Pump performance curves provide engineers with a wealth of information to guide pump selection, but for those new to the game, the sheer volume of data can often be daunting and overwhelming. Understanding pump curves is essential for anyone specifying circulating equipment.
Manufacturers publish all pump curves to reflect the thorough and rigorous testing carried out in the laboratory or at the factory. This testing allows the manufacturer to communicate how the pump curve will perform under a given set of conditions and which pumps will be the most effective solution for any given application.
The two main elements of the pump performance curve are total head and pump flow, which run on the vertical and horizontal axes of the pump curve graph.
The total head is shown on the y-axis. This number is expressed in feet or metres and represents the total pressure that the pump can overcome to achieve a specific flow rate. It encapsulates all resistance values, including friction head, static head, etc. The flow rate of the pump is shown on the x-axis. This number is used to measure the number of gallons or litres per minute (gpm or L/min) of fluid being pumped at any given point during operation.
Ceramic Slurry Pump
Flow and head are plotted on the main curve for the different impeller sizes, conveying how many gpm or L/min of fluid the pump will transfer depending on the total head conditions.
The pump efficiency line tells us the operating efficiency of a pump with a particular impeller size and head. Although the calculated efficiency of a pump will vary depending on the flow rate and head, the pressure in the system is usually lower than the value indicated by the pump curve. The optimum pump selection will therefore be slightly to the left of the optimum efficiency point (BEP). The preferred selection range is 85% to 105%.
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Impeller size - The impeller is the moving element that drives the fluid in the pump. The impeller size (also called "trim") on the pump performance curve helps engineers specify the best impeller size for their particular application.
Ceramic Slurry Pump
Brake Horsepower - Brake Horsepower (BHP) is the horsepower consumed by the pump at any point on the curve, as measured on a pony brake or dynamometer. These lines usually slope downwards from left to right on the curve and vary with impeller trim.
Required Net Positive Suction Head - The available net positive suction head (NPSHa) is the actual hydrostatic head pressure minus the total head loss in the system. If the NPSHa is less than that required for the pump to achieve the required flow and head (NPSHr), this means that the pressure at the impeller eye is lower than the vapour pressure of the fluid in the pump. NPSHr is shown as feet above the performance curve at each operating point and is positively correlated with flow rate.
NPSHr is an integral part of understanding the pump performance curve and is one of the most important calculations for specifying the correct pump for an application. All centrifugal pumps exert a negative pressure on the small space between the suction and impeller eyes due to sudden changes in velocity between the suction and discharge of the pump, changes in the direction of the fluid and increased turbulence.
The negative pressure is NPSHr - the minimum amount of pressure required at the pump suction inlet for the pump to operate properly without cavitation. The atmospheric pressure must also be taken into account when calculating the total head for the final NPSHr value.