Why can a centrifugal pump be operated with the impeller blocked for a short period of time?

1. A centrifugal pump can withstand pressure buildup because a check valve (foot valve) is installed at the inlet of the pump’s suction line. 2. Before starting a centrifugal pump, it is necessary to fill the pump casing and the suction pipe with water. Then the motor is started, causing the pump shaft to drive the impeller and the water to rotate at high speed; as a result, the water undergoes centrifugal force and is thrown toward the outer edge of the impeller, flowing through the channels in the volute-shaped pump casing into the pressure pipeline of the water pump. After pressure buildup, the flow rate is at its minimum, the work done by the impeller is minimal, and the motor current and power are also at their lowest. Therefore, for large centrifugal pumps, the outlet valve must be closed first at startup, and then gradually opened after the pump has started running

Generally, check valves are installed at the outlet of the pump. . . . . Could the senior please explain when it is installed at the inlet in such cases?

Installing a check valve at the outlet allows the outlet pressure of the pump itself to be measured. At the inlet, self-priming pumps are commonly used. The reason for this is that when the pump stops operating, the water inside the pump does not flow downward due to the large diameter of the pipes (which are vertical); this design allows it to be easier to refill the pump with water when it needs to be started again.

Hehe, right after I replied to the post, I thought of self-priming pumps! But personally, I think it’s not a good way to understand this issue; rather, it’s more appropriate to explain it from the perspective of energy conversion. If the shutdown time is extended indefinitely, the kinetic energy of the pump will be converted into potential energy, which is then released as heat – a situation that is detrimental to the proper operation of the pump. While short-term shutdowns do generate some heat, this amount is generally negligible!

Starting a centrifugal pump with the valve closed actually means forcing the pump to operate under pressure. In fact, ordinary mechanical devices have a margin of safety, so there’s usually no problem in the short term.

In a short period of time, the heat generated by the friction between the impeller and the liquid is not sufficient to vaporize the liquid. It also takes time for increased vibration to cause damage to the bearings. Therefore, a centrifugal pump can be operated with the pump shut off for a short period of time

Operating a centrifugal pump with a blocked impeller is equivalent to operating it at the far left end of its performance curve; the pump efficiency is at its lowest, and almost all of the shaft power is converted into heat in the fluid. Therefore, keeping the pump running without allowing it to stop will cause it to heat up more and more. If the pump is kept running for a short period of time, the temperature of the fluid won’t rise significantly enough to damage the pump’s components.

Pressure buildup in centrifugal pumps occurs after the pump starts operating when the outlet valve is not opened in time; it is a normal phenomenon. The drawbacks of pressure buildup are as follows: 1. Temperature rise – When pressure builds up, the impeller continues to do work and consume power. Although the motor’s power consumption isn’t very high at this point, this energy is not converted into pressure or kinetic energy of the fluid, but rather into heat energy within the fluid. The amount of heat generated in this situation is much greater than that during normal operation, resulting in a sharp rise in the fluid’s temperature. An elevated temperature has two negative effects on the fluid: (1) When the temperature reaches a certain level, cavitation can occur at the inlet of the impeller; (2) In the pump’s self-cleaning, self-circulating, and self-cooling systems, such as those found in mechanical seals, canned pumps, and magnetic drive pumps, self-cooling and self-lubrication functions are affected. A high fluid temperature is detrimental rather than beneficial for circulation. 2. Vibration – When the pump operates away from its optimal operating conditions, it generates significant vibration, which damages the pump itself. Combined with the vibration caused by cavitation, this can easily lead to damage to components such as sealing surfaces and rolling bearings. 3. Axial and radial forces: Surge is a brief condition that occurs when a pump operates far from its normal operating conditions. During this surge, the pump experiences high axial and radial forces, which puts significant stress on the bearings or shaft positioning mechanisms, potentially leading to damage to these components or a reduction in their service life. Therefore, pumps that are more robust can barely withstand such conditions; for those that are fragile and delicate, it is especially important to avoid keeping pressure on them for too long – the pressure should generally not be maintained for more than 5 minutes. In any case, this inevitable process – the occurrence of pressure buildup – should be minimized as much as possible. There are various reasons for pressure buildup in pumps; some of the ones that come to mind are as follows: Equipment issues: The outlet valve is damaged and cannot be opened, or the outlet pipeline is blocked. Space constraints: The outlet valve is located too far away, and the access path is difficult, taking a long time to reach it. Human factors: After starting the pump, other tasks get in the way, preventing the valve from being opened in time

Some suction pumps have a bottom valve at the inlet, which functions similarly to a one-way valve. But it doesn’t mean that one can hold back the pump because of this!