first Queensferry Crossing closure” (https://www.bbc.co.uk/news/uk-scotland-51456296)
Keeping such bridges open even in icing conditions is really not rocket science. What, to me anyway, is the obvious solution – to pass an electrical heating current through the bridge’s support cables – doesn’t seem to be “obvious” to other research scientists and engineers whose “Thermal Systems” for melting the ice are reviewed here. (https://backend.orbit.dtu.dk/ws/portalfiles/portal/4376052/ICSBOC2010+paper.pdf)
I suggested this simple solution, outlined the calculations required and warned of some dangers in an email to the Queensferry Crossing bridge authorities and contractors in March 2019, but as usual, the authorities ignore solutions until there is a political price to be paid for continuing to ignore solutions in a pig-headed, in-denial kind of way that politicians like to get away with, if they possibly can.
There follows a link to a PDF of the email I sent the bridge authorities last year – hopefully you can click the link and open and / or download the PDF so you can read it.
Queensferry falling ice hazard solution – electrically-heated cable stays (https://peterdow.files.wordpress.com/2020/02/queensferry-falling-ice-hazard-solution-electrically-heated-cable-stays.pdf)
Deicing power for 70km of cables
@ 100W/m = 7MW = household electricity within a 3 mile radius of the bridge.
@ 250W/m = 17.5MW = household electricity within a 5 mile radius of the bridge.
Cable strands
(https://peterdow.files.wordpress.com/2020/02/queensferrycrossingcablecrosssection55-strand_10803.jpg?w=500)
Some strands in the cable are better situated for heating the cable than other strands, depending on their position in the cable as I have labelled them alphabetically, beginning with the label “A” for the centre strand (which is the worst strand for heating the outside of the cable, where the ice would be) and labelling the outer strands last in alphabetical order, which are best for heating the outside of the cable.
The cable strands are by convention named here using the format – “(Number of strands in the cable)-(Letter)”. Thus the centre strand in the 55-strand cable is named as “55-A”, the 6 strands immediately surrounding the sole 55-A are all named of type “55-B”.
For each strand in the cable we can assign a factor of heating capacity.
(https://peterdow.files.wordpress.com/2020/02/qfccable55-heatingcapacityfactors_1080.jpg?w=500)
For the 55-strand cable, total heating capacity factor assigned is 48.
For the 55-strand cable, there are a total of 24 strands which have utility for heating the cable – 6 of the 55-F type name strands, 6 x 55-Gs and 12 x 55-Hs. The 31 other strands (the 55-A to 55-Es) are not needed for heating per se, though could carry electrical currents whether by design or otherwise.
We can tabulate for each strand label, the heating power fraction and percentage, according to each strand’s heating capacity factor as a fraction of the cable’s total heating capacity factor.
(https://peterdow.files.wordpress.com/2020/02/55heating.jpg)
(https://peterdow.files.wordpress.com/2020/02/61strandcable.jpg)
For the 61-strand cable, the total heating capacity factor assigned is 54.
For the 61-strand cable, there are a total of 24 strands which have utility for heating the cable – 6 x 61-Gs, 12 x 61-Hs and 6 x 61-Is. There are 37 other strands – the 61-A to 61-Fs.
(https://peterdow.files.wordpress.com/2020/02/61heating.jpg)
(https://peterdow.files.wordpress.com/2020/02/73strandcable.jpg)
For the 73-strand cable, the total heating capacity factor assigned is 54.
For the 73-strand cable, there are a total of 30 strands which have utility for heating the cable – 12 x 73-Hs, 6 x 73-Is and 12 x 73-Js. There are 43 other strands – the 73-A to 73-Gs.
(https://peterdow.files.wordpress.com/2020/02/73heating.jpg)
See my blog post (https://peterdow.wordpress.com/2020/02/11/electrically-de-icing-the-queensferry-crossing-cable-stayed-bridge/) for details for 85-, 91- and 109- strand cables.
VSL SSI 2000 Stay Cable System
There are a number of options available in the VSL SSI 2000 Stay Cable System (http://www.vsl.cz/download/48) so these figures cannot be confirmed without sight of the Queensferry Crossing engineering design specifications (or by actually measuring the cables, which I am unable to do!).
(https://peterdow.files.wordpress.com/2020/02/queensferrycrossingsun.jpg?w=500)
For now, I am assuming for simplicity that the required maximum heating power in watts/metre is the same as the stay pipe diameter in mm. This is not far off the maximum heat radiation from the sun on such a stay pipe, square on to the sun, at midday, midsummer, on a cloudless day – or more than enough heat to melt any ice in short order!
At this maximum heating power and after the cable cores warm up, they will emit 1000÷π = 318 Watts of heat energy per metre-squared of stay pipe surface area.
(https://peterdow.files.wordpress.com/2020/02/cablepower.jpg)
It is now possible to tabulate for each cable-label strand, the maximum heating power per metre and assuming a strand resistance of 0.001137 ohms per metre, what the maximum strand current would be.
(https://peterdow.files.wordpress.com/2020/02/strandpower.jpg)
Cable voltages and power
To calculate the cable voltages and power and to calculate the total maximum power to heat all the cables of the Queensferry Crossing accurately, I will need to know how many of each size of cable and their lengths.
Direct Current Heating
Those theoretical differences between strand situations only matter for direct current heating if it is possible electrically to isolate strands from each other. The strands are attached via steel wedges to a steel anchor head, which, for now,incidentally connects all the strands together electrically.
Please note, however, that when introducing a design requirement to conduct large electrical currents between strand pairs at the tower anchor heads (see DC Circuit Diagrams) the incidental electrical connection at the wedges may be of insufficiently or unreliably low resistance and should be supplemented with an ultra-low resistance connector between the strand ends, to avoid faults developing from excessive resistance heating at the wedges.
(https://peterdow.files.wordpress.com/2020/02/stay-cable-anchorages.jpg?w=768)
Cable anchorages
(https://peterdow.files.wordpress.com/2020/02/electrically-isolated-strands.jpg)
Teflon/PTFE-coated glass fibre fabric sheaths to electrically isolate the strands from the anchor head. The outer strands are for heating. The inner strands are for signals.
It should be possible to insert Teflon/PTFE-coated glass fibre fabric sheaths between the wedges which grip the strands we wish to insulate and to isolate from the anchor head and from each other, unless and until they are connected to electrical heating or signal circuits.
The signal circuits could be used to report to the power supply control electronics at one end of the cable, the output of heating current sensors at the other end of the cable, to help to detect current leakage faults in the cable strands’ insulation, to implement a residual current device, to trigger safety power-cut-outs or circuit-breakers, most notably.
Teflon (https://en.wikipedia.org/wiki/Polytetrafluoroethylene) is a good insulator and is used for thread seal tape (https://en.wikipedia.org/wiki/Thread_seal_tape) illustrating the properties of lubrication of the wedge to its housing cone required. The glass fibre fabric should provide strength under compression and a superior dimensional stability versus creep under load that a pure Teflon sheath may suffer from.
Clearly the sheath would have to remain thick enough to insulate against the highest voltage difference which might appear between the heating strands and the anchor head.
UPDATE 22/02/20
Such sheaths would likely not be available as an off-the-shelf product in the required dimensions, though general purpose PTFE-coated fibre glass cloth is commonly available and this expandable E-glass sleeving, expands from a relaxed internal bore of 15mm to a maximum bore of 38mm and insulates to 500V when not expanded (https://www.sleeveit.co.uk/products/protective-sleeving/vidaflex-xgr/), which is a useful size while relaxed to accommodate the strand and while expanded to accommodate the wedges.
The insulation should cope with the highest DC voltage of about 100 Volts, used to power the longest and highest heating capacity factor strands, albeit that this sleeving is inappropriately resin-coated and would therefore likely require to be custom adapted, the resin cleaned off and re-coated with PTFE, tested and proved in the laboratory. Perhaps wrapping the wedges in PTFE thread seal tape is all that is required to supplement the product as supplied for satisfactory performance? A promising avenue for research.
(https://peterdow.files.wordpress.com/2020/02/expandable-e-glass-sleeving.jpg)
DC Power Supplies
Not forgetting DC power supplies and I have noticed a comprehensive range of 3kW to 10kW DC power supplies here (https://lab-power.com/lab-dc-3-10kw-programmable-power-supply/) that I think will do nicely, an average of about a dozen power supplies per cable (more for the longer cables, fewer for the shorter cables), about 3500 power supplies required to de-ice all 288 cables.
(https://peterdow.files.wordpress.com/2020/02/high-power-lab-dc.jpg)
Where to store the cable power supplies?
Let’s examine the option of storing the cable heating power supplies in the towers, racked next to the anchorages of the cables which they will be heating. There might just be enough room to squeeze in another half a tonne of power supplies for the 4 cables per floor (https://www.youtube.com/watch?v=9HhcELxYK94&feature=youtu.be&t=371) (assuming their racks are securely attached to the tower walls), 12 tonnes worth of power supplies for all 24 floors per tower, for all 3 towers!
Even at 94% efficiency for switch mode power supplies, each tower’s cable power supplies could be generating at most about 0.4 MW of waste heat energy. A new massive extractor fan fitted into the roofs of the towers would be required to cool the inside of the towers while the DC power supplies are heating the cables.
Considering how cramped the insides of the towers are already, the daunting cooling problem, not to mention the risk of a tower fire destroying all of a tower’s power supplies at one time, it looks to be much the better option to install the cable power supplies on the deck, next to the deck anchorages to allow them to be supplied with power.
