Performance Characteristics of Centrifugal Pumps
Although the theory of centrifugal pumps gives many qualitative results, the most important indicator of a pump’s performance lies in extensive hydraulic testing.
In industry, characteristics of all pumps are usually read from its Q-H curve or performance curve (flow rate – height). As can be seen, the performance charts use a discharge – Q (usually in m3/h) and pump head – H (usually in m) as basic performance variables.
In the chapter on head loss, it was determined that both major losses and minor losses in piping systems are proportional to the square of the flow velocity. It is obvious the system head loss must be directly proportional to the square of the volumetric flow rate, because the volumetric flow rate is directly proportional to the flow velocity.
It must be added that the open hydraulic systems contains not only the friction head, but also the elevation head, which must be considered. The elevation head (static head) represents the potential energy of a fluid due to its elevation above a reference level.
In nuclear engineering most of hydraulic systems are closed hydraulic loops and these systems only have friction head (no static head).
Pump Head – Performance Curve
In fluids dynamics the term pump head is used to measure the kinetic energy which a pump creates. Head is a measurement of the height of the incompressible fluid column the pump could create from the kinetic energy, that the pump gives to the liquid. The head and flow rate determine the performance of a pump, which is graphically shown in the figure as the performance curve or pump characteristic curve. The main reason for using head instead of pressure to determine the performance of a centrifugal pump is that the height of the fluid column is not dependent on the specific gravity (weight) of the liquid, while the pressure from a pump will change. In terms of pressure the pump head (ΔPpump) is difference between system back pressure and the inlet pressure of the pump.
The maximum pump head of a centrifugal pump is mainly determined by the outside diameter of the pump’s impeller and the shaft angular velocity – speed of the rotating shaft. The head will also change as the volumetric flow rate through the pump is increased.
When a centrifugal pump is operating at a constant angular velocity, an increase in the system head (back pressure) on the flowing stream causes a reduction in the volumetric flow rate that the centrifugal pump can maintain.
The relationship between the pump head and the volumetric flow rate (Q), that a centrifugal pump can maintain, is dependent on various physical characteristics of the pump as:
- the power supplied to the pump
- the angular velocity of shaft
- the type and diameter of the impeller
and the used fluid:
- fluid density
- fluid viscosity
This relationship is very complicated and its analysis lies in extensive hydraulic testing of certain centrifugal pump. As can be seen from the picture below.
Operating Characteristics of a Hydraulic Loop
When we put together the frictional characteristics (system head) of a hydraulic loop and the performance curve the result will describe the characteristics of entire system (e.g. one loop of primary circuit). The following figure shows typical performance curve for a centrifugal pump related to the system frictional head.
Pump head, on the vertical axis, is the difference between system back pressure and the inlet pressure of the pump (ΔPpump). Volumetric flow rate (Q ), on the horizontal axis, is the rate at which fluid is flowing through the pump. As can be seen, the head is approximately constant at low discharge and then drops to zero at Qmax. At low discharge the characteristics can be unstable (with positive slope of pump head). This is undesirable characteristics, because an unstable pump may start to oscillate between the two possible combinations of flow rate and the pipeline can vibrate.
At flow rate Q1 the pump gains more head than consumes the frictional losses, therefore the flow rate through the system will increase. The flow rate will stabilize itself at the point, where the frictional losses intersect the pump characteristics.
In order to characterize the performance of centrifugal pumps, the following terms are defined:
- Shut-off Head
- Pump Efficiency
- Best Efficiency Point – BEP
- Brake Horsepower
- Net Positive Suction Head