How to Size a Rotary Gear Pump: Flow Rate, Pressure, and Viscosity Considerations

Step 1: Determine Required Flow Rate (GPM)

Flow rate requirement is the starting point for all pump sizing. Your required flow rate is determined by process requirements: how much fluid must you transfer, and in what timeframe?

Calculating Flow Rate from Process Requirements

Example: You need to load 1000 gallons of emulsion explosives into boreholes in 50 minutes during a blasting operation.

Required flow = 1000 gallons ÷ 50 minutes = 20 GPM

This is your baseline flow requirement. However, practical pump sizing adds a safety margin—typically 10-20%—to account for variations in operating conditions, pressure losses in hoses, and filter resistance.

Practical pump size = 20 GPM × 1.15 (15% safety margin) = 23 GPM

Understanding Pump Displacement and RPM

Gear pump manufacturers specify flow rate using a fundamental relationship:

GPM = (Displacement in cc/rev ÷ 61) × RPM × Volumetric Efficiency

Where:

• Displacement: Volume moved per complete revolution (in cubic centimeters), a fixed characteristic of each pump model.
• RPM: Revolutions per minute (rotational speed), typically 280 RPM or 190 RPM for NAPCO motors.
• Volumetric Efficiency: The percentage of theoretical flow actually delivered (typically 85-95% for gear pumps).
• 61: Conversion factor from cc/rev to GPM/RPM (metric to imperial).

NAPCO Pump Displacement Reference

NAPCO manufactures two standard gear pump sizes:

• PA300 Series: Larger displacement pump, suitable for higher flow applications.
• PA200 Series: Smaller displacement pump, appropriate for lower flow or high-pressure applications.

Motor speed selection depends on available motor supply (50 Hz or 60 Hz), facility electrical infrastructure, and required RPM for your flow target.

Step 2: Determine System Pressure

System pressure fundamentally affects the actual flow rate delivered by a gear pump. As pressure increases, internal leakage (slippage) between gear teeth and pump body increases, reducing volumetric efficiency and delivered flow rate. Understanding this pressure-flow relationship is essential for accurate sizing.

Pressure Sources

System pressure comes from multiple sources that must be considered together:

• Back Pressure: Pressure at the discharge line (pipe pressure or system relief setting), typically 10-50 PSI for transfer operations.
• Hydrostatic Head: Pressure from fluid column height; 0.433 PSI per foot of vertical rise. A 100-foot deep borehole adds approximately 43 PSI to system pressure.
• Line Friction Loss: Pressure loss due to hose resistance; typically 2-5 PSI per 100 feet of line for standard hose sizes at moderate flow rates.

Example: Loading a 500-foot deep borehole at 20 GPM with discharge line set to 20 PSI relief:

• Discharge line pressure: 20 PSI
• Hydrostatic head (500 ft): 500 × 0.433 = 216 PSI
• Line friction (estimated): 4 PSI
• Total system pressure: 20 + 216 + 4 = 240 PSI

How Pressure Affects Flow: NAPCO Performance Data

The following performance data demonstrates how NAPCO pumps behave across the pressure range. These values represent actual field performance at standard operating conditions:

Notice that the PA300S delivers 158 GPM at low pressure (10 PSI) but only 125 GPM at 150 PSI—an 80% retention. This is normal for gear pump behavior and must be factored into sizing decisions.

Sizing at Pressure: The Critical Decision

Always size gear pumps at the maximum expected system pressure, not at nominal discharge pressure. Using the borehole example above, a maximum system pressure of 240 PSI significantly exceeds the 150 PSI rating of standard NAPCO pumps. This application would require evaluation of alternative pump types or system redesign to reduce pressure (shorter hose, reduced depth, lower discharge pressure setting).

For applications within NAPCO's 150 PSI operating envelope, apply the pressure performance data to ensure delivered flow meets your requirement at maximum system pressure.

Step 3: Consider Fluid Viscosity

Fluid viscosity—measured in Saybolt Universal Seconds (SUS) or centistokes (cSt)—fundamentally affects pump performance. Viscosity influences volumetric efficiency, internal leakage, and wear characteristics.

Viscosity Impact on Performance

Thicker Fluids (Higher Viscosity): Thick fluids create better sealing between gear teeth and pump body, reducing internal leakage. Higher viscosity fluids typically exhibit better volumetric efficiency (90-95%) in gear pumps. However, higher viscosity also increases friction losses and heat generation.

