Wednesday, October 1, 2008
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Earlier post "Why bypass Non-Return Valve (NRV) ?" discussed the purpose of providing manual block valve across Non-Return Valve (NRV) on centrifugal pump discharge. Typical the purpose covers :
- Pump priming
- Pump warming
- NRV downstream section draining
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Now the question is focus on pump warming. What is the basis of setting this recycle flow rate ? Lets first define the purpose, how it is implemented and how to set the flow.
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
- fixed restriction orifice (RO) or;
- non-return valve (NRV) with hole or;
- globe valve
The bypass flow rate should be sufficient to cater for :
i) Start-up : pump and associate piping heat-up from minimum ambient to normal suction temperature within a reasonable time i.e. 2 hours
ii) Normal operation : heat leakage via insulation during normal operation
Generally above calculation are time consuming. 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.
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