Hazardous area & static electricity - polypropylene tank

[ScottyUK]

I'm looking at a design drawing for an ATEX zone 2 hazardous area release point on a polypropylene tank and a question has been raised about static as an ignition source. Polypropylene is one of the worse materials for building up static. The material is not carbon-loaded and is therefore insulating. Surface coatings are unlikely to be effective because they won't adhere to polypropylene. There's a motor-driven agitator on the vessel top which is earthed to the main earth bar.

The vessel content is a horrible brew, but for argument's sake lets call it diesel fuel as this is comparable in flashpoint. Any thoughts on whether this is a problem and if so some ideas on solving it would be welcome.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Wild ass suggestions:

1) Ground probe in the fluid.

2) Wrap the tank in wire mesh, fencing, or screen.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

I have some experience with Benzene, another liquid that has a static problem. The tank fill pipe is metal extending to near the bottom with a short piece of chain that touches bottom.
A hole is drilled through the fill pipe near the top of tank so it breaks any syphon.
In another case where liquid flows through glass lined pipe at every flange joint a grounding ring is installed.
Roy

 

[danw2]

These folks seem to specialize in grounding to eliminate static:

http://www.newson-gale.com/OurProducts/EarthRite.asp

 

[ScottyUK]

Thanks for the ideas guys - the company in the link looks worth a phone call.


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If we learn from our mistakes I'm getting a great education!

 

[roydm]

Scotty,
I think the Earthrite is just a device for checking that a tanker truck ground wire is connected.
As I said don't just let the product fall into the tank, run it to the bottom.
Have a grounding ring or use metal pipe for the fill connection.
If your fill connection is non metalic figure out a way to insert a ground rod inside it.
Roy

 

[ScottyUK]

Taking a step back, is there any realistic possibility of static being capable of igniting diesel vapour? If it were full of petrol (gasoline) I'd be concerned but diesel?


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If we learn from our mistakes I'm getting a great education!

 

[zappedagain]

IEC 60079-0 has the standards for non-metallic materials in hazardous environments. They have a list of seven ways that your non-metallic part can meet the requirements to be used in a hazardous location:

1. Surface resistivity less than 10^9 ohm
2. (tiny) surface area
3. bonding
4. minimal charge transfer
5. minimal capacitance
6. conductive coating
7. documentation for fixed locations

There are lots of details and fine print regarding all of these.


On your last question regarding the spark potential, as you dig into the standard definition you might find you'll meet the requirements for diesel. Although I don't see diesel fuel called out in the standard tables, so that may get back to your question of is this valid?

This link may be useful:

http://www.efsec.wa.gov/oplarchive/oplpft/Cca/jpw-2a.pdf

John D

 

[ScottyUK]

Thanks John,

I've been throught the nightmare that is BS EN 60079 - a series of documents surely written by lawyers - and options 1-6 are all infeasible in this instance. The tank is pre-existing, about 6' diameter and the surface resistivity is going to be 'high'. Which leaves 7, so I can defeat the static buildup with a sea of paper. The standard doesn't allow me to take credit for the film of conductive crud which the steelworks over the river will undoubtedly deposit onto the oily surface film. Frankly I could fill an extinguisher with the tank contents and use it to fight fires, but it's not so easy to prove definitively.


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If we learn from our mistakes I'm getting a great education!