Thin Fluids (Lower Viscosity): Thin fluids provide minimal sealing; internal leakage increases, reducing volumetric efficiency (typically 80-90%). Light fluids flow freely, minimizing friction and heat, but the reduced sealing results in higher slip losses at pressure.

Typical Fluid Viscosities

• Light petroleum: 32-100 cSt (light oils, fuel, low-viscosity hydraulic fluid)
• Emulsion explosives: 20-200 cSt (petroleum-based emulsions, thickness depends on sensitizer loading)
• Water-based adhesives: 50-1000 cSt (latex and PVA products, thicker formulations)
• High-viscosity adhesives: 1000-5000 cSt (hot-melt systems, thick polyesters)

Viscosity and Pump Selection

For NAPCO's standard operating envelope (fluids below 5000 cSt, pressures to 150 PSI), viscosity variations typically result in ±5-10% flow variations. Include viscosity in your sizing evaluation, but do not overweight its effect on NAPCO gear pump selection.

If your fluid exceeds 5000 cSt, consult NAPCO engineering—progressive cavity pumps may become more efficient than gear pumps for high-viscosity applications.

Step 4: Choose Pump Material (Body Construction)

NAPCO offers two material choices for pump bodies: ductile iron and stainless steel. This choice is independent of elastomer material (nitrile or Viton) and is based on environmental conditions and fluid chemistry as it affects metal surfaces.

Ductile Iron (Standard Selection)

Ductile iron is the standard body material for NAPCO pumps used in the majority of industrial applications. It offers excellent corrosion resistance to petroleum products, water-based emulsions, and most industrial fluids.

Select ductile iron for: Explosives loading, adhesive transfer, general industrial transfer, fuel petroleum service where moisture exposure is moderate and protective paint/coatings are maintained.

Stainless Steel (Corrosive/Chemical Environments)

Stainless steel pump bodies (typically 304 or 316 grade) provide superior corrosion resistance in aggressive chemical environments, marine applications, or where the pump exterior will be regularly washed or exposed to corrosive atmospheres.

Select stainless steel for: Chemical processing plants, outdoor mining operations, marine environments, applications with chloride exposure, systems where regular cleaning with corrosive agents is necessary.

Cost and Performance Comparison

Stainless steel pump bodies cost 15-25% more than ductile iron equivalents. Select stainless steel only when environmental conditions genuinely require it. For standard industrial applications with moderate moisture and reasonable protective measures, ductile iron is the cost-effective choice.

See our detailed article on Stainless Steel vs. Ductile Iron Pump Bodies for comprehensive guidance on material selection.

Step 5: Select Gear Material (Elastomer Type)

The final sizing decision is elastomer material—nitrile or Viton. This choice is based purely on fluid chemistry compatibility, independent of body material or displacement selection.

Quick Decision Framework

Select Nitrile if: Your fluid is petroleum-based, water-based, or standard industrial emulsion; operating temperature is below 240°F; cost-effectiveness is a priority.

Select Viton if: Your fluid contains aromatic solvents, chlorinated hydrocarbons, strong acids, or oxidizers; operating temperature exceeds 240°F; fluid chemistry is uncertain or potentially aggressive.

For detailed guidance on elastomer selection, see our article on Nitrile vs. Viton Gears.

NAPCO Pump Sizing Reference: Full Specifications

The following table provides complete specifications for NAPCO's standard pump lineup, enabling direct comparison across flow ranges and pressure conditions:

Additional configurations including Viton elastomers and custom displacement options are available. Contact NAPCO engineering for specifications beyond the standard lineup.

Practical Sizing Examples

Example 1: Explosives Emulsion Loading

Application Requirements:

• Load 1,500 gallons of emulsion explosive per operation
• Complete loading in 45 minutes
• Borehole depth: 200 feet
• Discharge line pressure: 15 PSI relief
• Fluid: Water-in-oil emulsion, ~100 cSt

Sizing Analysis:

• Required flow: 1500 ÷ 45 = 33 GPM
• Safety margin (15%): 33 × 1.15 = 38 GPM nominal
• System pressure: 15 + (200 × 0.433) + 3 = ~100 PSI
• At 100 PSI, PA300S delivers 140 GPM (exceeds 38 GPM requirement)
• Elastomer: Nitrile (standard for emulsions)
• Body: Ductile iron (adequate for emulsion service)

Recommendation: PA300S ductile iron with nitrile gears. At 100 PSI, this pump provides 140 GPM, well above the required 38 GPM, allowing margin for hose restrictions and pressure variations during the operation.

