Magnetic Drive Pump Sizing and Hydraulic Design Guide

Sizing Process Overview

Magnetic drive pump sizing follows standard centrifugal pump methodology with additional considerations for magnetic coupling losses.

Sizing Steps

1. Define Process Requirements (Flow, Head, Fluid)
        ↓
2. Calculate Hydraulic Power
        ↓
3. Add Magnetic Coupling Losses ← Unique to Mag-Drive
        ↓
4. Size Motor with Adequate Margin
        ↓
5. Verify NPSH and Operating Point
        ↓
6. Confirm Temperature Limits ← Unique to Mag-Drive
        ↓
7. Select Final Pump Model

Process Requirements

Flow Determination

Head Calculation

Fluid Properties Required

Hydraulic Power Calculation

Basic Hydraulic Power

P_hydraulic = (Q × H × ρ × g) / (3.6 × 10⁶)

Where:
P_hydraulic = Hydraulic power (kW)
Q = Flow rate (m³/h)
H = Total head (m)
ρ = Fluid density (kg/m³)
g = 9.81 m/s²

Simplified Formula (Water)

P_hydraulic = (Q × H) / 367

Where:
Q = Flow (m³/h)
H = Head (m)
Result in kW

Example Calculation

Given:
- Flow: 50 m³/h
- Head: 60 m
- Fluid: Water (SG = 1.0)

P_hydraulic = (50 × 60) / 367 = 8.2 kW

Shaft Power and Efficiency

Pump Shaft Power

P_shaft = P_hydraulic / η_pump

Where:
η_pump = Pump hydraulic efficiency (typically 0.60-0.80)

Efficiency Considerations

Example

P_hydraulic = 8.2 kW
η_pump = 0.70 (from curve)

P_shaft = 8.2 / 0.70 = 11.7 kW

Magnetic Coupling Losses

Loss Components

Eddy Loss by Shell Material

Calculating Total Magnetic Losses

P_magnetic = P_shaft × (Eddy% + Friction% + Slip%)

Example (Hastelloy shell):
P_magnetic = 11.7 × (0.10 + 0.03 + 0.02)
P_magnetic = 11.7 × 0.15 = 1.76 kW

Motor Sizing

Total Power Required

P_motor_min = P_shaft + P_magnetic

Example:
P_motor_min = 11.7 + 1.76 = 13.46 kW

Service Factor Application

Motor Selection

P_motor_rated ≥ P_motor_min × Service Factor

Example:
P_motor_rated ≥ 13.46 × 1.15 = 15.5 kW

Select next standard motor: 18.5 kW

Complete Power Flow

┌─────────────────────────────────────────────────────────────┐
│                     POWER FLOW DIAGRAM                       │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│  Motor Input    →   Motor Losses   →   Shaft Power          │
│  (18.5 kW)          (-1.5 kW)          (17.0 kW)           │
│                                                              │
│  Shaft Power    →   Magnetic Losses →  Available to Pump    │
│  (17.0 kW)          (-2.5 kW)          (14.5 kW)           │
│                                                              │
│  Available      →   Pump Losses     →   Hydraulic Power     │
│  (14.5 kW)          (-6.3 kW)          (8.2 kW)            │
│                                                              │
│  Overall Efficiency: 8.2/18.5 = 44%                         │
│                                                              │
└─────────────────────────────────────────────────────────────┘

NPSH Verification

NPSH Available Calculation

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

Where all pressures in absolute terms

NPSH Required

NPSH Margin Requirements

Operating Point Verification

Preferred Operating Region

Magnetic Pump Specific Concerns

Minimum Flow Calculation

Why Higher Minimum Flow?

Magnetic pumps require higher minimum flow because:

1. Bearing lubrication - Product-lubricated bearings need flow
2. Magnet cooling - Heat from eddy losses must be removed
3. Recirculation heat - Hydraulic energy converted to heat

Minimum Flow Determination

Temperature Rise at Minimum Flow

ΔT = (P_shaft × η_thermal) / (Q_min × ρ × Cp)

Where:
ΔT = Temperature rise (°C)
P_shaft = Shaft power (kW)
η_thermal = % power converted to heat (~30-40%)
Q_min = Minimum flow (m³/h)
ρ = Density (kg/m³)
Cp = Specific heat (kJ/kg·°C)

Example

P_shaft = 11.7 kW
Heat fraction = 35%
Q_min = 10 m³/h = 2.78 L/s
ρ = 1000 kg/m³
Cp = 4.18 kJ/kg·°C

Heat generated = 11.7 × 0.35 = 4.1 kW = 4.1 kJ/s

ΔT = 4.1 / (2.78 × 1.0 × 4.18) = 0.35°C

(Very low because Q_min is adequate)

Recirculation Line Sizing

When Required

• Pump may operate below minimum flow
• Process has variable demand
• No other flow path exists

Sizing Approach

Q_recirc = Q_min - Q_process_minimum

Where:
Q_min = Pump minimum stable flow
Q_process_minimum = Lowest expected process flow

Recirculation Options

Torque and Coupling Verification

Magnetic Coupling Torque

T_required = (P_shaft × 9550) / n

Where:
T_required = Torque (Nm)
P_shaft = Shaft power (kW)
n = Speed (rpm)

Torque Margin

Decoupling Prevention

T_coupling > T_max_load × SF

Where SF = 1.5 minimum per API 685

Summary Sizing Checklist

=== MAGNETIC PUMP SIZING CHECKLIST ===

□ Process Requirements
  □ Flow: Rated ___ m³/h, Min ___ m³/h, Max ___ m³/h
  □ Head: ___ m (includes margins)
  □ Fluid: ___ (SG: ___, Viscosity: ___ cP)
  □ Temperature: ___ °C normal, ___ °C max

□ Hydraulic Calculations
  □ P_hydraulic = ___ kW
  □ P_shaft = ___ kW (at η_pump = ___%)

□ Magnetic Losses
  □ Shell material: ___________
  □ Eddy losses: ___ kW (___%)
  □ Friction losses: ___ kW (___%)
  □ Total magnetic: ___ kW (___%)

□ Motor Selection
  □ P_minimum = P_shaft + P_magnetic = ___ kW
  □ Service factor: ___
  □ P_motor selected: ___ kW

□ NPSH Verification
  □ NPSHa = ___ m
  □ NPSHr = ___ m
  □ Margin = ___ m (≥ 0.5m? □)

□ Operating Point
  □ Duty point at ___% of BEP (70-120%? □)
  □ Minimum flow = ___ m³/h
  □ Recirculation required? □ Yes □ No

□ Temperature Verification
  □ Process temp: ___ °C
  □ + Eddy heating: ___ °C
  □ + Safety margin: ___ °C
  □ Total: ___ °C < Magnet MAT? □

□ Coupling Verification
  □ Required torque: ___ Nm
  □ Coupling rating: ___ Nm
  □ Margin: ___ (≥ 1.5? □)

Quick Reference Formulas