Piston Pump - Construction

A Piston Pump is very similar in construction to a Reciprocating Internal Combustion Engine. The basic pumping action is obtained by reciprocation of a piston in a cylinder. The cylinder has two valves, one inlet and one outlet valve. And they allow for only inwards and outwards movement of the liquid respectively. These valves are situated in inlet and outlet manifold respectively. The piston is connected to a crankshaft through a connecting rod. The Piston Pump has a liner made of leather or any other synthetic material to provide proper sealing between the moving surfaces of the piston and the cylinder.

 Piston Pump - Working

The reciprocating action of the piston is obtained by crankshaft arrangement and guide path provided by the cylinder. The piston is connected to the crankshaft through the connecting rod which is coupled with the crankshaft through a revolute joint situated at some distance from the crankshaft axis. This distance can be called as crank length or the eccentric distance. It is this crank length which determines the stroke length of a piston pump.
The crankshaft is coupled to an electric motor or an engine shaft. As the crankshaft rotates the piston reciprocates. In one cycle of the crankshaft the piston reciprocates once, that is, moves one stroke forward and one backward. The intake and discharge of the liquid by a piston pump is basically the same as reciprocating pumps in general as discussed in previous article under working of reciprocating pumps.

Piston Pump - Variants

The basic Piston Pump has single piston cylinder arrangement with intake and discharge of liquid through one side only. Piston Pumps are available is different configuration and each being suitable for particular application. The discharge of a piston pump can be changed by varying the stroke length of the piston. The common variants of the Piston or Plunger Pumps are:
Single Acting Piston Pump 
Having liquid chamber on only one side of the piston and valve arrangement on that side only. The liquid is discharged from one side of the cylinder once in a crankshaft cycle, only in the forward stroke of the piston.
Double Acting Piston Pump 
Both sides of the piston have liquid chamber and the valve arrangement. Liquid is discharged from both the sides, from one side in the first half of the cycle and from the other side in the second half of the crankshaft cycle. As the liquid is discharged in both the forward and backward stroke, the discharge is more per cycle and also smooth as compared to the Single Acting pump.
Duplex Pump 
There are two piston-cylinders assemblies. Both the pistons are coupled to the single crankshaft through separate connecting rod of each. The connecting rods are coupled to the crankshaft at an angular distance of 180 degrees from each other. Each piston-cylinder can be single acting or double acting.
Triplex Pump  
There are three piston-cylinder assemblies. All the three pistons are coupled to the single crankshaft through the connecting rod of each. There is an angular separation of 120 degrees between any two adjacent connecting rod and crankshaft couplings.

Classification Criterion

The most basic classification criterion for differentiating the pumps, whether it is for purpose of analysis or for commercial purpose, is the fluid flow pattern. There are also other important classifications based on the power rating range and fluid discharge rate. The flow supplied by a pump can be intermittent or continuous. The pumps with intermittent flow are all positive displacement pumps and pumps with continuous flow are all Roto-Dynamic pumps. These classifications of the pumps does not end here, these two broad categories have further different types of pumps having varying characteristics. To sum up the classification criterion:
Broad categories based on Flow Pattern:

1. Intermittent - Positive Displacement Pumps
2. Continuous - Roto-Dynamic or Turbo Pumps