Piston pump / plunger pump
The most well-known positive displacement pumps are piston or plunger pumps with linear stroke. Their displacement bodies are either discshaped pistons sliding to and fro within a hollow cylinder and having the sealing system attached to their outside diameter, or plungers which slide to and fro within a stuffing box. The motion of the piston/plunger increases and decreases the working volume. See Fig. 1 Positive displacement pump
Piston and plunger pumps are either controlled by a relatively simple bypass arrangement, which returns a greater or lesser portion of flow from the discharge to the suction side as required (the losses being quite acceptable on smaller units), or by the more economical infinitely variable speed control of pole-changing motors combined with a bypass arrangement.
Piston and plunger pumps are mainly used for handling very low flow rates at very high heads (pressure); they are suitable for very low specific speeds. Apart from applications requiring these conditions, piston and plunger pumps have almost completely been superseded by centrifugal pumps in their varied specialised design types (multistage pumps, peripheral pumps).
The advantages of piston and plunger pumps are their self-priming capability (self-priming pump) and high efficiency; disadvantages include their pulsating delivery and, especially in the case of large units, their space requirements, their large mass and their high investment costs per unit of pump output.