High Temperature High Pressure Petrochemical Pump: Thermal Expansion and Casing Stress Analysis

High-temperature, high-pressure (HTHP) pumping applications in petrochemical plants — including reactor feed pumps, fractionator bottoms pumps, hot oil circulation pumps, and hydrocracker charge pumps — operate at temperatures from 200°C to over 450°C and pressures from 20 to 200 bar. At these conditions, thermal expansion of the pump casing, shaft, and piping generates significant mechanical stress that can cause casing distortion, seal failure, bearing misalignment, and even catastrophic casing rupture if not properly addressed in the pump design and installation. Understanding thermal expansion behavior and casing stress is critical for selecting, installing, and maintaining reliable HTHP pumps.

This guide covers thermal expansion and casing stress analysis for high-temperature high-pressure petrochemical pumps, with focus on design features, installation requirements, and common failure modes.

HTHP Pump Applications

Thermal Expansion Fundamentals

Linear Thermal Expansion

When a material is heated, it expands. The amount of expansion is:

ΔL = L₀ × α × ΔT

where: ΔL = expansion (mm), L₀ = original length (mm), α = coefficient of thermal expansion (mm/mm/°C), ΔT = temperature rise (°C)

Expansion Coefficients for Pump Materials

Practical Expansion Example

Consider a BB2 pump casing (carbon-moly steel, α = 12.0 × 10⁻⁶ /°C) with the following dimensions:

• Casing length (suction to discharge centerline): 600 mm
• Operating temperature: 350°C
• Ambient (installation) temperature: 20°C
• ΔT = 330°C

ΔL = 600 × 12.0 × 10⁻⁶ × 330 = 2.38 mm

The casing expands by 2.38 mm in length. If the pump is rigidly bolted to a foundation that does not expand (concrete, α ≈ 10 × 10⁻⁶ /°C), the differential expansion creates stress in the casing, piping, and foundation.

Shaft expansion: A 316SS shaft (α = 16.0 × 10⁻⁶ /°C) of 1500 mm length at 350°C expands by:

ΔL = 1500 × 16.0 × 10⁻⁶ × 330 = 7.92 mm

This shaft expansion must be accommodated by the coupling (allowing axial movement) and the thrust bearing (setting the rotor axial position). If not accommodated, the shaft can bind, causing bearing failure or rotor rub.

Casing Stress Analysis

Sources of Casing Stress

1. Internal pressure stress: The casing contains high-pressure fluid, generating hoop stress (circumferential) and longitudinal stress. This is the primary design load and is addressed by pressure vessel design codes (ASME BPVC Section VIII, API 610).
2. Thermal stress (uniform temperature): If the entire casing is at uniform temperature, free expansion causes no stress. But if expansion is constrained (by piping, foundation, or supports), thermal stress develops.
3. Thermal gradient stress: Different parts of the casing are at different temperatures (e.g., suction vs. discharge, top vs. bottom). This causes differential expansion and bending stress. This is the most common cause of casing distortion in HTHP pumps.
4. Piping load stress: Piping connected to the pump flanges exerts forces and moments on the casing. At high temperature, piping expansion adds to these loads.
5. Bolt stress: Casing bolts (for split casing) must be tightened to a specific torque. At high temperature, bolt relaxation and differential expansion between bolt and casing can cause leakage or bolt failure.

Hoop Stress Calculation

For a cylindrical casing under internal pressure:

σ_hoop = P × D / (2 × t)

where: P = internal pressure (MPa), D = internal diameter (mm), t = wall thickness (mm)

Example: BB5 barrel, ID = 300 mm, P = 200 bar (20 MPa), t = 50 mm

σ_hoop = 20 × 300 / (2 × 50) = 60 MPa

Carbon-moly steel at 400°C has allowable stress ~120 MPa, so the design margin is 2:1. API 610 requires minimum safety factor of 4:1 for casing pressure containment (based on burst pressure).

