Purpose of Bypass in Discharge Lines

It could be used to depressurise and drain the line above it so that a Permit to Work can be raised, perhaps?

What does the installation's Standard Operating Procedure have to say about it?

What does the HazOp Study report have to say about it?

This pump like so many others in Process Plants are installed in sets of two (or more) and one normally is running and one is in "stand-by" mode. These paired pumps are alternated (between run and stand-by) on some 24 hour, 48 hour cycle.

The fluids in the stand-by pump and the pump body it self will cool down during the stand-by phase and can cause damage to the pump.

A small (1" in this case) "Warm-up" bypass in installed around the check valve to allow the discharge pressure from the "Run" pump to back flow around the check valve of the stand-by pump and keep the pump warm. This small flow is not enough to cause the pump to turn backwards which is undesirable for pumps.

Another way to allow a back flow is to drill a small hole in the disc of the check valve. The decision for a) is it needed and b) which method to consider is influenced by:

• The commodity
• The pump type
  
• The discharge line size
• The flow
• The temperature
• The method of operation
• The impact on maintenance
  
• The risk
  

The choice of "is it needed or not" is normally by the Process Engineer and the Pump Engineer.

The choice between the two methods (bypass or drilled check valve disc) is up to the Client.

...hence #1⇑.

This bypass also allow the check valve to open at specific crack pressure and it do not slamm back abruptly thus reduces water hammer.

"Reverse flow" is a key phrase used in HazOp Studies. The answer will be found in the output documentation from the HazOp Study for this plant.