Example 2: High-Viscosity Adhesive Transfer

Application Requirements:

• Transfer hot-melt adhesive at 20 GPM
• Adhesive viscosity: 3500 cSt at operating temperature
• Fluid contains aromatic resin components
• System pressure: ~40 PSI (short hose, low-pressure application)

Sizing Analysis:

• Required flow: 20 GPM
• Viscosity consideration: 3500 cSt is within gear pump range (excellent efficiency)
• System pressure: 40 PSI (low pressure)
• At 40 PSI, PA200S delivers ~67 GPM (exceeds 20 GPM requirement)
• Aromatic content: Nitrile may be at risk; evaluate Viton
• Body: Stainless steel recommended (adhesive service, potential outdoor exposure)

Recommendation: Specify either PA200C stainless with nitrile (if aromatic content is <10%) or PA200C stainless with Viton elastomers (if aromatic content exceeds 10%). At 40 PSI, PA200S/C delivers ~67 GPM, providing comfortable margin above the 20 GPM requirement.

Example 3: Low-Flow Chemical Transfer

Application Requirements:

• Transfer dilute chemical solution at 5 GPM
• Discharge pressure: 100 PSI (elevated pressure required)
• Fluid contains trace aromatic solvents and mild oxidizing agents
• Outdoor installation, potential corrosion exposure

Sizing Analysis:

• Required flow: 5 GPM
• System pressure: 100 PSI
• At 100 PSI, PA200S delivers 60 GPM (far exceeds 5 GPM)
• Chemical compatibility: Viton required (aromatic + oxidizers)
• Body: Stainless steel (outdoor, corrosion risk)

Recommendation: Specify PA200C stainless with Viton elastomers. At 100 PSI, this pump delivers 60 GPM, providing extreme oversizing for the 5 GPM requirement. While this creates excess capacity, it ensures Viton elastomer compatibility (chemical resistance) and stainless body (corrosion resistance) are the limiting factors. The large safety margin allows for future capacity expansion or higher-viscosity fluid options.

Common Sizing Mistakes to Avoid

Mistake 1: Sizing at Nominal Discharge Pressure Instead of Maximum System Pressure

Many engineers size pumps at the relief valve setting (e.g., 20 PSI) without accounting for hydrostatic pressure from depth or friction losses. A borehole adding 150 PSI of hydrostatic head will result in actual system pressure of 170 PSI—beyond the pump's rated capability. Always calculate total system pressure including all sources.

Mistake 2: Underestimating Pressure Derate Impact

A pump delivering 158 GPM at 10 PSI delivers only 140 GPM at 100 PSI—an 11% reduction. Underestimating this impact can result in a undersized pump that cannot meet flow requirements at operating pressure. Always reference flow at your expected operating pressure, not at low-pressure baseline.

Mistake 3: Inadequate Safety Margin

Sizing the pump to exactly match minimum theoretical flow requirements creates zero margin for hose friction, filter resistance, temperature variations, or future process changes. A 15-20% safety margin is industry-standard practice.

Mistake 4: Wrong Elastomer for Fluid Chemistry

Specifying nitrile for an application containing aromatic solvents results in rapid elastomer degradation and premature failure. Always verify fluid compatibility with elastomer type. When fluid chemistry is uncertain, Viton is the conservative choice.

Mistake 5: Ignoring High-Pressure Regime Pump Changes

Applications exceeding 150 PSI maximum operating pressure exceed NAPCO gear pump capability. Deep-well or extremely high-viscosity applications may require progressive cavity pumps. Consult NAPCO engineering before committing to applications beyond this envelope.

Related Technical Resources

• Nitrile vs. Viton Gears: Material Selection Guide
• Gear Pumps vs. Progressive Cavity Pumps
• Stainless Steel vs. Ductile Iron Pump Bodies
• PA300S Specifications
• PA300C Specifications
• PA200S Specifications
• PA200C Specifications
• Engineering & Technical Support