Mastering Centrifugal Pump vs Positive Displacement Pump Selection

In my 20-plus years of troubleshooting piping systems in refineries, chemical plants, and offshore platforms, I have seen millions of dollars wasted because of a simple, fundamental mistake: selecting the wrong pump type for the process fluid. The battle between centrifugal and positive displacement (PD) pumps is not about which machine is superior. Instead, it is about understanding how their contrasting operating principles interact with your piping system’s hydraulics.

A centrifugal pump is a dynamic machine that adds kinetic energy to the fluid, converting velocity into pressure head. A positive displacement pump, on the other hand, is a constant-volume machine that physically traps a fixed quantity of fluid and forces it into the discharge pipe. This basic difference changes how they handle viscosity, pressure changes, and flow control.

Evaluating Centrifugal Pump vs Positive Displacement Pump Hydraulics

To truly understand these machines, we must look at their performance curves. For a centrifugal pump, the Head-Flow (H-Q) curve is sloping. As you throttle the discharge valve to increase system resistance (head), the flow rate decreases. If you run a centrifugal pump against a completely closed valve (shut-off head), the pump will consume minimum power (for low specific speed pumps) but will rapidly heat up the trapped fluid, leading to flashing and mechanical seal failure.

Conversely, a positive displacement pump has a nearly vertical H-Q curve. The flow rate remains virtually constant regardless of the discharge pressure. The only slight drop in flow at high pressures is due to “slip”—the internal leakage of fluid from the high-pressure discharge side back to the low-pressure suction side through internal clearances.

The Impact of Fluid Viscosity

Viscosity is where these two technologies diverge dramatically. Centrifugal pumps rely on high-speed fluid shear. When viscosity increases, the frictional losses inside the impeller passages skyrocket. This causes a severe drop in head, a massive reduction in flow rate, and a steep decline in mechanical efficiency. According to the Hydraulic Institute (HI) standards, once fluid viscosity exceeds 100 Centistokes (cSt), you must apply heavy derating factors to centrifugal performance.

Positive displacement pumps thrive on viscosity. Because they operate at lower internal velocities and rely on tight mechanical clearances, high-viscosity fluids reduce internal slip. This actually increases the volumetric efficiency of rotary gear, screw, and lobe pumps. While a centrifugal pump becomes useless when pumping heavy lube oils, polymers, or resins, a PD pump handles them with ease.

Mathematical Relationships and Calculations

For centrifugal pumps, the relationship between speed, flow, head, and power is governed by the Affinity Laws:

• Flow Rate (Q) is directly proportional to speed (N): Q2 / Q1 = N2 / N1
• Head (H) is proportional to the square of speed: H2 / H1 = (N2 / N1) squared
• Brake Horsepower (BHP) is proportional to the cube of speed: P2 / P1 = (N2 / N1) cubed

For positive displacement pumps, the theoretical flow rate is calculated based on displacement per revolution:

Q_theoretical = Displacement per Revolution * Pump Speed (RPM)

The actual flow rate is then determined by subtracting the internal slip:

Q_actual = Q_theoretical – Q_slip

Where slip is inversely proportional to fluid viscosity and directly proportional to the differential pressure across the pump.

The following table provides a direct engineering comparison between centrifugal and positive displacement pumps under standard industrial operating conditions. These parameters should guide your initial selection process.

This matrix maps specific pump sub-types to their governing industry standards, typical applications, and key design limitations.

Applying Centrifugal Pump vs Positive Displacement Pump Checklists

Before you press the start button on any newly installed pump, you must perform a series of rigorous field checks. In my experience, skipping these steps is the leading cause of infant mortality in rotating equipment. The checklist below highlights the critical verification points for both pump categories.

Field Case Study: Real-World Application

Direct Engineering Recommendation: When dealing with fluids whose viscosity varies significantly due to temperature changes, do not attempt to oversize a centrifugal pump to overcome the friction losses. Instead, select a rotary positive displacement pump that will maintain a constant flow rate and operate efficiently across the entire viscosity spectrum.