(https://peterdow.files.wordpress.com/2020/02/queensferry-crossing-on-deck-23-550x365-1.jpg)
The stay cables penetrate the surface of the deck, as can be clearly seen in this next photograph, taken during construction.
(https://peterdow.files.wordpress.com/2020/02/queensferrycrossingstaycablesinsidedeck.jpg)
Therefore best access to the anchor heads, to attach the cable heating power supplies, may be from inside the deck, where the power supplies themselves should be stored too.
5/3/20
(https://peterdow.files.wordpress.com/2020/02/deck-below-cables.jpg)
Update 25/2/2020
DC Circuit Diagrams
Locating all the electrics at the deck anchorages, while leaving the strands earthed at the tower anchorages, offers advantages for design, development, installation, commissioning and servicing.
(https://peterdow.files.wordpress.com/2020/02/qfc-circuit-diagram.jpg)
Circuit Diagram – 2 heating strands, 1 power supply
(https://peterdow.files.wordpress.com/2020/02/currentbalancedetector.jpg)
Heating strands pair current balance detector
The window detector circuit (https://en.wikipedia.org/wiki/Window_detector) compares the isolated power supply’s potential with respect to earth to detect the expected balance of current and voltage in the heating strands pair. If an imbalance fault develops then the safety switch is used to cut the power.
DC Summary
So isolating the strands for DC heating purposes presents technical challenges. It would be very convenient if the outer strands could be preferentially used for heating purposes without having to isolate the strands electrically etc. but to achieve that we must consider using not direct current but alternating current instead.
Alternating Current Heating
The skin effect (https://en.wikipedia.org/wiki/Skin_effect) observed with alternating current changes matters in that with increasing frequency the heating current will tend to distribute towards strands nearer the surface of a cable. However if too great a frequency is used then the skin effect will increase the resistance of even the most superficial strands so much that inappropriately high and difficult to insulate against voltages would be required to obtain the required heating power.
Assuming that the appropriate AC frequency can be determined for preferentially heating the superficial strands of the Queensferry Crossing stay cables, although there would be no need to isolate the strands from the anchor head, there then presents the challenge of isolating the anchor heads and anchorages so that the current is not dissipated through the bridge instead of heating the cables as required.
Having isolated the cables for heating purposes, one may then wish later to reconnect the cables electrically to the rest of the bridge and disconnect the heating power supplies for lightning protection purposes. Certainly, one would not wish to encourage a lightning strike to find its way to ground via the bridge’s cable deicing power supplies!
Tower ice
To prevent the bridge piers or towers (with non-conducting concrete surfaces) from icing up, they could be surface fitted with new electrical heating trace cables (https://en.wikipedia.org/wiki/Trace_heating) which are then appropriately electrically-powered for deicing when necessary.
Ideally, such additional heating elements would have been embedded into the surface of the piers at construction time. Too late for that now.
Another option to consider is heating the hollow piers from within. However, considering the considerable mass and thickness of the piers their surfaces would have to be kept above freezing temperature all winter long. Heating the piers from within, there simply wouldn’t be time to allow the piers to get freezing cold because there was no icing then suddenly heat them from the inside to deice a sudden incidence of icing.
So heating from within bridge piers would use more electricity, though the cost shouldn’t be prohibitive – surplus grid electricity is a common occurrence at times of high wind power generation, so the electricity grid managers should offer a very low price for such electricity (just the grid connection charge) – plus it should be a lot safer upgrade from the point of view of bridge users – far less chance of things falling onto the road during the fitting of the piers’ internal heating elements.
Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of towers in between the open stairways and scaffolding.
Post by: Bored chemist on 18/02/2020 19:43:52
100 Watts would melt about 1/3 grams of ice per second.
A metre of cable 100 mm in diameter has an area - as you calculate- of roughly a third of a square meter
I am assuming for simplicity that the required maximum heating power in watts/metre is the same as the stay pipe diameter in mm.
So, if snow is falling at an equivalent of more than 1/3 grams per sec on 1/3 m^2 i.e. 1 gram per m2 per sec, you aren't going to melt it.
Assuming that the met office rainfall map is sensible
https://www.metoffice.gov.uk/public/weather/observation/rainfall-radar#?map=Rainfall&fcTime=1582010400&zoom=5&lon=-4.00&lat=55.01
we can expect precipitations up to 32 mm/hr
32mm of rain on 1 m^2 is 3.2*100*100 grams per hour
That's 32 kg/hr
which is about 9 g/m^2/sec
So you need 10 times more power just to melt the ice that lands on it. (Please don't waste time telling me the wires are vertical)
You need even more if it's going to have to heat it in air that's above freezing.
I'm also intrigued by the notion that it matters much which wires you heat.
Metals conduct heat quite well.
If you heat the ones in the middle, the heat will diffuse to the outside.
If you heat the ones round the perimeter, the heat will diffuse into the core of the cable.
Fundamentally, the only place that (most of) the cable can lose heat is from the surface, so it hardly matters which bits you heat.
Also, have fun with galvanic corrosion.
Certainly, one would not wish to encourage a lightning strike to find its way to ground via the bridge’s cable deicing power supplies!Lightning goes exactly where it damned well pleases.
the maximum heat radiation from the sun on such a stay pipe, square on to the sun, at midday, midsummer, on a cloudless day – or more than enough heat to melt any ice in short order!Snow is noted for not melting very quickly when the sun shines on it and, to be useful, your scheme has to melt it in real time.
Post by: Peter Dow on 19/02/2020 00:49:20
The heat of fusion of ice is 333.55 J/g.If the ice was at freezing temperature and didn't have to be warmed up from below freezing - 100/333 = 0.3 grams of ice per second.
100 Watts would melt about 1/3 grams of ice per second.
A metre of cable 100 mm in diameter has an area - as you calculate- of roughly a third of a square meter(https://4.bp.blogspot.com/-fCN9zyLLM-U/Wo6ap9NfchI/AAAAAAAAALk/iTPdygJ8bBMWPHI0_kS1-r8eaGqool2vQCLcBGAs/s1600/pipe.jpg)I am assuming for simplicity that the required maximum heating power in watts/metre is the same as the stay pipe diameter in mm.
So the surface area of a pipe per metre of length of pipe of diameter 100mm is 0.314 m2 but don't take my word for it - here is an on-line calculator.
https://vodoprovod.blogspot.com/p/area-pipe.html
So, if snow is falling at an equivalent of more than 1/3 grams per sec on 1/3 m^2 i.e. 1 gram per m2 per sec, you aren't going to melt it.Well snow wouldn't fall on the underside of the pipe but if that much snow fell on the pipe then not all of it could be melted, no, but no worries, because we don't have to melt all the snow that falls onto the pipe, just enough of the bottom layer of snow or ice so that the rest slides off.
(https://images.prismic.io/avalancheca/03cd8dab4b4fac05957853cb79f1fe31203228a4_inclinevsavalanchefrequency_lorizacaruk.jpg?auto=compress,format)
https://www.avalanche.ca/tutorial/avalanche-terrain/slope-angle
Assuming that the met office rainfall map is sensibleNo, because you don't need to melt rain and hail will bounce off and snow will slide off.
https://www.metoffice.gov.uk/public/weather/observation/rainfall-radar#?map=Rainfall&fcTime=1582010400&zoom=5&lon=-4.00&lat=55.01
we can expect precipitations up to 32 mm/hr
32mm of rain on 1 m^2 is 3.2*100*100 grams per hour
That's 32 kg/hr
which is about 9 g/m^2/sec
So you need 10 times more power just to melt the ice that lands on it.
All you need to do is to keep the pipe surface at or above freezing temperature - keep the pipe surface wet, not frozen, and gravity will do the rest.
(Please don't waste time telling me the wires are vertical)I won't but it is worth mentioning that the shallowest slope of pipe is 19 degrees.
(https://peterdow.files.wordpress.com/2020/02/cable-shallow-slopes.jpg)
Quote
You need even more if it's going to have to heat it in air that's above freezing.You mean air that's below freezing? More, admittedly, but not too much more.
I'm also intrigued by the notion that it matters much which wires you heat.(https://peterdow.files.wordpress.com/2020/02/cablesectionpartlandscapecrop540.jpg)
Metals conduct heat quite well.
If you heat the ones in the middle, the heat will diffuse to the outside.
If you heat the ones round the perimeter, the heat will diffuse into the core of the cable.
Fundamentally, the only place that (most of) the cable can lose heat is from the surface, so it hardly matters which bits you heat.
The metal conductors are insulated so there's nothing to be gained by heating the inner strands, and much to lose if you melt the insulation.
Also, have fun with galvanic corrosion.The current in the strands won't effect corrosion, which is limited by the flow of oxygen and water to the conductor surface, not electrons.
Lightning rods have something to say about that. However, if the lightning strikes a cable then best that one at least of the strands offers a conducting path to ground and that none offer a path to a power supply.Certainly, one would not wish to encourage a lightning strike to find its way to ground via the bridge’s cable deicing power supplies!Lightning goes exactly where it damned well pleases.
The pipe will melt the ice and snow and heat the water in contact with the pipe surface by efficient conduction, not by snow's inefficient solar radiation absorption.the maximum heat radiation from the sun on such a stay pipe, square on to the sun, at midday, midsummer, on a cloudless day – or more than enough heat to melt any ice in short order!Snow is noted for not melting very quickly when the sun shines on it and, to be useful, your scheme has to melt it in real time.
Post by: Bored chemist on 19/02/2020 07:21:32
The current in the strands won't effect corrosion, which is limited by the flow of oxygen
No, water is sufficient.
