It consists basically of two parts:
1. The pressure difference between discharge side and suction side tank. It is zero for open tanks and closed circulation systems.
2. The height difference between the liquid levels of discharge side and suction side tanks respective the system inlet and outlet. It is zero for closed circulation systems.
This means for circulation systems the static head is always zero, 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

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.

System Curve

The system performance curve is composed of a static and a dynamic component. Hsystem = Hstat + Hloss(Q) The static component Hstat is independent of the flow velocity (and thus of the flow rate). It contains the geodetic height difference as well as the pressure difference between suction and pressure vessel or inlet and outlet point of the system under consideration. In closed circuits (e.g. heating circulation) the static heed is always zero. The dynamic part of the performance curve describes the piping losses, which depend on the flow rate. In the case of turbulent flow of NEWTON liquids with constant loss coefficients of the system components, the characteristic curve results in a square parabola. If the static head and the duty point are known, the system curve can be shown with sufficient accuracy.

Dimensioning criterias

The top four criteria are: WHAT for a medium? –> Pumping medium HOW MUCH Amount? –> Flow rate WHERE, how far, how high? –> Pipe geometry WHICH pump unit should be used? –> Delivery unit If the flow rate and pipe geometry are known, the head can be calculated with the help of the pressure loss calculation. Flow rate and head together form the duty point for the pump design.

Operating Point of a Centrifugal Pump

“It indicates the values of Flow and Head which will be obtained at stationary operation with the respective speed-related pump H-Q curve.” The specified duty point is defined to be that point on the system H-Q curve for which a pump is to be selected in line with the calculated hydraulic design criteria. The objective of the selection is (apart from other criteria, such as maximum efficiency) to minimise the deviation between the specified and the actual duty points. The actual duty point is always located at the intersection of pump H-Q curve and the actual system H-Q curve. At constant speed it moves up the pump H-Q curve with increasing friction losses towards a lower flow rate. The duty point should be chosen as close as possible to the point of optimum efficiency.