API 610 OH2 Pump for Refinery Desalter Service: Material and Seal Strategy

Crude oil desalting is the first major processing step in every refinery. Before crude oil enters the atmospheric distillation column, it must be treated to remove salt (primarily NaCl, MgCl₂, CaCl₂), water, and suspended solids. If not removed, these contaminants cause severe downstream problems: chloride hydrolysis produces HCl which corrodes distillation overhead systems; salt deposits foul heat exchangers and furnace tubes; solids cause erosion and catalyst deactivation. The desalter uses water washing, chemical demulsifiers, and a high-voltage electrostatic field to separate water and salt from the crude oil. Pumps in desalter service handle a challenging mixture of crude oil, emulsified water, salt, and abrasive solids — making material selection and seal strategy critical for reliability.

This guide covers API 610 OH2 pump selection for refinery desalter service, with focus on material compatibility and mechanical seal strategy.

Desalter Process Overview

Single-Stage and Two-Stage Desalting

Most modern refineries use a two-stage desalting system for maximum salt removal:

1. Crude preheating: Crude oil is heated to 100–150°C (typically 120–140°C) using heat exchangers and a fired heater. Higher temperature reduces viscosity and improves water-oil separation.
2. Water injection: Wash water (4–10% of crude volume) is injected into the crude stream. The water dissolves salt and picks up suspended solids. Typically demineralized water or stripped sour water is used.
3. Mixing valve: A pressure-drop mixing valve (or static mixer) creates a turbulent emulsion, ensuring intimate contact between crude and wash water. Pressure drop is typically 0.5–1.5 bar.
4. Desalter vessel: The emulsion enters a large horizontal vessel where a high-voltage electrostatic field (10–30 kV AC or DC) coalesces water droplets. Water and salt settle to the bottom (brine), clean crude exits the top.
5. Second stage (optional): For high-salt crudes or strict salt specifications (<1 PTB), a second desalter stage provides additional washing. Effluent water from stage 2 is reused as wash water for stage 1 (countercurrent).

Pumps in Desalter Service

Why API 610 OH2 for Desalter Service

The API 610 OH2 (overhung, horizontal, centerline-mounted) pump is the standard choice for desalter feed and desalted crude service because:

1. Centerline mounting: The pump casing is supported at its centerline, allowing thermal expansion without misalignment. Critical for 120–150°C crude oil service.
2. Back pull-out design: The rotating assembly (impeller, shaft, seal, bearings) can be removed without disconnecting piping. Reduces maintenance time in critical refinery service.
3. Heavy-duty construction: API 610 requires minimum casing thickness, shaft stiffness, and bearing life (L10 ≥ 25,000 hours). Desalter service is demanding — the pump must handle emulsions, solids, and thermal cycling.
4. Wide hydraulic coverage: OH2 pumps cover flows from 10 to 500 m³/h and heads from 30 to 200 m, suitable for most desalter applications.
5. Standardized seal chamber: API 682 seal chamber dimensions allow use of standard cartridge seals with various flush plans.

Our API 610 OH2 process pump range is designed and manufactured to API 610 11th edition standards, suitable for refinery desalter and crude oil service. For large-volume crude oil transfer, an API 610 BB1 double-suction pump may be more efficient at flows above 300 m³/h.

Material Selection Strategy

Wetted Parts

Material Class Selection by Service

• Desalted crude pump (clean crude): API 610 S-5 (carbon steel casing, 316SS impeller) is usually sufficient. Crude after desalting has <1 PTB salt and minimal water.
• Desalter feed pump (crude + water emulsion): API 610 S-6 (carbon steel casing, 316SS impeller and shaft sleeve) minimum. Consider S-8 (316SS casing, duplex impeller) for high-salt crudes or frequent water carryover.
• Wash water pump (sour water): 316SS or duplex. Sour water contains H₂S, ammonia, and sometimes cyanide — carbon steel is not acceptable. If H₂S >50 ppm, consider duplex or Alloy 20.
• Brine effluent pump: Duplex stainless steel or Hastelloy C. Brine is hot (100–140°C), high chloride (5,000–50,000 ppm), and contains oil and solids. 316SS will pit in this service. Duplex (2205 or 2507) is the minimum; Hastelloy C for severe service.

Chloride Stress Corrosion Cracking (CLSCC)

The biggest material concern in desalter service is chloride stress corrosion cracking in austenitic stainless steel (304SS, 316SS):

• Conditions for CLSCC: Chloride >50 ppm, temperature >60°C, tensile stress (residual or applied).
• Desalter brine: Chloride 5,000–50,000 ppm at 100–140°C — ideal conditions for CLSCC.
• Prevention: Use duplex stainless steel (resistant to CLSCC) or Hastelloy C for brine service. For 316SS in crude service (lower chloride), ensure stress relief and avoid stagnant brine pockets.
• Inspection: During turnaround, inspect 316SS components for cracking (dye penetrant or ultrasonic). Pay special attention to welds and seal areas.

