Why can’t the pressure and flow rate of this pump be increased?

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The last edit to this post was made by He Xiaowo on 2017-9-1 at 12:08. The image shows the test bench for the pump we developed, which is used primarily to measure the pump’s flow rate and head. It seems that there is no problem with the pump itself. The pipes used are of DN100 diameter. The pump’s rated head is 23.2 meters, and its rated flow rate is 76 m3/h. However, during testing, it was found that neither the head nor the flow rate could reach these values. When the outlet valve was fully open, the maximum flow rate recorded was 23 m3/h. When the outlet valve was completely closed, the maximum pressure reached was only 0.02 Mpa, which corresponds to a head of just 2 meters. There is a significant difference between these values. Additionally, it was observed that the pump’s temperature rose significantly during testing. This issue occurred with several different pumps. I would like to know where the problem might lie. Thank you everyone. I’m not sure what type of pump this is; “Transformer oil pumps are of fully sealed design, with a three-phase asynchronous motor driving a centrifugal or axial-flow vane pump directly.” This information was found online. The flow and pressure sensors also seem to be functioning properly. The blue-green device in the image is the pump, and wave-shaped expansion joints are installed at both ends to compensate for any displacement. The liquid level in our tank is at 2/3 of the tank’s height, which is above the level of the pump itself. Under normal conditions, the inside of the pump should be filled with water. The lid of the tank is open. Regarding the issue of cavitation mentioned by some people, since the initial liquid level was already above the pump’s level, once the pump was connected, water flowed naturally into the pipes and within the pump, and the liquid level remained well above the pump’s level. Therefore, there shouldn’t be any problem with cavitation. Additionally, through subsequent tests, we removed the elbow at the outlet and replaced it with a long hose, and both flow rate and pressure returned to normal; it still seems that the problem lies with the elbow at the outlet.