Plunger Pump vs. Diaphragm Pump: A Performance Comparison

The main structural difference between plunger pumps and diaphragm pumps lies in the liquid end, as shown below.

Diaphragm pump	Plunger pump

A diaphragm pump uses a diaphragm to separate the fluid from the hydraulic chamber. The hydraulic chamber provides the power that drives the diaphragm in a reciprocating motion, creating the volume change that transfers the fluid. A plunger pump, by contrast, drives the fluid through the reciprocating motion of a plunger. In terms of construction, on a hydraulic diaphragm pump the part in contact with the fluid is the diaphragm, which is sealed statically. As a result, the pump can achieve completely leak-free fluid handling. Moreover, because there is no relative motion at the diaphragm seal, the diaphragm is not subject to wear. Replacing the diaphragm periodically is purely a matter of accounting for material fatigue. Since wear is not an issue, the diaphragm replacement interval remains essentially stable across different fluids. The plunger inside the hydraulic chamber is sealed with metal piston rings or another type of seal, and it is always well lubricated by the hydraulic oil, giving it a service life of 8–10 years.

On a plunger pump, the parts in contact with the fluid are the plunger and the packing. Because the packing relies on a dynamic seal, wear is unavoidable, and some fluid will seep through. The packing replacement interval is affected by several factors. The first is the lubricity of the fluid. If the fluid does not lubricate well, the packing and plunger run almost in a "dry-grinding" state, which accelerates packing wear. If the fluid contains impurities, the wear becomes even more severe, and the plunger itself is also at risk of rapid wear. Because different fluids have different lubricity and corrosivity, plunger pumps wear out over different periods of time. With a very low-viscosity fluid, wear can occur faster than you might expect — the packing may be completely worn out within a month. The second factor affecting packing wear is how tightly the packing is "torqued down." This is a somewhat contradictory issue: if you want to reduce the amount of fluid leaking from the plunger pump, you have to torque the packing tighter, which accelerates packing wear. If the fluid also has poor lubricity, these two factors combine to accelerate wear of both the packing and the plunger. In that case, wear happens very quickly and unpredictably, and operators and maintenance staff have no way of knowing when the packing will wear out.

For a plunger pump, then, taking both factors above into account, the timing for maintenance and spare-part replacement is uncertain. In our experience, if the fluid has good lubricity and the packing is torqued correctly, and the equipment is inspected daily by maintenance staff, the packing typically needs replacing after about six months and the plunger after about a year. If both factors are at their worst, however, the plunger and packing are at risk of failing almost immediately.