pressure drop. On the other hand, it accepts water when the pump opens,
i.e. under positive transient conditions.
Between the pump and air vessel (Fig. 13.11) a check valve has to be
provided, which closes quickly after a power failure as the pump, during its
slowing down, cannot deliver against the head maintained by the air
vessel. The compressed air delivery is triggered by the predetermined
maximum and minimum water levels in the air vessel.
Analogous to surge tank operations, the simplified solution of
dynamic and continuity equations of an air vessel for a sudden closure
(Q0; check valve closure) without entrance losses and slow isothermic
pressure variations yields
HH 0 Q 0 (LH 0 /gAV 0 )1/2 (13.19)
where the positive sign is taken for Hmaxand the negative sign for Hmin, and
suffix ‘0’ refers to steady state conditions.
Design charts based on the assumption of complete closure, a ratio
of head losses at entrance to air vessel in both directions of 2.5 at the air
vessel base and a gas law of HV1.2constant are available. The dimension-
less surges for the total head loss coefficient Ksfor a flow into the air vessel
of 0.3 are shown in Fig. 13.12. Similar design charts are available for other
values of Ks.
H 0 and 0 are the steady-state pressure head in the air vessel and the
velocity in the pipeline respectively, and V 0 is the corresponding air
volume in the air vessel.
For a given pipeline (diameter D, length Land wall thickness e) and
dischargeQ 0 the pressure wave celerity ais calculated by
a1/[ (1/KD/eE)]1/2 (13.20)
564 PUMPING STATIONS
Fig. 13.11 Layout of an air vessel
