Understanding the net positive suction head of a pump
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The net positive suction head of a pump is divided into effective net positive suction head and required net positive suction head. The net positive suction head that must be available at a given speed and flow rate is known as the required net positive suction head, denoted by NPSHr. Also known as the net positive suction head available, NPSHr is related to the internal flow within the pump and is determined by the pump itself. Its physical meaning is to indicate the degree of pressure drop at the pump’s inlet, and it serves to ensure that cavitation does not occur in the pump. The net positive suction head must be independent of the device parameters, and depend only on the dynamic parameters of the pump’s inlet section; these dynamic parameters are determined by the geometric parameters at a given speed and flow rate. In simple terms, NPSHr is determined by the pump itself. For a given pump, such as the LHF fluoroplastic centrifugal pump, regardless of the medium used (with the exception of cases where high viscosity of the medium affects the gravitational flow rate), for example, in the case of the LHF50-32-125 model with a flow rate of 12.5 cubic meters and a head of 20 meters, since the inlet pressure velocity is the same, the pressure remains constant; thus, its NPSHr value is also the same. The lower the inlet pressure, the lower the NPSHr value will be.
The effective net positive suction head, usually denoted as NPSHa, also known as the plant net positive suction head, is the excess energy per unit weight of liquid at the pump inlet provided by the suction system, which exceeds the vaporization pressure head. That is, it can be understood as being related to the material of the pipeline. The larger the NPSHa, the greater the effective net positive suction head, and the less likely the pump is to experience cavitation. Put simply, the hydraulic loss of the suction device is proportional to the square of the flow rate; therefore, NPSHa decreases as the flow rate increases.
Is there a better way to explain it? Many people can’t understand it
This post was last edited by 3983596_FPPZ on 2019-1-12 at 16:28. Headroom is an energy value, which represents the difference in energy above the vaporization pressure of the medium (it is also referred to as excess pressure energy in many sources). Reference: Vapour pressure of the medium. Pump inlet pressure – vaporization pressure = NPSHA ； The NPSHA of a device refers to the energy level at the pump inlet that is higher than the vaporization pressure of the medium; it is determined by all the equipment in the system connected to the pump as well as the actual process parameters, and represents the margin available to prevent cavitation in the pump. Inlet pressure – vaporization pressure = NPSHA, the net positive suction head of the pump. As the flow velocity of the fluid entering the impeller increases, the ability of the blades to do work has not yet been fully utilized, yet the pressure decreases (according to Bernoulli’s equation). At a certain distance behind the blades at the inlet, there is inevitably a point where the pressure is at its lowest; this creates an additional decrease in pressure energy between the pump inlet and that point with the lowest pressure. If the pressure energy at this point is lower than the vaporization pressure of the fluid, cavitation occurs. Therefore, to prevent cavitation, the pump inlet must be supplied with an energy value (pressure energy) that is greater than the additional loss of pressure energy due to the blades; this pressure energy is the difference between the device’s net positive suction head available (NPSHA) and the vaporization pressure. The higher the pump inlet pressure (the greater the NPSHA), the more pressure head is available for the pump, and thus the less likely it is to suffer from cavitation. The lower the lowest pressure point on the blades (the smaller the NPSHR), the less likely the pump is to experience cavitation, and it can operate at a lower supply pressure. In summary, the pressure drop between the pump inlet and the impeller blades is the cause of pump cavitation, and it also serves as a criterion for measuring the pump’s cavitation margin. The greater the pressure drop, the larger the cavitation margin; consequently, a higher inlet pressure is required to prevent cavitation.
NPSH is the absolute total suction head based on the vapor pressure of the liquid, measured from a reference level. NPSH is the head of the liquid being pumped, expressed in meters (feet). The net positive suction head available (NPSHa) of a device is also known as the effective net positive suction head. It is a parameter that depends on flow rate and pumping temperature; it represents the remaining pressure head of the liquid after subtracting the vaporization pressure head from the pressure head of the liquid at the pump’s inlet. The net positive suction head required, which is determined by the buyer based on that pump system (under rated flow rate and normal pumping temperature), is also known as NPSHr. This required net positive suction head refers to the pressure necessary when liquid enters the impeller inlet from the pump’s inlet; under the effect of the centrifugal force exerted by the impeller blades, the velocity of the liquid increases while its pressure decreases. The required net positive suction head is the head value at the suction side (such as the pump inlet) necessary to prevent fluid cavitation (provided by the manufacturer). Several names for the required net positive suction head: NPSHi (i.e., Cavitation inception): it takes the occurrence of the first bubble as the criterion for cavitation, and is more stringent than NPSH3 and NPSH0. It is easy to define, but the required measurement techniques are complex and extremely costly. It is commonly used in high-suction-energy pumps with high inlet flow velocities. NPSH0: The point at which the head begins to drop rapidly, but drops to 0%, is used as a criterion for cavitation. It is easy to define, but difficult to measure. It is commonly used in axial flow pumps and mixed-flow pumps. NPSH3: A 3% decrease in head (the head of the first stage) is used as the criterion for cavitation; it is easy to measure and relatively reliable. It is widely used in various centrifugal pumps. NPSHFC (Full cavitation): At a certain suction head, the head loss decreases very sharply.