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centrifugal pump minimum flow control

centrifugal pump minimum flow control

(OP)
What would be the criteria to select the use of an automatic flow control valve vs. an restriction orifice installed on the spill-back line?

RE: centrifugal pump minimum flow control

The use of a recirculation valve results in less energy usage and it reduces the volumetric flow requirement for the pump. In some cases the impact of the these variable is significant and in other cases it's not. Evaluate your case and decide.

There's one other factor that sometimes comes into play, and that is plugging tendency. If it's a potentially plugging service, then a continuous recirculation flow (e.g. using an RO) is less likely to become plugged.

RE: centrifugal pump minimum flow control

An FIC - FCV loop works for a reasonably wide range of differential pressures across the FCV, while the RO will only work when the dp across the RO matches the design case dp.

RE: centrifugal pump minimum flow control

If your pump is operating a large percent of the time and at flows greater than minimum then an orifice is just throwing energy away. Your pump needs to be bigger to cope with the constant recirc plus the design flow. The extra cost for an arv is well worth it.

If only used occasionally, low power and most of the time in standby/low flow mode then an orifice is a cheap and simple solution.

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RE: centrifugal pump minimum flow control

(OP)
Thanks for your feedback, don1980 / georgeverghese. My case is a system already built but not commissioned yet, to be used for pumping waste HC and oily water (2 independent pumps) from a closed drain system drum to offsite (submergible pumps), using a continuous recirculation spill back on each going back to the drum. I am puzzled on whether these systems would not need a globe valve on their spill back lines to regulate this flow in order to ensure we can get forward flow to offsite. The spill back lines are immediately after the pump’s discharge before the check valve and the globe valve to be manipulated to send the flow towards offsite. The pumps are: Waste HC (discharge 3”x2”/ rated flow, 18 m3/hr. (minimum 12 m3/hr), Power, 16 kw); Oily water (discharge= 2” x 3” / rated flow=19 m3/hr. (minimum 9), power= 9 kw). Thanks for your insights. Regarding the RO potential plugging won’t it be MORE likely to get plugged if used on a potentially plugging service? Thanks!

RE: centrifugal pump minimum flow control

(OP)
Thanks LittleInch. My pumps run on intermittent service. They run whenever the material level hits a set value, so I guess an RO has been well specified. My question now remains as per the convenience of installing a globe valve on these spill backs. Thanks.

RE: centrifugal pump minimum flow control

But why is the "spill back" there at all?

What is stopping forward flow?

I've seen a lot of "spill backs" - I hate that term - installed just because that's what they did last time.

If your pump is intermittent on a level switch, why do you need this waste of energy? What causes the flow to go below min continuous flow or no flow for long periods of time?

The orifice or valve should be sized such that the outlet head is maintained and the pressure drop taken across the orifice.

Those flows seem quite high for min flow. It means you have a pump 30 to 50% Bigger than you need.

Strange sounding design.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: centrifugal pump minimum flow control

(OP)
LittleInch, the line to offsite is connected to other delivering systems, so there is a potential for some back pressure to be exerted on the line coming from the concerned pumps when they get started by the level function in the drum. I suppose that’s the reason for the recirculation flow back to the drum, to ensure the pump’s manufacturer ’s specified pump-protecting minimum flow, while the operator takes action to adjust the forward flow to offsite through the globe valve installed on the forward line.

RE: centrifugal pump minimum flow control

Still sounds like you don't really need it, but hey, that's your issue not mine. Just be aware that there are alternatives next time around.

I can't see enough of your design and operation to know how it might work without it, but often designers are just lazy and stick one in and don't concern themselves about the electricity bill you get...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: centrifugal pump minimum flow control

1)If the spillback lines are manifolded together well before the spillback return line into the CD drum, then pressure in the return line will vary depending on the actual spillback rate, and that will affect the actual spillback flow from each pump.

2) A globe valve on each pump discharge may work for a limited range of built up backpressures in the common dishcharge line going offsite. If you find your forward flows vary beyond the operating range for the globe valve, then would suggest a PIC - PCV on each pump discharge.

RE: centrifugal pump minimum flow control

(OP)
Greetings to all!

I attach one sketch to explain better the case. Hope it helps. Regards!

RE: centrifugal pump minimum flow control

(OP)
Does anyone know about a formal/official specification/engineering standard documenting or providing guidelines for selecting an specific pump recycle system? Meaning when to select: Continuous recycle (Use of ROs), Control Loop (Use of Control Valve). API 610 does not address this issue, as far as I know. Thanks!

RE: centrifugal pump minimum flow control

Not that I'm aware of as there are just too many variables.

Your sketch is interesting, but doesn't really tell us anything about the d/s system, how it works, what flows and pressures etc are present.

I can only assume that whoever devised this system thought that there was a potential that if the other users all used the system at the same time, the back pressure would be enough to either stop flow from your pump or severly reduce it.

However your idea of the globe valve shouldn't be required if the orifice is sized correctly. The differential pressure across the orifice plate should be sized based on realtively low flow through the pump, hence a high head. The globe valve won't help to increase pressure, but would, of course, mean that more of the flow through the pump went down the line instead of back into the tank.

If you enough flow then simply put an isolation valve there and turn it off if this is a manual operation.

Like I've been saying, unless you really understand the operation, flow, pressure etc being or likely to be experienced you can't make judgements or design it properly. If things keep changing you can't just set an orifice or a globe valve and expect it to magically change when things change. That's why ARV valves do this automatically without any electrical or instrumentation input, Then you get "spillback" when you need and more flow though the system when you don't. This sort of thing is what they were invented for.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: centrifugal pump minimum flow control

Install a backpressure control valve on the spillback line for each pump, if you have a pump Q-h curve which is not flat at around the operating point. Unless there are issues with cavitation d/s of the spillback PCV, the RO will then not be required.

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