Is there a negative pressure inside the pump casing when a centrifugal pump is started and in operation?

If it’s negative pressure, how can there be a leak from the mechanical seal that causes spraying?

It’s not negative pressure, but a region of varying pressure. If a water pump can draw water from above the water surface, there is negative pressure; the water that is then pumped out has pressure

This post was last edited by A Le on 2016-1-14 08:27. It depends on the pressure of the equipment ahead of the pump inlet. For example, in the case of a liquefied gas pump, assuming that the inlet pressure of the upstream tank is 1.0 MPa, after the pump operates, the saturated pressure of the liquefied gas becomes very high; it is therefore impossible for there to be a negative pressure.

Before a centrifugal pump starts operating, the suction pipeline and the pump itself must first be filled with the liquid to be transported. When the motor drives the impeller to rotate, the blades cause the liquid inside the impeller to spin, and the liquid gains energy as it is ejected from the impeller. The liquid thrown out from within the impeller flows through the pump casing’s flow channel and diffuser before being discharged through the outlet pipe. At the same time, a vacuum is created inside the impeller, drawing the liquid into it. Because the impeller rotates continuously and evenly, the liquid is continuously and evenly thrown out and drawn in. This is the explanation in the book; there is a passage stating that a vacuum is created inside the impeller, which refers to the initial stage of operation. Once the liquid starts to be transported continuously, there should be no negative pressure anymore.

I think it should be lower than the inlet pressure at first; it can’t be considered a vacuum

There are high-pressure areas and low-pressure areas; negative pressure does not exist, unless it is a vacuum pump ? ? ? Search for the book \"Pumps and Fans\" online; it explains it clearly

It should be local intermittent negative pressure!

The explanations in your book lean more towards self-priming pumps, or in other words, centrifugal pumps that operate by suction (i.e., the liquid level is below the pump inlet). The pressure before this pump is usually at atmospheric pressure; since there is a column of liquid in the pipeline leading to the pump inlet, negative pressure exists at the pump inlet (even during operation). This is also merely the negative pressure at the inlet of the impeller; since the impeller generates pressure through its operation, the pump casing is in a high-pressure area, so there is positive pressure as well at the pump casing. However, in the case of backflow pumps, when the liquid level at the suction side is above the pump inlet or there is a certain pressure in the system ahead of the pump, the medium will automatically fill the pump chamber and pipes even if the pump is not running. Even during operation, there is no such thing as negative pressure.

Why can a pump increase the pressure of a fluid? How does its energy conversion take place? It is recommended that you refer to Equation 1-26 in \"Pumps and Compressors\" (2nd edition), edited by Qian Xijun and Chen Hong, to understand the meaning of this formula before discussing whether there is negative pressure inside the pump chamber. Sharing my personal opinion, the pressure within the pump chamber can be divided into static pressure and dynamic pressure. Dynamic pressure is the actual pressure measured when a fluid is in flow. To use a vivid analogy, for instance, several holes are drilled at designated positions on the pump chamber, and the pressure actually measured is… ； Static pressure, on the other hand, refers to the energy level that results from converting all the kinetic energy associated with fluid flow into potential energy (pressure), in addition to the dynamic pressure that has been measured. When discussing the poster’s proposition, it is very important to distinguish between the aforementioned concepts, because the perception that there are errors in the book stems from a failure to understand these concepts. Assuming that the flow area from the pump inlet to the tip of the pump impeller remains constant, and assuming no friction between the fluid and the pump impeller, the dynamic pressure remains unchanged at any point within the flow channel, while the static pressure increases. Assuming that the flow area from the pump inlet to the end of the pump impeller remains constant, but taking into account the frictional losses between the fluid and the impeller, the dynamic pressure decreases along this flow path from the pump inlet to the impeller, while the static pressure increases (unless the pump has a negative head) ； Assuming that it is necessary to take into account both the change in the flow area from the pump inlet to the tip of the impeller (which generally increases) and the frictional losses within the fluid domain between the impellers, theoretically, the dynamic pressure along this flow path from the pump inlet to the impeller should decrease; otherwise, how can the fluid flow from one direction to another? Otherwise, how can fluid flow from a low-pressure area to a high-pressure area? Or, at least, the two should cancel each other out. Returning to the original question, it can be broken down into two questions: 1/ Is there a negative static pressure inside the pump chamber? 2/ Is there a negative dynamic pressure in the pump chamber? First question: Unless negative pressure occurs at the pump inlet, there will be no negative static pressure inside the pump chamber ； Second question: This needs to be explained in three cases, based on the relationship between the inlet pressure and the frictional losses from the pump inlet to the impeller. If you understand the meaning above, there is no need to repeat it here.