There are many reasons why the outlet pressure of the multistage pump decreases, basically are the general points:
(1) Motor reversal
As a result of wiring, the rotation direction of the motor will be opposite to the actual requirement of the centrifugal pump. In this way, the rotation direction of the pump should be observed before starting. If the rotation direction is reversed, any two wires on the terminal of the motor should be exchanged for wiring.
(2) Deviation of working point to large flow and low head
In general, the multistage pump has a continuous downward performance curve, and the flow rate gradually increases with the decrease of the head. In the process of operation, due to some reasons, the back pressure of the pump decreases, and the working point of the pump passively shifts with the curve of the device to the point of low head and large flow, which will cause the head to decrease. In fact, this is caused by the change of external factors such as the device, which has no special relationship with the pump itself. At this time, as long as the pump back pressure is increased, such as closing a little outlet valve, the problem can be solved.
(3) Speed reduction
The important factors that affect the head of centrifugal pump are the outer diameter of impeller and the speed of pump. Under other conditions, the head of pump is in a positive proportion to the second power of speed. It can be seen that the speed has a great influence on the head. Sometimes, because of some external reasons, the speed of pump decreases, the head of pump will be reduced accordingly. At this time, check the speed of the pump. If the speed is not enough, check the cause and solve it reasonably.
(4) Cavitation at inlet
If the suction pressure of the multistage pump is too low, which is lower than the saturated steam pressure of the pumped medium, cavitation will be formed. At this time, check whether the inlet pipeline system is blocked or the opening of the inlet valve is too small, or increase the liquid level of the suction pool.
(5) Internal leakage
When the clearance between the rotating part and the static part of the pump exceeds the design range, it will cause internal leakage, which is reflected in the decrease of the discharge pressure of the pump, such as the clearance between the impeller ring and the interstage of the multistage pump. At this time, corresponding disassembly and inspection shall be carried out to repair or replace the parts with excessive clearance.
(6) Impeller passage blocked
If part of the flow passage of the impeller is blocked, the work of the impeller will be affected and the outlet pressure will drop. Therefore, it is necessary to dismantle the pump to check and remove foreign matters. In order to prevent this problem from happening again, filter device can be added in front of the pump inlet if ne
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