how to sizing control valve on the pump minimum kickback line? 1

[nli2]

Hello,
How to sizing control valve on centrifugal pump minimum kickback line? [not using restriction orifice].
Say,Pumpflow on main discharge line (rated/normal/turndown) is 400/360/180 m3/hr, minimum stable flow is 100 m3/hr per pump vendor. What is max/normal/min flow for this kickback control valve?

Thanks & BR,

nli1972

 

[JLSeagull]

Is this a motor driven pump or pumps? Are multiple pumps automatically started?

Start with the pump curve. Your objective is to keep the pump or pumps within the safe operating part of the curve.

 

[TD2K]

As JL said, look at your pump curve and pump data sheet, most manufacturers will now provide a continuous minimum flow. If it's not on the documentation, contact them.

I assume you'll be measuring the pump discharge flow rate. As you feed more forward to the process or wherever it's going, you need less flow through the bypass to keep the specified minimum flow through the pump.

The minimum flow thus becomes your maximum sizing case. There really is no normal or minimum flow (IMO) since depending on your process, the required minimum flow to keep the pump at its vendor recommended minimum can go all the way to essentially zero and no valve can be sized for an infinite turndown. Which is why you don't want to oversize your valve, performance on the low end will suffer. This is one case where I'd be happy if the valve was almost wide open.

Now, if your pump's minimum flow and process requirements are such that you will always usually be recyling, one could call that the 'normal' flow through the recycle valve but that would be unusual.

For the inlet and outlet pressure, that depends on your system. The dP is the pump head at the minimum flow. If you are taking suction from a tank, the outlet pressue can vary from the correponding value at minimum level to maximum level. I always want to check the valve operation with the tank at minimum level for cavitation. If the dP and vapor pressure is high, you can have cavitation issues. That can be addressed with speciality trim like Cavitrol but you better have clean fluids, they make a great filter.

 

[nli2]

thanks!very helpful!
We pump the heavy naphtha out of plant to downstream tankage.It is motor-driven pump.Do check the pump NPSHA per naphtha stripper column Low-low level,sizing the pump suction line per rated flow. Protect pump with the minimum kick back line to the source column with FIC to control the kick-back valve. After read this post, i talked with instrument engineer, we put FIC set point at ~110 m3/hr, if flow bigger than the set point, the kick-back control valve will keep close to save money. valve will receive FIC signal only Flow rate decrease to the set point. control valve will be sizing based on the pump minimum stable flow as the normal case, take 1.1 factor as maximum case, 0.3 factor as minimum case with corresponding calculated delta P.
Thanks again!