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Sizing a pump spillback line 3
- Thread starter OilBoiler
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3
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I assume you are talking about a centrifugal pump and not a spillback line for a PD pump. For a PD, the spillback should be sized for the pump's full capacity or you run the risk of popping the PSV on the discharge.
For a centrifugal, the flow rate should be based on the minimum stable flow recommended by the vendor. Some companies will take credit for the forward flow to whatever destination the pump is sending the liquid to. I don't favor that as I usually find that sooner or later that flow you are counting on stops and then the pump is dead-headed or running below the minimum recommended flow. A lot depends on how long it might run at a low or no flow (no flow can not be tolerated for very long due to heating of the fluid inside the pump).
Recirc streams used to be sized in a lot of cases for thermal protection where the recir flow through the pump in a no forward flow case would limit the dT across the pump to 10F to 20F. Very little flow is needed for this basis but you typically have pump problems due to thrust issues. My experience is that rotating equipment engineers today want a lot more flow than that.
I've found that a lot of pumps recommended min flow is between 20% to 30% of the flow at BEP. Some are higher, up to 40% or more. Best thing, talk to the vendor.
Spillback can be done through something like an ARC valve, a flow meter opening a recycle valve or simply a restriction orifice (usually only justified on smaller pumps as the continuous flow increases energy costs and has to be included in the pump sizing). For sizing the spillback line itself, I tend to run towards the upper end of the velocity limit since I'm going to be taking most of the pump head across the recycle device (ARC valve or control valve). 12 to 15 ft/sec wouldn't bother me.
I much prefer going back to the source tank rather than right into the pump suction with the recir to allow the heat introduced by the pump to be dissapated before the fluid re-enters the pump.
For a centrifugal, the flow rate should be based on the minimum stable flow recommended by the vendor. Some companies will take credit for the forward flow to whatever destination the pump is sending the liquid to. I don't favor that as I usually find that sooner or later that flow you are counting on stops and then the pump is dead-headed or running below the minimum recommended flow. A lot depends on how long it might run at a low or no flow (no flow can not be tolerated for very long due to heating of the fluid inside the pump).
Recirc streams used to be sized in a lot of cases for thermal protection where the recir flow through the pump in a no forward flow case would limit the dT across the pump to 10F to 20F. Very little flow is needed for this basis but you typically have pump problems due to thrust issues. My experience is that rotating equipment engineers today want a lot more flow than that.
I've found that a lot of pumps recommended min flow is between 20% to 30% of the flow at BEP. Some are higher, up to 40% or more. Best thing, talk to the vendor.
Spillback can be done through something like an ARC valve, a flow meter opening a recycle valve or simply a restriction orifice (usually only justified on smaller pumps as the continuous flow increases energy costs and has to be included in the pump sizing). For sizing the spillback line itself, I tend to run towards the upper end of the velocity limit since I'm going to be taking most of the pump head across the recycle device (ARC valve or control valve). 12 to 15 ft/sec wouldn't bother me.
I much prefer going back to the source tank rather than right into the pump suction with the recir to allow the heat introduced by the pump to be dissapated before the fluid re-enters the pump.
For a complete rundown on Centrifugal Pump Minimum Flow Control Systems I'd recommend reading Chapter 2.3.4 by William O'Keefe, and Centrifugal Pump Bypass Chapter 8.2 by J.P. Messina, both in the Pump Handbook by Igor J. Karassik et al., published by McGraw-Hill, ISBN 0-07-033302-5.
Oilboiler,
I would like to second TD2K's detailed explanation and fine suggestions. I have learned musch from his posts over the years.
Based on your handle, I would like to ask if your question is based on a boiler feedpump application ? Boiler feedpump recirc lines are notorious for noise, vibration, erosion etc due to the extreme pressure drop.
If this is a boiler feedpump, I would like to add that the recirc for such a flow must return back to the pumped source (typically the Deaerator) and that the deaerator must be designed for such a flow. Additionaly, there are a choice of several recirc devices manufactured by third parties (not the pump vendor) for this extreme service
My opinion only..
MJC
I would like to second TD2K's detailed explanation and fine suggestions. I have learned musch from his posts over the years.
Based on your handle, I would like to ask if your question is based on a boiler feedpump application ? Boiler feedpump recirc lines are notorious for noise, vibration, erosion etc due to the extreme pressure drop.
If this is a boiler feedpump, I would like to add that the recirc for such a flow must return back to the pumped source (typically the Deaerator) and that the deaerator must be designed for such a flow. Additionaly, there are a choice of several recirc devices manufactured by third parties (not the pump vendor) for this extreme service
My opinion only..
MJC
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