Thermal Gradient Stress

During startup or transient operation, the casing outer wall heats up slower than the inner wall (which is in contact with hot fluid). This creates a temperature gradient through the wall thickness:

• Inner wall: Hot, expands more → compressive stress (constrained by cooler outer wall)
• Outer wall: Cooler, expands less → tensile stress (pulled by expanding inner wall)

If the temperature gradient is too steep (rapid startup), the tensile stress at the outer wall can exceed the material's yield strength, causing permanent deformation or cracking. This is called thermal shock.

Mitigation:

• Limit startup rate to 50–100°C per hour (depending on casing thickness)
• Use a warming line (small bypass from discharge to suction) to pre-heat the casing
• Insulate the casing to reduce heat loss and temperature gradients
• Use centerline-mounted design (allows free expansion)

Casing Distortion Effects

Even if stress is within allowable limits, thermal distortion can cause operational problems:

• Bore distortion: The casing bore (where the impeller operates) can become non-circular, causing impeller rub and efficiency loss.
• Seal chamber distortion: The seal chamber bore distorts, causing the mechanical seal to bind or leak.
• Flange face distortion: Suction/discharge flanges warp, causing gasket leakage and piping stress.
• Bearing housing misalignment: The bearing centerline shifts relative to the casing, causing shaft misalignment and bearing failure.

Design Features for HTHP Service

Centerline Mounting

API 610 OH2 and BB2 pumps use centerline mounting — the casing is supported at its horizontal centerline, not at the bottom feet. This allows the casing to expand equally upward and downward as temperature increases, keeping the shaft centerline at the same elevation. This is critical for maintaining coupling alignment and preventing piping stress.

Foot-mounted (OH1) pumps expand upward from the feet, raising the shaft centerline by several millimeters at high temperature. This causes coupling misalignment and piping stress. OH1 pumps are limited to <150°C service by API 610.

Our API 610 OH2 process pump range features centerline mounting for high-temperature service up to 400°C. For higher pressures and flows, our API 610 BB2 between-bearing pump range provides centerline-mounted, radially split construction for HTHP applications.

Radially Split Casing (BB2/BB5)

For HTHP service, a radially split casing (no horizontal joint) is preferred:

• No horizontal gasket: The horizontal joint in axially split (BB1/BB3) pumps is a potential leak path at high pressure and temperature. Gasket creep and bolt relaxation can cause leakage.
• Circular cross-section: Radially split casings have a circular cross-section, which is optimal for pressure containment (uniform hoop stress).
• Thicker walls: Radially split casings can be made with thicker walls for high pressure, without the complexity of a horizontal joint.
• Back pull-out: The rotating assembly can be removed from the end (BB2) or as a cartridge (BB5) without disturbing piping.

Casing Material Selection

Important: When casing and impeller/shaft materials have different expansion coefficients (e.g., Cr-Mo casing with 316SS impeller), differential expansion must be accounted for in the running clearances (wear rings, seal clearances). At operating temperature, clearances change — if not designed correctly, the impeller can rub the casing or the seal can bind.

Shaft and Coupling Design

• Shaft material: 4140/4340 for general service, 17-4PH or F6NM for corrosive, Inconel 718 for HTHP + corrosive. Must have high creep resistance at operating temperature.
• Shaft stiffness: API 610 requires shaft deflection at seal <0.05 mm (0.002 in). At high temperature, shaft stiffness decreases (modulus of elasticity drops). Larger shaft diameter may be needed.
• Coupling: Must accommodate axial thermal expansion of the shaft (up to 8 mm for long shafts). Use a flexible coupling (disc or diaphragm type) with adequate axial travel. Gear couplings are not recommended for HTHP (lubrication fails at high temp).
• Thrust bearing: Must accommodate shaft thermal expansion while maintaining impeller position. Use a Kingsbury tilting-pad thrust bearing with L10 life ≥ 25,000 hours.

