Using relief valve as minimum bypass for centrifugal pump

[Olidin]

Hello,

I have a question about some set-ups that use relief devices as minimum bypass valves. On PD pumps, I understand why this is necessary since PD can overpressure the line or itself if blocked in. However, for centrifugal pump, it's unlikely pump will overpressure itself during shut-in (if that was possible then every start-up would be very scary for the operators).

So what are the benefit of using relief valves for minimum bypass? How would you determine the valve set pressure? On the other hand, what are the advantage for using a minimum bypass line instead of a relief valve?

If the current set-up uses relief valves, what are the incentives to use a minimum bypass line?

Thanks for your help.

 

[Latexman]

I see no benefit. I put in a small line from discharge to supply tank (preferably) with an orifice sized for min. flow at deadhead pressure.

Good luck,
Latexman

 

[Propacket]

The line you are looking is the relief line for the pump because in most cases PSV discharge is sent to the pump suction. There should be a separate minimum flow line for the pump.

 

[MortenA]

The insentative to have a minimum flow "by pass" or return or is that some pumps may not be able to handle shut-in operation for long (e.g. if they are cooled by the fluid the pump). But this would normally be handle with some sort of control valve. Are you sure its a regular PSV (with a safety function)? A "spring loaded PSV type" valve would be practical since it wont require flow or pressure instruments etc. But normally a PSV would suffer from frequent operation, start to leak. There are special min. flow. mechanical valves - maybe its one of those types?

Best regards

Morten

 

[moltenmetal]

I've seen a relief valve used for this purpose before.

A back pressure regulator works more reliably for longer.

These approaches are not foolproof but have the benefit of sending flow back to suction only when needed. That's handy when you have a pump whose minimum flow is a significant fraction of the operating point.

 

[SNORGY]

Use the restriction orifice idea per Latexman, and go back to pump suction if it is impractical to go back to the supply tank. That's bullet-proof and it will pretty much always work.

 

[don1980]

Relief vslves are not an ideal device for ensuring min flow from a centrifugal pump. A better device is an ARC valve (automatic recirculation valve) which is made for this purpose. ARC valves are true modulating devices.

Alternatively you can install a bypass line with an orifice, as discussed above. I don't think there's a general answer for which is better. It depends on the specific application. A continuous bypass is simple and reliable, but the downside is that it wastes energy. However, if the consequences of failure are bad enough, then that msy be the right answer.

For small PD pumps, I think a PSV is the best solution.

 

[psafety]

Concur with Latexman.