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Pump Minimum Recirculation Line

09/11/2020 2:45 AM

Can the minimum recirculation line in a pump lining operate without an orifice or a valve?

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#1

Re: Pump minimum recirculation line

09/11/2020 2:57 AM
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#2

Re: Pump minimum recirculation line

09/11/2020 3:07 AM

There is clearly a lack of understanding about the need.

What about a thought experiment? Look at the pump characteristic curve and conceive a minimum recirculation line around it. Think about what happens when the <...orifice or valve...> is present, and when the same is absent. What happens to the pump's performance in each case?

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#3

Re: Pump minimum recirculation line

09/11/2020 3:15 AM

If sized just right, yes; but that would be quite unusual.

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#4
In reply to #3

Re: Pump minimum recirculation line

09/11/2020 7:22 AM

<...sized just right...>

An interesting exercise for the Original Poster would be to calculate what the pipe length and diameter would be so as to be correctly sized for the <...minimum recirculation...> flow at the intended operating point on the pump's pressure/flow characteristic curve, and see whether the calculated diameter corresponds with any standard diameter of pipe that might be available. The Darcy-Weisbach equation is the correct tool to use.

The outcome of that calculation would determine the practicality of leaving out the <...valve...> or <...orifice...> in the said pipe, or going to a larger, practicable, size and including one.

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#5

Re: Pump Minimum Recirculation Line

09/11/2020 10:37 PM

Yes, just size the pipe to deliver the amount of liquid to be passed using the differential pressure of the pump discharge to suction.

This is not an uncommon solution where people don't want to worry about someone closing off a valve incorrectly, or corrosion/scaling/erosion caused by a small orifice / cavitation in a large pipe. It is not likely you'll get an accurate bypass flow, so an orifice/orifice union is useful on a tight pump specification. Otherwise, a 1/2" or 3/4" pipe on a larger pump would not be unusual.

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#6

Re: Pump Minimum Recirculation Line

09/12/2020 9:46 AM

One could always mount a pressure relief valve downstream of the pump, and the need for a <...minimum recirculating line...> would then disappear.

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#7
In reply to #6

Re: Pump Minimum Recirculation Line

09/12/2020 2:07 PM

That is incorrect. The Q-min line is not about limiting discharge pressure; it is about maintaining adequate flow to prevent recirculation of fluid within the pump, which can be another source of cavitation.

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#8
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Re: Pump Minimum Recirculation Line

09/12/2020 7:24 PM

If the pressure goes high, it is an indicator of low flow, all other factors being normalized. This is commonly applied on PD pumps, maybe not so common on centrifugal pumps, but it would work, and be more efficient than the steady parasitic load of a bypass line.

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#9
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Re: Pump Minimum Recirculation Line

09/12/2020 10:48 PM

On a centrifugal pump, it would be pretty hard to set a pressure relief valve accurately enough to make sense. In the low-flow region, the curve is nearly flat.

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#10
In reply to #9

Re: Pump Minimum Recirculation Line

09/12/2020 11:13 PM

Some curves are flatter than others, lower impeller speeds tend to drop off fast. So difficult maybe, not out of the question...

You may well have an application you know never is supposed to run at or near shutoff head, so you can conserve energy and not open the bypass until you’re at or close to shutoff.

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#11
In reply to #10

Re: Pump Minimum Recirculation Line

09/13/2020 6:09 AM

That could certainly work, by various solenoid valve logic, for instance. I haven't seen it done, though.

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#12
In reply to #11

Re: Pump Minimum Recirculation Line

09/13/2020 12:38 PM

Nor have I...

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#14
In reply to #6

Re: Pump Minimum Recirculation Line

09/17/2020 4:13 AM

Not a workable option for any normal centrifugal pump.

Head rise to shutoff can be very small over a large portion of the pump curve or worse still you could have a curve that droops as it approaches shutoff.

You need either a fixed orifice back to the suction vessel, which is very wasteful as you would be constantly pumping 30 to 50% of the pumps capacity in circles with resultant energy loss and you would need to install a bigger than necessary pump and motor. Works fine and is very simple and cheap to install.

Or you need a flow control valve or system where the flow is measured and when it drops below the set point a control valve opens in the line from pump discharge to the suction vessel and controls the minimum flow.

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#15
In reply to #14

Re: Pump Minimum Recirculation Line

09/17/2020 12:52 PM

Each installation of course has considerations, it is unlikely there is one best solution for every situation. Your solution is pretty complicated, expensive, and considerably less reliable than a simple orifice. It could be the best solution in certain cases, not likely the best for all. We usually shut the pump down if the speed is too low on a VFD or the control valve hits a predetermined floor limit. Also, there are plenty of normal pump curves that drop off head quickly as flow increases, maybe not so many with 2 pole or 4 pole drive motors.

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#16
In reply to #15

Re: Pump Minimum Recirculation Line

09/17/2020 9:03 PM

I agree that there is no one best solution. However, I suggested two.

One very simple and cheap to install, but inefficient - fixed orifice.

The second is more complex and more expensive up front, but much more efficient - low flow control valve.

Head rise to shutoff is a function of impeller design not driver speed, but the reason for showing the "typical" curve was to indicate that the use of a relief valve was not practical with such small pressure change.

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#17
In reply to #16

Re: Pump Minimum Recirculation Line

09/17/2020 10:39 PM

The OP wondered if you could design a bypass line without an orifice, which is indeed possible.

If you look at the pump curve for that same pump you gave as an example, I believe you will find a significant difference in the shutoff characteristic at 1200rpm, and even more pronounced at 900 rpm. Same impeller.

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#18
In reply to #17

Re: Pump Minimum Recirculation Line

09/21/2020 5:08 AM

A bypass line acts in the same way as an orifice.

You are absolutely correct that the head rise to shutoff will be much more pronounced at lower speeds with the same impeller as head varies as the square of speed.

I should have said that head rise to shutoff is governed by the geometry of the impeller and volute assuming fixed design speed.

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#13

Re: Pump Minimum Recirculation Line

09/17/2020 1:51 AM

Just switch it off when it’s not needed. Saves a lot of farting around...

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