Mechanical Seal for HTHP

• Seal type: Metal bellows seal (no elastomer that can fail at high temp) or pusher seal with FFKM (Kalrez) elastomers.
• Seal faces: SiC vs SiC or SiC vs carbon. SiC vs SiC preferred for high temp (no carbon oxidation above 350°C).
• Flush plan: Plan 23 (recirculate + cooler) to keep seal faces below 80°C, or Plan 32 (external clean flush) for dirty/hot service.
• Seal cooling: A seal cooler or quenching (Plan 62) may be needed to keep the seal environment below 200°C.
• Thermal expansion: The seal must accommodate shaft and sleeve expansion. Metal bellows seals are inherently flexible and handle expansion well.

Installation Requirements

Piping Stress Analysis

Before installing an HTHP pump, a piping stress analysis (using CAESAR II or similar) must be performed to ensure that piping forces and moments at the pump flanges are within API 610 limits:

Key piping design measures:

• Use expansion loops or bellows to absorb thermal expansion
• Minimize rigid piping runs between pump and first anchor
• Use spring hangers or constant force supports for hot piping
• Ensure piping is self-draining (no liquid traps that can cause thermal shock)
• Perform hot alignment check after 24–48 hours of operation

Foundation and Grouting

• Baseplate: Heavy-duty steel baseplate, grouted to concrete foundation. The baseplate must be stiff enough to maintain alignment under thermal load.
• Grout: Epoxy grout (not cementitious) for HTHP pumps. Epoxy grout has better adhesion, vibration damping, and chemical resistance.
• Foundation: Mass concrete foundation, weight ≥ 3x pump/motor weight. Isolated from surrounding structure to prevent vibration transmission.
• Anchor bolts: Properly sized and torqued. Allow for baseplate thermal expansion (slotted holes or spring washers).

Startup Procedure

1. Pre-heat: Open warming line (bypass from discharge to suction) and gradually heat the pump to 100–150°C. Hold for 30–60 minutes to equalize temperature.
2. Ramp temperature: Increase temperature at 50–100°C per hour until operating temperature is reached. Monitor casing temperature with thermocouples (inner and outer wall).
3. Hot alignment: After reaching operating temperature (24–48 hours), check coupling alignment and adjust if needed. Thermal expansion can shift alignment by 0.5–2 mm.
4. Seal flush: Ensure seal flush plan is operating before starting the pump. Verify seal cooler flow.
5. Start pump: Start at minimum flow (bypass open), then gradually open discharge valve. Monitor vibration, temperature, and seal leakage.

Common Problems and Solutions

Problem 1: Casing gasket leakage (axially split pump). At high temperature, gasket creeps and bolts relax, causing leakage at the horizontal joint. Solution: use radially split (BB2/BB5) casing for HTHP (no horizontal gasket). If axially split is used, upgrade gasket to spiral-wound with inner ring, use controlled-torque bolt tightening, and re-torque bolts after first hot shutdown. Consider casing clamp rings for additional joint pressure.

Problem 2: Impeller rub / efficiency loss. Thermal expansion changes running clearances (wear rings, impeller-to-casing). If clearances are too tight at operating temp, the impeller rubs. Solution: calculate thermal growth of all wetted components and set cold clearances to account for hot expansion. Use non-galling wear ring materials (316SS vs PTFE, or SiC vs SiC). Monitor vibration (rub causes high-frequency vibration).

Problem 3: Seal failure within months. High seal chamber temperature, thermal distortion of seal chamber, or inadequate flush. Solution: use metal bellows seal (no elastomer) or pusher seal with FFKM. Verify Plan 23 cooler is functioning (seal temp <80°C). Check seal chamber bore distortion (measure during overhaul). Use Plan 32 external flush if process fluid is too hot/dirty. Ensure seal is rated for operating temperature (not just ambient).

