Diver said:
I could not find the suction head performance curves for the TF30 but I have linked the charts for the Onga GPP40, a similar pump. If you look at the suction head litres per minute (lpm) and the rated head/lpm and by way of example, you will see that with a 3 metre suction head, lpm is 28 lpm but the rated head less 3 metres is 56 lpm.
Relocating the pump to reduce the suction head's impost will make a considerable difference and allow the pump to operate to its optimum efficiency.
http://www.onga.com.au/objectlibrary/817?filename=GPP40%20-%20Brochure.pdf
Firstly, many thanks for finding the data for the Onga GPP40 pump, which is exactly what is needed to anwer the question as to if and how much pump performance is reduced due to a suction head. Theoretical predictions are all very well, but measured data in the real world is what ultimately matters.
However, what I find is that the Onga data confirms what I said previously.
Firstly, let me reiterate what I said previously, but in more detail. For small suction heads (say <2m, where nasty effects such as cavitation should not come into play), conservation of energy tells us that the TOTAL pump head, defined as the difference in pressure between inlet and outlet ports, should not be a function of whether the pump inlet is above or below the surface of the water being pumped, but should only be a function of the flowrate. To put that another way, for the situation described by Warnsey, the pressure and flow available at any given discharge height should not depend on whether the pump is located just above the top of his 2m high tank, or at the base of the tank.
Now to the Onga data, which appears to confirm what physics predicts. The Onga graph of head vs Flow is plotted for zero meters of suction head, so a good first step is to see if the tabulated point (0m suction, 35 lpm, 140kPa) really does lie on the graph. Onga are not helping us by mixing pressure units of meters head and kPa, but the conversion is 1kPa=0.102meters. Thus, 140kPa is 14.3kPa. Look on the graph, and observe that the point (35 lpm, 14.3kPa) does indeed lie on the graph. Excellent.
Next, we need to think about what the tabulated suction data actually means. The tabulated data, at a variety of suction depths, is all taken at a fixed 'operating pressure' of 140kPa (14.3m). 'Operating pressure' as specified in the table presumably means the gauge pressure measured at the outlet port, which corresponds to the vertical head above the pump body. To this must be added the vertical suction head, being the vertical height from the surface of the water being pumped to the pump inlet on the body of the pump. What the table is showing us, is that as the total head is increased by way of increased suction head, the available flowrate decreases. That is exactly what we should expect. Furthermore, if the 'physics' prediction is correct, the flowrate should depend only on the TOTAL head, being the height from the surface of the water being pumped, to the height at which the water is discharged. For example, the flowrate should be the same for a suction head of 3m plus a +ve head of 14.3m (total head 17.3m), compared to a suction head of 0m plus a +ve head of 17.3m (total head 17.3m). The Onga data shows that this is indeed the case.
Case1.
Suction head = 3m
Positive head = 14.3m (140kPa)
Flow = 28 lpm (from the tabulated data, for a suction depth of 3m)
Case2.
Suction head = 0m
Positive head = 17.3m
Flow ~ 28 lpm (from the graph, check for yourself)
The physics, and what I said previously, are just fine.
I'm sure some of the confusion can be traced to the term 'suction head's impost'. There is no 'suction head impost', in Warnsy's situation, except of course for cavitation and friction loss in the inlet pipework, which are both negligible for a 2m suction height. Let me explain when there is a 'suction head impost' (from the suction height), and when there is not.
Case1 - Impost from increased suction height.
Consider the tabulated suction data for the Onga pump, where it is observed that the available flow decreases with increasing suction height. The physical situation here, is that the height from the pump body to the point of discharge is kept constant, and the suction height is increased by raising the level of the pump above the height of the water. The decreased flowrate at large suction depth is totally expected, because this results in a greater overall height from the surface of the water, to the point of discharge.
Case2 - No significant impost from increased suction height.
This is the case pertaining to the original posting by Warnsey, where the overall height from the surface of the water to the point of discharge remains constant, regardless of whether the pump is placed at the bottom of the tank, or at the top.
Re the benefits of a pressure vessel on the pump outlet, I agree completely, and may talk more about it in another posting.
Posted Monday 16 Jan 2012 @ 3:08:41 am from IP
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