Mechanical Seal Strategy

Seal Selection

Desalter feed pumps handle a crude oil-water emulsion with solids — one of the most difficult services for mechanical seals. The standard approach is:

API 682 Seal Configuration

For desalter feed pumps, the recommended API 682 seal arrangement is:

• Seal type: Cartridge mechanical seal, pusher or metal bellows. Pusher seals are more common for crude oil service; metal bellows for high temperature (>150°C) or coking service.
• Seal faces: Silicon carbide (SiC) vs SiC for abrasive service. Carbon vs SiC may be used for clean crude, but SiC vs SiC is preferred for emulsion/solids.
• Springs and metal parts: Hastelloy C (alloy C-276) for chloride resistance. 316SS springs can corrode in brine service.
• Elastomers: FKM (Viton) for crude oil service (up to 200°C). FFKM (Kalrez) for sour service or high aromatic crudes (which swell FKM). EPDM is NOT compatible with crude oil.
• Seal chamber: API 682 standard large bore chamber with throat bushing. The large chamber improves circulation and reduces solids accumulation at the seal.

Flush Plan Details

Plan 32 (External Flush) — Most Common for Desalter Feed:

• A clean fluid (usually desalted crude from downstream, or diesel/gas oil) is injected into the seal chamber at a pressure 1–2 bar above seal chamber pressure.
• The flush fluid flows across the seal faces, preventing emulsion and solids from reaching the seal.
• Flow rate is typically 5–20 L/min, controlled by an orifice or needle valve.
• A flow indicator and pressure gauge are installed to verify flush is operating.
• Critical: If the flush fails, the seal will fail in hours or days due to emulsion and solids. The flush must be reliable and monitored.

Plan 53A (Pressurized Barrier Fluid) — For Brine Effluent:

• A double seal with a pressurized barrier fluid (usually a clean oil or water-glycol mixture) between the inboard and outboard seal.
• Barrier fluid pressure is 1–2 bar above process pressure, ensuring any leakage is barrier fluid into process (not process into environment).
• Barrier fluid is circulated by a pumping ring and cooled by a heat exchanger.
• A reservoir with level and pressure indicators provides monitoring.

Seal Support System

• Flush fluid reservoir: For Plan 32, a dedicated flush oil drum with level control ensures continuous flush supply.
• Filters: Flush fluid should be filtered (50–100 micron) to remove particles that could damage seal faces.
• Cooling: For high-temperature service (>150°C), a seal cooler (Plan 23 or 62) may be needed to keep seal faces below 80°C.
• Monitoring: Install seal leak detection (API Plan 65A/B) to alert operators of seal failure before a major leak occurs.

Hydraulic Design Considerations

Emulsion Handling

Crude oil-water emulsions have different properties than clean crude:

• Viscosity: Emulsion viscosity can be 2–10x higher than clean crude (water-in-oil emulsion is more viscous than oil alone). Verify pump performance at actual emulsion viscosity.
• Specific gravity: Emulsion SG is between crude (0.85–0.95) and water (1.0), typically 0.9–0.98. This affects pump power and motor sizing.
• Shear sensitivity: High shear at the impeller can stabilize the emulsion, making it harder to separate in the desalter. Use a moderate specific speed impeller (Ns 20–50) to minimize shear.
• Gas entrainment: Crude may contain dissolved gas that comes out of solution at the pump suction. Ensure NPSHa is adequate and use a gas-handling impeller if gas content >5%.

Solids Handling

Desalter feed contains sand, rust, scale, and catalyst fines from upstream production:

• Suction strainer: Install a temporary cone strainer (1–3 mm) during startup, then a permanent basket strainer. Clean regularly.
• Impeller passage: Use an impeller with large passages (at least 3x max particle size) to prevent clogging. Avoid very small specific speed impellers with narrow passages.
• Wear rings: Use non-galling materials (316SS vs glass-filled PTFE, or SiC vs SiC) to prevent seizure if solids enter the ring clearance.
• Erosion: Solids erode the impeller eye and volute cutwater. Hardened impellers (28% HRC minimum) or duplex materials extend life.

Operating Point

• Run near BEP: Desalter pumps should operate at 80–110% of best efficiency point (BEP). Operation below 50% BEP causes recirculation, vibration, and seal damage.
• Minimum flow: Install a minimum flow bypass line (or automatic recirculation valve) to protect the pump during low-flow conditions. Minimum flow is typically 25–30% of BEP flow.
• Parallel operation: If two pumps operate in parallel, ensure both have identical curves and that the system curve is stable. Mismatched pumps can cause one to operate at shut-off.

