External PSV required when Internal available

[controlnovice]

We have a PD pump with internal PSV. Relieves from discharge back to suction. Set at 125#.

Is an external PSV required on the discharge of the pump? Process engineers are saying that if the internal PSV opens up for a period of time, that temperature will build up and it needs to be relieved, requiring an external PSV. External PSV would be piped back to the tank.

I agree that temperature may build up, but what will the additional PSV do that the internal already isn't doing?


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This is normally the space where people post something insightful.

 

[SNORGY]

Depending on rates, temperatures and pressures involved, it may (or may not) take a substantial period of time to add enough heat to the recycled fluid to damage the pump (or piping). If the recycle via the PSV is 100% of the flow, such as with a closed discharge valve, it's more of a concern, but I would question why operation in that mode would continue for any extended period of time. Conversely, if the recycled flow is less than 100%, such as with discharge throttling, then you might already be rejecting enough heat to effectively limit the temperature rise.

Regards,

SNORGY.

 

[psafety]

Years ago we did some research and at that time I think Viking was the only manufacturer that relieved 100%. Others relieved only a portion of pumped flow, we did have a problem with an internal relief. I honestly do NOT know where it stands today. Check with the pump manufacturer to get it right.

 

[psafety]

The external PSV is always better "if" you can relieve back to the supply vessel. If only to the suction you only gain a degree or two, and its soon dissipated.

 

[ggordil]

One thing to keep in mind is that internal PSV's are not inspected and maintained in most plants. Therefore, most of my clients chose to add external PSV's that can be inspected and maintained to ensure that if a blocked outlet were to occur, the equipment is protected.

 

[tickle]

I would consider that the internal relief does not have the integrity of an external releif valve. It may get blocked and only protects against over pressure developed by the pump.

The internal releif in an ebsray sliding vane pump is similar in configuration to a standard PSV, but the set point can easily be changed.

If recycling and temperature build up is considered a problem, then add a temperature switch and shutdown

 

[Chance17]

I was once responsible for a positive displacement pump with an internal relief valve.
After several overpressure incidents on the discharge I discovered the
internal valve was rated for significantly less than total pump flow.
I later discovered the relief valve intension was to balance pressures across shaft bearings.

Moral: Find out the intension of the internal relief.

I think you will find an API standard - any pump 15-hp motor or greater must have an external relief valve.

 

[vloor]

Yitbos,

Would you point out where in API that specify a 15-hp motor or greater must have an external relief valve?

Thank you,
Sharon