graphite filled seals and fire safe for ball valves

[chinwar]

I had lots of discussion with my suppliers in regard of fire safe and choice of seals materials for ball valves recently. But I found their technical support could not give me a good answer of why graphite filled PTFE has to be used for body seals for fire safe purpose.

I am curently preparing MRs for those ball valves. Because my project is for a pharmaceutical plant, my specification asks all seals used for ball valve must be PTFE and free of carbon or graphite component, and all ball valve must be fire safe to EN ISO 10497 (BS 6755-2). But my suppliers said if we choose virgin PTFE seals, their ball valves can not be fire safe graded.

My understanding is the design of secondary metallic seal shoud be for fire safe purpose. For fire-testing temperature up to 980 DegC, no seals materials can stand for.

Appreciated someone can explain what is fire safe herein and why we have to use graphite for fire safe?

 

[BSQUAREDBUCKEYE]

Chinwar,

I'm not an expert in this field but "fire safe" can be a tricky thing. Some companies say their valve are "built to" fire safe standards, while others say they are "tested to" fire safe standards. Be sure to ask for a copy of their fire test report to qualify the difference.

Fire safe ball valve can have virgin Teflon seats and still pass a fire test. The graphite used in the body seal and usally on top of the lower stem bearing are actually "insulators" to try and prolong the vaporizing of the Teflon. Anything left in these areas of the valve will help reduce the leakage to atmosphere when pressurized during the test.

 

[chinwar]

BSQUAREDBUCKEYE:
Thanks for your reply and it is really help me a lot.

So it should be clear to say that 'fire safe' means leakage during fire-testing is not exceed what codes require. (While one supplier told me fire safe means valves still can be closed functionally after or during fire).

 

[NGiLuzzu]

chinwar,
I think that your Suppliers are right and, probably, the "fire safe" requirement for a pharmaceutical plant does not make much sense. In fact, the usual reference standards (see, for example, API 607 and the many threads within this forum about this issue...) have been developed for the oil and gas (and in particular refinery) applications.

There is no PTFE grade able to withstand temperatures over +250°C whitout degrading (into dangerous compounds, maybe...).
Graphite instead may be used up to +450°C (and even above, in ambients without oxydant compounds) and also after a fire may mantain a consistency sufficient to be compressed and give tightness. In addition, graphite is more often coupled or reinforced with metal (think to seating elements such as spiral wound gaskets and "lamellar" seal rings...).

Hope this helps, 'NGL

 

[BSQUAREDBUCKEYE]

Chinwar,

I suspect your project is occuring in the "ethical chemical" side of the pharmaceutial plant. Otherwise you would be talking about electro-polishing and probably not ball valves.

The whole intent of fire safe valves is to limit feeding the fire in the event of one. With a closer inspection you may find that the graphite used in the lower stem bearing area, and in the body seal area is for the most part encapsulated and not likely to enter the process stream in great quantity like the seat material.

Since you are trying to limit carbon or graphite from the process, graphite seats may not be a good idea. Please be aware that if considering metal seats there typically has to be a lubristic coating like chrome. This may not be acceptable in the process either.

 

[Sircrashalot]

For all of the ball valves that I have used for offshore oil & gas, the primary process seals have always been a soft material such as PTFE for seats and Viton or HNBR or PTFE for the body and stem seals. However, to obtain their fire safe rating (tested in all my cases), the valves have all had graphite or graphite/316 sprial wound fire safe secondary seals. Except for the seals on the floating seats of trunnion mounted valves, these only see process fluid if the primary seal fails (hopefully only because of a fire). As long as your primary process seal does not have any graphite, you should be OK to have a second back up fire seal.

Andy

 

[Thevalveman]

Chinwar,

Fire-safe testing of ball valves involves two main tests, integral test (to proof the body,trunnion,stem seals) and a function test ( to proof the ability of the valve to close and seal) in the event there is a fire.

The primary seals (elstomers) of the valve can be materials of such to meet your process requirements, the secondary seal or fire safe seal must meet API 607or your applicable code that is to say, will not burn or leak for a specific time frame. These seals are usually not in the process area of a valve.

As far as the seat in the valve, they also can be materials of such to meet your process requirements, by using various fillers in the PTFE seats it's only added to increase stability and reduce friction (prevent or reduce cold flow, reduce operating torques) of the seat under process conditions. When the valve is subjected to the high heat or a fire, the seat material melts and the ball when closed seals on the fire safe seating area of the seat

 

[chinwar]

Thanks all!

I read BS fire testing code and find the testing temperature is 980 DegC holding for 30 minutes. If such, I think all seals have burnt out without doubts.

Recently I had a discussion with the client. They said failure due to ball seats are higher to 20% to 40% (I am not going to suspect my client are not using the best suppliers in the world since it could be the biggest one.), so it looks that it is not a good choice to use graphite filled PTEFE for body seals.

If we talking about 980 DegC testing temperature, I do think PTFE or graphite filled PTFE can give us much difference. So my understanding is that body design instead of choise of seals is the key to commit to fire safe. We can find all soft sealed ball valves have a secondary metallic seal for fire safe purpose.

But anyway, it looks I can not get any more info from my vendors since they all keep quiet or just give you some other explanation about fire safe. They just insist on that you have to go for graphite if you want fire safe.