Screw Pump Selection Guide - Type Selection and Sizing Criteria

Selection Decision Process

Step 1: Confirm PD Pump is Appropriate

Choose Positive Displacement (Screw) when:

Choose Centrifugal instead when:

Step 2: Select Screw Pump Type

                      START
                        │
                        ▼
            ┌───────────────────────┐
            │ Fluid contains solids │
            │      or fibers?       │
            └───────────────────────┘
                   │         │
                  YES        NO
                   │         │
                   ▼         ▼
           ┌──────────┐   ┌───────────────────────┐
           │  SINGLE  │   │ Viscosity > 10,000 cSt│
           │  SCREW   │   └───────────────────────┘
           └──────────┘          │         │
                               YES        NO
                                │         │
                                ▼         ▼
                        ┌──────────┐   ┌───────────────────┐
                        │  SINGLE  │   │ Pressure > 100 bar│
                        │  SCREW   │   └───────────────────┘
                        └──────────┘          │         │
                                            YES        NO
                                             │         │
                                             ▼         ▼
                                     ┌──────────┐   ┌──────────┐
                                     │  TRIPLE  │   │   TWIN   │
                                     │  SCREW   │   │  SCREW   │
                                     └──────────┘   └──────────┘

Type Selection Matrix

Application-Based Selection

Performance-Based Selection

*Special designs only

Sizing Criteria

Required Process Data

Viscosity Considerations

Critical: Viscosity changes with temperature!

Design for worst case:

• Sizing: Use viscosity at operating temperature
• Starting: Check viscosity at ambient (cold) temperature
• Motor: Size for maximum viscosity condition

Flow Rate Determination

Required Flow = Process Demand × (1 + Margin)

Typical Margins:
- Standard service: 10-15%
- Critical service: 20-25%
- Variable demand: Size for maximum

Pressure Determination

Required Pressure = System Pressure + Friction Loss + Safety Margin

Components:
- Static head (elevation)
- Vessel/equipment pressure
- Pipeline friction losses
- Control valve pressure drop
- Safety margin: +10-15%

Speed Selection

Speed vs Viscosity Guidelines

Speed Impact on Performance

NPSH Verification

NPSH Calculation

NPSHa = (P_suction - P_vapor) / (ρ × g) + H_static - H_friction

Where:
P_suction = Absolute pressure at suction source
P_vapor = Vapor pressure of fluid at temperature
ρ = Fluid density
g = Gravitational acceleration
H_static = Static head (positive if flooded)
H_friction = Suction line friction losses

NPSH Requirements for Screw Pumps

NPSH Margin

NPSHa ≥ NPSHr × 1.5 (minimum)

For critical service: NPSHa ≥ NPSHr × 2.0

Material Selection

Standard Materials by Component

Material Selection by Service

Motor Sizing

Power Calculation

P_shaft = (Q × ΔP) / (600 × η)

Where:
P_shaft = Shaft power (kW)
Q = Flow rate (L/min)
ΔP = Differential pressure (bar)
η = Pump efficiency (typically 0.7-0.9)

Motor Selection

P_motor = P_shaft × Service Factor

Service Factors:
- Normal duty: 1.15
- Heavy duty: 1.25
- Variable viscosity: 1.30
- Cold start: Consider startup torque

Cold Start Considerations

Vendor Selection Criteria

Technical Evaluation

Must-Have Requirements

• Flow tolerance within ±3%
• Pressure rating ≥ specified + 10%
• NPSHr < NPSHa with margin
• API 676 compliance (if specified)
• Material certificates for wetted parts
• Performance curves at operating viscosity
• Bearing life calculation (L10 ≥ 15,000 hrs)

Quick Selection Reference

By Flow Rate

By Viscosity

By Pressure