Positive displacement, as used in pumps, is a term that refers to how fluids are moved. Each cycle of operation of a positive displacement pump moves the same precise amount of fluid from its inlet to its outlet having thus ‘displaced’ a definite volume with each action. The fact that it works on this principle is why reciprocating pump is called “positive displacement” pump.
Reciprocating pump’s basic components comprise of cylinder, piston, inlet and outlet valves and sometimes other parts like crankshaft and connecting rod. When functioning, the piston reciprocates within the cylinder or moves in backward and forward motion. Thus during the moving away period from the inlet, a vacuum is created by it which draws the liquid through into the cylinder via an opened inlet valve. When moving towards the exit, on one hand the entrance valve shuts but simultaneously opening an exit valve allowing piston propel out that space occupied by fluid inside driving it into system or pipeline.
The amount of fluid moved by each stroke will be constant since size and travel distance of piston are fixed hence determined by geometry of pistons and cylinders. A Key characteristic feature about positive displacement pumps is their exactness at displacing precise volumes per cycle.
Positive Displacement Pumps such as Reciprocating Pump have an advantage when there is need for consistent volumetric flow rate which must be accurately attained. For example in situations like chemical processing,metering or dosing where precision in amounts transferred matters most.The pumps also can generate high pressures because they pumping effect does not depend much on outlet pressure.On other hand centrifugal pumps which are not really positive displacement tend to move at different rates depending on system resistance/pressure changes.
Also Viscosity handling features among applications where positive displacement pumps outperform others: In comparison with other types of these devices thick liquids can be moved more efficiently due to their unit operations being oscillatory.Though this may mean serious mechanical assemblies involving some wear and tear and increased frequency of maintenance.
More importantly, it implies that flow rates in positive displacement pumps are not only uniform but can be easily modified by changing the speed at which piston reciprocates.This provides an additional level of control for situations where differentiable flow rates have to remain accurate.
Another point to note is risk of hydraulic shock (commonly known as “water hammer”) in reciprocating pumps. This pulsating rather than continuous fluid flow calls for specific design features as well precautions in operation aimed at damping its effects by use of dampeners or accumulators.
To sum up, reciprocating pumps are called positive displacement pumps because they displace similar, fixed amounts of fluid during each cycle. This characteristic makes them particularly useful for tasks that require high pressures and precise, consistent volumetric flow rates, although it also brings specific design and operational challenges.
