Dissimilar Check Valves on Pump Discharge

[Sawsan311]

Dear All,

I am seeking your technical views on the requirement of providing two dissimilar check valves at centrifugal pump discharge despite knowing that these check valves are not internally inspected and hence are not qualified for overpressure protection. These check valves are located downstream the minimum flow recycle line connection. Do you agree that the primary reason for providing such check check valves (including non-slam type) is to protect from the reverse rotation of the pump's impeller and shaft after sudden shutdown and as a result of the inertia of the downstream system?

Thanks

regards,

 

[LittleInch]

A bit of that, but usually its to protect the lower pressure rated inlet system from seeing the high pressure on the pump outlet.

Often comes up in a HAZOP as an action to prevent reverse flow or high pressure.

Each system is different so if e.g. there is a high static head on the pump discharge this means you could get high pressure or high flow before you can close the isolation valve which might not be good or could over pressure the inlet system.

That's why you do HAZOPS.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Yes, a little bit of that. Some pumping systems don't have inlet systems such as wet wells. Without check valves, the pumps and wet well of a lift station can be subject to damage from backflow. Backflow occurs when the lift station pumps stop operating and the water in the force main and discharge piping flows back into the wet well. In virtually every case, the backflow creates high levels of hydraulic pressure that pumps and wet wells are not designed to handle. Check valves also keep the pipeline from draining and filling with air.

 

[Sawsan311]

Thank you all for your inputs,

I understand that you are not referring to the check valves as measures for overpressure protection since as per API 521, irrespective to the number of check valves in series, there is no role of overpressure protection unless they are internally inspected.

I understand from your response is that the role of these non slam check valves to mitigate reverse flow which can cause damage to the pumps and upstream system.
Thanks again,
Regards,

 

[georgeverghese]

Yes, if also used as overpressure protection ( for system upstream of the pump) in lieu of PSV, then at least one of them must be inspected in accordance with plant PM schedule. This check valve must also be identified as a high integrity CV on the process drawings.

 

[Sawsan311]

Dear All,

thank you for feedback, just a small question, in case the objective is to minimize reverse flow into the system/pump, why do we recommend two dissimilar check valves? is it for reliability to ensure that at least one type is functional?

Regards,

 

[LittleInch]

sawsan,

to be honest I'm not completely sure, but usually this turns up in a HAZOP when the chairman won't allow a single check valve to be used as a mitigation to a hazard for reverse flow unless there are two different check valves.

Now why and where that became a "thing" I don't know but suspect it came from something like nuclear or some other high risk location where you really really don't want back flow.

So basically yes, it is to get a higher level of reliability that your protective device (the check valve in this case) is going to protect you from the hazard.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.