Pipe Friction Loss for Parallel Pumps

All each pump can do is to follow its flow/pressure characteristic curve.

Therefore the system characteristic curve is of primary importance, as it determines where the system will operate.

The system will operate where the system characteristic curve and the pressure/flow characteristic curve for two pumps intersect. Just because one runs two pumps in parallel doesn't mean one obtains twice the flowrate (unless they are positive displacement pumps, of course).

End of story.

...which means that in order to get 60m³/h at 14m, two pumps each producing around 30m³/h at 14m will be needed. If each pump is only 30m³/h at 4m, then they aren't going to achieve 60m³/h at 14m, if that is what the system characteristic indicates. The only solution is bigger pumps.

60m³/h at 4m requires only 0.8kW or so, split between 2 pumps. 60m³/h at 14m requires 2.7kW or so split between 2 pumps. So, are the existing pumps big enough?

The 2 pumps are equal and both is pumping into the same system?

Remember

Flow 1 + Flow 2 = Flow total

Pressure 1 = Pressure 2 = Pressure total = 14m friction + static + other.

The same is true for unequal pumps but the split will have to be determined on the curves.

Also note that a single pump will deliver more than 50% of the total. That will also have to be determined by superimposing the pump curve on the system curve.

Absolutely.

Hi Rahhal

I think PWSlack diagnosed your problem accurately in post 3

You now seem to have 4 options

1 = buy 2 pumps of 30m3 x (x+14)m

2 = keep the one pump at 30 x (x+4)m for small flows and

buy one pump at 60m3 x (x+14)

3 = keep the one pump at 30 x (x+4)m for small flows and

buy two pump at 30m3 x (x+14)

options 2 or 3 may be better because the efficiency of the smaller pump would most probably be higher than using a bigger pump at a reduced flow. (Check the pump curves for that)

4 = Doubling your pipe must also be considered. The additional capital investment of pipe - pump(s) will be saved over time as running costs.

There is an answer to your question, I am sure. However, I am not sure that you have correctly posed the question. Let me see if I understand what you are saying. Bear with me a moment.

You designed or calculated the friction loss for your piping system and determined that the loss at 60cuM/hr would be 14m, and for 30cuM/hr it would be 4m. These were, again calculated numbers as I read your post.

With this in mind, the first observation is that these numbers are based on flow, not the number or size of the pumps. If the flow and friction conditions are 60cuM/hr and 14m friction loss, that could be the product of four pumps operating or just one pump. The same result would be obtained. If the characteristics are 30cuM/hr and 4m friction, that doesn't imply, as your post states, only one pump operating. It is simply the product of the system into which you are pumping.

If, however, there are two pumps pumping into the system and producing 60cuM/hr, and they are identical pumps, it is allowable to assume that both are pumping 30cuM/hr, more or less (two identical pumps are never EXACTLY identical. There would still be some small, insignificant differences in them). They would both definitely be pumping against a friction loss of 14m because that is what the system is "seeing" at that time.

With the flow of 30cuM/hr you have calculated a friction loss of 4m. If that is so, it doesn't necessarily mean that one operating pump will deliver that amount. As a matter of fact, it is probable that with one pump (of the size and configuration you are using) operating, the resultant flow would be closer to 40 cuM/hr because, if the flow at 14m is 60cuM/hr, it is not possible for one pump to pump 30cuM/hr at 4m since it has already demonstrated that it will produce 30cuM/hr at 14m. Something would have to change.

If your dilemma is to get more flow from the two existing pumps, you have only one option: increase the "tip speed" of the pump impeller. There are two ways to accomplish that fete. One is to increase the rotational speed of the pump and the other is to increase the diameter of the impeller. Either one of these would produce more flow but you must be mindful of the horespower requirements. They would also increase.

You have posed an interesting question. If you need to have further information, you might be best to contact a good pump salesman. You mentioned static head and outlet pressure. These values will impact your sloution as well. If you can supply the length of the forcemain, the static head, the diameter and type of the pipe and the medium being pumped, it is quite simple to calculate the system head requirements. Then, as stated by previous answers, you simply impose your system head curve on the manufacturer's pump characteristic curve and the delivery point will be the point of intersection of the two curves. Good luck. Let me know if I can help further.