Parallel and single use of centrifugal pumps
Thread Content
I’m completely puzzled by the following question; I’m seeking help from all fellow netizens! Replace the centrifugal pump, which is essentially a regular clean water pump. Originally, there were two identical pumps connected in parallel, with pump parameters of a flow rate of 8 m3/h and a head of 30 m. It is changed to use a single pump, with a flow rate of 18 m3/h and a head of 40 m. The pressure gauge at the pump outlet shows an increase from 0.15 MPa on the original two pumps to 0.3 MPa, but the flow rate is lower than before... What’s going on? :Q Please help me fill in the gaps in my basic knowledge……
This is a problem with the pump curve; the curve has humps, which makes parallel operation unsuitable!
This situation is probably not a problem with the pump. The modified pump exhibits an increased flow rate and head compared to when it was connected in parallel before; according to the principles of water pumps, the flow rate will be higher than the rated value, as the increased head leads to a higher flow velocity, and a higher flow velocity results in an increased flow rate. This will not happen unless you reduce the outlet valve. There are a few doubts now: 1. Is the valve at the pump outlet or on the pipeline closed too tightly? Is there any damage to the valves? 2. Is there a bypass or something that directs the water to another place? 3. Check whether the pipes are blocked, such as due to scaling; this is likely to occur with hot water.
Reply to 5# trampman: Mm-hmm~ I’ll take note! But... in the entire system now, only the pump and a small section of piping at its inlet and outlet have been modified; everything else remains as it was. If there is scaling in other pipelines and systems, then the same issue of scaling occurred when those two pumps were used in parallel earlier; it shouldn’t suddenly appear after just a few days of shutdown... The pump outlet valve... shouldn’t present any problems... Also, I would like to ask: in theory, it’s better if the diameter of the pipe leading into the pump is larger than that of the pump’s outlet pipe. Since the diameters of the inlet and outlet pipes of this pump are the same, will that have any impact? How big of an impact is it likely to have?
Each case needs to be analyzed on its own, after all I can’t see the situation on site. It has no effect on the pump if the inlet and outlet are the same size.
This post was last edited by zjywwjg on 2010-6-30 08:52. Generally, the inlet diameter of a pump is larger than its outlet diameter; this helps to increase the head at the pump’s outlet and also raises the pressure there. Since the flow rate remains constant, the smaller the diameter, the higher the flow velocity and the greater the pressure. With a constant pump power, flow rate is inversely proportional to flow velocity; the smaller the flow rate, the greater the flow velocity, and consequently the higher the head and pressure. Personally, I think the pump outlet diameter may have decreased after the modification.
The parameters provided by the poster are as follows: when two pumps are used in parallel, each pump has a flow rate of Q=8 m3/h and a head of H=30 m. However, during use the pressure gauge showed H=0.15 MPa; after switching to a pump with parameters of Q=18 m3/h and H=40 m, H became 0.3 MPa, but the flow rate was lower. When pumps are used in parallel, the flow rate increases significantly, but the head increase is not substantial. The parameters provided by the original poster aren’t quite correct. If there are no significant changes in the pipes (including fittings), valves, and pipeline resistance within the system. The following problems may exist: 1. The motor connected to the pump does not have sufficient power, 2. There are air leaks in the pump’s inlet pipeline, resulting in insufficient flow rate. 3. There are issues with the manufacturing quality of the pump itself; the sealing ring between the pump body and the pump cover is damaged, causing the liquid to flow back into the inlet. This is just my personal opinion, for the original poster’s reference.
Originally, there were two identical pumps connected in parallel, with pump parameters of a flow rate of 8 m3/h and a head of 30 m. It is changed to use a single pump, with a flow rate of 18 m3/h and a head of 40 m. The pressure gauge at the pump outlet shows an increase from 0.15 MPa on the original two pumps to 0.3 MPa, but the flow rate is lower than before... What’s going on? The head is 30 meters, and the pressure gauge at the outlet reads 0.15 MPa – why is the pressure drop so large? There’s a problem with the pump, right?
When two identical pumps are connected in parallel, the flow rate is less than twice that of a single pump; the head is determined by the pipeline characteristics