should the diaphragm in spring-diaphragm actuator always be designed for maximum air supply pressure

[undo]

the maximum air supply pressure is 120 PSIG. the maximum pressure the diaphragm can withstand is 87 psig. The vendor proposed a relief valve in the pneumatic circuit to limit the pressure to 80 psig.
Is this a good engineering practice? or should I stick to my initial requirement in case of relief valve failure?

 

[hydtools]

Is there a good reason you can not get what you want? Will the other parts of the valve tolerate 120 psi?

Ted

 

[MikeHalloran]

A regulator would be a good choice for limiting the local supply pressure to protect the diaphragm. A relief valve in the line direct to the diaphragm would be insurance against the regulator failing open.

Diaphragms suffer shortened life when use near their limiting pressure.
If the local supply pressure can be reduced below 80 psi and still have the actuator work satisfactorily, that would be even better for diaphragm life. Of course the actuator will exert less force and operate more slowly; whether that is acceptable is up to you.

Simply specifying a diaphragm suitable for your full supply pressure may not solve the problem; such diaphragms may not exist.

This would be a good time for the actuator supplier's application engineer to get involved in helping you; call and ask for one.





Mike Halloran
Pembroke Pines, FL, USA

 

[PeterIgg]

I would concur with Mike Halloran's comments. I would comment further:

1. Presumably the system is designed to operate with only 80 psi. Charging the dome with 120 psi merely increases your response time, 40 psi shall have to be vented before you see any change in position of the valve being controlled. Once the valve is open/closed, increasing the pressure to the actuator does not make it more so!

2. With a regulator and associated relief valve in the circuit, what are the chances of a concurrent failure in both units if the system is well maintained? Minimal in my experience.

 

[mk3223]

Something to be considered in your air supply line operation. Typically, the pressure of 120 psig is at main header of the air supply, which is reduced when it enters to the specific process unit with the pressure regulators. The air supply pressure for instrument and actuators are operated normally at 40 psig and 80 psig max.

 

[hydtools]

Even though normal operating pressure may be 40 or 80 psi, there is nothing wrong having the actuator capable of handling 120 psi without failure.

Ted

 

[gerhardl]

Almost all is said above. Seen from an European (and practical) side 6 bar (very near 87 psi) is a normal air supply pressure often used for air supply for valve-actuators and instrumentation. A lot of actuators is designed operated at this pressure, and will require, say, 4-6 bars for a normal valve operation.

On the other hand a normal EU class for valves is 10 bar (150 psig), and the whole valve, including actuator and membrane is very often designed for this, but not always.

The choice for a limitation of the membrane (allowed) operational pressure may ether be practical, based on engineering/design reasons, or based on price. What does the vendor say? Size, experience and availability of material might also be factors here.

Again: you will not necessarily get a poorer product with a lower expected life-time by choosing a membrane with higher allowable working pressure, you might need a less elastic material with different qualities.