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.... Special Pumps

failure. Consequently, the use of power monitors is recommended for
all applications in which magnetic drive pumps are used.

.3.4 Failure costs
When failures do occur, magnetic drive pumps are usually much more
expensive to repair than conventional pump designs with mechanical
seals. The parts that frequently need replacement can cost a very high
percentage of the entire pump replacement value.
Many end users approach the magnetic drive pump as a cure-all for all
pump ills. That is not true. In fact, they have less tolerance for
misapplication and process upsets than conventional pumps. However,
with an understanding of their limitations and unique advantages, they
can provide reliable operation.

9.4 Positive displacement pumps

Positive Displacement (P.D.) pumps operate with a series of working
cycles where each cycle encloses a certain volume of fluid and moves it
mechanically through the pump into the system. Depending on the
type of pump and the liquid being handled, this happens with little
influence from the back pressure on the pump.

While the maximum pressure developed is limited only by the
mechanical strength of the pump and system and the driving power
available, the effect of that pressure can be controlled by a pressure
relief or safety valve.
A major advantage of the P.D. pump is that it can deliver consistent
capacities because the output is solely dependent on the basic design of
the pump and the speed of the driver. This means that, if the liquid is
not moving through the system at the required flow rate, it can always
be corrected by changing one or both of these factors.
To varying degrees, positive displacement pumps are suitable for
handling highly viscous liquids. They are also self-priming and therefore
have the ability to handle liquids with a certain volume of entrained air.

All discharge valves installed with any PD pump must be open before
that pump is started. This will prevent any fast buildup of pressure that
could damage the pump or the system.

9.4.1 The piston pump
One of the earliest and most basic type of PD pump, the Piston Pump
uses a plunger or piston to force liquid from the inlet side to the outlet
side of the pump.

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