Positive Displacement Pumps

Overview

Engineering reference data for Positive Displacement Pumps in pumps.

Key Formulas

Pump Power

$P = \frac{Q \cdot H \cdot \rho \cdot g}{\eta}$

Hydraulic power / efficiency.

NPSH Available

$NPSH_a = \frac{P_s}{\rho g} + \frac{v_s^2}{2g} - \frac{P_v}{\rho g}$

Net Positive Suction Head available.

Affinity Laws

$Q \propto n, \quad H \propto n^2, \quad P \propto n^3$

Flow, head, power vs speed.

Variables

Pump Classification

Positive displacement pumps are divided into two main classes:

• Reciprocating pumps: plunger pumps, diaphragm pumps, piston pumps
• Rotary pumps: gear pumps, lobe pumps, vane pumps, progressive cavity pumps, peripheral pumps, screw pumps

Operating Characteristics

A positive displacement pump operates with an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the suction cavity expands, and flows out as the discharge cavity collapses.

Key operational properties:

• Constant volume delivery: Each cycle displaces a fixed volume regardless of discharge pressure
• No shut-off head: Unlike centrifugal pumps, positive displacement pumps cannot operate against closed discharge valves — this will cause pressure buildup until line failure or pump damage
• Relief valve required: A safety/relief valve on the discharge side is mandatory. External relief valves with return lines to the suction side or supply tank are highly recommended; internal valves should serve only as backup safety devices

Reciprocating Pump Operation

Plunger pumps use a cylinder with a reciprocating plunger. During the suction stroke, the plunger retracts and the suction valve opens. During the forward stroke, the plunger pushes fluid out through the discharge valve.

Single-cylinder plunger pumps produce pulsating flow — maximum flow during mid-stroke and zero flow at end positions. This causes energy waste, vibration, and potential water hammer effects. Multi-cylinder configurations with offset phasing compensate for these issues.

Diaphragm pumps use pressurized hydraulic oil to flex a diaphragm, making them suitable for pumping hazardous and toxic fluids.

Reciprocating Pump Speed Correction Tables

Rotary Pump Speed Correction Table

Viscosity Correction Example

Fuel oil No. 2 at 20°C has a viscosity of approximately 150 SSU, requiring no speed reduction. Fuel oil Type 5 at 100°C has a viscosity of approximately 4000 SSU, requiring approximately 30% speed reduction for a reciprocating pump.

Interactive Charts

References

• Original Source