Environmental Engineering Reference
In-Depth Information
are the focus of our discussion. In the wastewater industry, positive-
displacement pumps are most often found as chemical feed pumps. It
is important to remember that positive-displacement pumps
cannot
be
operated against a closed discharge valve. As the name indicates, some-
thing must be displaced with each stroke of the pump. Closing the dis-
charge valve can cause rupturing of the discharge pipe, the pump head,
the valve, or some other component.
3.4.1 Piston Pump or reciprocating Pump
The
piston
or
reciprocating pump
is one type of positive-displace-
ment pump. This pump works just like the piston in an automobile
engine—on the intake stroke, the intake valve opens, filling the cylinder
with liquid. As the piston reverses direction, the intake valve is pushed
closed and the discharge valve is pushed open; the liquid is pushed into
the discharge pipe. With the next reversal of the piston, the discharge
valve is pulled closed and the intake valve pulled open, and the cycle
repeats. A piston pump is usually equipped with an electric motor and a
gear-and-cam system that drives a plunger connected to the piston. Just
like an automobile engine piston, the piston must have packing rings to
prevent leakage and must be lubricated to reduce friction. Because the
piston is in contact with the liquid being pumped, only good-grade lubri-
cants can be used for pumping materials that will be added to drinking
water. The valves must be replaced periodically as well.
3.4.2 diaphragm Pump
A
diaphragm pump
is composed of a chamber used to pump the
fluid, a diaphragm that is operated by either electric or mechanical
means, and two valve assemblies—a suction assembly and a discharge
valve assembly (see Figure 3.17). A diaphragm pump is a variation of
the piston pump in which the plunger is isolated from the liquid being
pumped by a rubber or synthetic diaphragm. As the diaphragm is moved
back and forth by the plunger, liquid is pulled into and pushed out of the
pump. This arrangement provides better protection against leakage of
the liquid being pumped and allows the use of lubricants that otherwise
would not be permitted. Care must be taken to ensure that diaphragms
are replaced before they rupture. Diaphragm pumps are appropriate for
discharge pressures up to about 125 psi, but they do not work well if
figure 3.17 Diaphragm pump.