Post by: Petrochemicals on 19/02/2020 23:32:53
Post by: syhprum on 20/02/2020 06:28:35
Post by: Peter Dow on 20/02/2020 08:43:16
Surface coat the cables and uprights in something that ice does not cling. Surely teflon would have an effect.If anti-icing chemical was painted or sprayed on would some of it drip or be sprayed onto the road, making for slippery, treacherous driving conditions?
If it was stuck on with sticky tape, would the sticky tape stay on, or come off?
Wouldn't the coating need re-doing every winter, or more often? Presumably the coating would suffer weathering? How long would it be good for?
They seem to use both de-icing and anti-icing chemicals for aircraft, though Wikipedia doesn't mention "Teflon" or "PTFE".
https://en.wikipedia.org/wiki/De-icing
Create all surfaces with a friction factor low enough to shed iceThat's what heating the cable will do.
Post by: Bored chemist on 20/02/2020 19:52:11
If anti-icing chemical was painted or sprayed on would some of it drip or be sprayed onto the road, making for slippery, treacherous driving conditions?https://en.wikipedia.org/wiki/Millennium_Dome
If it was stuck on with sticky tape, would the sticky tape stay on, or come off?
Wouldn't the coating need re-doing every winter, or more often? Presumably the coating would suffer weathering? How long would it be good for?
Post by: Peter Dow on 20/02/2020 20:20:01
If anti-icing chemical was painted or sprayed on would some of it drip or be sprayed onto the road, making for slippery, treacherous driving conditions?https://en.wikipedia.org/wiki/Millennium_Dome
If it was stuck on with sticky tape, would the sticky tape stay on, or come off?
Wouldn't the coating need re-doing every winter, or more often? Presumably the coating would suffer weathering? How long would it be good for?
Quote from: Wikipedia: Millennium Dome
The canopy is made of PTFE-coated glass fibre fabric,
insert Teflon/PTFE-coated glass fibre fabric sheaths between the wedges which grip the strandsSnap.
It's one thing to coat a surface of glass fibre (melting point 1400 - 1600 °C) with PTFE on a production line in a factory and quite another on the surface of a HDPE (melting point 120 - 180 °C) stay pipe covering a bridge cable while in use.
Then again there is the option of wrapping the existing stay pipes with PTFE-coated fibre glass cloth. That's more of an option to consider.
Post by: Bored chemist on 20/02/2020 20:40:57
Is the Millennium Dome an example of a heated surface which melts ice?No.
I think it is too late to try the PTFE coating option...Google finds 21 million hits for "ptfe spray".
I'm not saying this is a good idea, I'm just saying it's less impractical than wiring the bridge to the grid.
Post by: Peter Dow on 20/02/2020 20:54:31
Without the quotes. With the quotes it is only 153,000 hits.I think it is too late to try the PTFE coating option...Google finds 21 million hits for "ptfe spray".
I'm not saying this is a good idea, I'm just saying it's less impractical than wiring the bridge to the grid.Well why don't they cover aircraft with PTFE and why doesn't that work to solve all aircraft's deicing problems?
Covering the pipes with PTFE-coated glass fibre fabric might help but I would simply point out that everyone's house is connected to the grid but few roofs are covered with PTFE. There must be a practical reason for that, other than to stop the roof repair man from sliding off! :D
Post by: Bored chemist on 20/02/2020 22:06:33
Without the quotes. With the quotes it is only 153,000 hits.Do you not feel that 153000 is enough to choose from?
Well, you can add a few more hundred thousand by using teflon instead of ptfe and changing the word order.
The fact is, of course, it just needs 1
There must be a practical reason for that,There are obvious reasons.
Nobody cares much if there's snow on the roof.
People can't run their computer using a PTFE roof.
Well why don't they cover aircraft with PTFE and why doesn't that work to solve all aircraft's deicing problems?There are a few ways I know of that they use for deicing planes.
Inflatable rubber bits, trace heating and antifreeze sprays.
So far as I know, nobody does it by rebuilding the wing with weak points made of PTFE so that they can electrically isolate the leading edges and pass huge electrical currents through them.
So, given that doing so isn't practical for planes- because there are better ways to do it- perhaps that why the people you set your ideas to thought there were better ways to do it for a bridge.
Post by: Peter Dow on 20/02/2020 22:33:29
Spraying PTFE all over the bridge is a non-starter.Without the quotes. With the quotes it is only 153,000 hits.Do you not feel that 153000 is enough to choose from?
Well, you can add a few more hundred thousand by using teflon instead of ptfe and changing the word order.
The fact is, of course, it just needs 1
A factory-produced cover for the pipes which comes with PTFE already securely attached in the factory is a possibility but access for the workers to attach the covers will be very difficult.
So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.Well why don't they cover aircraft with PTFE and why doesn't that work to solve all aircraft's deicing problems?There are a few ways I know of that they use for deicing planes.
Inflatable rubber bits, trace heating and antifreeze sprays.
So far as I know, nobody does it by rebuilding the wing with weak points made of PTFE so that they can electrically isolate the leading edges and pass huge electrical currents through them.
The PTFE sheaths to isolate the strands don't add a weak-point but we'll be sure to test your "weak-point" theory in the laboratory, making such adaptations as are necessary to prove that strength is maintained.
So, given that doing so isn't practical for planes- because there are better ways to do it- perhaps that why the people you set your ideas to thought there were better ways to do it for a bridge.They don't have a better way and they know it.
Post by: Petrochemicals on 20/02/2020 22:36:31
To put it simply engineering is about finding a way. Infact its most tardy of them (engineers) they have not incorporated this into bridges for at least 100 years. Surface finish shapes eg dimples like a golf ball or the reverse, mini pyramids, paint etc. As you have demonstrated an electrical system of heating is unfeasably complicated and therefore expensive to be retrofitable and therefore maintainable. Its possible to go to the moon, yet beyond apollo we have not bothered for 50 years, until now for some reason, everything is a balance between cost and worth.Surface coat the cables and uprights in something that ice does not cling. Surely teflon would have an effect.If anti-icing chemical was painted or sprayed on would some of it drip or be sprayed onto the road, making for slippery, treacherous driving conditions?
If it was stuck on with sticky tape, would the sticky tape stay on, or come off?
Wouldn't the coating need re-doing every winter, or more often? Presumably the coating would suffer weathering? How long would it be good for?
They seem to use both de-icing and anti-icing chemicals for aircraft, though Wikipedia doesn't mention "Teflon" or "PTFE".
https://en.wikipedia.org/wiki/De-icingCreate all surfaces with a friction factor low enough to shed iceThat's what heating the cable will do.
https://www.technologyreview.com/s/408558/mining-the-moon/
https://philosophia.uncg.edu/phi361-matteson/module-1-why-does-business-need-ethics/case-the-ford-pinto/
They could build extremley safe cars for both passengers and pedestrians, problem is no one would buy them and they would be expensive.
Post by: Peter Dow on 20/02/2020 22:55:10
As you have demonstrated an electrical system of heating is unfeasably complicated and therefore expensive to be retrofitable and therefore maintainable.Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.
Engineering designs are de rigueur full of lists of numbers in mind-numbing detail. It's what makes a good design foolproof not "complicated".
A terrible design plan that was no plan at all would leave lots of decisions to the construction workers to choose an arbitrary length, a thickness, a material, a strand, a voltage or a current and that would be a recipe for disaster because without the detail they don't know what to do, even where to begin.
Architects can't simply pass an artists impression to labourers and invite them to have a go. ::)
Post by: vhfpmr on 21/02/2020 11:41:55
The glass fibre fabric should provide strength under compression and a superior dimensional stability versus creep under load that a pure Teflon sheath may suffer from.Good luck with that one, the compressive strength of something like Duroid is about a tenth that of typical steels.
Post by: vhfpmr on 21/02/2020 11:53:28
So you need 10 times more power just to melt the ice that lands on it.And then by the time you've got ~1kW/m^2, you'll have a temperature rise of something like 100-150K when the cable's dry.
Post by: Peter Dow on 21/02/2020 11:56:39
DuroidI'm not familiar with Duroid, so I searched for it
Quote
Rogers RT/duroid® high frequency circuit materials are filled PTFE (random glass or ceramic) composite laminates for use in high reliability, aerospace and defense applications.When they say "random" what does that mean? If it doesn't mean "glass fibre fabric" but short isolated glass or ceramic fibres or particles or something then that's a different material with different flow behaviour under compression altogether than a cloth woven from long fibres.
https://www.rogerscorp.com/advanced-connectivity-solutions/rt-duroid-laminates
Post by: Peter Dow on 21/02/2020 12:05:44
I specifiedSo you need 10 times more power just to melt the ice that lands on it.And then by the time you've got ~1kW/m^2, you'll have a temperature rise of something like 100-150K when the cable's dry.
Quote
318 Watts of heat energy per metre-squared of stay pipe surface areaand I refuted the suggestion that more power was needed because there is no need to melt all the ice, but only the surface layer, so that the rest of the ice slides off.
So 0.318 kW/m2 is the maximum power I have suggested.
Post by: Bored chemist on 21/02/2020 12:47:35
Spraying PTFE all over the bridge is a non-starter.They paint bridges...
The PTFE sheaths to isolate the strands don't add a weak-point but we'll be sure to test your "weak-point" theory in the laboratory, making such adaptations as are necessary to prove that strength is maintained.In the very real sense that PTFE isn't weaker than steel.
So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.I have never tried it, have you?
I'm guessing that neither of us has, so your claim that "you and I know that..." is not true.
