Questions regarding the net positive suction head of the device
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I don’t know much about pump technology, yet sometimes I’m responsible for liaising with pump manufacturers. Today, I contacted pump manufacturers to request selections and quotes. They asked me to provide the net positive suction head of the installation. I roughly understand how to calculate the installation height of a pump. For example, under standard atmospheric pressure, 10.33 meters minus the pump’s required net positive suction head minus a 1-meter safety margin gives the pump’s installation height. I provided the pump salespeople with examples, such as when the water level in the sump is 0.5 meters or 2 meters higher than the pump inlet; conversely, when the water level in the sump is 2 meters lower than the pump inlet, meaning it’s a 2-meter suction lift. As I understand it, when pump manufacturers make their selections, they can reasonably determine the required net positive suction head based on the installation height of the pump that I provide. For example: 10.33 – 0.5 – 1 = 9.83 meters; 10.33 – 2 – 1 = 11.33 meters; 10.33 – 2 – 1 = 7.33 meters. However, the sales representatives from the pump manufacturer keep saying that I failed to clearly specify the required net positive suction head for the system, leading to various disputes and problems. Excuse me, is the installation height of the pump that I provided correct or reasonably close to the actual value? I really can’t provide any design-related net positive suction head values. Thank you for your reply!
It is recommended that this issue be referred to the manufacturer, preferably. You’re only responsible for providing a schematic diagram indicating the medium and conditions... Leave the rest to the pump manufacturer to handle. It’s a buyer’s market now; don’t forget that you’re God, dude! ;P
Calculation of the net positive suction head available at the pump (NPSHa): The formula for calculating the effective net positive suction head provided by the input system to a centrifugal pump is as follows: Where: NPSHa = (P1 – Pv) / (9.81γ) ± H1 – (ΔP1 + ΔPe1) × K2 / (9.81γ). NPSHa represents the effective net positive suction head provided by the input system to the pump, measured in meters of liquid column； P1 — Minimum normal operating pressure of the equipment on the pump suction side, (kPa) ； Pv – Saturation vapor pressure of the liquid at the pump inlet (at section S-S), (kPa). The calculated temperature is the temperature under the operating conditions of the input system. If the liquid is a hydrocarbon mixture, Pv must be measured using the bubble point method; however, during the design process, the saturated vapor pressure of the medium at the transport temperature is often used instead. γ — relative density of the liquid at the transport temperature ； g – acceleration due to gravity, g = 9.81 (m/s²) ； H1 – Minimum liquid level at the pump suction side to the geometric center of the pump* (m of liquid column). * H 1: Minimum liquid level at the suction side equipment to the geometric center height of the pump. If unknown, in engineering practice, the vertical distance from the minimum liquid level at the suction side equipment to the top surface of the pump’s foundation may be used as a substitute. In calculating this formula, when the liquid level is below the center of the pump (suction), H1 takes a “–” value; when the liquid level is above the center of the pump (priming), H1 takes a “+” value. △P1 — Pressure drop due to pipeline friction at normal flow rate between the suction-side vessel and the pump suction inlet (including fittings, valves, etc.), (kPa) ； △Pe1 — Sum of pressure drops across equipment on the pump suction side under normal flow conditions (kPa) ； K – The flow safety factor of the pump; it is the ratio of the pump’s design flow rate to its normal flow rate. This is used when the flow rate is calculated based on the normal flow rate ； If the design flow rate is used in the calculations, then K=1. Precautions regarding the available net positive suction head provided by the pump inlet system: 1) The available net positive suction head at the pump inlet, NPSHa, must be greater than the required net positive suction head for the selected pump, NPSHr, in order to ensure the safe and stable operation of the pump. When the material on the inlet side is under reduced pressure, or when the operating temperature is close to the boiling point of the conveyed material, the cavitation safety factor of pumps used in such situations (such as pumps at the bottom of reduced-pressure towers) should be greater than or equal to 1.3. 2) Note: The required allowable net positive suction head (NPSHr) for the pump, as provided by the pump manufacturer, is determined through cavitation tests conducted using water as the medium. When the material system and test conditions of the installation differ from those used in the tests, the required NPSHr value must be recalculated accordingly. 3) When the available net positive suction head NPSHa provided by the pump input system is relatively low and fails to meet the required allowable net positive suction head NPSHr for the pump, the commonly used methods are: a) Increase the liquid level difference; for example, raise the liquid level in the container on the suction side ； b) Where possible, lower the installation height of the pump, etc. In short, it is necessary to ensure that the available net positive suction head (NPSHa) provided by the pump’s inlet system is greater than the required net positive suction head (NPSHr) for the selected pump. (3) Relationship between the required and available net positive suction heads: To guarantee the normal operation of the pump without any cavitation, the available net positive suction head (NPSHa) must be greater than the required net positive suction head (NPSHr). Generally, the net positive suction head available (NPSHa) for a pump must be at least 0.3 meters greater than the net positive suction head required (NPSHr): (NPSHa) > 0.3m + (NPSHr) ； If the temperature of the fluid being transported is close to the boiling point of that liquid, the following condition must be met: (NPSHa) ≥ 1.3(NPSHr).
Generally, it is the design institute or the owner who provides the net positive suction head of the equipment; this value can also be roughly calculated. However, the inlet pressure or the height of the inlet liquid level, as well as the medium temperature and saturation vapor pressure, must be provided； It’s a minor issue; under normal operating conditions, cavitation won’t occur. Just pay attention to it in high-temperature environments
Yes, I provided the data you mentioned. Thank you!
Thank you! Pretty much the same way I provide information.
For ordinary water pumps, providing this much information is usually sufficient for manufacturers to make a recommendation. However, when it comes to purchasing high-end brand pumps, if the manufacturer has no idea what kind of medium will be used or what operating conditions apply, they dare not make the sale—they can’t afford to bear the costs in case of any problems. Domestic customers often sign contracts that consist of just one page; whenever issues arise, they blame the pump itself and expect the pump manufacturer to resolve all problems
What manufacturers need is the required net positive suction head or effective net positive suction head. The effective net positive suction head refers to the net positive suction head when the pump is installed. Manufacturers select the required net positive suction head for the pump based on this effective value； Or you can simply provide the required net positive suction head of the pump. When calculating the effective net positive suction head, it’s necessary to know the resistance losses in the pump’s suction piping. Without knowing the size and length of the pipes, this cannot be determined.