8 replies to this topic

[JEBradley]

Im just doing some work for a client who have requested we use diaphragm valve extensively across their plant. The reasoning behind this is that they can be used as a pressure relief device to protect against hydraulic pressure inside lines.
Anyone got any thoughts about this - are any codes being broken???

[pleckner]

Pipe is governed by ASME B31.x and NOT ASME Section VIII so the rules are different. But you still can't exceed pipe design pressures so if there is a way of over pressurizing a pipe, it must be protected with a ASME Section VIII certified PSV. For process piping, see ASME B31.3.

For emergency over pressure protection, I would not want to use standard diaphragm valves.

[Art Montemayor]

JE:

If what you mean by a “diaphragm valve” is a type that I am very familiar with (the “Saunders” type of valve), then I think Phil and I have different interpretations of your query.

The Saunders type of diaphragm valve was a unique development that I believe originated in the UK by the Saunders company; later, it was licensed in the USA to such valve manufacturers as Hills-McCanna. I think this started in the 1940’s or ‘50s. Regardless of their origin, these valves are unique because they are, in my recollection, the original “soft-seat” technology in block valves. And that is what they were developed to do: establish a 100% block in a line that was near-bubble tight. They were certainly not meant to throttle a stream and have never been intended to do that. For example, I’ve yet to hear of a process control valve (not an air solenoid valve) that had the body of a diaphragm valve. The Cv for these types of valves must be something that is very difficult to predict; I’ve never seen a Cv for these valves published – nor have I heard of one. Additionally, these valves are relatively very heat sensitive and can’t take temperatures in excess of 150 – 200 ^oF without suffering diaphragm decay or degradation. I’ve used them extensively – but only in relatively low pressure and ambient temperature environments. They work great in clean or “mushy” fluids – even liquids with solids in them. But I would never rely on them as safety devices – mainly because of the temperature limits and the inability to predict the Cv. Besides, I doubt that there are PSVs made in the style of a diaphragm. I realize that you could take a diaphragm solenoid valve and employ it as a safety device – but that is not a PSV. Am I on the right wave length in trying to understand your query?

I don’t know where your application is; I suspect it is in the UK. If so, I am unfamiliar with what codes or local ordinances exist for PSVs and their applications. I know you have regulations very similar to the USA.

The great majority of diaphragm valves being used in the USA are of the plastic variety – a lot of PVC and other plastics meant for low pressures. They are used in conjunction with plastic pipe. I used a lot of cast iron body Saunders and these are rated for a higher pressure – 75 to 100 psig I believe. Can you help us by clearing up the questions and the assumptions I have made? Just exactly what do you mean by the phrase “used as a pressure relief device”?

Await your reply.

[JoeWong]

JE,

...client who have requested we use diaphragm valve extensively across their plant. The reasoning behind this is that they can be used as a pressure relief device to protect against hydraulic pressure inside lines.

Are you referring to normal pressure regulation, overpressure protection OR surge pressure ?

i) If pressure regulation...yes.

ii) If overpressure protection...
one of the critical issue would be is the diaphragm valve certified as ultimate overpressure protection device by pressure code where the installation located e.g. ASME B13.3 in US, BS5500 in UK, GB150 in CHINA, etc ?
If not, i really think the authority would have problem to authorize the operation of the system...

iii) If surge pressure protection...the response may not fast enough...


Thank to Mr. Montemayor. I have made some net search...Some findings :
1) SAUNDER actuated diaphragm valve
2) Guide to valves type & Comparison between diaphragm,ball, butterfly, globe, gate, plug
3) CV for different type of SAUNDER diaphragm valves

• A TYPE - FLOW COEFFICIENTS DN250 AND ABOVE
• A TYPE FLOW COEFFICIENTS UP TO DN200
• K TYPE FLOW COEFFICIENTS
• KB TYPE - FLOW COEFFICIENTS DN250 & ABOVE
• KB TYPE - FLOW COEFFICIENTS UP TO DN200

4) Gas Sizing and Liquid Sizing


JoeWong

[pleckner]

No Art, I think we are thinking of the same thing.

There are no ASME certified diaphragm PSVs.

I wouldn't use one of these for pressure reduction either.

[JEBradley]

Ok first of all I should confirm it is the saunders type diaphragm valve.

The whole question arose during a HAZOP. We were looking at the 'more pressure' possibilities in a line pumping a fluid (nonafluorobutane). It was decided more pressure could occur if the stretch of line was valved off and the liquid then allowed to warm up - thermal expansion causing a hydraulic pressure that would rupture the line. I was surprised to find this had happened a number of times in the past.

To militate against this problem it was decided to replace the ball valves with diaphragm valves. The diaphragm would represent the weak point in the line and would lift, allowing enough liquid through the valve body to lower the pressure.

I have numerous issues with this - first I dont think the relief pressure of the valve can be guaranteed (insofar as a manufacturer wont give us certification) so it shouldnt be used as a safety device. Therefore it's use as a mitigant in the HAZOP was invalid.

Secondly - no thought was made to the downstream relief line. It would just blow out of the valve (creating another hazard).

Im a junior engineer and feel a bit awkward telling the client they're wrong The question was really if anyone knew of any legislation or guidance on how these things should be used. Perhaps someone could proposed an alternate method??

Thanks for the help guys.

[Art Montemayor]

JE:

Thanks for the detailed explanation. This puts a new light on the subject for me. My comment is: Don’t let the others have their way. They are creating a hazard, rather than curing it. I think both Joe and Phil will agree with me on this one.

First of all, do not allow yourself to be intimidated by the client. You are there to lend credible, professional, and logical engineering advice. I sincerely believe that you owe the client the truth about the erroneous belief that the Saunders valve will form a sort of predictable rupture device. It certainly will not do that. If you are familiar with the internals of a Saunders valve, you can understand what I mean. The diaphragm is positively held down by the screw mechanism on the valve stem that is imposing pressure on the diaphragm which, in turn, is held against the valve’s seat. This type of mechanism is not suited for a predicted failure.

Additionally, the use of a ball valve in a line where you have a possibility of thermal expansion in the liquid fluid is another problem. Unless you have the ball drilled towards the upstream side, you will cause thermal expansion and possible valve seat failure in a ball valve when the hydraulic pressure starts to increase inside the shut-off ball valve. Ball valves in this type of service should be specified as having a drilled relief hole in the ball, facing the upstsream side.

Perhaps if you sit down with a representative of the client and explain your concerns – based on facts and logic – you can arrive at a safe and reasonable resolution. I would certainly give it try. The client deserves your best service since that is what you are being paid to do – whether the client likes the answer or not.

Good Luck.

[pleckner]

I totally agree with Art.

Review ASME B31.3 paragraph 301.2.2 and Part 6, Systems, Section 322. I think you will find that you need to use an ASME Section VIII certified PSV.

[JoeWong]

Don’t let the others have their way. They are creating a hazard, rather than curing it. I think both Joe and Phil will agree with me on this one.

Yeap..i am in agreement with Mr. Montemayor.

Never tolerate on uncertified methodology & devices when you are dealing with SAFETY.

JoeWong