Analysis of the reasons why high-speed centrifugal pumps fail to deliver flow

The possible reasons why a centrifugal pump does not deliver adequate flow are as follows: 1. Blocked flow path; 2. Clogged inlet filter; 3. Inadequate exhaust leading to air locking; 4. Cavitation during operation; 5. Mechanical failures, such as a detached impeller. Based on the owner’s description, if it’s not air locking, then it’s likely a mechanical failure – the owner should analyze the situation further

I think the most likely reason is that the gas inside the pump has not been completely expelled. Typically, during exhaust, any gas present inside the pump, such as in the inlet or within the pump’s flow channels, even in small amounts, will affect the pump’s operation and cause cavitation. And the gas at the pump outlet does not cause cavitation in the pump. Pump cavitation will affect the operating performance of the pump, reducing its flow rate and head. You said that it’s fine to wait 2 or 3 hours before starting the pump; this means that when the pump is stopped, the small amount of air trapped inside the pump gradually rises to the outlet as the fluid comes to rest, and starting the pump at that time will prevent cavitation. Furthermore, based on what you have said and since the parameters of the pump have not been provided, I would estimate from my personal experience that the pump’s minimum NPSH is close to the NPSH required under its operating conditions; therefore, even a small amount of gas inside the pump can cause cavitation. To avoid this issue, and given that the current conditions of the pump cannot be changed, it is necessary to pay special attention to the starting, stopping, and switching operations of the pump, as well as to closely monitor the inlet pressure and the composition of the fluid, in order to prevent conditions that could lead to cavitation. Regarding your experience with pump venting, my experience is that the method of venting must be determined based on the properties of the medium involved, and it’s also necessary to use some experience to determine whether the venting has been thorough. For liquid hydrocarbons, the exhaust gas is usually discharged into a venting system; the outlet exhaust valve can be opened to release the gas, then closed after a while, and this process is repeated several times. The temperature of the pipe behind the exhaust valve should be lower than that of the medium inside the pump. Whether venting is indeed necessary depends on individual operational experience ； In general, for media that are close to water, it is possible to first vent at the inlet and then at the outlet. During venting at the outlet, ventilation should be carried out for a certain period of time before closing the vent valve and allowing it to rest for a while before venting again. This process should be repeated several times; whether complete ventilation has been achieved ultimately depends on the actual conditions on site and individual experience. It is essential to accumulate experience at work and keep summarizing what has been learned. In the field, there are situations that require the on-site operator’s own experience to make judgments; theory and the experiences of others merely provide possibilities for making such judgments on site. The above are things I saw on the forum; I think they are quite classic.

1. There is air inside the pump or in the gasoline. Remedial action: Open the backflow valve to completely exhaust the air. 2. The suction head is insufficient. Remedial action: Increase suction pressure and turn on the backflow exhaust. 3. Blockage in the inlet and outlet pipelines. Remedial action: Remove the blockage. 4. There is a foreign object in the impeller. Remedial action: Inspect the impeller and remove foreign objects. 5. The gasoline viscosity exceeds the specified value. Action taken: Check the gasoline components and take appropriate action. 6. Wear of the mouth ring. Remedial action: Repair or replace. 7. The speed of the prime mover is insufficient. Remedial action: Increase the speed of the prime mover.

The poster should check whether there are any high points in the inlet piping from which air cannot be discharged. In a vertical high-speed pump, the inlet pipeline is located at a higher level than the suction inlet of the impeller; therefore, an exhaust port must be installed at the highest point of the inlet pipeline, and the air there must be removed or evacuated before starting the pump. We have encountered this kind of thing many times.

It is also possible that the diameters of the inlet and outlet pipelines of the pump are insufficient, especially the inlet pipeline. High-speed centrifugal pump impellers rotate at high speeds; if the liquid pumping rate is insufficient, it can have an impact on the impeller, ultimately resulting in an inability to pump liquid

Last time, the centrifugal pump in our factory failed to deliver the required flow rate because the gap between the impeller and the volute was too large; reducing that gap slightly resolved the issue

The inlet filter is clogged, the liquid level in the inlet tank is too low, the inlet pipe is too narrow, there is excessive pipe resistance, and not all of the gas was expelled after pumping

1. Inlet pipeline resistance (filter clogging). 2. Low inlet pressure. 3. Mouth ring clearance.

Is there a leak in the inlet pipeline, causing air to be drawn in?