Common Problems and Solutions

Problem 1: Seal failure within months of installation. Emulsion and solids reaching the seal faces, or flush plan not functioning. Solution: verify Plan 32 flush is operating (check flow indicator and pressure). Ensure flush fluid is clean (replace filter). Upgrade to SiC vs SiC seal faces. Verify seal chamber pressure is below flush pressure. Consider double seal (Plan 53A) if single seal fails repeatedly.

Problem 2: Casing pitting and corrosion. Water carryover from desalter exposes carbon steel casing to hot brine, causing corrosion. Solution: upgrade casing to 316SS or duplex (S-8 material class). Inspect casing during turnaround (ultrasonic thickness). Ensure desalter is operating correctly (reduce water carryover). Use corrosion inhibitor in crude (if compatible with downstream).

Problem 3: Impeller erosion from solids. Sand and catalyst fines erode impeller vanes and volute, reducing performance. Solution: install and maintain suction strainer (clean weekly during operation). Use hardened or duplex impeller. Reduce impeller speed if erosion is severe. Inspect impeller during annual maintenance — replace if vane thickness is reduced by >20%.

Problem 4: Pump cavitation (noise, vibration, flow fluctuation). Crude temperature too high (vapor pressure), or suction pressure too low, or gas entrainment. Solution: verify crude temperature (reduce if above design). Check suction pressure and NPSHa. Ensure suction strainer is clean. Degas crude upstream if gas content is high. Use a low-NPSHr impeller or larger pump. Avoid throttling suction valve.

Problem 5: Desalter efficiency drop (salt in desalted crude). Pump flow or pressure is incorrect, affecting mixing valve pressure drop and desalter operation. Solution: verify pump flow (use flow meter or clamp-on). Check impeller condition (wear reduces flow). Verify mixing valve pressure drop (should be 0.5–1.5 bar). Ensure pump is operating near BEP. Calibrate instruments.

Problem 6: Bearing failure (high vibration, temperature). Misalignment from thermal expansion, or lubrication failure, or shaft deflection. Solution: verify pump is centerline-mounted (OH2) — allows thermal expansion. Check coupling alignment (hot alignment after 24 hours of operation). Verify bearing lubrication (oil level, oil quality). Check shaft deflection at seal (should be <0.05 mm). Use bearing housing with temperature and vibration monitoring.

Problem 7: Flush fluid contamination of crude. Plan 32 flush fluid (diesel or gas oil) enters the crude stream, affecting product quality. Solution: use desalted crude as flush fluid (from downstream of desalter) instead of diesel. Minimize flush flow rate (use minimum needed for seal protection — typically 5–10 L/min). Monitor flush flow and crude quality. If product contamination is critical, consider double seal with barrier fluid (no process contamination).

Problem 8: Thermal shock during startup. Cold pump suddenly receives 140°C crude, causing casing distortion and seal damage. Solution: pre-heat pump by circulating warm crude through the casing (bypass line) before opening main valve. Gradually increase temperature (50°C per hour maximum). Ensure pump is centerline-mounted (reduces thermal stress). Use warming line (small bypass from discharge to suction) to maintain pump at operating temperature during standby.

Selection Summary

1. Define service: desalter feed, wash water, brine effluent, or desalted crude
2. Select API 610 OH2 pump for desalter feed and desalted crude (centerline-mounted, back pull-out)
3. Choose material class: S-5/S-6 for clean crude, S-8 (316SS/duplex) for emulsion, duplex/Hastelloy for brine
4. Verify chloride SCC risk: use duplex for brine service (>50 ppm Cl at >60°C)
5. Select mechanical seal: SiC vs SiC, Hastelloy C springs, FKM or FFKM elastomers
6. Specify flush plan: Plan 32 (external clean flush) for desalter feed; Plan 53A (double seal) for brine
7. Ensure flush system is reliable: reservoir, filter, flow indicator, pressure gauge
8. Size pump for emulsion viscosity (2–10x clean crude) and SG (0.9–0.98)
9. Use large-passage impeller for solids; hardened or duplex for erosion resistance
10. Install suction strainer and minimum flow protection
11. Verify NPSH for hot crude and gas entrainment (NPSHa - NPSHr ≥ 2 m)
12. Specify pre-heat/warming line for standby pumps
13. Require API 610 performance test and hydrotest before shipment
14. Plan preventive maintenance: seal inspection annually, impeller/casing inspection during turnaround (2–3 years)
15. For flows above 300 m³/h, consider API 610 BB1 double-suction pump for higher efficiency

For refinery desalter pump selection, our API 610 OH2 process pump range provides centerline-mounted, API 610-compliant construction for crude oil and emulsion service. For material selection and seal strategy consultation, talk to our engineers with your crude assay, desalter operating conditions, and salt specification.