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5.8 Laboratory Test of a Centrifugal Pump
adjusted with a knob on the controller and is read out digitally. After adjustment,
the controller keeps the rotational speed constant, independent of the motor load.
• Set six to eight points to the characteristic by constricting the delivery pipes.
Keep the constriction valve on the suction side completely open. Provide for
exactly equal pressures on both delivery pipes. Start the test by closing the valves
on the delivering pipes. This provides maximum pressure. Then open the valves
completely. This provides minimum pressure. Distribute the operating points approximately equally over pressure range. Perform the initial actions with the
pump supplying to the suction reservoir. First open all valves when switching
the bypass pipe valves to the suction reservoir. The position of the handles allows one single person to do this. The pump is not damaged by closing all valves
suddenly, but water hammer may be generated within the delivery pipes, and
this is best avoided. Let the pump deliver to the suction reservoir after flow rate
measurement and empty the discharge reservoir by opening the connecting valve
between the reservoirs (e.g. let in 300 l). The discharge reservoir is sufficiently
large, making it impossible to overflow it. The suction pipe inlet in the suction
reservoir comes out of the water with a completely filled discharge reservoir.
5.8.4 Calculations
• Determine for each operating point: flow rate, delivery pressure, suction pressure, torque. Calculate the mechanical energy rise and the overall efficiency. Plot
these two parameters: ΔE m = f( Q); η = f( Q).
• Transform, by similitude (Chap. 7), mechanical energy rise, flow rate and efficiency to n = 4500 rpm. Apply Pfleiderer’s formula (7.14) for correction of the
overall efficiency with changing Reynolds number. Apply the correction to the
overall efficiency, even though the formula is intended for the internal efficiency.
We do this because we are unable to determine the mechanical efficiency and
the volumetric efficiency during the test. Corrections of the head and the flow
rate are applied with the formulae of Casey and co-authors (7.10–7.12). Plot the
converted results: ΔE m = f( Q); η = f( Q).
• Determine the work coefficient and the specific speed for the operating point
with maximum efficiency (interpolate between the results found, if necessary).
When calculating the work coefficient, determine the rotor work from the shaft
power and the flow rate, assuming that the mechanical and volumetric efficiencies equal unity. With a radial pump, the volumetric efficiency is close to unity
(wear rings, labyrinth seals and shaft seals are applied to seal the rotor from
the stator, both for internal and external leakage), but the mechanical efficiency
may be significantly lower than unity (friction within the seals and disc friction
are not negligible).We thus overestimate the work coefficient (by about 10 %).
Evaluate the values found for work coefficient and specific speed.
5.8 Laboratory Test of a Centrifugal Pump
adjusted with a knob on the controller and is read out digitally. After adjustment,
the controller keeps the rotational speed constant, independent of the motor load.
• Set six to eight points to the characteristic by constricting the delivery pipes.
Keep the constriction valve on the suction side completely open. Provide for
exactly equal pressures on both delivery pipes. Start the test by closing the valves
on the delivering pipes. This provides maximum pressure. Then open the valves
completely. This provides minimum pressure. Distribute the operating points approximately equally over pressure range. Perform the initial actions with the
pump supplying to the suction reservoir. First open all valves when switching
the bypass pipe valves to the suction reservoir. The position of the handles allows one single person to do this. The pump is not damaged by closing all valves
suddenly, but water hammer may be generated within the delivery pipes, and
this is best avoided. Let the pump deliver to the suction reservoir after flow rate
measurement and empty the discharge reservoir by opening the connecting valve
between the reservoirs (e.g. let in 300 l). The discharge reservoir is sufficiently
large, making it impossible to overflow it. The suction pipe inlet in the suction
reservoir comes out of the water with a completely filled discharge reservoir.
5.8.4 Calculations
• Determine for each operating point: flow rate, delivery pressure, suction pressure, torque. Calculate the mechanical energy rise and the overall efficiency. Plot
these two parameters: ΔE m = f( Q); η = f( Q).
• Transform, by similitude (Chap. 7), mechanical energy rise, flow rate and efficiency to n = 4500 rpm. Apply Pfleiderer’s formula (7.14) for correction of the
overall efficiency with changing Reynolds number. Apply the correction to the
overall efficiency, even though the formula is intended for the internal efficiency.
We do this because we are unable to determine the mechanical efficiency and
the volumetric efficiency during the test. Corrections of the head and the flow
rate are applied with the formulae of Casey and co-authors (7.10–7.12). Plot the
converted results: ΔE m = f( Q); η = f( Q).
• Determine the work coefficient and the specific speed for the operating point
with maximum efficiency (interpolate between the results found, if necessary).
When calculating the work coefficient, determine the rotor work from the shaft
power and the flow rate, assuming that the mechanical and volumetric efficiencies equal unity. With a radial pump, the volumetric efficiency is close to unity
(wear rings, labyrinth seals and shaft seals are applied to seal the rotor from
the stator, both for internal and external leakage), but the mechanical efficiency
may be significantly lower than unity (friction within the seals and disc friction
are not negligible).We thus overestimate the work coefficient (by about 10 %).
Evaluate the values found for work coefficient and specific speed.