Hello. I want to ask about the yellow-green cable that is placed between two pipes which are flange-connected. Why do we connect the external surface of the pipes with cable? Maybe to avoid the electrical corrosion or to ensure the same potential between the pipes?
More than likely the cable is there for minimization of static electricity effects.
William(Bill) I. Mostia, Jr. P.E.
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These opinions are my own and are offered on the basis of Caveat Emptor.
This is often done in hazardous areas to reduce the risk of static charges building up on isolated pieces of metal.
You can lose electrical continuity without a bolt being loose. However, I tend to agree that with as many bolts being in the bolt circle that its almost certain there is good continuity, especially where SS piping and bolts are uses. Usually where I have seen the jumper used it is fastened to the flanges itself by drilling and tapping the edge of the flange so the jumper is completely independant of the bolts holding the piping together.
To assure electrical continuity - because the electrical continuity across the flange cannot be guaranteed
We have a similar installation on our hydrocarbon lines within our petroleum product pipeline pump-station. We had an argument regarding this subject recently. I wished to differ on precisely this point. Why should a steel flange connected by 10 or more steel fasteners with a spirally wound metallic gasket in between not be able to guarantee electrical continuity?
Could somebody enlighten me about the same?
Because it has 10 bolts which can be loose and who knows when someone might sub in a non-conductive gasket. And unless the bolts are really tight (gas tight) they can corrode. The jumper assures that you will have continuity and you won't get sparks at the flange if they are seperated. Anyone who has an old car can tell you how reliable steel connections are. Great when fresh, but 10 years from now? Of course, copper to steel has its own set of problems.
The other issue of course is that its something visible that can be inspected externally. You can't inspect the electrical continuity of the metallic gasket or fasteners without disassembling it.
Dear Bob and Curt,
Thanks for the replies. But still my point is that with a pipeline rated at 100 kgf/cm2 pressure, with the line being always pressurized at a minimum of 40 kgf/cm2, loose bolts would be evident very fast. Moreover other types of gaskets are not used due to pressure related problems.
It's always good practice to hedge your bets. And with some workloads the method of finding that your joint is leaking and that it's sparking is because the area no longer exists. What you say is fine when it works. The wire is there for when it doesn't
The flange and bolts will have an oxide layer and be poor conductors on their surfaces. Low voltage and low current contacts need to be specially designed for the task. This is even true for relay contacts - which is why most relays have a minimum as well as maximum current rating.
The ground cable connection can be designed to act as a ground connection, while the flanges and bolts can be designed to hold the pipe together.
London, Ont. Canada
To the original poster and others:
The wire is there to insure a solid ground connection between two separate metallic components. Just as an enclosure door should have a grounding bond wire from the main body of the enclosure to the door, yes there a metal hinge pin there, but what happens after is has sat outside for a while??? Sure you can measure that the two are grounded with your volt meter, but a little oxidation goes a long ways in reducing the ability to keep these two items at the same potential.
Moving fluids, gases and such do cause the build up of static charges, a little corrosion, some oxidation, a small leak and things go boom.
I have seen tanks of natural gas that was being blown off without a ground strap to solid earth ground cause flash ignition of plumes. The ignition source was a static discharge spark.
I have also seen these bonding straps on water pipes, geez no explosion potential there, but oxidation, corrision and galvonic actions are still at work.
The bottom line, if the bonding wire is installed, inspect it, keep it clean and in good repair. If you take the flange apart to replace the gasket, put a new one back on... Metal to metal, if required or allowed use a little graphite grease to prevent the bare metal surfaces from oxidizing or corroding. You can paint over the bonding points in most cases, sometimes you can not. Building, electrical and fire ispectors and industry codes / rules will dictate what is to be done and what methods are acceptable.
Oh, and so there is no doubt, there is no such thing as a perfectly leak free seal. All seals leak, it is just a question of how much.