Why say it?
Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.So, what you are saying is that 200 years of experience says we don't need it.
Time to close the thread then
Post by: Petrochemicals on 21/02/2020 12:55:01
You could fit external heating covers easier than teflon coated strands, they would be easier to engineer around and retrofit, to replace one is going to be a hasstle. But lets say you can run the national grid through the existing cables, each cable in entirity is insulated. Its the only way to make it uncomlicated and reliable enough. Each cable is going to need to be heated to keep its temerature above freezing.As you have demonstrated an electrical system of heating is unfeasably complicated and therefore expensive to be retrofitable and therefore maintainable.Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.
Engineering designs are de rigueur full of lists of numbers in mind-numbing detail. It's what makes a good design foolproof not "complicated".
A terrible design plan that was no plan at all would leave lots of decisions to the construction workers to choose an arbitrary length, a thickness, a material, a strand, a voltage or a current and that would be a recipe for disaster because without the detail they don't know what to do, even where to begin.
Architects can't simply pass an artists impression to labourers and invite them to have a go. ::)
You will need
Temperature difference
Rate of loss
Ammount of steel
Ammount of water to be heated
Then you will need :
the steel cross section (run it up and down alternate cables, you would not achieve the required resistance with square metres of steel)
Resistance heating ammount
Voltage
Length
See whether it will add up.
Post by: Peter Dow on 21/02/2020 13:45:47
Painting the Forth Road Bridge is a never ending dangerous, labour-intensive job. I don't want to inflict the same burden on the Queensferry Crossing whose stay cables are supposed to be maintenance-free.Spraying PTFE all over the bridge is a non-starter.They paint bridges...
The PTFE-coating per se doesn't need great strength. The PTFE-coating serves to lubricate the wedges in compression against the glass fibre cloth. The compressive strength is provided by the glass fibres, which are stronger in compression than many grades of steel.The PTFE sheaths to isolate the strands don't add a weak-point but we'll be sure to test your "weak-point" theory in the laboratory, making such adaptations as are necessary to prove that strength is maintained.In the very real sense that PTFE isn't weaker than steel.
Presumably, if PTFE was a perfect solution for anti-icing aircraft then it would have replaced all other methods. "PTFE doesn't work perfectly" is a reasonable deduction for a reasonable person to make.So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.I have never tried it, have you?
I'm guessing that neither of us has, so your claim that "you and I know that..." is not true.
Why say it?
The ice-fall on the Queensferry Crossing which closed the bridge for the best part of two days is the recent experience which says that we do need a good solution.Actually, I've demonstrated that an electrical system of heating is feasible and what for others was too complicated to design in more like 200 years since the first wire cable bridges https://en.wikipedia.org/wiki/Suspension_bridge#Wire-cable is for me something I can do in a week.So, what you are saying is that 200 years of experience says we don't need it.
Time to close the thread thenIf you are bored with this thread it doesn't mean that others are. I'm having a whale of a time with this thread! ;D
Post by: Peter Dow on 21/02/2020 15:09:19
You could fit external heating covers easier than teflon coated strands, they would be easier to engineer around and retrofit, to replace one is going to be a hasstle.Thanks for that suggestion.
But lets say you can run the national grid through the existing cables, each cable in entirity is insulated. Its the only way to make it uncomlicated and reliable enough.Heating with DC means that each of the 55 to 109 strands in each cable will have be electrically isolated from other strands. The strands are already isolated as shown in this cross-section model.
(https://peterdow.files.wordpress.com/2020/02/cablesectionpartlandscapecrop540.jpg)
The only issue with completing the isolation of the cable strands from each other is where they are attached to the anchor head, where the insulation is stripped off and it is metal strand compressed by metal wedges into a metal anchor head housing cone - that's the bit that needs to have insulation added - I suggest by adding Teflon/PTFE-coating glass fibre fabric sheaths, illustrated in this diagram coloured red.
(https://peterdow.files.wordpress.com/2020/02/electrically-isolated-strands.jpg)
Each cable is going to need to be heated to keep its temerature above freezing.0°C should be sufficiently warm for the temperature of the cable surfaces - higher is OK, lower would not be OK.
You will needThe maximum rate of heat loss is a key design objective which determines all the other heating current calculations - I have suggested 318 Watts/m² of stay pipe surface area would be plenty for all eventualities of temperature and amount of water/snow/ice that needs to be heated.
Temperature difference
Rate of loss
Ammount of steel
Ammount of water to be heated
Then you will need :
the steel cross section (run it up and down alternate cables, you would not achieve the required resistance with square metres of steel)
Resistance heating ammount
The strand resistance I have assumed to be 0.001137 ohms per metre, following a calculation I made last year (https://peterdow.files.wordpress.com/2020/02/queensferry-falling-ice-hazard-solution-electrically-heated-cable-stays.pdf).
VoltageThe voltage and power I have quoted per metre and to calculate for each cable I will need the length data for each cable which will be specified in the Queensferry Crossing engineering design plans which I have requested sight of.
..
Length
The strands have quite a variety of lengths and current requirements and whilst all will require to be paired to facilitate current flow in both directions, some of the lowest-power, shortest strands could have up to 3 sets of strand pairs to form one heating circuit.
See whether it will add up.It adds up.
Post by: Bored chemist on 21/02/2020 17:37:05
Painting the Forth Road Bridge is a never ending...No it isn't.
Were you being wrong about the road bridge
https://www.scotsman.com/news-2-15012/forth-road-bridge-to-get-first-paint-job-since-1964-1-4669130
or the rail bridge?
https://www.bbc.co.uk/news/uk-scotland-edinburgh-east-fife-14789036
Thanks for that suggestion.You are welcome
There are a few ways I know of that they use for deicing planes.
Inflatable rubber bits, trace heating
Post by: Bored chemist on 21/02/2020 17:41:15
"PTFE doesn't work perfectly"
Is not the same as
So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.
And pretending it was is a fallacy.
https://en.wikipedia.org/wiki/False_dilemma
Post by: Petrochemicals on 21/02/2020 18:40:44
Loading insulation is a no no. Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.
Post by: Peter Dow on 21/02/2020 21:44:47
"PTFE doesn't work perfectly"
Is not the same as
Quote from: Peter Dow on Yesterday at 22:33:29
So you and I know that covering the ice-accumulating surface of planes with PTFE doesn't work.
Here is a quote which puts some numbers on how good anti-icing chemicals are.
Quote from: Science Daily
Their innovation, described in the Journal of Materials Chemistry, is a gel-based, soft coating made out of PDMS (polydimethylsiloxane), a silicone polymer gel with already widespread industrial use. Their experiments were supported by careful analysis of ice adhesion mechanics.So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.
The performance measure of de-icing coatings is called ice adhesion strength -- the shear stress necessary to remove ice from a surface -- and is measured in kilopascals (kPa). Kota's group demonstrated ice adhesion strength for their coating of about 5 kPa. By contrast, soft coatings available on the market have ice adhesion strength of about 40 kPa (lower is better). Other types of de-icing coatings made of rigid materials like Teflon typically perform at around 100 kPa.
https://www.sciencedaily.com/releases/2016/11/161117150436.htm
But Teflon/PTFE is not the anti-icing agent of choice, for a spray, it seems.
Teflon/PTFE is maybe better suited for a more durable non-stick surface on frying pans and glass fibre fabric?
All those sprays will wear off and have to be reapplied every so often.
Post by: Peter Dow on 21/02/2020 21:54:47
Loading insulation is a no no.Spark plugs are made of aluminum oxide ceramic insulator which is loaded by the compression of the ignited fuel air mixture.
That "loading insulation" is a yes yes if you want to travel anywhere using a petrol engine.
Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.An exterior heated (or non-stick) sheath would have complications of its own - large surface area - 55,000 m².
70,000 metres length of cables with an average diameter of, say, 250mm,
check my sums yourself https://vodoprovod.blogspot.com/p/area-pipe.html
Post by: Petrochemicals on 21/02/2020 23:17:57
compressive loading tension loading, as seen in such things as bridges. Obviously electrically and thermally they work as sparkplugsLoading insulation is a no no.Spark plugs are made of aluminum oxide ceramic insulator which is loaded by the compression of the ignited fuel air mixture.
That "loading insulation" is a yes yes if you want to travel anywhere using a petrol engine.
It will be roughly the same area ? Slighly bigger, but being as you will not be heating the solid mass of the cable, considerably more efficient. Still a technical maintenance and expense nightmare.Incredibly complicated. You just seem to be adding complication to a simple idea of adding an exterior heated sheath.An exterior heated (or non-stick) sheath would have complications of its own - large surface area - 55,000 m².
70,000 metres length of cables with an average diameter of, say, 250mm,
check my sums yourself https://vodoprovod.blogspot.com/p/area-pipe.html
Post by: Bored chemist on 22/02/2020 10:08:51
So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.Then use it on the bridge.
Problem solved.
Post by: Peter Dow on 22/02/2020 11:46:27
So they reckon their PDMS anti-icing agent is 20 times more slippy than Teflon.Then use it on the bridge.
Problem solved.
Quote from: Wikipedia
It is important to understand the science of these coatings before attempting to use this technology:So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.
Instead of using fluorine atoms for repellence like many successful hydrophobic penetrating sealers (not super hydrophobic), superhydrophobic products are a coating—they work by creating a micro- or nano-sized structure on a surface which has super-repellent properties.
