Positive Displacement Principle
Screw pumps are positive displacement (PD) pumps, fundamentally different from centrifugal pumps.
PD vs Centrifugal
| Characteristic | Positive Displacement (Screw) | Centrifugal |
|---|---|---|
| Flow generation | Trapped volume displacement | Kinetic energy transfer |
| Flow vs pressure | Constant (at constant speed) | Decreases with pressure |
| Flow vs speed | Linear (Q ∝ N) | Non-linear |
| Viscosity effect | Improves efficiency | Degrades efficiency |
| Self-priming | Yes | No |
| Pulsation | Low to none | Higher |
Key Principle
Flow Rate = Displacement × Speed
Q = V × N
Where:
Q = Flow rate (L/min)
V = Displacement volume (cc/rev)
N = Rotational speed (RPM)The flow is essentially independent of discharge pressure - a key advantage for process applications.
Screw Pump Types
Single Screw (Progressive Cavity)
Construction:
- One helical rotor rotating inside elastomer stator
- Stator has double-helix internal profile
- Creates progressing cavities that move axially
Operating Principle:
Rotor rotation → Cavities form at suction
→ Cavities seal and progress
→ Cavities discharge at outletCharacteristics:
| Parameter | Typical Range |
|---|---|
| Viscosity range | 1-1,000,000 cP |
| Pressure capability | Up to 48 bar |
| Flow rate | 0.1-500 m³/h |
| Speed | 100-500 RPM |
Best Applications:
- Highly viscous fluids (bitumen, polymers)
- Fluids with solids content
- Shear-sensitive products
- Sludge and wastewater
Twin Screw Pump
Construction:
- Two intermeshed helical screws
- Screws do NOT contact each other
- Synchronized by timing gears
- Close-fitting housing
Operating Principle:
Counter-rotation → Gaps expand at suction (fluid enters)
→ Fluid trapped between threads
→ Gaps contract at discharge (fluid expelled)Characteristics:
| Parameter | Typical Range |
|---|---|
| Viscosity range | 1-2,000 cP (up to 1M cP special) |
| Pressure capability | Up to 30 MPa (300 bar) |
| Flow rate | 0.3-1,000 m³/h |
| Speed | 1,400-2,800 RPM |
Best Applications:
- Industrial oils and lubricants
- Petroleum products
- Chemical transfer
- High-pressure systems
Triple Screw Pump
Construction:
- One driving screw + two driven screws
- All three screws intermesh within housing
- Driven screws rotate opposite to driver
- Pressure-balanced design
Operating Principle:
Power screw drives → Two idler screws rotate
→ Sealed cavities form between all three
→ Fluid transported with minimal pulsationCharacteristics:
| Parameter | Typical Range |
|---|---|
| Viscosity range | 10-1,000 cSt |
| Pressure capability | Up to 35 MPa (350 bar) |
| Flow rate | 1-500 L/min |
| Speed | Up to 3,600 RPM |
Best Applications:
- Hydraulic power units
- Lubrication systems
- Fuel transfer
- High-pressure, precision flow
Flow Characteristics
Flow vs Speed Relationship
┌─────────────────────────────────────────────────────────────┐
│ Flow Rate │
│ │ │
│ │ Screw Pump │
│ │ ╱ │
│ │ ╱ (Linear) │
│ │ ╱ │
│ │ ╱ │
│ │ ╱ │
│ │ ╱ Centrifugal │
│ │ ╱ ╱ (Non-linear) │
│ │ ╱ ╱ │
│ │ ╱ ╱ │
│ └──────────────────────────────────────── Speed │
└─────────────────────────────────────────────────────────────┘Flow vs Pressure Relationship
| Pump Type | Flow Behavior |
|---|---|
| Screw Pump | Nearly constant regardless of pressure |
| Centrifugal | Decreases as pressure increases |
Pulsation Comparison
| Pump Type | Pulsation Level | Cause |
|---|---|---|
| Single screw | Low-medium | Cavity progression |
| Twin screw | Very low | Continuous intermeshing |
| Triple screw | Minimal | Pressure-balanced design |
| Gear pump | High | Gear tooth engagement |
| Reciprocating | Very high | Piston stroke |
Internal Sealing Mechanism
How Sealing Works
Screw pumps rely on tight clearances between:
- Screw threads and housing
- Adjacent screw threads (in twin/triple)
Slip (Internal Leakage)
Slip = Fluid that leaks from discharge back to suction
Slip ∝ (Pressure Differential) / (Viscosity × Clearance²)Key Relationships:
| Factor | Effect on Slip |
|---|---|
| Higher pressure | Increases slip |
| Higher viscosity | Decreases slip |
| Tighter clearances | Decreases slip |
| Higher speed | No direct effect |
Why Viscosity Helps
Low Viscosity Fluid: High Viscosity Fluid:
┌──────────────────┐ ┌──────────────────┐
│ Discharge │ │ Discharge │
│ ↓ │ │ ↓ │
│ ← Slip → │ │ ← Minimal → │
│ ↓ │ │ ↓ │
│ Suction │ │ Suction │
└──────────────────┘ └──────────────────┘
More leakage Almost no leakage
Lower efficiency Higher efficiencyVolumetric Efficiency
Definition
ηv = (Actual Flow / Theoretical Flow) × 100%
Where:
Theoretical Flow = Displacement × Speed
Actual Flow = Measured outputTypical Efficiency Ranges
| Pump Type | Viscosity Range | Volumetric Efficiency |
|---|---|---|
| Twin screw | < 100 cSt | 85-90% |
| Twin screw | 100-500 cSt | 90-95% |
| Twin screw | > 500 cSt | 95-100% |
| Triple screw | Standard | 90-95% |
| Single screw | Standard | 85-90% |
Higher viscosity = Higher efficiency (opposite of centrifugal pumps!)
Pressure Generation
Pressure Stages
Each complete screw thread creates a “stage” of pressure:
- More threads = Higher total pressure capability
- Pressure rises progressively from suction to discharge
Pressure Distribution
Suction ────────────────────────────────── Discharge
│ │
P1 → P2 → P3 → P4 → P5 → P6 → P7 → P8 → P9 │
│ Progressive pressure increase │
└───────────────────────────────────────────┘Maximum Pressure by Type
| Screw Type | Maximum Pressure | Notes |
|---|---|---|
| Single screw | 48 bar | Limited by stator elastomer |
| Twin screw (standard) | 30 bar | General purpose |
| Twin screw (HP) | 300 bar | Heavy-duty design |
| Triple screw | 350 bar | Highest pressure capability |
Self-Priming Capability
Why Screw Pumps Self-Prime
| Factor | Explanation |
|---|---|
| Positive displacement | Creates vacuum at suction |
| Sealed cavities | Prevent air from bypassing |
| No impeller | No need for liquid momentum |
Priming Performance
| Pump Type | Self-Priming | Suction Lift |
|---|---|---|
| Single screw | Excellent | Up to 8 m |
| Twin screw | Good | Up to 6 m |
| Triple screw | Good | Up to 5 m |
| Centrifugal | No | Requires flooded suction |
Speed and Flow Control
Variable Speed Operation
Screw pumps are ideal for VFD (Variable Frequency Drive) control:
Flow Control:
- Reduce speed → Proportional flow reduction
- No minimum flow concern (unlike centrifugal)
- Excellent turndown ratio
Turndown Ratio = Max Flow / Min Flow
Typical: 10:1 to 20:1Speed Selection Factors
| Viscosity | Recommended Speed |
|---|---|
| < 100 cSt | Higher speeds OK |
| 100-1000 cSt | Moderate speeds |
| > 1000 cSt | Lower speeds |
| > 10,000 cSt | Very low speeds (100-300 RPM) |
Advantages Summary
| Advantage | Explanation |
|---|---|
| Constant flow | Independent of pressure changes |
| High viscosity handling | Performance improves with viscosity |
| Self-priming | No need for foot valves or priming systems |
| Low pulsation | Smooth, continuous flow |
| Gentle pumping | Low shear - good for sensitive fluids |
| Reversible | Can run in both directions |
| Wide speed range | Excellent for VFD control |
Limitations
| Limitation | Impact | Mitigation |
|---|---|---|
| Higher cost | 2-3× centrifugal | Justified by application needs |
| Sensitive to solids | Wear, damage | Use filters, single screw for solids |
| Lower max flow | <1000 m³/h typical | Multiple pumps if needed |
| Viscosity minimum | Below 5 cSt, excessive slip | Use centrifugal instead |