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The Practical Pumping Handbook ........................ iiii::: ............................. ................................. IF ....

4.8.2 Centrifugal pump with priming tank
This priming tank acts in a manner similar to that of the suction trap in
a self-priming pump, and must be sized so that it contains 3 times the
volume of the suction line. When the pump starts to empty the tank, it
creates enough of a vacuum in the priming tank to draw the suction
line full again before the tank empties. During this time it will also be
capable of supplying sufficient NPSH to the pump.

4.8.3 Air ejector system
An air ejector system can be automated to use available compressed air
to vacate the entrained air in the suction line and pump prior to the
pump startup. By creating a vacuum in the pump, it will draw the liquid
into the suction line and fill the pump. At that point the pump will start
in a fully primed condition.

Air
Ejector

i.i.i-iii.i-17..77..7.11-i.7i7:7.11~.i.i?

ii;iii!iiii;:!ii!:!ii:!!:!!i!:i!!
~ 21"s163
iiiiii~iiii!ii:.iiiiiiiiiiiiiiiii!ii
Figure 4.13: Air ejector priming system

4.9 Submergence

Submergence is the static elevation difference between the free surface
of the liquid and the centerline of the impeller in a vertical shaft pump.
Inadequate submergence causes random vortices that permit air to be
drawn into the pump, causing increased vibration and reduced life. This
required submergence is completely independent of the NPSH
required by the pump.
Inadequate sump design frequently causes serious pump problems but
many sources, such as the Hydraulic Institute Standards, provide

m 68