Screw Pump Viscosity Handling Guide - High Viscosity Design

Viscosity Fundamentals

Understanding Viscosity

Viscosity Reference Table

Viscosity Effects on Performance

Slip Behavior

Slip ∝ Pressure / Viscosity

Higher viscosity → Lower slip → Higher volumetric efficiency

Example:
At 50 cSt:    Slip = 10%  → Efficiency = 90%
At 500 cSt:   Slip = 3%   → Efficiency = 97%
At 5000 cSt:  Slip = 1%   → Efficiency = 99%

Efficiency vs Viscosity

Power Consumption

At higher viscosity:
✅ Volumetric efficiency increases (less slip)
❌ Friction losses increase
❌ Power required increases

Net effect: Power usually increases with viscosity

Temperature-Viscosity Relationship

Critical Understanding

Temperature is the most important factor affecting viscosity

Typical Viscosity Changes

Example Calculation

Starting viscosity at 25°C: 500 cSt
Temperature coefficient: -6% per °C

At 40°C: 500 × (1 - 0.06)^15 = 500 × 0.40 = 200 cSt
At 60°C: 500 × (1 - 0.06)^35 = 500 × 0.11 = 55 cSt

Viscosity drops dramatically with temperature!

Temperature Impact Summary

Design for Variable Viscosity

Cold Start Considerations

Motor Sizing for Cold Start

Required starting torque = f(cold viscosity, pressure)

Rule of thumb:
If cold viscosity > 3× operating viscosity:
  Motor SF = 1.25 minimum
  Consider VFD with boost

If cold viscosity > 10× operating:
  Pre-heat fluid to reduce viscosity
  Size for startup condition

Hot Operation Considerations

Speed Selection for Viscosity

Speed Guidelines

Why Speed Matters

High Viscosity + High Speed = Cavitation Risk

At suction:
- Fluid cannot fill cavities fast enough
- Vacuum pockets form
- Noise, vibration, damage

Solution: Reduce speed proportionally with viscosity increase

Speed Selection Process

1. Identify operating viscosity range (min/normal/max)
2. Select speed for highest viscosity (cold start)
3. Verify flow meets requirements at that speed
4. Size pump displacement accordingly

Suction System Design

High Viscosity Suction Requirements

Suction Line Sizing

Recommended approach:

Suction pipe diameter ≥ 1.5 × pump inlet diameter

For high viscosity (>500 cSt):
Suction pipe ≥ 2 × pump inlet diameter

NPSH Considerations

Solutions:

• Larger suction lines
• Flooded suction preferred
• Pre-heating to reduce viscosity
• Slow pump speed

Heating and Cooling

When to Heat

Heating Methods

When to Cool

Viscosity Monitoring

Why Monitor Viscosity

Monitoring Methods

Control Strategy

Temperature Control → Viscosity Control

If temperature increases → Viscosity decreases → Flow increases
If temperature decreases → Viscosity increases → Flow decreases

Use temperature to infer viscosity changes
Adjust speed (VFD) to compensate

Specification Considerations

Datasheet Requirements

VISCOSITY SPECIFICATION:

Operating Conditions:
Temperature (°C):    Min ___    Normal ___    Max ___
Viscosity (cSt):     ___        ___           ___

Cold Start:
Ambient temperature: ___ °C
Fluid temperature at start: ___ °C
Viscosity at start: ___ cSt

Temperature-Viscosity Data:
□ Viscosity curve provided
□ ASTM D341 slope calculated

Heating Requirements:
□ None required
□ Tank heating to ___ °C
□ Line heating required
□ Pump jacket heating

Vendor Requirements

Summary Guidelines

Design Checklist

□ Define full viscosity range (cold to hot)
□ Specify viscosity at each temperature
□ Calculate cold start viscosity
□ Verify motor can start at cold viscosity
□ Select appropriate pump speed
□ Size suction line for highest viscosity
□ Specify heating if cold viscosity > 10,000 cSt
□ Consider VFD for viscosity compensation
□ Verify efficiency acceptable at min viscosity
□ Plan for temperature/viscosity monitoring

Quick Reference