How to Set Hot Pump Non-return Valve bypass Flowrate ?

Can someone suggest the flow rate through the Restriction Orifice (RO) on a pump warm-up bypass line for the standby pump, maybe a certain percentage of the rated flow of the running pump ?

Above question could be tackled in simple or difficult way. However lets start to understand the purpose, how it is implemented and how to set the flow.

Purpose
The main purpose of the bypass line is to maintain a minimum temperature different between the pump (and associate piping ) and the pump suction fluid temperature to avoid temperature shock in the event of standby pump is started-up automatically.

How it is implemented ?
The bypass can be a fixed RO, non-return valve with hole or a globe valve. It generally install across the standby pump discharge non-return valve.

How much flow ?
The bypass flow rate should be sufficient to cater for :

i) pump and associate piping heat-up from minimum ambient to normal suction temperature within a reasonable time i.e. 2 hours
ii) heat leakage via insulation during normal operation

Generally above calculation are time consuming.

Tips
Experience based approach may be taken where setting the RO / NRV hole size as 6-8 mm or install a one (1) in globe valve.

Reference :
i) Why bypass Non-Return Valve (NRV) ?