Minimium flow of pump 1

[sujins]

1.Can any expert help explain, what is the minimum required flow rate of pump?

2.In the situation that we have to throttle the discharge flow because the rated flow is not at the intersection of pump curve and system curve(the rated flow is less than the flow at the intersected point, do we need to have the recirculation line to absorb for the excess flow rate?And do the system curve change after throtting of discharge control valve?

3. Does the recirculated flow rate in item 2 equal to the flow rate at the intersected point minus the flow rate at rated point?

4. The role of recirculation line and its meaning.???
Thanks

 

[JJPellin]

The concept of minimum flow for a centrifugal pump can be complicated. A number of standards have been used. Sometimes the number is provided as minimum flow for thermal stability. This means that the temperature rise in the fluid as it passes through the pump will not cause the fluid to vaporize (cavitate). There can be a minimum flow for hydraulic stability. Below this flow rate the pump may experience suction recirculation or discharge recirculation that may result in cavitation, vibration and inefficiency. Frankly, I am not sure what method is used for the minimum flow value published on an API datasheet. We have changed our method for determining minimum flow over the past few years. Both our former method and our current method involve calculating Specific Speed and Suction Specific Speed for the impeller. Then a graph or table specifies what percentage of Best Efficiency Point (BEP) flow is required as minimum. I believe that our original criteria was based on a Hydraulic Institute standard. Our current criteria is based on a system published in a book by Heiz Bloch and Allan Budris titled "Pump User's Handbook". If the total flow is throttled with a valve on the pump outlet, all that is needed is for the total flow from the pump to be greater than the calculated minimum. If the flow is above minimum, it should not be necessary to use a recirculation line. If the total flow needs to be pinched down below the minimum, the use of a recirculation line can be used to get the total back above minimum. Pinching down on the discharge side of the pump changes the system curve. The pump will always operate at the intersection of the pump curve and system curve. One a recirculation line is installed, it can be used to make the pump more reliable. It can be used to run the total flow from the pump closer to BEP than is required to satisfy the minimum flow requirements. The cost of doing so is the extra energy needed to recirculate more flow. But if the reliability of the pump is critical, it may be worth it. A pump running close to BEP will be more efficient, with lower vibration, lower temperature rise, less shaft deflection and probably have better seal and bearing reliability. We occasionally use our recirculation lines to improve reliability in this manner.

 

[25362]

Sometimes pump makers provide pressure-balancing holes through the impellers, which are supposed to enable working at flow rates below 50% of the BEP. I believe it is always advisable to get the relevant data from the pump supplier. From past experience I know Goulds supplies this kind of information upon request.

 

[rzrbk]

If you go to the vendor for information, ask for 'Minimum Continuous Stable Flow'.

 

[sujins]

Could anyone please explain more!! I haven't yet cleared the point.

1. When we throtting the control valve at the outlet to receive the required flow rate,do the pump regulate itself to the new flow rate and that control valve will absorb for the higher pressure drop? Then the recirculation line is no need anymore, is it right or wrong??

2. For the min. flow rate of pump, is it come from the design of pump manufacturer??

3. At what percentage that we can operate far from the BEP in order to acheive the good efficiency of pump?

Thanks in advance.

 

[Artisi]

1. In forcing the pump to run at a higher head by closing a valve on the discharge is not a particulary good practice -long term - if maybe ok for short time to solve an immediate problem. The correct solution is either reduce the impeller diameter to suit the actual duty, use a recirculation line if the pump needs to operate at a higher flow at different times or change the pump to one suitable for the duty.
2. Yes - ask the manufacturer for the minumum allowable continous flow rate.
3. Once you move away from the BEP the pump efficiency starts to reduce- refer to the pump curve for this information.
4. My suggestion is to employ a consultant pump engineer to advise you on this problem.

Naresuan University
Phitsanulok
Thailand

 

[JJPellin]

Pinching a control valve on the downstream side of the pump is the most common way to regulate the flow of a centrifugal pump. In a continuous process, it is impractical to size every pump perfectly for the flow rate needed. The most efficient way to control the flow would be to have instrumentation change the speed of the pump, either with a governor on a steam turbine or a variable speed drive on a motor. But it is still more common to measure the flow and have a flow controller adjust a control valve to pinch the flow down to the desired rate. In fact, many pumps do not run well unless they have a certain minimum amount of back pressure from a pinched valve. Allowing a pump to run-out too far on the curve can produce high vibration, inefficiency, heat, shaft deflection and a reduction in reliability from bearings and seals.

Just because you are pinching on a downstream valve you don't necessarily need a recirculation flow. Once the valve is pinched, the pump will automatically adjust to the new system curve (higher pressure drop) and flow accordingly.

The pump manufacturer should provide the minimum stable continuous flow. But this number is sometimes not conservative enough. We normally use a number that is larger than the one provided by the manufacturer. As a rule of thumb, most low-energy, single stage pumps can run at flows as low as 25% of BEP. But we also have pumps that will not run well unless the flow is at least 75% of BEP. It is different for every pump

 

[TD2K]

Sujins

For your first question, the control valve (as said) takes up the excess pressure between what the pump puts out at the required flow and what the process needs (as shown by the system resistance curve). Do you still need a minimum flow line? If the reduction in flow that you expect decreases below the vendor's recommended minimum flow you need to consider providing minimum flow protection.

2. The vendor is the best person to provide this flow. Various companies have procedures to estimate the flow if that's no longer an option. I would also take a look at the book suggested above by Heiz Bloch, his emphasis is on reliabity.

3. BEP stands for Best Efficiency Point. By definition therefore, any change from this flow means a drop in efficiency. You need to look at the pump curves for your pump to quantify for a given change in flow how much the efficiency will change, I can't give you a rule of thumb. If you have a copy of the Gould's pump manual in your office, looking at some of the various curves will demonstrate this. Older pump curves will commonly not show the efficiency curves though you can estimate the efficiency knowing the flow, head and Hp and back-calculate the efficiency at various flows.

 

[checman]

Sujins

Question 1: The minimum required flow rate of pump is the lowest continuous flow rate that does not do immediate or catastrophic damage to the pump. It should be the same as the OEM’s minimum recommended flow. Watch out for old manuals about this, before the 1990’s most pump companies just used product thermal rise (15 deg.F) as a guide they did not compensate for specific suction speed. The normal range of minimum flow for process pumps is between 10% to 25% of BEP.

Question 2: Pump curves and the system curve are independent of each other. The system curve is the system curve no matter what pump is in it. And the pump curve is the pump curve no matter what system it is in. Throttling a pump may not be a bad thing you may be throttling it into its BEP if the system does not supply enough back pressure. Only plotting the pumps performance on its curve will tell.

Question 3: The recirculation line is the line that sends product from the discharge side of the pump to the suction side. Ideally it normally goes back to the supply tank. The flow rate of the recirculation line is normally based on the minimum recommended flow of the pump.

Regards Checman