During the commissioning of the centrifugal pump, the flow rate kept decreasing

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Colleagues, during the commissioning of skid-mounted equipment for a project recently, I encountered an issue where the flow rate kept dropping after the centrifugal pump started up. I would appreciate it if you could help analyze this problem. The process is as shown in the CAD diagram: the expansion tank is located at a higher position, with a capacity of around 1.5 m3, while the centrifugal pump is situated at a lower position. Its flow rate is 160 cubic meters per hour, and its head is 20 meters. The outlet of the centrifugal pump enters the shell side of the tube heat exchanger, where it is used to exchange heat with other fluids flowing in the tube side. After exiting the tube heat exchanger, the fluid goes into the plate-fin heat exchanger, and finally returns to the expansion tank; the on-site equipment is shown in the other two diagrams. The problem now is: 1) Since no valve is installed at the outlet of the centrifugal pump, only a butterfly valve is installed at the outlet of the shell-and-tube heat exchanger, before it enters the plate-fin heat exchanger; therefore, before starting the pump (after filling it with fluid), I first closed this valve, and at that time the pressure indicated by the pressure gauge was around 0.35 Mpa. After the valve was fully opened, the flow rate rose to 80 cubic meters per hour, but it began to decrease gradually, eventually dropping to 4 cubic meters per hour, with a pressure of 0.05 Mpa. 2) Initially, I suspected that there was a problem with the electromagnetic flow meter; therefore, to verify the discharge flow rate, I opened the outlet leading back to the expansion tank, and found that the flow rate indeed decreased and was below the optimal level. 3) There is a conical filter installed in the pipeline; I removed it and cleaned it as well, but that did not solve the problem. 4) I once again considered whether the flow rate of the centrifugal pump was too high, the volume of the expansion tank was too small, and the pumping time was too short, all of which could lead to cavitation. So I tried to reduce the valve opening and lower the flow rate in the pipeline, in an attempt to find a stable equilibrium point. However, the butterfly valve does not have strong regulating capabilities; if the valve opening is too small, the flow rate is too low, and while increasing the opening slightly raises the flow rate, it starts to drop again very quickly. 5) At the same time, I wonder if the head pressure is too low; the pressure drop across the plate-fin heat exchanger is relatively high. Therefore, I plan to disconnect the plate-fin heat exchanger tomorrow and have the fluid flow directly from the tube heat exchanger back to the expansion tank, in order to verify whether the flow rate will decrease further. Regarding the above debugging process, I would appreciate it if colleagues could analyze what other factors might be causing the current situation and share their insights