Explosive Atmospheres - Static - incendive discharge - hydrogen - pipe earthing and bonding

[Doove117]

Having some differences of opinion with a client and a supplier (...I can hear you grinning from here...)

IEC 60079-14 paragraph 6.4
In the design of electrical systems, steps shall be taken to reduce to a safe level the effects of static electricity.

I have 5% HCl acid line (PTFE lined stainless pipe, exterior 400 microns painted) in a zone 2 area. Zoned due to hydrogen producing electrolyers nearby.
Exterior marine environment.

PTFE liner isolates from conductive liquid. All equipment ICEEx rated.

Do I need bonding across pipe flanges, specifically to reduce risk of incendive discharge from static build up? The client is suggesting wind blowing over the 2" pipe spools will accumulate static charge on painted exterior surface.

I note, nothing has been mentioned of static present on operators discharging against earthed pipe and structure. Nor have we been asked to make entire surface of painted pipe conductive, just provide bonding. Nor has anything been mentioned about the whole steel structure being painted in same paint system, presumably also non-conductive and exposed to same wind. Nor has anything been mentioned about the huge non-conductive GRP tanks adjacent.

To be fair any hydrogen escaping from the elctrolysers about 3m away is going to go straight up and not hang about. I don't really know why the area is zone 2, but that is not my area and I am not challenging that at all.

I have been referred to IEC 60079-14 para 6.4 (which I have quoted above) which is not really helpful. And also IEC 60092-502 sub-section 5.5 which I don't yet have and am waiting for the office to receive.

Anyone else come up against this? I can't see how bonding the pipe across flanges gets rid of static in middle of pipe spool. Surely all the spool needs to be conductive? And if all pipe spools surfaces then all painted steelwork surfaces? i.e. the paint system should be conductive?

 

[Compositepro]

Grounding across flanges has to do with flow through the pipe and electrically connecting the vessels at both ends of the pipe. It doesn't have any thing to do with what is outside the pipe that I am aware of.

 

[RoboCop16]

My understanding is that it is correct that wind blowing past a pipe can cause static electricity, in particular if the wind carries dust or water particles. Another mechanism that can create high potentials between pipe and structure - and hence a risk of sparks - is thunderstorms,

Static electricity wouldn't normally be able to build in a pipe carrying conductive liquid, but in this case the liquid is insulated from the pipe by the liner.

In the industry where I work, upstream oil and gas, it is not common to ground pipe or to bond flanges, whatever the media in the pipe. Presumably because the flanges bolts and metallic spiral wound gaskets are supposed to give enough conductivity.

But may it is anyway prudent to ground the pipe back to the structure - say at 20 meter intervals - and make sure that there is electric conductivity across flanges by bonding them. The bonding and earthing has to be metal-to-metal, not to painted surfaces, to ensure conductivity. The idea is to ensure that all parts of the piping system and supporting structure has the same electric potential, so that there can be no sparks. The electric charge is conducted away through the metallic pipe, also from the middle of the spool.

See also these threads:

http://www.eng-tips.com/viewthread.cfm?qid=302126

http://www.control.com/thread/1026194337

 

[Doove117]

Thanks for the responses.
Terp16, the thread links were also useful, I have emailed Erico to ask their advice.

I have started to organize some of the info in my head as follows:
Reasons for bonding / earthing
1. electrical safety - if a powered device faults or cable ruptures and causes connected pipe to become live (assuming pipe is conductive); also important for any non-current carrying metallic structure (like pipe support, or machinery base etc). This is the primary reason in my mind.

2. Static Discharge in Hazardous Zones - By equi-potential bonding we stop any sparking between near proximate surfaces including those of valve disks to pipe, or flanges being separated, people touching the pipe.
Should you then have an earth connected to the equi-potential plane.
It is important to note that the surface must be conductive.
It is also interesting to note that non-conductive fluids (such as fuel oils) can build up static charge, particularly when filling tanks, passing through valves and filters or going through mixers. There are many recorded incidents of static discharge igniting fuel vapours, causing explosions or deflagrations during fuel tank filling and emptying. All such procedures mandate the use of equipment that is bonded together and earthed.
If pipework is lined then any static in the fluid cannot be earthed through the pipe so conductive earth rings should be installed in the line.
It is also interesting to realize if an operator accumulates a static charge then touches an surface of different potential or bonded to earth then the charge on them will also spark as it discharges to earth. Think of when you get out of your car and get a shock as the static accumulated from your clothes rubbing on the car seat earths through the car body. (although if the operator is at same potential then no spark).

Lloyds Register note that there must be a maximum of 1 MOhm in the path between non-current carrying structures and the ship's hull, for control of static. However I think this considers the required voltage for a fuel gas incendive spark i.e. a spark with sufficient energy to ignite fuel gas.

As I have hydrogen potentially knocking about I must consider the energy required to ignite hydrogen is an order of magnitude lower, i.e. I can't afford much voltage.

So - how do I quantify the maximum charge that accumulates on the pipe due to wind or to thunderstorms?
Assuming I am right about the bonding not actually dissipating static charge except for the static immediately around the bond strap, does this concern extend to the painted surfaces of the steel structure?
The location is offshore, well above sea-level (approx. 70m aMSL).

Only other thing is that the hydrogen won't really collect in this area as hydrogen is very light and rises about 6 times faster than helium. It's not going to hang about on an open deck.