Tag: System Curve

Total static head

Posted on by Jens-Uwe Vogel
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  

Calculating the System H-Q Curve

Posted on by bs
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.    

Operating Point of a Centrifugal Pump

Posted on by bs
“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.

Flow

Posted on by bs
The flow rate for the duty point of a pump is determined from the application, for example for heating systems from the heat requirement calculation or for wastewater systems from statistical parameters for the maximum expected wastewater volume. National and international standards exist for many applications. The performance curves of a centrifugal pump (e.g. head, power consumption, efficiency) are given as a function of the flow.

Head

Posted on by bs
The head for the duty point of the pump is composed of
  • the static head (static = independent of the flow rate)
    • height difference between suction side and discharge side liquid level (geodetic head)
    • pressure difference between discharge side and suction side tank (for closed tanks)
    • the required outlet pressure, if any
  • the friction loss head from the pressure losses in the piping system as a function of the flow rate
The useable mechanical work transferred from the pump to the fluid being pumped, related to the weight force, is called head H of the pump. At constant speed n and constant flow Q, it is independent of the density of the pumped liquid, but dependent on its viscosity. It can be calculated by the pressure difference divided by the density of the pumped fluid and the local gravitational constant. For Newtonian fluids one can consider the head independent from the pumped fluid for kinematic viscosities less than 20 mm²/s. By this reason it is especially suitable to present the characteristic curve of centrifugal pumps. For pumping water the value is equal to the pressure given in meters of water column.

Operating Point of a Centrifugal Pump

Posted on by bs
It indicates the values of Flow Q and Head H 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 duty point and the actual operating point. The operating 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 best efficiency point.