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Centrifugal Pump Selection and Sizing

Centrifugal Pump Selection and Sizing

A centrifugal pump must be selected according to the actual characteristics. Once we know the GPM and pump head, we can start the centrifugal pump selection. Pumps may be selected either manually using pump curves, or with software. To use software intelligently, it is first necessary to understand the manual selection procedures.

The steps for a correct design are the following:

Determination of the Flow Rate (Q):
Amount of liquid charged by the pump in the unit of time, normally expressed in (m3/ h) or (gal/min).
Determination of the Total Head or the Total Manometric Height (Hmt):
It is the sum of the geometric height in the fluid levels and the losses caused by internal friction that form the passage of liquid in the pipes, pump and hydraulic accessories.

The expression to identify it is the following:

Hmt = Hg + ∆pc   (m liquid column)

Hg = Hga + Hgp

Hga: is  suction geometric height.
 ( the height between the water surface and the axis of the pump, with negative value if the pump is located above the surface of the water)

Hgp: is the geometric height discharge.

∆PC: is sum losses in the installation based on the following elements:

1. Diameter, length and material of the suction and discharge pipes.

2. Quantity and type of the bends in the pipes and hydraulic accessories such as strainer,          valves, check valve, filters.. etc.
3. Nature of the liquid (if it is not water), temperature, viscosity and density.

Pump Characteristics:
After determining the values of flow rate Q and the total manometric height installation Hmt, to determine the power input P of the pump the following formula must be applied:

Q = flow rate in m3/h
H = height in meters
 = density of the liquid (for the water = 1000 kg/m3)
ηp = pump performance (Ex. performance pump 68% = ➩ηp 0.68)

Calculation example for the selection of a centrifugal pump

Q (flow rate) = 42 m3/ h.
    Hga (suction geometric height) = 3.5 m.
      Hgp (geometric height) = 39 m discharge.
        Suction hose 5 m long diameter DN 100 mm with 1 elbow and 1-foot valve.
          70 m long diameter DN 80 mm discharge pipe with 1 check valve, 1 valve and 3 elbows.

            Hg = Hgp + Hga = 39 + 3.5 = 42.5 m geometrical height of installation

            ∆PC = sum losses

            Losses at Suction:
            5 m of pipe Ø100 pc = 0.12 m
            1 elbow pc = 0.045 m
            1 pc foot valve = 0.46 m

            Losses at Discharge:
            70 m of hose Ø 80 pc = 5.25 m
            1 check valve pc = 0.5 m
            1 valve pc = 0.05 m
            3 elbows pc = 0.09 m

            Total ∆pc = 6.5 m.

            Given that the calculation has been made with new pipes, we must bring an increase of 15 / 20% for aging.

            so the Total ∆pc is around 8 m.

            The head that the pump must reach is therefore:
            Hmt = Hg + ∆p = Hgp + Hga + ∆pc = 39 + 3.5 + 8 = 50.5 m total

            You can choose the NM 50/20AE (see pump diagram) 
            Table 1 Losses in pump accessories
            Table 2 Losses in steel pipes

            Pump charactaristics curves / diagram