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The Practical Pumping Handbook ..............................................................

through a connection close to the discharge nozzle of the pump, either
manually or by an automated system. At that point the pump can then
be started without difficulty and will operate under normal conditions.
While priming the pump is always essential on installation and initial
startup, it can also be necessary when the pump is operating on
intermittent duty. Under these conditions the pump will occasionally
stop, thus leaving the liquid in the suction line to the mercy of gravity
that will tend to empty that line. To prevent such an occurrence, a foot
valve is fitted to the bottom of the suction line in the sump. These
valves are usually designed with a flap or ball positioned in such a way
that, the normal flow direction of the liquid will hold the valve open.
However, when the flow through that valve reverses, the flap or ball
will close against the seat, thus holding the liquid in the suction line.
Unfortunately, if a small piece of debris is caught between the seat and
the flap or ball, the valve will stick open and the suction line will drain
out into the sump and the pump loses its prime. For occasional use
where the pump is not an important part of the process, and operators
are readily available, this repetitive priming may be quite acceptable.

9.1.4 Air operated double diaphragm pump
A variety of positive displacement pumps have the ability to prime
themselves, and information on them can be seen in Chapter 9.4.

Figure 9.3: Double diaphragm pump (Reproduced with permission of Warren Rupp Inc., a Unit
of IDEX Corp.)

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