These very tiny structures are by their nature very delicate and very easily damaged by wear, cleaning or any sort of friction; if the structure is damaged even slightly it loses its superhydrophobic properties.[citation needed] This technology is based on the microstructure of the hairs of a lily pad which make water just roll off. Rub a lily leaf a little and it will no longer be superhydrophobic. Unlike a lily leaf, which can heal and grow new hairs, a coating will not do this.
As a result, unless advancements can resolve the identified weakness of this technology its applications are limited. It is used mainly in sealed environments which are not exposed to wear or cleaning, such as electronic components (like the inside of smart phones) and air conditioning heat transfer fins, to protect from moisture and prevent corrosion.
https://en.wikipedia.org/wiki/Superhydrophobic_coating#Applications
This new paper suggests mixing the anti-icing liquid with a resin to make a more durable anti-icing coating.
Facile One-Step Method to Fabricate a Slippery Lubricant-Infused Surface (LIS) with Self-Replenishment Properties for Anti-Icing Applications
https://www.mdpi.com/2079-6412/10/2/119/htm
Which would be great if it solved the durability problem, but it is possibly too early for this product to have got to market yet.
It sounds like it is worthy of research but I think it is for someone else to advocate for this anti-icing coating solution.
I will promote my electrical heating solution and we will see which (or both) will find favour with the bridge owners.
Post by: Peter Dow on 22/02/2020 22:47:57
Teflon/PTFE-coated glass fibre fabric sheaths to electrically isolate the strands from the anchor head.
I've modified my OP with the following additional ..
Such sheaths would likely not be available as an off-the-shelf product in the required dimensions, though general purpose PTFE-coated fibre glass cloth is commonly available and this expandable E-glass sleeving, expands from a relaxed internal bore of 15mm to a maximum bore of 38mm and insulates to 500V when not expanded (https://www.sleeveit.co.uk/products/protective-sleeving/vidaflex-xgr/), which is a useful size while relaxed to accommodate the strand and while expanded to accommodate the wedges.
The insulation should cope with the highest DC voltage of about 100 Volts, used to power the longest and highest heating capacity factor strands, albeit that this sleeving is inappropriately resin-coated and would therefore likely require to be custom adapted, the resin cleaned off and re-coated with PTFE, tested and proved in the laboratory. Perhaps wrapping the wedges in PTFE thread seal tape is all that is required to supplement the product as supplied for satisfactory performance? A promising avenue for research.
(https://peterdow.files.wordpress.com/2020/02/expandable-e-glass-sleeving.jpg)
DC Power Supplies
Not forgetting DC power supplies and I have noticed a comprehensive range of 3kW to 10kW DC power supplies here (https://lab-power.com/lab-dc-3-10kw-programmable-power-supply/) that I think will do nicely, an average of about a dozen power supplies per cable (more for the longer cables, fewer for the shorter cables), about 3500 power supplies required to de-ice all 288 cables.
(https://peterdow.files.wordpress.com/2020/02/high-power-lab-dc.jpg)
Where to store the cable power supplies?
Let’s examine the option of storing the cable heating power supplies in the towers, racked next to the anchorages of the cables which they will be heating. There might just be enough room to squeeze in another half a tonne of power supplies for the 4 cables per floor (https://www.youtube.com/watch?v=9HhcELxYK94&feature=youtu.be&t=371) (assuming their racks are securely attached to the tower walls), 12 tonnes worth of power supplies for all 24 floors per tower, for all 3 towers!
Even at 94% efficiency for switch mode power supplies, each tower’s cable power supplies could be generating at most about 0.4 MW of waste heat energy. A new massive extractor fan fitted into the roofs of the towers would be required to cool the inside of the towers while the DC power supplies are heating the cables.
Considering how cramped the insides of the towers are already, the daunting cooling problem, not to mention the risk of a tower fire destroying all of a tower’s power supplies at one time, it looks to be much the better option to install the cable power supplies on the deck, next to the deck anchorages to allow them to be supplied with power.
(https://peterdow.files.wordpress.com/2020/02/queensferry-crossing-on-deck-23-550x365-1.jpg)
The stay cables penetrate the surface of the deck, as can be clearly seen in this next photograph, taken during construction.
(https://peterdow.files.wordpress.com/2020/02/queensferrycrossingstaycablesinsidedeck.jpg)
Therefore best access to the anchor heads, to attach the cable heating power supplies, may be from inside the deck, where the power supplies themselves should be stored too.
...
Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.
Post by: Peter Dow on 25/02/2020 18:06:06
DC Circuit Diagrams
Locating all the electrics at the deck anchorages, while leaving the strands earthed at the tower anchorages, offers advantages for design, development, installation, commissioning and servicing.
(https://peterdow.files.wordpress.com/2020/02/qfc-circuit-diagram.jpg)
Circuit Diagram – 2 heating strands, 1 power supply
(https://peterdow.files.wordpress.com/2020/02/currentbalancedetector.jpg)
Heating strands pair current balance detector
The window detector circuit (https://en.wikipedia.org/wiki/Window_detector) compares the isolated power supply’s potential with respect to earth to detect the expected balance of current and voltage in the heating strands pair. If an imbalance fault develops then the safety switch is used to cut the power.
Post by: Bored chemist on 25/02/2020 19:06:10
So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.If you knew it wouldn't work, why did you bring it up?
Post by: Bored chemist on 25/02/2020 19:07:15
Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.How about the waste heat from the PSUs?
Post by: Peter Dow on 25/02/2020 19:14:24
Did I? I thought I brought up my electrical heating proposal and I was responding to other people's alternative suggestion of anti-icing coating, quoting references to support my responses.So on the bridge, such a coating would presumably quickly be weathered off and need reapplying, which is hard and dangerous enough to do once, but to have to keep doing it, would be a pain.If you knew it wouldn't work, why did you bring it up?
Post by: Peter Dow on 25/02/2020 19:19:20
What preceding part of my very post you were replying to, didn't you understand?Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.How about the waste heat from the PSUs?
I had just explained that the waste heat from the PSUs is too much to store them in the towers, so the PSUs have to go on the deck.
Want to try reading that again?
Where to store the cable power supplies?
Let’s examine the option of storing the cable heating power supplies in the towers, racked next to the anchorages of the cables which they will be heating. There might just be enough room to squeeze in another half a tonne of power supplies for the 4 cables per floor (assuming their racks are securely attached to the tower walls), 12 tonnes worth of power supplies for all 24 floors per tower, for all 3 towers!
Even at 94% efficiency for switch mode power supplies, each tower’s cable power supplies could be generating at most about 0.4 MW of waste heat energy. A new massive extractor fan fitted into the roofs of the towers would be required to cool the inside of the towers while the DC power supplies are heating the cables.
Considering how cramped the insides of the towers are already, the daunting cooling problem, not to mention the risk of a tower fire destroying all of a tower’s power supplies at one time, it looks to be much the better option to install the cable power supplies on the deck, next to the deck anchorages to allow them to be supplied with power.
The stay cables penetrate the surface of the deck, as can be clearly seen in this next photograph, taken during construction.
Therefore best access to the anchor heads, to attach the cable heating power supplies, may be from inside the deck, where the power supplies themselves should be stored too.
Post by: Bored chemist on 25/02/2020 20:22:34
How about the waste heat from some of the PSUs?Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.How about the waste heat from the PSUs?
Post by: Peter Dow on 25/02/2020 22:17:00
Winter-long heat for inside the towers is needed more often, less intensely and when free surplus wind power is available than the fewer times when the PSUs are needed on demand, paying full price electricity to provide a more intense heat for the cables.How about the waste heat from some of the PSUs?Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.How about the waste heat from the PSUs?
The PSUs in the tower anchorages would be high up the tower and can only heat the top of the tower without a lot of forced recirculating of heat.
So trying and failing to use some PSUs to heat the tower would unnecessarily complicate the design to no good purpose.
Post by: Peter Dow on 23/04/2021 13:15:52
I’ve written this spreadsheet to calculate the specifications for the electrical power supplies to heat the strands of stay cables of the Queensferry Crossing (https://peterdow.wordpress.com/2020/02/11/electrically-de-icing-the-queensferry-crossing-cable-stayed-bridge/) or of other bridges prone to icing with similar cables of an arbitrary number of strands up to 109 strands.
This is a screenshot – click to view a high-resolution image (https://peterdow.files.wordpress.com/2021/04/strand-heating-capacity-calculator-screenshot-1080.jpg).
(https://peterdow.files.wordpress.com/2021/04/strand-heating-capacity-calculator-screenshot-960.jpg)
Download QFC Electrically-heated Cables (https://peterdow.files.wordpress.com/2021/04/qfc-electrically-heated-cables_v2.xlsx).
It’s an Excel .xlsx file but it was developed in Google Sheets (for free!). I don’t have Excel on my computer so it has never been tested in Excel. If it doesn’t work on Excel then try uploading it to Google sheets. That should work but any problems let me know
Email: peterdowaberdeen@gm…….
and if necessary I can let you share my Google sheets version with viewer access which I can authorise to any Gmail account and then you can copy it directly into your Google sheets file space.
Instructions
Select a strands sheet with the closest number of strands to the cable you want to calculate for, choosing between 45, 55, 73, 91 or 109 strands. Best to duplicate the sheet for back-ups.
The strands arrangement is represented by coloured cells where the cell formula calculates that strand’s heating capacity.
To re-arrange and / or change the number of strands, first drag off the strand circles overlay image then add strands by selecting an existing strand cell, Edit Copy (ctrl-C) then select another coloured cell for the new strand and Edit – Paste special – Paste formula only. To remove a strand just delete the formula from its coloured cell.
