Overpressure Protection of Valve Actuators on Instrument Air Systems 1

[Fac_Eng]

Hi all,

My plants instrument air header is designed for 150 psi and operates at around 100 psi. The IA is then typically let down to 50 psi or so by a small regulator when used for valve actuators or pneumatically driven pumps. I assume that if you connect a typical control valve diaphragm actuator like a Fisher 667 to pressure source and slowly increase the pressure, at some pressure it is going to fail i.e. it has a maximum pressure rating. But in googling and reading product datasheets, I do not see the "MAWP" of a diaphragm actuator discussed anywhere.

I'd like to understand why we don't include PSVs on the downstream side of instrument air regulators that feed into things like valve actuators. If I don't know the MAWP of the end device how can I know that it cant be over pressured? I don't think I have ever seen a PSV installed between the letdown regulator and an end device so clearly I am missing something.

Thanks!

 

[LittleInch]

Did you look for this ?

https://www.emerson.com/documents/a...actuator-sizes-30-30i-76-76i-87-en-122368.pdf

Table 1 gives you maximum pressures no?



I've seen small pressure relief valves between regulators and actuators

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Fac_Eng]

Well shoot, I glossed over that line in Table 1 apparently. I have not seen PSVs between regulators and actuators, but as you can probably tell from my initial question there are many things I have not seen. That gives me enough information to sort out my issue though. Thanks!

 

[bimr]

If you are considering a PSV, I believe that calls for at least an informal Hazop since you will have to note the change on your P&ID. Your air receiver tank should also be protected with a PSV. The changes should undergo an informal Hazop review.

Do you need the air regulator or can the pressure be reduced? The moment you have a regulator in a pneumatic system you will have to consider:

1) what happens if the regulator fails (open) and;

2) what happens if a technician will try to fix a problem by adjusting the correct set value (provided it was documented anywhere and readible from a reliable gauge)?

Maintaining the regulator costs money as well. Creating too much air pressure costs money. My personal opinion: The best regulator is no regulator.

 

[Tuckabag]

Why would you NOT run a PSV/s on your IA lines where the supply pressure is higher than the MAWP of any equipment connected to it?
IA will generally serve a number of different processes all requiring different MAWP's so regulators and associated PSVs are really the most economical way to do it.
As bimr mentioned, you will want to size the regulator for full flow of the regulator capacity at fail wide open condition.
Maintenance is a reality (for all equipment), but if you have some form of filtration/conditioning you should many years of reliable service.