Most pump problems are caused by suction problems. In all the years in the pump business, I only found one instance of a pump problem related to discharge, except of course when the pump closed the discharge valve during operation. Closing the drain valve on the pump will cause the residual fluid in the pump to become very hot and damage the casing, bushings, seals, etc. Therefore, the focus of this article is on the correct pump installation for pump suction conditions. I will first introduce a few basic rules, and then introduce some other ideas or methods to ensure low maintenance costs and low-cost operation of the pump.

　　Before discussing the first rule of providing sufficient NPSH for pumps, we need to discuss the concept of "Head Feet of Head". The pump does not suck, but the pump pushes or throws the fluid out of the pump, creating a partial vacuum. Then (usually) atmospheric pressure pushes the fluid into the pump. For centrifugal force, the unit of this force is "feet of the head". The atmospheric pressure at sea level is 14.7 psia. At sea level, it can also be represented by 29.94 inches of mercury (barometer) or 33.9 feet of water. Therefore, at sea level, we can say that a storage tank has a pressure of 14.7 psia from atmospheric pressure, or that it has a head of 33.9 feet. The convention with centrifugal force is to measure pressure within the first foot. A quick formula to convert between "head and foot" and "psi":

　　Where does 2.31 come from? Divide the 33.9-foot head by 14.7 pounds per square inch. The specific gravity of water is 1.0. Therefore, the formula is always valid. If the specific gravity of the pumped fluid is known, the pressure gauge reading can be converted to "in feet", which is useful for determining the operating position of the centrifugal pump on its pump curve. A final note before discussing NPSH. The pressure of centrifugal pumps (inlet and outlet) is measured in feet of head, while the pressure of positive displacement pumps is usually measured in psi. One of the exceptions is the pneumatic diaphragm (AOD) pump, which is a positive displacement pump whose discharge pressure is measured in units of "head".

　　Rule 1. Provide enough NPSH

　　Simply put, if there is not enough inlet pressure, the pump will not operate normally and the pump will cavitation. Cavitation is caused by the rapid formation of vapor pockets (bubbles) in the liquid flowing in a very low pressure area and collapse in the high pressure area. It is usually a common cause of structural damage to the propeller or other parts of the pump. NPSHR or the required net positive and negative pressure head is a technical term used to determine the pressure energy (pounds per square inch or pound force) required to fill the pump inlet without cavitation. NPSHR is based on the design of the pump. This is a characteristic that mainly varies with pump speed and fluid viscosity.

　　The available NPSHA or net positive suction heads are based on the design of the system around the pump inlet. During operation, the average pressure (in pounds per square inch) measured at the inlet or minus the vapor pressure of the liquid at operating temperature. It indicates the amount of useful pressure energy available to fill the pump. What we want to ask is whether the system provides enough pressure to completely fill the pump without cavitation (considering the pump design, speed, fluid viscosity, etc.).

　　Where Ha = Atmospheric head is the pressure head or the pressure on the surface of the liquid in the tank we want to pump out (pressure is in head feet). In an open system like this, it will be atmospheric pressure, 14.7 psi or 34 feet of water.

　　Hs = the vertical distance (in feet) from the free surface of the liquid to the centerline of the pump impeller. If the liquid is lower than the pump, the value is negative.

　　Hvp = the vapor pressure of the liquid at the pumping temperature, in units of head.

　　Hf = friction loss in the suction pipe, in units of head.

　　Simply put, NPSHA is the result of the atmospheric head (pressure) pushing fluid into the pump. If the liquid level is higher than the pump inlet, the pump gets additional inlet head or pressure, and if the liquid level is lower than the pump, it gets a negative head. The weight of the fluid creates pressure. The pump loses its inlet head or pressure due to the friction loss of the fluid flowing through the suction pipe (small or long pipes have high friction). Finally, the inlet head or pressure is reduced by the steam pressure. If the fluid evaporates easily or the temperature is high, problems will arise. So NPSHA is positive or negative for atmospheric pressure

　　A final note about the pump NPSHR. Many pump manufacturers provide NPSHR curves for their pumps. The curve is determined in the laboratory using the method proposed by the Hydraulic Research Institute. The various points on the curve are determined by restricting the inlet pressure with a valve. The restricted inlet pressure can cause flow loss or cavitation. The NPSHR curve is drawn based on the pump losing 3% of its rated flow. At each flow point, a vacuum reading will be obtained at the inlet of the pump. These points are plotted below the pump curve and show the minimum inlet pressure required by the pump, but by definition, this lost flow is actually vapor bubbles and the pump is damaged. When installing the pump, make sure that the inlet conditions are much higher than the NPSHR requirements of the pump.