To use the Strands calculator spreadsheet with data for another bridge, you will have to edit or create a new “Cable Data” sheet with the new data and modify the strands calculator sheets’ (hidden) cell X61 so that it fetches the cable data from the new sheet with the appropriate table range.
All my Queensferry Crossing blog posts. (https://peterdow.wordpress.com/category/queensferry-crossing/)
Post by: alancalverd on 23/04/2021 16:57:13
The other thing that bothers me is the idea of replacing cable anchorages with insulators. You need at least to de-stress each cable in turn, which means finding some way of supporting the cable's weight or removing it and recabling from scratch. And then you need an insulator that doesn't creep, and matches the cable thermal expansion. Probably feasible, but is it economic? How many hours a year is the bridge closed for de-icing?
The trick on airplanes is to fit an inflatable rubber boot on the leading edge of the wing. When you have acquired a few millimeters of ice, you inflate the boot and the stuff cracks off. If you do it too soon the ice just accumulates on the inflated surface and the boot pumps up and down in a void beneath the ice - with potentially fatal consequences, so it's a bit of a critical art. But you could encase each cable with a boot and do the same. No problem if you get a void - just close the bridge anyway!
Post by: Bored chemist on 23/04/2021 17:37:55
I’ve written this spreadsheet to calculate the specifications for the electrical power suppliesGlad to know you have found something to keep yourself occupied during lockdown.
I re-glazed my greenhouse.
Post by: Peter Dow on 24/04/2021 00:21:17
The other thing that bothers me is the idea of replacing cable anchorages with insulators. You need at least to de-stress each cable in turn, which means finding some way of supporting the cable's weight or removing it and recabling from scratch.Didn't you notice that these bridge cables are comprised of multiple parallel individually insulated strands?
(https://peterdow.files.wordpress.com/2020/02/queensferrycrossingstaycablesinsidedeck.jpg)
Well those strands are individually attached and can be individually detached for replacement etc.
And then you need an insulator that doesn't creep,Fibre glass.
(https://peterdow.files.wordpress.com/2020/02/expandable-e-glass-sleeving.jpg)
and matches the cable thermal expansion.Fibre glass stretches but it probably needs to be made into a glass fibre reinforced plastic insulating sheath to exclude water ingress defeating the insulation.
One could use a suitably lowish co-efficient of thermal expansion epoxy resin (https://www.masterbond.com/properties/epoxies-low-coefficient-thermal-expansion) to arrive at an insulating sheath with suitable thermal expansion properties to match the steel parts it is insulating.
Probably feasible, but is it economic? How many hours a year is the bridge closed for de-icing?It seems to be averaging at a few days closed per year.
The trick on airplanes is to fit an inflatable rubber boot on the leading edge of the wing. When you have acquired a few millimeters of ice, you inflate the boot and the stuff cracks off. If you do it too soon the ice just accumulates on the inflated surface and the boot pumps up and down in a void beneath the ice - with potentially fatal consequences, so it's a bit of a critical art. But you could encase each cable with a boot and do the same. No problem if you get a void - just close the bridge anyway!It works out at about 25 tonnes of ice per mm over the bridge's 70km of cables. I'm looking for a solution that doesn't require any bridge closures.
Post by: Bored chemist on 24/04/2021 00:30:22
As far as I can tell the set of strands that make up the cable are all covered by the same cladding.
And if that's the case, then it doesn't matter which particular strand(s) you choose to heat.
The overall effect will be the temperature rise of the outside of the cladding will be related to the total power dissipation in the cable. (and the relation will be very close to "proportional to").
So, in that case, what does your spreadsheet tell us?
What model does it give for whatever that white coating is?
Post by: Peter Dow on 24/04/2021 00:31:10
I didn't start programming the spreadsheet until 9 days ago when the Scottish Government agency in charge of the bridge - Transport Scotland - finally agreed to release to me the cable data - lengths, numbers of strands, stay pipe diameter - I had been asking for, for more than a year.I’ve written this spreadsheet to calculate the specifications for the electrical power suppliesGlad to know you have found something to keep yourself occupied during lockdown.
I re-glazed my greenhouse.
If this country is being held back it is not by the competence of our computer scientists.
Post by: Bored chemist on 24/04/2021 00:38:58
I'm also intrigued by the notion that it matters much which wires you heat.A bit of plastic won't matter much- it's not as if we are trying to get things "hot", just keep them above freezing.
Metals conduct heat quite well.
If you heat the ones in the middle, the heat will diffuse to the outside.
If you heat the ones round the perimeter, the heat will diffuse into the core of the cable.
Fundamentally, the only place that (most of) the cable can lose heat is from the surface, so it hardly matters which bits you heat.
And, if a bit of plastic does matter then the really important one is the overall wrapping of the cable.
So, why do you think it matters which of the hundred-and-odd strands we heat?
Post by: Bored chemist on 24/04/2021 00:40:36
I didn't start programming the spreadsheet until 9 days agoThat's OK, I waited until January before I refurbished my greenhouse.
If this country is being held back it is not by the competence of our computer scientists.I wasn't aware that anyone had suggested it might have been.
Post by: Petrochemicals on 24/04/2021 00:57:47
And then you need an insulator that doesn't creep, and matches the cable thermal expansion.You are probably on to something there Alan, as metal gains and releases heat faster, probably due to internal conduction, this problem could probably be solved to an acceptable level with insulation.
Post by: Peter Dow on 24/04/2021 01:00:42
Correct. The set of strands are surrounded by a HDPE "Stay Pipe".bridge.png (376.19 kB . 627x354 - viewed 4436 times)What happens here?
As far as I can tell the set of strands that make up the cable are all covered by the same cladding.
And if that's the case, then it doesn't matter which particular strand(s) you choose to heat.It does matter because you can't heat the inner strands because they will get too hot, the HDPE insulation will melt and then the cables will get damp air about them, rust or suffer hydrogen embrittlement and break and we couldn't have that, could we? ::)
The overall effect will be the temperature rise of the outside of the cladding will be related to the total power dissipation in the cable. (and the relation will be very close to "proportional to").The ideal overall effect will be when the heat flows evenly from the exterior strands that are being heated to heat the stay pipe evenly.
So, in that case, what does your spreadsheet tell us?
It tells you the specification for the electrical power supply required for each heating strand pair, as shown in this image (https://peterdow.files.wordpress.com/2021/04/strand-heating-capacity-calculator-screenshot-1080.jpg) previously helpfully linked to.
This is a screenshot – click to view a high-resolution image. (https://peterdow.files.wordpress.com/2021/04/strand-heating-capacity-calculator-screenshot-1080.jpg)
What model does it give for whatever that white coating is?The heat flow characteristics across the thickness of the HDPE stay pipe can indeed be modelled but the heat transfer is relatively efficient compared to that between the strands across a mostly air gap to the stay pipe. So the temperature of the inside of the stay pipe is only 5 or so degrees hotter than the outside of the stay pipe. It is the much higher temperature of the strand sheaths that you have to watch.
These are complex calculations with multiple factors and mechanisms of heat transfer - conduction, radiation and convention, only some of which I have provisionally calculated so far.
Anyway, I'd want to confirm such calculations experimentally before committing to a certain heating power design - hence the power figures given are nominal and subject to a percentage factor yet to be determined.
(https://peterdow.files.wordpress.com/2020/02/cable-heating-control-50.jpg)
Post by: Peter Dow on 24/04/2021 01:08:10
It's been a while, and you still haven't answered this properly.All that plastic which surrounds the metal strands is not in the least suitable for transferring heat so it matters very much to be careful to moderate the rise in temperature within the cable with the best design possible.I'm also intrigued by the notion that it matters much which wires you heat.A bit of plastic won't matter much- it's not as if we are trying to get things "hot", just keep them above freezing.
Metals conduct heat quite well.
If you heat the ones in the middle, the heat will diffuse to the outside.
If you heat the ones round the perimeter, the heat will diffuse into the core of the cable.
Fundamentally, the only place that (most of) the cable can lose heat is from the surface, so it hardly matters which bits you heat.
And, if a bit of plastic does matter then the really important one is the overall wrapping of the cable.
So, why do you think it matters which of the hundred-and-odd strands we heat?
Post by: Petrochemicals on 24/04/2021 02:27:19
https://www.powerandcables.com/impact-mitigation-of-icing-on-power-network-equipment/
Post by: Bored chemist on 24/04/2021 11:59:49
The heat flow characteristics across the thickness of the HDPE stay pipe can indeed be modelled but the heat transfer is relatively efficient compared to that between the strands across a mostly air gap to the stay pipe.The HDPE is surrounded by a much much bigger air gap.
That's my point.
The biggest thermal resistance is the loss from the cable "as a whole". The temperature gradient within the pipe is quite small.
In any event, we can make a simplifying assumption.
Imagine we just heat the outermost layer of wires.
The inner ones will be less hot than the (the inner side of) outer ones, so they will be OK.
If you can't heat the outer ones to a point where the cable sheds ice without damaging the outermost layer of wires, then the task is impossible anyway, and we can abandon it.
Post by: Peter Dow on 24/04/2021 13:10:47
The biggest thermal resistance is the loss from the cable "as a whole".That really depends on whether it is raining ice cold water onto the cable or not. If it is then the thermal resistance of the cable will be very low. Lots of power and the cable surface temperature won't rise much above freezing.
On the other hand if it is a dry freezing night then the thermal resistance of the cable will be a lot higher - but that's actually a good thing, because it makes it a lot easier to heat the cable to above freezing, with just 10 watts per square metre - check out the "Ambient if dry" line plotted in my "Cable Heating Control" graph.
