Several concepts related to pump net positive suction head

When the pump is in operation, vapor is generated at the inlet of the impeller due to a certain vacuum pressure. The vaporized bubbles, driven by the impact of liquid particles, cause erosion on metal surfaces such as those of the impeller, thereby damaging them. This vacuum pressure is known as the vaporization pressure. The net positive suction head is the excess energy per unit weight of liquid at the pump’s inlet, above the vaporization pressure. The unit is indicated in meters, as (NPSH)r. The suction lift is equivalent to the required net positive suction head Δh: it represents the vacuum level at which the pump can draw in liquid, or in other words, the maximum installation height allowed for the pump, measured in meters. Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters); the standard atmospheric pressure can create a vacuum in the pipeline up to 10.33 meters. Cavitation phenomenon: At a certain temperature, when the pressure of a liquid is reduced to its vaporization pressure at that temperature, bubbles form in the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation, as they move to areas of higher pressure, shrink in size and eventually burst. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse. While the pump is in operation, if the absolute pressure of the liquid being pumped in a certain local area of its flow path – usually somewhere slightly downstream of the inlet to the impeller blades – drops to the vaporization pressure of the liquid at that temperature for some reason, the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. As the bubbles coalesce and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect, which strikes the metal surface at a high frequency. The impact stress can reach several hundred to several thousand atmospheres, while the impact frequency can be in the tens of thousands of times per second. In severe cases, this can cause the wall thickness to be breached. The process in which bubbles are formed and burst in a water pump, causing damage to the flowing components, is known as cavitation in water pumps. After cavitation occurs in a water pump, in addition to causing damage to the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance; in severe cases, it can even disrupt the flow of liquid within the pump, preventing it from functioning properly. NPSH: Refers to the difference between the total head of the liquid at the pump inlet and the pressure head at the point where the liquid vaporizes. It is expressed in meters (of water column) and denoted as (NPSH). It can be categorized as follows: NPSHa – also known as available NPSH; the higher this value, the less likely cavitation will occur ； NPSHr —— pump net positive suction head, also known as the required net positive suction head or the dynamic pressure drop at the pump inlet; the lower this value, the better the pump’s resistance to cavitation ； NPSHc —— Critical net positive suction head, refers to the net positive suction head at which the pump’s performance declines by a certain amount ； NPSH — allowable net positive suction head, which is the net positive suction head used to determine the operating conditions of a pump; it is usually taken as (1.1～1.5)NPSHc.

Although it’s some basic knowledge, it’s extremely important. It’s explained very clearly; these are arguably the most fundamental and crucial concepts in pump calculation. This is my first post, shared here with everyone – please feel free to give me your advice!

The analysis on the 2nd floor is very thorough; thank you so much!

“The suction lift is equivalent to the required net positive suction head Δh: it represents the vacuum level at which the pump can draw in liquid, or in other words, the maximum installation height allowed for the pump, measured in meters. I’m still not quite clear about the sentence \"Suction lift = Standard atmospheric pressure (10.33 meters) – NPSH – Safety margin (0.5 meters)\". If the suction lift represents the required NPSH, then why is another value related to NPSH subtracted from it in the formula? What does the net positive suction head available at the back refer to?