9.12 Pump behavior power outage

Pump behavior power outage

In case of a power failure it’s important to understand pump behavior at a power outage. The pump is sometimes compelled to operate at abnormal operating points especially in the case of pump installations without a non-return (check) valve. This occurs in the event of power failure for example. In as much as reversal of flow or direction of rotation are occurrences which must be considered, a knowledge of pump curve behavior under these conditions is advisable.

Figure 9.12a Complete performance curves for a centrifugal pump (H =delivery head, Q = flow, n = speed, M = shaft torque).

Figure 9.12a shows that a normal centrifugal pump possesses a certain pumping capacity even rotating in the reverse direction and that it functions exceptionally well as a turbine. An important consideration from the point of view of water hammer is that of the rapidly increasing pressure differential across the pump which occurs with reverse flow and pump speed. Another way of illustrating a possible pump operating condition is shown in figure 9.12b.

Figure 9.12b Possible centrifugal pump operating conditions.

Pump stoppage without non-return (check) valve

The transient process in the event of power failure is illustrated by an “extreme example”. The installation is equipped with a slow-closing valve which remains open during the process.

When the driving torque fails, the impeller is braked by the hydraulic torque. The decreasing speed also reduces Q and H for the pump. The flow through the pump changes direction whilst the speed is still positive. The pressure in the pump reaches its maximum value. The direction of rotation also changes a few seconds later. Very high pressures occur in the pump when retarding reversed flow at high reversed speed.

Figure 9.12c Transient process in the event of power failure, conditions at pump,

he pressure oscillations are eventually dampened out and a steady flow condition is obtained. The pump reaches its so-called “runaway” speed.

Figure 9.12d Steady and initial and final conditions in the event of power failure.