Problem 4: Bearing failure (high temperature, vibration). Thermal expansion causes shaft misalignment, or bearing lubrication fails at high temp, or thrust load increases due to thermal growth. Solution: verify centerline mounting (casing expands evenly). Check coupling alignment hot (after 24h). Use oil-lubricated bearings with oil cooler (grease fails above 120°C). Verify thrust bearing setting (shaft expansion must not overload thrust bearing). Monitor bearing temperature (alarm at 90°C, trip at 100°C).

Problem 5: Casing cracking (thermal fatigue). Repeated thermal cycling (startup/shutdown) causes fatigue cracking at stress concentrations (nozzles, casing feet, bolt holes). Solution: limit startup/shutdown rate (50–100°C/h). Use warming line to minimize thermal gradients. Inspect casing during turnaround (MPI or dye penetrant at high-stress areas). Consider stress-relieving the casing after fabrication. Avoid rapid temperature changes (e.g., cold water ingress into hot pump).

Problem 6: Piping stress / nozzle load failure. Piping thermal expansion exerts excessive force on pump flanges, causing casing distortion or nozzle cracking. Solution: perform piping stress analysis (CAESAR II) before installation. Add expansion loops or bellows. Use spring supports for hot piping. Verify nozzle loads are within API 610 limits. Re-check piping after hot alignment.

Problem 7: Shaft binding / coupling failure. Shaft thermal expansion is not accommodated by coupling, or shaft and casing expand at different rates causing rotor lockup. Solution: use flexible coupling with adequate axial travel (disc or diaphragm type). Verify coupling axial travel rating exceeds shaft thermal expansion. Ensure thrust bearing allows shaft growth (proper setting). Check shaft/casing material expansion coefficients — differential expansion changes impeller position.

Problem 8: Bolt failure (casing or flange). At high temperature, bolts relax (loss of preload) or creep, causing leakage or fatigue failure. Solution: use high-temperature bolt material (B7M, B16, or Inconel 718). Apply anti-seize compound (copper or nickel based). Use controlled torque tightening (torque wrench or hydraulic tensioning). Re-torque after first hot shutdown. Inspect bolts during turnaround (replace if stretched or corroded).

Selection Summary

1. Define operating conditions: temperature, pressure, fluid, flow rate, head
2. Select pump type: OH2 (medium temp/pressure), BB2 (HTHP, single stage), BB5 (HTHP, multistage)
3. Require centerline mounting for T >150°C (mandatory per API 610)
4. Use radially split casing (BB2/BB5) for P >40 bar or T >300°C
5. Select casing material: carbon-moly (200–400°C), Cr-Mo (400–550°C), 316SS/duplex (corrosive)
6. Calculate thermal expansion of casing, shaft, and piping; verify clearances at operating temp
7. Select shaft material with high creep resistance; verify stiffness at operating temp
8. Use flexible coupling (disc/diaphragm) with axial travel ≥ shaft expansion
9. Select mechanical seal: metal bellows or FFKM pusher, Plan 23/32 flush with cooler
10. Perform piping stress analysis; ensure nozzle loads within API 610 limits
11. Use epoxy grout and heavy baseplate; verify foundation mass
12. Develop startup procedure: pre-heat, ramp rate 50–100°C/h, hot alignment check
13. Require API 610 performance test at operating temperature (or cold test with correction)
14. Plan preventive maintenance: seal inspection annually, casing inspection (MPI) during turnaround, bolt re-torque after hot shutdown
15. For HTHP service, our API 610 OH2 process pumps and API 610 BB2 between-bearing pumps provide centerline-mounted, high-temperature construction

For HTHP pump selection and thermal expansion analysis, our API 610 OH2 process pump range and API 610 BB2 between-bearing pump range are designed for high-temperature petrochemical service. For pump sizing, material selection, and piping stress consultation, talk to our engineers with your operating conditions, fluid properties, and piping layout.