The temperature gradient within the pipe is quite small.You don't seem to have studied and understood my aforementioned graph, which suggests that the temperature gradient between the strand sheaths and the inside of the stay pipe could be of the order of as much as 60 degrees centigrade!
Take another, longer look.
(https://peterdow.files.wordpress.com/2020/02/cable-heating-control-50.jpg)
In any event, we can make a simplifying assumption.In simple terms, that's my plan. There are a lot of complicated details to do that in the most effective way, but in a nutshell, correct.
Imagine we just heat the outermost layer of wires.
The inner ones will be less hot than the (the inner side of) outer ones, so they will be OK.
If you can't heat the outer ones to a point where the cable sheds ice without damaging the outermost layer of wires,Well my calculations suggest that we can.
Post by: alancalverd on 24/04/2021 13:37:01
Next project: find some way to prevent closures due to high winds!
Post by: Bored chemist on 24/04/2021 13:48:49
There are a lot of complicated details to do that inthe most effectivean unduly complicated way,
Post by: Bored chemist on 24/04/2021 13:50:04
Next project: find some way to prevent closures due to high winds!Put a building round the whole damned thing.
Post by: Peter Dow on 24/04/2021 19:36:46
A simpler idea that doesn't involve dismantling the cables, is to place a heating sheath over the entire cable.Placing anything over the cable presents access difficulties.
We used to wrap nichrome wire around exposed water pipes - it's called "trace heating".I mentioned "trace" "heating" in my OP.
Tower ice
To prevent the bridge piers or towers (with non-conducting concrete surfaces) from icing up, they could be surface fitted with new electrical heating trace cables which are then appropriately electrically-powered for deicing when necessary.
Then Bored Chemist mentioned it
There are a few ways I know of that they use for deicing planes.
Inflatable rubber bits, trace heating and antifreeze sprays.
In my own "Electrically de-icing the Queensferry Crossing cable-stayed bridge (https://peterdow.wordpress.com/2020/02/11/electrically-de-icing-the-queensferry-crossing-cable-stayed-bridge/)" blog post "trace" heating is mentioned in 3 places.
1
Already quoted in my OP.
2
Quote
Trace heating is a low-maintenance industry standard.
Using the bridge’s own cable strands to carry the trace current is innovative but equally low maintenance.
3
Then someone commenting on my blog mentioned it.
Quote
Trace heating is used as I’m sure you are aware, on oil pipelines in extremely cold environments, much colder than even the lowest temperatures we have in Scotland.
Or you can use a plastic boot and blow hot air up the tube.The air will be too hot at one end of the cable and too cold by the time the air flow gets to the middle of the cable - 200 metres away in the case of the longest 400 metre cables.
Wither technique has the advantage that you can also heat the surface of concrete and mass steel structures with it, so yoe can keep the entire bridge free from ice, not just the cables.I am proposing heated air for warming the bridge towers from the inside, but not for the cables.
Quote
Another option to consider is heating the hollow towers from within. However, considering the considerable mass and thickness of the towers, their surfaces would have to be kept above freezing temperature all winter long. Heating the towers from within, there simply wouldn’t be time to allow the towers to get freezing cold because there was no icing then suddenly heat them from the inside to deice a sudden incidence of icing.
So heating from within bridge towers would use more electricity, though the cost shouldn’t be prohibitive – surplus grid electricity is a common occurrence at times of high wind power generation, so the electricity grid managers should offer a very low price for such electricity (just the grid connection charge) – plus it should be a lot safer upgrade from the point of view of bridge users – far less chance of things falling onto the road during the fitting of the towers’ internal heating elements.
Heating the towers may be as simple as a big electric heater on the ground floor, the warm air rising up the insides of the towers, in between the open stairways and scaffolding.
Next project: find some way to prevent closures due to high winds!The Forth Road Bridge (opened 1964) has to close for high winds but the Queensferry Crossing (opened 2017) has wind shields that protect the traffic from high winds and so can cope with high winds much better.
So the Scottish Government's agency - Transport Scotland - have employed Balrick to come up with a cunning plan to divert / revert traffic in all weather conditions. ;)
(https://peterdow.files.wordpress.com/2021/01/qfc-cunning-plan-50.jpg)
Post by: Peter Dow on 24/04/2021 19:45:05
an unduly complicated way,Complexity is duly required to optimise the design to allow for the greatest heating power to be safely used.
A simple design wouldn't be able to heat as powerfully or de-ice as quickly without the danger of melting the strand sheaths and causing damage to the cable.
Post by: Bored chemist on 24/04/2021 20:21:33
Post by: Peter Dow on 24/04/2021 20:50:29
So, the simple design would work OK as long as someone checked the weather forecast.No. Someone "checked the weather forecast" on the Titanic.
Post by: alancalverd on 24/04/2021 20:59:05
Post by: Peter Dow on 24/04/2021 21:27:04
Access is no problem for the trace heating idea. You can use a motorised crawler to walk up the cables, or hoist a bloke up with a rope. Much better than dismantling and rebuilding the anchorages: "if it ain't broke, don't fix it"!The stay cable system is boasted by its manufacturers to be designed so as to individually replace the strand cables, without breaking the anchorage, the cable, or the flow of traffic.
Whereas the stay pipe which covers the cable is just a 6mm to 10mm thick of HDPE pipe - which isn't yet broken - but it isn't designed to take the weight of a "motorised crawler" or a "hoist" plus all the people, tools and parts they would have to lift up there to get the job of fitting the trace heating done.
So the design of crawlers and hoists would not only have to provide access for inspection, which is easy enough, but also to do all the work required, which would be problematic to be sure to not damage the relatively fragile stay pipe.
So if the stay pipe "ain't broke" then don't break it and then have to fix it.
I think there is more chance of the stay pipe getting broken or the crawler breaking down and getting stuck or a bloke falling out of his hoist or some tool or part getting dropped from a great height and breaking something or someone on the roadway below more severely than any lump of ice can do.
Post by: Bored chemist on 24/04/2021 21:38:24
That doesn't make any real sense.So, the simple design would work OK as long as someone checked the weather forecast.No. Someone "checked the weather forecast" on the Titanic.
Post by: Peter Dow on 24/04/2021 21:47:22
Having "someone checking the weather forecast" isn't a foolproof recipe for avoiding dangerous problems with ice, whether that's ice falling from a bridge or an iceberg a liner steams full speed into.That doesn't make any real sense.So, the simple design would work OK as long as someone checked the weather forecast.No. Someone "checked the weather forecast" on the Titanic.
In order to have systems that work and are foolproof there needs to be a lot more complexity than you suggest.
Post by: Bored chemist on 24/04/2021 21:58:57
And what you propose would add even more complexity.Having "someone checking the weather forecast" isn't a foolproof recipe for avoiding dangerous problems with ice, whether that's ice falling from a bridge or an iceberg a liner steams full speed into.That doesn't make any real sense.So, the simple design would work OK as long as someone checked the weather forecast.No. Someone "checked the weather forecast" on the Titanic.
In order to have systems that work and are foolproof there needs to be a lot more complexity than you suggest.
Unless you plan to leave your system heating the wires all summer, it also needs to look at the weather forecast.
Post by: Peter Dow on 24/04/2021 22:30:06
And what you propose would add even more complexity.Sure. Complexity is good.
Unless you plan to leave your system heating the wires all summer, it also needs to look at the weather forecast.What my heating system needs is to have sensors which measure the surface temperature of the cables and to have computer control adjust the heating power according to the measured temperature.
As my oft-posted bar chart and line graph proposes, the cable heating control would adjust the "Heating Power" (pink bars) - Watts of heat energy per metre-squared of stay pipe surface area - according to the Temperature of the Stay Pipe Surface in degrees Celsius.
(https://peterdow.files.wordpress.com/2020/02/cable-heating-control-50.jpg)
So helping you to read that graph, the heating control regime it proposes would be, from left to right of the graph -
- at a temperature of stay pipe surface of -1.4°C, the heating power is the maximum of 318 Watts/metre-squared.
- at a temperature of stay pipe surface of 0°C, the heating power is 150 Watts/metre-squared.
- at a temperature of stay pipe surface of 1°C, the heating power is zero, or off.
Of course there are many other possible control regimes that may serve better in practice than that one.
No doubt you could switch the system off altogether during the summer months.
What you couldn't risk doing is to simply employ a worker with an on-off switch with instructions from management to "check the weather forecast". You need something more complex, more automated, more foolproof than that.
Post by: alancalverd on 25/04/2021 17:01:57
You need something more complex, more automated, more foolproof than that.The word you need is "reliable". There is a complex, automated, foolproof system to prevent the 737MAX from stalling on rotation. So far it has killed around 350 people.
Post by: Bored chemist on 25/04/2021 17:16:44
Sure. Complexity is good.No.
Just , no..
It's also a pity that you didn't understand that I was kidding when I said it should be a person who checked the weather.
People are extremely complicated and dreadfully unreliable. That was part of my point.
My point was that there isn't a need to heat the cables quickly, because the forecast (or even the current temperature and rate of change) will let you turn on the heating in good time. (And you would need to do that, no matter what heating system you used.)
And, since the "big selling point" of your complicated system is that it is quick, -which isn't necessary- your system is,as I said, overly complicated.
Post by: Peter Dow on 25/04/2021 23:29:40
Reliable is good. The UK has an unreliable Prime Minister whose foolish misgovernment of the pandemic so far has killed around 120,000+ British people.You need something more complex, more automated, more foolproof than that.The word you need is "reliable". There is a complex, automated, foolproof system to prevent the 737MAX from stalling on rotation. So far it has killed around 350 people.
