Hydraulic Structures: Fourth Edition

13.8.1 Flywheels

A flywheel coupled with the pump unit provides additional inertia so that
the pump continues to rotate even after a power cut. This reduces the
pressure transients (analogous to slow valve closure).
The rate of change of speed of a pump is given by

dN/dt 900 gQH/π^2 IN (13.18)

whereIis the moment of inertia of the rotating parts of the pump and
motor and is the pump efficiency at discharge Q, head Hand speed N.
This method of providing protection without any ancillary equipment is
recommended for small installations.

13.8.2 Bypasses and pressure relief valves

When a pump stops suddenly a low-pressure wave in the delivery pipe and
a high-pressure wave in the suction pipe develop, resulting in a discharge
from the suction side to the delivery side through a non-return valve
(Fig. 13.10) in the bypass. In addition (or alternatively) pressure relief
valves and air inlet valves may be provided in the pipeline system.

13.8.3 Surge tanks and air vessels

In contrast to a hydropower plant, it is usually not practicable to construct
an open surge tank in pumping installations to suppress the surges.
Instead, a closed air vessel with an air compressor is provided close to the
pump. When a negative transient occurs due to pump ‘tripping’ the air
vessel supplies water under pressure into the pipeline, thus reducing the

SURGES 563

Fig. 13.10 Bypass arrangements