2 PSV's or 1 PSV

[MechEngr2007]

hi everybody
i want to know is there any specific criteria to have two PSV other than one in the industry. or it depends on the criticality. because my contractor is providing 2 PSV for each releifing system.

 

[BigInch]

There is no code requirement to have two relief valves. It should be decided based on the estimated reliability of the valve, the required frequency for maintenance and testing, the criticality of the process and the magnitude of possible consequencial damages from excessive pressure due to the valve's failure to operate.

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[terje61]

Besides the redundancy that Biginch mentions, it might be possible that the capacity of one relief valve is not sufficient and your contractor has decided to install two small PSV's instead of 1 large relief valve.

Gr.
Terje

 

[JimCasey]

The ASME VIII code allows multiple valves with staggered set pressures. Accumulation of 16% is allowed instead of the normal 10% allowed with a single valve. If there is just a small upset, possibly only one valve will pop. If there is a progressively greater upset, more valves pop until the relief capacity meets the flow of the upset. If there was only as small overpressure flow and a single valve, the valve would only operate at a fraction of its capacity. This could cause chatter and the valve would leak after the event. So you would have more reliability because of both redundancy and because of matching the capacity of specific events. And two small valves are less money than an equivalent large valve usually, so the multivalve installation is more economical.

 

[MechEngr2007]

yes also it depends on the facility O&M philosiphy.
thanks all.

 

[dcasto]

On some storage tanks they install 2 PSV's as a 100% spare so you never have to take the tank out of service to repair or inspect the valves. (old school stuff)

 

[MortenA]

Most of the times where i recommend two valves it is as biginch says a question of operability. The PSV must be tested on a regular basis and even with a warehouse spare replacing the valve would require a shut down and isolation of the unit in question (or possibly the whole plant).

With tandem valves you may take one out and still continue to operate. But if you only have a batch service or for any other reasons shuts down planned then one could be justified.

Best regards

Morten

 

[Newmanite]

To have a secondary relief valve in a circuit is a great idea it can be set higher than your main relief and set closer to the load so the pressure shock wave doesnt travel all the way back to the main relief of the circuit having the ability to burst lines etc.
They can be set so the liquid can go straight to the low pressure side of a cylinder or motor.Or vent straight to atmosphere for air systems