(https://peterdow.files.wordpress.com/2021/04/who-killed-more-civilians.jpg)
Post by: Peter Dow on 25/04/2021 23:36:15
Dreadfully, especially BoJo the Clown.Sure. Complexity is good.No.
Just , no..
It's also a pity that you didn't understand that I was kidding when I said it should be a person who checked the weather.
People are extremely complicated and dreadfully unreliable. That was part of my point.
My point was that there isn't a need to heat the cables quickly, because the forecast (or even the current temperature and rate of change) will let you turn on the heating in good time. (And you would need to do that, no matter what heating system you used.)Well for heating the cables whose stay pipe can cool down very quickly, it may be more efficient to be able to heat quickly and powerfully - like the power profile for doing a washing machine cycle - rather than like that for leaving the heating on all day.
And, since the "big selling point" of your complicated system is that it is quick, -which isn't necessary- your system is,as I said, overly complicated.
I don't really see the purpose in dumbing down the design to the simplest you can get away with.
Post by: alancalverd on 25/04/2021 23:45:41
In the early days of manned space flight, NASA spent huge sums designing a pen that could write in zero-g, and freeze-dried food that could be reconstituted with hot water in a pouch. The Russians used pencils and sandwiches.
Post by: Petrochemicals on 26/04/2021 02:21:09
Post by: Bored chemist on 26/04/2021 08:33:11
The UK has an unreliable Prime Minister whose foolish misgovernment of the pandemic so far has killed around 120,000+ British people.That may be the only sensible thing you have said in this thread, but I don't think it's related to the topic.
Post by: Bored chemist on 26/04/2021 08:38:11
In the early days of manned space flight, NASA spent huge sums designing a pen that could write in zero-g, and freeze-dried food that could be reconstituted with hot water in a pouch. The Russians used pencils and sandwiches.https://www.snopes.com/fact-check/the-write-stuff/
Post by: Peter Dow on 26/04/2021 11:10:41
I hate to be a kill joy, but what about a mesh grid sloped above the road, it would reflect the ice and let the wind pass ?A reasonable suggestion but
1. A large part of the reason for building a bridge rather than a tunnel is for the architectural art of it - the form, not just the function. No doubt many would object to a net in that it would "spoil the look of the bridge", especially when that net got dirty with bird droppings, wind-blown garbage - plastic bags etc. There may not be suitable anchor points to attach the net in the correct position and so adding these may be glaring bolt-ons which again may detract from the bridge as art. The ice net would serve as a constant visual reminder not of the wisdom of the Scots as bridge-builders but as a reminder of the error of the Scots in the initial mistake in not designing to prevent ice fall from the beginning. Whereas my solution once completed is pretty much invisible from the outside.
2. How big would be mesh be? Too big and dangerous lumps of ice could fall through, too small and it could ice up, form a ice sheet which would present a wind loading problem.
Post by: Petrochemicals on 26/04/2021 21:08:46
The mesh should not succumb to ice too easily as it is not as high, the wind I suspect has a lot to do with superchilling water and/or the surfaces it lands on. I suppose the angle may also be effectual as water will drip if too shallow angle isI hate to be a kill joy, but what about a mesh grid sloped above the road, it would reflect the ice and let the wind pass ?A reasonable suggestion but
1. A large part of the reason for building a bridge rather than a tunnel is for the architectural art of it - the form, not just the function. No doubt many would object to a net in that it would "spoil the look of the bridge", especially when that net got dirty with bird droppings, wind-blown garbage - plastic bags etc. There may not be suitable anchor points to attach the net in the correct position and so adding these may be glaring bolt-ons which again may detract from the bridge as art. The ice net would serve as a constant visual reminder not of the wisdom of the Scots as bridge-builders but as a reminder of the error of the Scots in the initial mistake in not designing to prevent ice fall from the beginning. Whereas my solution once completed is pretty much invisible from the outside.
2. How big would be mesh be? Too big and dangerous lumps of ice could fall through, too small and it could ice up, form a ice sheet which would present a wind loading problem.
Asked for. The mesh, perhaps in metal, would need to stop the ice falling, reducing its speed, so a certain size ice piece from a height of 60ft would be able to be calculated as to the damage potential it has.
Post by: Peter Dow on 28/04/2021 22:05:55
Or you can use a plastic boot and blow hot air up the tube.(https://peterdow.files.wordpress.com/2020/02/strand-sheath-temperature-vs-heat-power-standard-compact-50.jpg)
FORCED CONVECTION
Other bridges subjected to much icier weather may require more powerful heating so strand overheating may be a greater risk and then in order to enhance the heat transfer from strands to stay pipe, one could force convection in the air space between the strands and stay pipe, by oscillating an air flow up and down the cable and creating air turbulence within the pipe.
I don’t propose to spend any time soon researching exactly how forced convection could be engineered because the Queensferry Crossing strands won’t overheat according to my calculations.
Post by: Petrochemicals on 29/04/2021 03:55:36
Post by: Bored chemist on 29/04/2021 08:50:36
What is the corrosion danger from electrically charging a bridge structure ?Nil, because the cables strands are coated in plastic, and the rest of the bridge isn't connected.
How do they do it for power lines ?Often by ... coating the lines in plastic.
With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.
Post by: Colin2B on 29/04/2021 10:22:46
as well as that, they also use Al for some overhead which doesn't have the same corosion problems as the steel in bridges.How do they do it for power lines ?Often by ... coating the lines in plastic.
With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.
Post by: Bored chemist on 29/04/2021 10:26:49
They generally use a combination of aluminium and steel, which provides its own set of challenges.as well as that, they also use Al for some overhead which doesn't have the same corosion problems as the steel in bridges.How do they do it for power lines ?Often by ... coating the lines in plastic.
With some power lines they use air as the insulator but that's not a problem. To get electrolytic corrosion, bot polarities ( + and - ) must be in the same body of water.
Post by: Peter Dow on 29/04/2021 15:03:00
What is the corrosion danger from electrically charging a bridge structure ?A reasonable concern shared by the Bridge Manager of the Queensferry Crossing, Jason Cheetham who wrote to me stating
Quote
"We would be concerned that running an electrical current through the wires would increase the
possibility of hydrogen being emitted within the sheath, with the potential to lead to broken wires."
HDPE Sheathed Stay Cable Strands
Other than at the anchorages where it is stripped off so the wedges can grip onto the wires, the strands are protected by an HDPE sheath with good electrical insulator properties – more than enough to insulate the cable heating voltages required – a maximum of about 100V for the longest cables.
(https://peterdow.files.wordpress.com/2020/02/hdpe_stay_cable_strands-750.jpg)
No Cathodic Hydrogen Charging
The integrity of the HDPE sheath prevents a corrosion process whereby moisture from the outside air could in theory penetrate any wear-and-tear gaps in the strand’s HDPE sheath to react slowly with the zinc galvanic coating to liberate hydrogen atoms from the water which could threaten to embrittle the steel wires over time with the potential eventually to lead to broken wires.
Likewise, the same integrity of the HDPE sheath would also prevent any possibility of the establishment of a cathodic hydrogen charging circuit (https://www.aimspress.com/fileOther/PDF/Materials/matersci-03-01350.pdf) through non-existing electrolyte-infiltrated gaps in the sheath.
How do they do it for power lines ?
So in normal operation, the heating voltages and currents would be unable to emit hydrogen within the sheath – for the same reason that domestic electrical power cables do not emit hydrogen within their insulating sheaths either.
However, should there ever be a failure of the HDPE sheath integrity and consequential ingress of damp, salty air (or any other electrolyte) which establishes a new fault or leakage circuit then the heating system’s fault or leakage current detection circuits must be able to detect the fault condition, immediately cut the power to avoid any risk of exacerbating mechanisms of hydrogen emission and embrittlement and report the fault condition so as to prompt remedial maintenance, of course.
DC Circuit Diagrams
Locating all the electrics at the deck anchorages, while leaving the strands earthed at the tower anchorages, offers advantages for design, development, installation, commissioning and servicing.
(https://peterdow.files.wordpress.com/2020/11/qfc-circuit-diagram-v.jpg)
(https://peterdow.files.wordpress.com/2020/11/voltagebalancedetector.jpg)
Heating strands pair voltage balance detector
The window detector circuit (https://en.wikipedia.org/wiki/Window_detector) compares the isolated power supply’s potential with respect to earth to detect the expected balance of resistance and voltage in the heating strands pair. If an imbalance fault develops then the safety switch is used to cut the power.
Post by: Peter Dow on 29/04/2021 15:21:50
and the rest of the bridge isn't connected.The rest of the bridge will be earthed and you should be able to see from my circuit diagram that I have connected the earth to all the heating circuits at the tower anchorages.
(https://peterdow.files.wordpress.com/2020/11/qfc-circuit-diagram-v.jpg)
So the rest of the bridge is indeed connected via the earth and that's worth remembering because the power supplies and power lines at the deck anchorages must be kept isolated from the earth for the heating circuits to work properly and for safety reasons. One can't afford to ignore the risk for someone at the deck to touch a power line and touch the bridge earth otherwise the heating voltage may decide it is easier to go through the person than through the cables! Recognising that risk is why I have designed this safety switch which will cut the power if there is any imbalance in the 2 voltages dropped over each of the strand pair which carry the heating current.
(https://peterdow.files.wordpress.com/2020/11/voltagebalancedetector.jpg)