Discharge of Centrifugal Pump

[PAN]

I always install check valve at the discharge of pump. I understand that it is a general practice for pump installation.

However, someone said that check valve is not required in case of no chance of back flow to damage the pump. They did not tell me in detail about such cases. Please advise me about this matter.

 

[TBP]

In the case of larger pumps, if they are started while the pump is rotating backwards due to reverse flow, the torque can shear the shaft off. Very nasty. I worked in a central station where the main feedpumps (16,400 HP turbine drive) had a reverse rotation interlock that prevented the pump from being started if that condition was sensed.

I can't recall seeing a pump without a check valve on the discharge.

The biggest problem with check valves is that they are routinely oversized. People tend to size them off the pump discharge pipe diameter. They should be sized for the flow. An oversized check valve will flap like a flag in the wind, and wear out long before it should. (This is why swing checks wear at the point where the shaft goes through the flapper.) I've seen pumps with 4" discharge flanges that only required a 1-1/2" check valve.

 

[dcasto]

They may be refering that there are some applications that would allow a check valve on the suction or where there is an open ended discharge. Other than that TBP is on target.

 

[MJCronin]

PAN,

The question of the need for a check valve is related to the number of pumps performing the same service (i.e. the installed spare situation). Most pump installations have two pumps (each at 100%) or three (or more) pumps each with a percentage of the duty.

When there are multiple pumps installed in parallel, there must be a check valve installed to prevent reverse flow to and protect the pump or pumps at rest.

If you have a single pump, only used occasionally without a path for reverse flow, then isolation valves upstream and downsteam of the pump will suffice.

I am aware that there is a lot of opinion in cyberspace about this subject, and some will not agree with me..... nonetheless the single-pump system can be operated adequately.

Any ideas from other people ????>>>>>>>>



MJC

 

[krishnamoorthi]

In addition to preventing back flow the presence of check valves provide the following advantages.
1. A pump fitted with check valve can be left with both suction and discharge valves wide open. This allows the operator to just switch on and bring the pump on line within a short span of time.
2. If a standby pump is to be a autostart pump then check valve is a must.
3. Presence of discharge check valve with bypass line and valve allows the pump to kept hot.
Thanks

 

[troubleshooter]

Need for check valves:

Check valves are necessary to prevent the backflow that would occur in most systems due to downstream pressure reversing the flow with a sudden loss of input pressure from the pump. Most systems do not suddenly depressurize, and would tend to backflow.

Leaving pump on check valves with discharge and suction open:
This is hazardous. Check valves are NOT positive seal valves and the pump can rotate backwards due to downstream pressure. Also, if a seal failed, the fluid (often hazardous) can leak out if the valves are left open.

 

[electricpete]

MJCronin - I would not limit the concern to systems with more than one pump. There are other factors that can cause the pump to rotate backwards. Let's say the pump takes a suction on an open resevoir and pumps to a system at higher level or higher pressure. Pump trips off... all water starts flowing backwards through pump from system to reservoir. Operator responds quickly to restart the pump. Sheared shaft per TBP scenario.