Magnetic Drive Pump Containment Shell Design - Materials and Efficiency

Containment Shell Function

The containment shell (also called containment can or rear casing) is the critical hermetic barrier in magnetic drive pumps.

Primary Functions

Design Requirements (per API 685)

Material Options

Metallic Shells

Non-Metallic Shells

Eddy Current Losses

What Are Eddy Currents?

When the outer magnet rotates, it creates a rotating magnetic field that passes through the containment shell. In electrically conductive materials, this changing magnetic field induces circulating electric currents (eddy currents) that:

1. Generate heat (I²R losses)
2. Consume shaft power
3. Create opposing magnetic field (reduces coupling efficiency)

Eddy Loss Formula

P_eddy ∝ (B² × f² × t² × σ) / ρ

Where:
B = Magnetic field strength
f = Rotational frequency (speed)
t = Shell thickness
σ = Electrical conductivity
ρ = Material resistivity

Eddy Loss by Material

Temperature Rise from Eddy Losses

This temperature rise must be added to process temperature when selecting magnet grade.

Shell Thickness Design

Thickness vs Performance Trade-off

Typical Shell Thickness

Thickness Calculation Factors

Shell Thickness = f(Design Pressure, Temperature, Material Strength, Safety Factor)

Per ASME Section VIII:
t = (P × R) / (S × E - 0.6P)

Where:
t = minimum thickness
P = design pressure
R = inside radius
S = allowable stress at temperature
E = joint efficiency (1.0 for seamless)

Secondary Containment (API 685)

API 685 Section 9.1 Requirement

“A secondary containment shall be provided…”

Secondary Containment Options

Shell Design Configurations

Standard Designs

Shell Mounting Methods

Efficiency Optimization

Strategies to Minimize Eddy Losses

When Efficiency Matters Most

Energy Cost Impact Example

Motor: 50 kW
Operating hours: 8,000 hr/year
Energy cost: $0.10/kWh

Shell Option A (Hastelloy): 10% eddy loss = 5 kW loss
Annual cost: 5 × 8,000 × $0.10 = $4,000/year

Shell Option B (Titanium): 5% eddy loss = 2.5 kW loss
Annual cost: 2.5 × 8,000 × $0.10 = $2,000/year

Savings: $2,000/year → Payback titanium premium in 2-3 years

Pressure Rating

Design Pressure Determination

Pressure Rating by Material

Corrosion Considerations

Material Selection for Corrosive Service

Corrosion Allowance

Specification Format

=== CONTAINMENT SHELL SPECIFICATION ===

Design Conditions:
- Design Pressure: ___ barg at ___°C
- Design Temperature: ___°C
- Test Pressure: ___ barg (1.5× design)

Shell Construction:
- Material: _________ (Grade: ___)
- Type: Metallic / Non-metallic
- Nominal Thickness: ___ mm
- Minimum Thickness: ___ mm

Performance:
- Estimated Eddy Losses: ___ kW (___ % of shaft power)
- Temperature Rise at Min Flow: ___°C

Secondary Containment:
- Type: _______________
- Detection Method: _______________

Certification:
- ASME Section VIII: □ Required
- PED (European): □ Required
- Other: _______________

Vendor Data Requirements

Request from Vendor

Review Checklist

□ Shell material matches specification
□ Thickness adequate for design pressure
□ Eddy losses included in motor sizing
□ Temperature rise added to magnet selection
□ Secondary containment per API 685
□ Leak detection provision included
□ Material test reports available
□ Weld procedures qualified (if welded)

Summary: Shell Selection Guide