4 Pumps in parallel
4 Pumps in parallel
(OP)
I have been reviewing threads on pumps running in parallel here and on other sites. I seem to find a lot of information on 2 pumps running in parallel and then the information on 3 pumps running in parallel and how it is pretty much useless……I have 4 pumps, all identical performance characteristics, all running at the same speed, all discharging into a common header and am attempting to develop a set of curves that accurately depicts what is happening as 2 pumps are running, 3 pumps are running and 4 pumps are running. It would seem though that after two pumps there is little to no change in performance. It this true or am I making to broad an assumption? The fact that the additional two pumps are not increasing my capacity or head of any appreciable quantity would not surprise me. I have been an engineer at this steel mill for a little over a year and have found some INTERESTING engineering solutions. Thanks in advance for any input you folks can provide.
RE: 4 Pumps in parallel
RE: 4 Pumps in parallel
RE: 4 Pumps in parallel
You will find that vanstoja is right with a couple possible exceptions.
1) If the intake of the four pumps is one common line or source when the third and fourth pumps start you may be starving them. You need to review the intake configuration.
2) The four pumps are additive. The system curve (friction loss etc.) will change with Q. If one pump will produce 10 GPM into a common system two pumps may only produce 19 GPM due to the system limits or changes.
I have made a spreadsheet in the past that calculates four pumps connected in parallel. One pump by itself would produce a little over 900 GPM through a 11” pipe, 11000 feet long. With all four pumps operating total Q was about 3000 GPM due to the increased system losses. This sheet was a prediction before the pumps were installed. The actual operation was very close to my prediction.
Good Luck
D23
RE: 4 Pumps in parallel
Then if you draw on your system curve it will be clear what the problem is - too much system friction. You are trying to push too much liquid down the pipe because the pumps develop insufficient head to do it. You will see that the curves show each pump adds progressively less additional head - the combined curve gets flatter. If the system had negligible friction (e.g. pumping into an elevated reservoir through a really big pipe), the system curve would be flat and the flow would be proportional to the number of pumps running. However in your case, increasing flow also requires increasingly more head to deliver it.
Either reduce system resistance, look at speeding up the pumps or fitting bigger impellers (and probably motors). Or look at running the pumps in two parallel trains of two pumps each.
Cheers
Steve McKenzie
RE: 4 Pumps in parallel
The head loss component of the total head on the pumps increases as you bring more pumps online, due to the fact that the pipeline friction loss increases as the square of the flow rate change, ie., double the flow rate increases the friction loss by a factor of 4.
So lets assume that 1 pump will deliver 100 gpm and the friction head in the pipeline is 2 feet loss, the second pump increases the flow to 200 gpm the friction head is now 8 feet loss, the 3rd pump increases flow to 300 gpm the head loss is now 18 ft the 4th pump increases the flow to 400 gpm and the friction loss becomes 32 ft loss.
As Steve has pointed out, because of the increased friction loss plus your static head(read total head)it is forcing the pumps left along the performance curve reducing your flow rate.
I suggest you draw the combined pump and system curve so you see the real situation, it's always an interesting exercise and tells all.
International College
Naresuan University
Phitsanulok
Thailand
RE: 4 Pumps in parallel
But that does not mean that running 3 or 4 pumps in parallel --- is pretty much useless…….
I have worked on a number of stations with 6 pumps in parallel. - The advantages of using larger number of pumps are:
1) Allows the use of fixed speed rather than variable flow pumps to match variable demands.
2) Lower starting currents.
3) Lower voltage.
4) reduced stand by capacity (eg if you have only one duty and a stand-by the satnd-by capacity is 100%. If you have 3 duty and one stand-by the stand-by capacity is 33%).
Disadvatages include increased capital costs.
It is a question of selecting the right number of pumps for the system and operating conditions taking account of available voltage, voltage drop, capital and running costs etc.
RE: 4 Pumps in parallel
International College
Naresuan University
Phitsanulok
Thailand