5 replies to this topic

[fallah]

'Def110', on 27 May 2010 - 3:42 PM, said:

I have posted earlier in student section but I think this is the right section to get the feedback on the issues.

A centrifugal pump is provided with a minimum flow recycle line that routed back to the suction line. The minimum flow recyle line is provided with a flow control valve. The pump discharge is around 60 barg whereas the suction line is only rated for 19 bar (150# class)upstream of pump suction valve. The question is; it possible if the flow control valve on recycle line fail open/stuck open then the suction line will be overpressure and relief valve is required to protect the line? Inlet line of pumps is from a separation vessel and have an check valve on the line from vessel to pumps. This check valve is located upstream of recyle line tie-in to suction line. I have seen many system that have similar configuration is not provided with RV for this case.

It is not common to routed back the pump by pass line to the suction line.

Is that min flow recycle line continuous min flow or it is intermittent (opened in the case of flow shortage)?

Is the 60 barg pump shut off pressure?
Is there any possibility of pump dischage blockage leading to pump shut off?

By providing a simple sketch and more information as above you can get better assistance on your query.

[chemsac2]

Def110,

I have never seen NRV on pump suction line.

I would formulate my explaination first assuming no NRV in suction line.

Irrespective of if control valve fails open or is in normal opening, control valve outlet pressure is same as pump suction pressure. Pump suction pressure is equal to suction vessel pressure plus static head.

Note that a control valve does not have pressure drop of its own. It merely connects two pressure systems. Thus irrespective of % opening of control valve (normal opening or fail open), control valve outlet pressure would be equal to pump suction pressure only. Hence, no need to have PSV on suction line. Increased # rating for suction is for different reason i.e. to protect stand-by pump suction system from overpressure developed by parallel pump considering stand-by pump suction valve closed, NRV latent failure and discharge valve open.

System with NRV in suction line:

This situation reminds me of the phrase "circle has no end". Pump adds head to suction pressure. Control valve decreases head to bring it back to suction pressure when it enters the pump. Thus, a balance is reached. This is possible for any value of suction pressure. I understand your question as what is the value of this suction pressure. Is it likely to be higher than normal suction pressure?

My answer would be - no.

Agreed that due to NRV, pump suction pressure can be higher than vessel pressure plus static. However, suction pressure same as vessel pressure plus static is the starting condition of the system. When control valve fails open, pump just adds different differential head to this pressure. Thus, pump operating point shifts to right of normal operating point with increased flowrate.

Regards,

Sachin

[Lowflo]

Def110,

The only pressure you get on the suction line of the pump is that caused by the pressure in the suction vessel, static head, and the friction loss of the suction pipe. Using an extreme example, let's suppose the entire capacity of the pump is routed back to the suction. In that case, the pressure in the suction pipe will only increase by the amount caused by the additional flow resistance in the suction line. And, once the pump reaches the end of it's curve, the differential head from the pump is ~0. Pumps, by themselves, are incapable of generating pressure unless there is resistance in the system.

So, there's no need to be concerned that this pump could cause the suction pressure to be anywhere close to that of the normal discharge pressure.

[M S Miglani]

PSV on suction line is not required. Control valve which connects the discharge to suction takes care of pressure drop. Only problem in this case could be heating of fluid which is being circulated from suction to discharge, Normally the minimum flow bypass is connected to suction vessel to avoid heating. In the sketch there is no NRV in the suction and it is not required also as there is no possibility of suction fluid going to the discharge side because of pressure difference. There must have been a pressure control mechanism if it is a vapour liquid separator or feed surge drum or atmospheric tank.

Only issues I have seen where safety valve is provided on the suction vessel especially for very low pressure vessel like tank for possible reverse flow from high pressure system in case of pump trip and in many occassions a NRV is provided downstream of tap-off for Minimum flow bypass line.

I disagree that incase suction pressure increases the pump will see its shut off as centrifugal pumps work on concept of differential pressure.