Welcome Tips for Sizing a Pump MiniFlow Line?

[brochon]

Can anyone please help in sizing/specifying a pump mini flow line?

The pump is in service, I have the pump spec sheets, know the pump normal operating conditions and have the vendor's recommended continuous minimum flow. Full capacity is required at times, therefore a control valve is desired over a orifice.

Questions that come to mind;

Recommended velocity through pipe (e.g. 3-5 fps)?
Any preferred type of control valve (i.e. make, model, flow characteristics etc.)?
How much over design to include in valve sizing?
Any special requirements for pipe configuration (e.g. minimum distance from pump discharge/suction etc.)?
Any other suggestions you may have!
Any good references that may be of help?

Thx Brian

 

[TD2K]

I use a much higher velocity for sizing a minimum flow line
- you are going to take the pressure drop across the valve anyway, putting in a larger pipe to reduce line losses just increases the dP for the valve.
- minimum flow is usually not a continuous operation, thus again a higher velocity is used.

The upper limit for me would be 125% of the common erosion limit 100/(density, lb/ft3)^0.5. If your fluid is dirty or has abrasive material in it (catalyst fines for example), you might want to use a lower velocity.

3 to 5 ft/sec is fine for a pump suction line, it's small for a recirc line.

I try to always route the minimum flow back to the tank or suction vessel to allow the heat of pumping to be dissapated. If you go directly into the pump suction, and sometimes you have to, you will have problems on continuous recirculation if the heat of pumping can't be dissipated to the atmosphere and I've never been able to get much heat losses through those lengths and sizes of piping.

I doubt I would 'oversize' the valve much over what the vendor has recommended. Get involved with the instrumentation person who is specing the valve, look at your conditions (no net forward, all flow through recycle at the vendor's recommended amount) and then see for the valve you select what it will pass wide open. If it will pass another 30% or so, I'd say that is sufficient. If you oversize the valve, you just make it potentially more difficult to control the bypass valve as net forward flow increases, causing the bypas valve to close. At some point, it's going to be just off the seat giving you poor control, you want to make sure that operation isn't where you expect/need to operate but that you will simply pass through that point either to no recycle (forward flow will increase) or higher rates of recycle (less forward flow).

As for the type of valve, it depends on the head of the pump, the type of fluid, is it clean or dirty? Can't answer with the information you've provided.

 

[scalleke]

You have flow curves and the pump is in service. The pump supplier must have given a min flown requirement also. follow this requirement, there is a margin in it.

I agree with TD2K, avoid going in the suction, if you cannot avoid that try to go as far upstream of the suction as you can to reduce turbulence and to avoid vapor bubbles that might have formed during the expansion over the recycle valve from entering the pump suction.