Positive Displacement Pumps

Positive Displacement Pumps

Positive displacement (PD) pumps move fluid by trapping a fixed volume and forcing it into a discharge pipe. Unlike centrifugal pumps, they deliver nearly constant flow regardless of discharge pressure.

Types of Positive Displacement Pumps

Gear Pumps

External Gear Pumps

• Two meshing gears rotate in opposite directions
• Fluid trapped between gear teeth and casing
• Simple, reliable, good for clean, viscous fluids
• Flow proportional to speed
• Typical efficiency: 80-95%

Internal Gear Pumps

• One gear inside another
• Crescent-shaped seal between gears
• Smoother flow, less pulsation
• Good for viscous fluids
• Compact design

Piston Pumps (Reciprocating)

Single-Acting Piston

• Fluid displaced on one stroke only
• High pulsation
• Simple construction

Double-Acting Piston

• Fluid displaced on both strokes
• Reduced pulsation
• Higher efficiency

Multi-Piston (Triplex, Quintuplex)

• Multiple pistons offset in phase
• Smoother flow
• Common in high-pressure applications
• Typical efficiency: 85-95%

Diaphragm Pumps

Air-Operated Double Diaphragm (AODD)

• Two flexible diaphragms
• Air pressure drives operation
• Self-priming, can run dry
• Excellent for slurries and solids
• Lower efficiency (~30-70%)

Mechanically Driven

• Diaphragm actuated by mechanical linkage
• Higher efficiency than AODD
• Good for metering applications

Screw Pumps

Single Screw (Progressive Cavity)

• Rotor rotates within stator
• Continuous, non-pulsating flow
• Excellent for viscous, shear-sensitive fluids
• Self-priming

Twin/Triple Screw

• Two or three intermeshing screws
• Low pulsation
• Good for high-pressure applications
• Typical efficiency: 75-90%

Lobe Pumps

• Two or more lobes rotate in opposite directions
• Gentle handling of product
• Common in food, pharmaceutical industries
• Easy to clean (sanitary designs)
• Typical efficiency: 50-80%

Key Characteristics

Constant Flow Behavior

Ideal Behavior:

• Flow rate independent of discharge pressure
• Flow proportional to speed only
• Q = N × V_d

Where:

• Q = volumetric flow rate
• N = pump speed (rpm)
• V_d = displacement per revolution

Real Behavior:

• Flow decreases slightly with pressure (slip)
• Efficiency varies with operating conditions

Volumetric Efficiency

Volumetric efficiency accounts for internal leakage (slip):

ηv = Qactual / Q_theoretical

ηv = (Qtheoretical - Qslip) / Qtheoretical

Factors affecting volumetric efficiency:

• Clearances and wear
• Fluid viscosity (higher = better sealing)
• Differential pressure (higher = more leakage)
• Operating speed

Slip and Leakage

Slip Flow: Q_slip = C × ΔP / μ

Where:

• C = slip coefficient (depends on clearances)
• ΔP = differential pressure
• μ = dynamic viscosity

Implications:

• Viscous fluids: less slip, higher efficiency
• High pressures: more slip, lower efficiency
• Worn pumps: increased clearances, more slip

Pulsation

Causes:

• Discrete volume displacement
• Reciprocating motion
• Gear tooth engagement/disengagement

Pulsation Index: PI = (Qmax - Qmin) / Q_avg × 100%

Typical Pulsation Levels:

• Single piston: Very high (100%+)
• Duplex piston: High (~50-60%)
• Triplex piston: Moderate (~10-20%)
• Gear pumps: Low to moderate (~5-15%)
• Screw pumps: Very low ( 10-20 bar
• PD pumps maintain efficiency at high pressure
• Centrifugal pumps become impractical

1. Viscous Fluids

• μ > 100 cP
• PD efficiency improves with viscosity
• Centrifugal efficiency drops dramatically

1. Constant Flow Required

• Metering and dosing
• Flow independent of pressure variations
• Predictable delivery

1. Low Flow, High Pressure

• Centrifugal pumps inefficient at low flow
• PD pumps excel in this range

1. Self-Priming Required

• Suction lift needed
• Air entrainment possible
• Dry-run capability

1. Shear-Sensitive Fluids

• Food products, polymers
• Use lobe or progressive cavity pumps
• Gentle handling

Choose Centrifugal When:

1. High Flow, Low Pressure

• Q > 100 m³/h, ΔP NPSH_required + margin
• Size suction piping appropriately

1. Evaluate Pulsation:

• Calculate pulsation index
• Determine if dampening needed
• Size dampener if required

1. Verify Operating Range:

• Minimum/maximum speed
• Pressure limitations
• Viscosity range

Performance Monitoring

Key Parameters to Track:

• Flow rate vs. speed (check for increased slip)
• Discharge pressure
• Power consumption (increased = wear)
• Vibration levels
• Temperature

Indicators of Wear:

• Reduced flow at same speed
• Increased power consumption
• Increased noise/vibration
• Reduced volumetric efficiency

Maintenance Planning:

• Replace seals/gaskets per schedule
• Monitor clearances in gear pumps
• Check valve seats in piston pumps
• Inspect diaphragms regularly

Summary

Positive displacement pumps are essential for:

• High-pressure applications
• Viscous fluid handling
• Metering and constant flow
• Self-priming requirements

Key design considerations:

• Account for volumetric efficiency (slip)
• Size for actual flow needed
• Consider pulsation dampening
• Match pump type to application

Trade-offs vs. centrifugal:

• Higher pressure capability
• Better viscosity handling
• Pulsating flow
• Higher initial cost
• More maintenance

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Soljourner
• Source: Soljourner/claude-engineering-skills
• License: MIT

Install and usage instructions live in the source repository linked above.