Figure 6-8.Operation of a gate valve.
closed manually by means of a handwheel. They are
used primarily to start or stop the flow of liquid
through the pump during certain phases of operation.
Thus stop valves are often placed on suction and
discharge lines, so the pump is isolated or sealed off
from the rest of the liquid system. Figure 6-8 shows the
operation of a gate valve. A gate valve is a type of stop
valve. A gate, or wedge, is raised or lowered by turning
the handwheel. Some types of stop valves are used for
throttling purposes; that is, to regulate the flow of
liquid. However, gate valves are never recommended
for throttling service because the flow of liquid past the
partially opened gate can rapidly erode the gate face.
Instead, the gate valve can be replaced with a tapered
needle valve (another type of stop valve) which
gradually opens or closes through the valve seat.
A third type of valve generally found on most types
of pumps is the RELIEF VALVE. As you can see from
figure 6-9, most relief valves are similar in their design
to check valves. These valves are designed to open
when the liquid pressure in the pump becomes
dangerously high. In most cases, the outlet of the relief
valve is connected to a recirculating line that passes the
excess liquid back to the suction side of the pump.
Almost all pressure relief valves are fitted with an
adjusting nut or screw that permits the spring tension to
be regulated. In this way, the pressure at which the
valve is opened can be varied.
TYPES OF PUMPS
Pumps are classified according to the type of
movement that causes their pumping action. The five
broad categories of pumps are rotary, reciprocating,
centrifugal, air lift, and jet pumps.
Figure 6-9.Relief valve.
All rotary pumps use the principle of entrapment
and displacement of fluid by the rotating elements of
various designs. These rotating parts, which may be
gear teeth, screws, lobes, or vanes, trap the fluid at the
suction inlet and remove it to the discharge outlet.
Instead of throwing the water as in a centrifugal
pump, a rotary pump traps it, pushes it around inside a
closed casing, and discharges it in a continuous flow.
Since rotary pumps move liquid by this method, they
are often classified under the broad heading of positive
Most rotary pumps have stuffing boxes provided
at the rotor shafts to prevent excessive leakage at the
shaft joint. In addition, various types of bearings can
be fitted at the ends of the rotor shaft to minimize
Generally, rotary pumps are self-priming; that is,
the pump end need not be filled with liquid to initiate
pumping action. Instead, the movement of the rotating
elements creates a partial vacuum sufficient to lift or
draw liquid into the pump and begin the pumping
process. Note that self-priming and good suction lift
are characteristics of the whole class of positive
displacement pumps. Rotary pumps are less expensive
and considerably simpler in construction.
utilities field, rotary pumps are used for pumping fuel
oil in boiler houses, for pumping chemical feed in
water purification systems, for priming larger pumps,
and for special applications, such as emergency pumps
at fire-fighting stations.
TYPES OF ROTARY PUMPS.The classifi-
cation of a rotary pump is determined by the type of
rotating element it has. However, no matter what form
of rotating element is used, the basic principles of
pump operation remain the same. In this section, two