## Pump Performance Curve

The graph of the curve is shown dropping from top left to bottom right with increasing rate of flow. The slope of the curve is determined by the pump construction and particularly by the design of the impeller. The characteristics of the pump duty curve is the inter-dependent relationship between capacity and head. Each change of head effects a consequential variation in the rate of flow. High rate of flow -> low head Low rate of flow -> high head Though it is the frictional resistance of the installed pipe system which determines a given pump capacity, the respective pump can take up only one duty point on its curve. This duty point is the intersection of pump H-Q curve with the system H-Q curve. In addition to the Q-H performance curve, the following performance curves are often found for centrifugal pumps:
• Power
• Shaft power P2(Q)
• Electrical power consumption P1(Q)
• Efficiency
• Hydraulic efficiency ηhydr(Q)
• Total efficiency ηtot(Q)
• NPSH required NPSHreq(Q)
• Speed n(Q)

## Calculating the System H-Q Curve

The required pumping head in a branchless pipeline is determined from BERNOULLI’s equation for one-dimensional, stationary flow of incompressible fluids: pin, pout = pressures on suction respectively discharge liquid levels ρ = fluid density g = gravity (9.81 m/s2) Hgeo = static height difference between suction and discharge liquid levels Hl,tot = total pipe friction loss between inlet and outlet areas vin, vout = mean flow velocities at inlet and outlet liquid level areas The mean flow velocities at the inlet and outlet areas are, based on the Continuity Law, mostly insignificantly small and can be neglected, if the tank areas being relatively large compared to those of the pipe work. In this case, above formula will be simplified to: The static portion of the system H-Q curve, that part that is unrelated to the rate of flow, reads: For closed circulating systems this value becomes zero. The total friction losses are the sum of the frictional losses of all components in the suction and delivery piping. They vary, at sufficiently large REYNOLDS numbers, as the square of the flow rate. g = gravity (9.81 m/s2) Hl,tot = total friction loss between inlet and outlet areas vi = mean flow velocities trough pipe cross-section area Ai = characteristic pipe cross-sectional area ζi = friction loss coefficient for pipes, fittings, etc. Q = flow rate k = proportionality factor Under the above stated premises the parabolic system H-Q curve can now be drawn: The proportionality factor k is determined of the specified duty point. The intersection of the system H-Q and the pump H-Q curves defines the actual operating point.

## Best efficiency point of the pump

It is determined by the flow rate and head at the corresponding operating speed. When pumping highly viscous media, the pump characteristic curves and thus also the design point are shifted in relation to the characteristic curve recorded with water.

## Duty Point

The point is composed of the volume flow Q and the flow rate H. To calculate the design point, the required volume flow (flow rate of the pump) is first determined. Depending on the application, this can depend on various variables (e.g. heat requirement for heating systems, volume of wastewater produced, etc.). The calculated volume flow is then used to determine the frictional losses of the pipeline, which together with the static head then gives the total head of the pump. If a minimum flow velocity is specified for the application and this is not reached for the calculated flow rate, the rated flow rate is adjusted so that the minimum flow velocity is reached. The pump then runs in off mode (intermittent). The duty point of the system is the required operating point for the pump selection. The standard pumps usually have a deviation between the desired duty point and the actual operating point. The permissible deviation depends on the field of application and is partly regulated by applicable standards. With speed-controlled pumps, the speed of the pump is modified so that the set operating point is approached exactly. Especially in systems that are operated in different load conditions (e.g. heating system), this enables efficient operation. Depending on the design of the pump, there are further possibilities for adapting the pump performance curve to the duty point. In addition to changing the speed, the following methods are widely used:
• Impeller trimming
• Throttling
• Bypass

## Net positive suction head (NPSHr)

Generally recognizable is the strong dependence on the pump speed. If the construction is unchanged: High speed -> High holding pressure head Low speed -> Low holding pressure head In order to take account of any uncertainties in the design of the duty point, these values must be increased by a safety margin of 0.5 m when selecting the pump. By definition a minimum cavitation is permissible at NPSH, whereby the following conditions are allowed:
• The head of the pump at the nominal point is reduced by 3%.
• No material damage impairing the function and service life occurs.
Due to the permissible cavitation, cavitation noises can still occur, some of which are perceived as disturbing. In order to eliminate the residual cavitation, it is necessary to provide the calculated minimum inlet head with a surcharge of approx. + 1 to + 5 m. The minimum inlet head must be calculated with a surcharge of approx. This surcharge depends on the speed and the operating point of the pump.

## Best flow rate

This point is called the best efficiency point (BEP) of the pump. The position of the point changes if the hydraulic parameters of the pump, such as the impeller diameter or the speed resp. the viscosity of the pumped liquid, change. The aim of an optimal pump selection is for the pump to operate at the BEP so that it achieves its maximum efficiency.