return pump cavitating

[Mike321]

I am running my return pump externally, and the input is most likely to restrictive, causing the pump to cavitate. The pump isn't spitting out too many bubbles and probably wouldn't be worth trying to redo the input side plumbing to the pump. I was wondering if I but a valve on the output side of the pump to cut the flow rate down would this help the pump produce less micro bubbles?

[Indymann99]

Below is a document that I came across a while ago. While not directly addressing your question, I believe the principles still apply.


An interesting read.

Sorry the graphs didnt copy

CONTROLLING CENTRIFUGAL PUMPS
© Walter Driedger, P. Eng., 2000 May 20. walter(at)driedger(dot)ca


First published in Hydrocarbon Processing , July 1995.

This Adobe® file is available for download.

INTRODUCTION. The centrifugal pump is one of the simplest pieces of equipment from the controls and instrumentation point of view. It is a two port device with a well defined characteristic. Its purpose is to provide the necessary pressure to move liquid at the desired rate from point A to point B of the process. Figure 1-1 shows a 'generic' process with a centrifugal pump connected to deliver liquid from A to B.

The intersection of the two curves defines the operating point of both pump and process. It would be fortunate indeed if this operating point is the one actually specified for the process. It is impossible for one operating point to meet all desired operating conditions since the operating point is, by definition, exactly one of an infinity of possible operating points. In fact the entire point of controlling the pump is to modify its characteristic so that its actual operating point is the one that is required at every instance in time.


Assuming the pump is more than adequate for the process requirements at the moment, what is the best way to trim it back to the desired operating point, P1, Q1? There are three possible locations to place a valve: At the discharge, at the suction, and as a recycle valve. Each will be discussed in turn.

DISCHARGE THROTTLING. Since the pump exists to serve the requirements of the process, and one of the primary purposes of instrumentation is to adapt the equipment to the process, let us consider the pump from the point of view of the process. It can be viewed as a constant pressure device with an internal restriction. It is the restriction that gives it the "curve". It seems natural to put a valve on the discharge to further restrict the pump. This has the effect of rotating the curve of the pump/valve system clockwise around Ppm, as can be seen in Figure 1-3.

At this point I must warn the reader that we are about to encounter a paradigm shift. (!) The combination of pump and valve will be presented as a "black box" with a single characteristic curve which I shall term the "modified" pump curve.

The more traditional way of looking at the situation is from the point of view of the pump. It sees the process system curve as having rotated counter clockwise around Plm. Figure 1-3 shows that the flow, Q1, is the same for both cases. The difference between the two pressures is the Delta P across the valve. Since the purpose of the pump is to serve the process requirements, and the purpose of the valve is to adapt the pump to the process, it makes sense to consider the valve to be part of the pump system and to use the modified pump curve rather than the modified system curve in our discussion. In any case it can be seen that a discharge valve can be used to achieve any operating point on the system curve so long as the point is below the pump curve.

SUCTION THROTTLING. The second possibility for control using valves is to place the valve in the pump suction line. This would have an identical effect on the characteristic curve, but the method has a fatal flaw – cavitation. Cavitation is a phenomenon that occurs when the pressure of a liquid is reduced below its vapour pressure and brought back up above the vapour pressure again. Bubbles of vapour form in the liquid and then collapse upon arriving at the higher pressure region. The collapse occurs at sonic speed ejecting minute jets of extremely high velocity liquid. Wherever these jets impinge on a solid surface extreme erosion occurs. Over time even the hardest materials will be destroyed. Therefore it is of utmost importance that this pressure reduction never occurs. It is prevented by having sufficient pressure available at the pump suction so that the pressure drops that occur as the liquid is drawn into the eye of the impeller are at all times above the vapour pressure of the liquid at its current temperature.

Anything that would reduce the net positive pressure at the pump inlet below the NPSHR must be absolutely avoided. Thus suction throttling is never used to control pump flow.

RECYCLE CONTROL. The third remaining possibility for pump control with valves is to bleed some of the discharge flow back to the pump suction or to some other point on the supply side. Once again we can view the result as a modified system curve or as a modified pump characteristic. Figure 1-4 shows both. Each curve is a rotation of the original: The modified system curve as a clockwise rotation around Plm. Note the little "tail" at the left of the modified system curve. This represents the flow through the recycle valve before the discharge check valve opens to the process. The modified pump curve has a counter clockwise rotation around the hypothetical intersection of the pump curve with the flow axis.

This family of curves shows several problems with recycle control. Firstly, the pump is not rated to discharge more than the flow rate at the end of the curve. It is possible, of course, to run the pump with a wide open discharge, minimum D P, but it is unhealthy for this particular pump to run at such a high rate. Excessive flow may cause cavitation damage. (Excess flow cavitation is not caused by NPSH problems but by high velocity within the internal passages of the pump.) This restriction means that the minimum discharge pressure may not be lower than the one corresponding to the maximum flow. In other words, the modified pump curve cannot reach all points on the system curve.

Secondly, although many pumps are capable of operating near zero discharge pressure, the very flat pressure vs. flow curve for much of the lower range for most pumps means a change of flow has very little effect on the discharge pressure. Thus it would take a very large amount of flow to produce a small drop in pressure. In control terms this means that control would be very 'sloppy'. Discharge throttling on the other hand, allows the pump to develop the head that 'suits' it. The unwanted pressure is dropped across the valve. (Note that the curve for this particular pump rises rather steeply. It will be more easily controlled than most.)
Thirdly, this method is often inefficient. Figure 1-5 shows a system curve, a pump characteristic, a discharge modified characteristic, and a recycle modified characteristic. Above these is a pump power requirement curve. In the case of discharge control, the pump is adapted to the process by dropping its discharge pressure. If one follows the flow line vertically to the actual pump curve and then beyond to the power requirement curve one arrives at its power requirement. In the case of recycle control, the pump is adapted by reducing the discharge flow. Following the pressure line to the right to the actual pump curve and then upwards to the power requirement curve one arrives at the power requirement for recycle control. Note that the power requirement curve tends to slope upward as flow increases. Therefore recycle control consumes more pump horsepower than discharge throttling when both achieve the same operating point. This is not always so. If the power requirement curve were flat, there would be no difference. Notice on the curve that there is a slight drop in horsepower near the right hand end. If circumstances were such that the operating point corresponded to a downward sloping power curve, recycle control would be more efficient. This is rare.

[tozzi]

I think it would help. U always want the feed for the pump atleast the same size or bigger than the output. So if the feed is being restricted some how u can choke the pump back with a valve to balance it out better