Each pump made by the manufacturer has different characteristics in accordance with the function and design of the manufacturer. This pump characteristic curve is influenced by the size and design of the pump, the diameter of the impeller, as well as the speed of its operating rotation. Pump characteristics are shown through a head capacity vs discharge pump curve.
The above characteristic curve of the pump is also known in engineering and industrial world as the Pump Performance Curve.
If a particular pump is kept constant in its rotation speed, then we can shift the performance curve by varying the size of the impeller diameter as below.
Similarly, if we keep the pump impeller diameter in constant condition, then we vary the speed of the pump rotation, then we can also shift the pump performance curve to the right or left.
The variation of pump conditions above does seem less prevalent. But in the industrial world it is a common thing. In Steam Power Plant for example, the main pump that supplies water to the boiler must be able to vary the discharge water flow in accordance with the needs of water vapor that will be produced by the boiler. Changes in electrical load then the need for water vapor is also different. The variation of pump rotation speed becomes a reasonable solution for use in this industry.
Additional Components of Pump Characteristics Curves
There are other things we need to know about some of the parameters that are usually included in the pump characteristics curve. The first is the Brake HorsePower (BHP) information required to operate the pump. BHP, also known as pure engine power, is a unit of power designation of a machine before it’s reduced by losses due to system design or other losses.
Keep in mind that the BHP information on the pump characteristics curve is for a water fluid that has a specific gravity value = 1. If the pump will be used for another fluid, then the BHP value must be calculated first. For example the fluid to be used is gasoline with a specificity value of 0.72, then the required value of BHP is:
5 bhp x 0,72 = 3,6 bhp
Other information provided with the pump characteristics curve is usually the point of its hydraulic efficiency. Best Efficiency Point (BEP) / hydraulic efficiency is the pump efficiency that has been reduced by losses due to hydraulic effect.
The third parameter is Net Positive Suction Head Required (NPSHR). NPSHR is a pump parameter which the value was obtained from the lab test. The NPSHR is a quantity to indicate the losses of the internal pump. The magnitude is determined by the pump design, its size, and its rotational operation.
Large NPSHR is affected by the rotation speed of the pump when used on the system. While the pump rotation depends on the design of the system itself. Another case with NPSH whose value is directly influenced by system design. The actual NPSH (Net Positive Suction Head) value must always be higher than this NPSHR value.
The last information on the pump characteristics curve we need to consider is the ability of the pump to lift the water from the inlet side to the outlet. This term we know as the priming lift.
On the curve above, the ability of the pump to lift water from a certain depth at each impeller diameter. This information is very important especially when we choose pump to be used on a deep system.