GNF Screw Pump for Heavy Fuel Oil and Residue Handling in Refineries

Refineries handle large volumes of heavy, viscous fluids — residual fuel oil (RFO), vacuum residue, asphalt, bitumen, heavy cycle oil (HCO), and slurry oil — that cannot be pumped efficiently by conventional centrifugal pumps. These fluids have viscosities ranging from 100 cSt at 100°C to over 10,000 cP at ambient temperature, and often contain abrasive catalyst fines or coke particles. A GNF-type three-screw pump (also called a progressive cavity screw pump or triple screw pump) is a positive displacement pump designed specifically for these heavy, viscous, and often abrasive services. It provides smooth, pulse-free flow, high efficiency on viscous fluids, and excellent suction performance — making it the standard choice for heavy fuel oil and residue handling in refineries.

This guide covers GNF screw pump selection for refinery heavy fuel oil and residue service, with focus on viscosity, temperature, and abrasive handling.

Refinery Heavy Oil Services

How a GNF Three-Screw Pump Works

A GNF screw pump uses three intermeshing screws — one power rotor (male, driven by the motor) and two idler rotors (female, free-spinning) — housed in a close-fitting liner. As the screws rotate, they form sealed cavities that move continuously from suction to discharge, trapping and conveying the fluid without turbulence or shear.

Key Design Features

• Three-screw design: One central power rotor and two idler rotors. The idler rotors are driven hydraulically by the fluid pressure (no timing gears), reducing mechanical complexity and wear.
• Sealed cavities: The screws form continuous sealed chambers that move axially. Flow is smooth and pulse-free — critical for metering and for reducing piping vibration.
• Internal pressure balancing: The screw profile is designed so that hydraulic forces are balanced, reducing bearing loads and extending bearing life.
• Replaceable liner: The screws operate inside a removable liner (bronze or cast iron). When worn, only the liner and screws need replacement — not the entire casing.
• Jacketed casing (optional): For heavy oil service, the casing can be steam-jacketed or electrically traced to maintain fluid temperature and prevent solidification.

Advantages for Heavy Oil Service

1. High viscosity efficiency: Screw pumps maintain high efficiency (70–85%) even at viscosities above 1000 cP. Centrifugal pump efficiency drops sharply above 100 cP.
2. Self-priming: Positive displacement action creates strong suction lift (up to 8 m water gauge), making it suitable for underground tanks and sumps.
3. Pulse-free flow: No pressure pulsation — reduces piping vibration, noise, and wear on downstream equipment.
4. Low shear: Gentle handling of shear-sensitive fluids (asphalt, polymers, emulsions).
5. Constant flow regardless of pressure: Flow rate depends only on speed and displacement, not on discharge pressure (within slip limits).
6. Good suction performance: Can handle fluids with entrained gas or vapor without losing prime.

For lighter refinery services (diesel, gasoline, kerosene) where viscosity is below 20 cSt, an API 610 OH2 process pump is typically more efficient and cost-effective. For large-volume cooling water or circulating water service, an API 610 BB1 double-suction pump is the standard choice. Screw pumps are specifically for heavy, viscous service where centrifugal pumps cannot operate efficiently.

Sizing Parameters

Flow Rate

GNF screw pump flow is determined by displacement and speed:

Q (m³/h) = Displacement (m³/rev) × Speed (rpm) × 60 / 1000 × Volumetric Efficiency

Typical flow ranges for refinery service:

• Fuel oil burner feed: 1–10 m³/h
• Tank to tank transfer: 10–50 m³/h
• Residue to coker / hydrocracker: 20–100 m³/h
• Asphalt terminal loading: 30–150 m³/h
• Slurry oil circulation: 5–30 m³/h

Pressure

• Standard GNF pumps: up to 16 bar (230 psi)
• High-pressure models: up to 40 bar (580 psi) for burner feed or high-head transfer
• Typical refinery service: 5–16 bar

Important: Screw pumps are positive displacement — they will generate whatever pressure is needed to overcome system resistance, up to the point of mechanical failure. A pressure relief valve (internal or external) is MANDATORY to protect the pump and piping from overpressure.

Speed Selection

Speed is critical for viscous service:

• Low viscosity (<100 cP): 1500–3000 rpm (direct drive or gear increaser)
• Medium viscosity (100–1000 cP): 750–1500 rpm (gear reducer)
• High viscosity (1000–10000 cP): 200–750 rpm (gear reducer or VFD)
• Very high viscosity (>10000 cP): 50–200 rpm (large gear reducer)

Running too fast on high viscosity fluid causes:

• Excessive suction pressure drop (cavitation)
• Increased slip (internal recirculation)
• Higher power consumption
• Premature wear

Viscosity Correction

Unlike centrifugal pumps, screw pumps do NOT lose flow significantly with increasing viscosity — in fact, volumetric efficiency IMPROVES with viscosity (less slip). However, power consumption increases:

Hydraulic power (kW) = Q (m³/h) × P (bar) / 36

Shaft power = Hydraulic power / Mechanical efficiency (typically 80–90%)

Example: 20 m³/h, 10 bar, viscosity 500 cSt, mechanical efficiency 85%

Hydraulic power = 20 × 10 / 36 = 5.6 kW

Shaft power = 5.6 / 0.85 = 6.6 kW

Select 7.5–11 kW motor (add 15–20% margin for startup and cold fluid).

Startup power: When the pump is cold (fluid at ambient temperature, viscosity much higher), startup power can be 2–3x normal operating power. The motor must be sized for cold startup, or the system must pre-heat the fluid before starting.

NPSH for Viscous Fluids

NPSH requirements increase with viscosity because of higher friction losses in the suction line and pump inlet:

• For water: NPSHr typically 1–3 m
• For 100 cP: NPSHr increases by ~30–50%
• For 1000 cP: NPSHr can be 2–3x the water value

Rules for heavy oil NPSH:

• Mount pump below suction tank (flooded suction) — preferred for all heavy oil service.
• Keep suction piping short and large (minimize friction).
• Pre-heat suction piping and pump before startup (steam tracing or jacket).
• Maintain NPSHa - NPSHr ≥ 2 m (preferably 3 m for very viscous fluids).
• Use a larger pump (lower speed) if NPSH is marginal.

Temperature and Heating

Why Heating Is Required

Heavy oils are solid or semi-solid at ambient temperature. To pump them, they must be heated to reduce viscosity:

• Residual fuel oil: Heat to 80–120°C to reduce viscosity to pumpable range (100–500 cSt).
• Vacuum residue: Heat to 120–180°C.
• Asphalt/bitumen: Heat to 150–200°C. Must maintain temperature throughout the system — any cold spot causes solidification and blockage.

Heating Methods

• Steam-jacketed casing: The pump casing has a jacket for steam or hot oil circulation. Most common for asphalt and heavy residue service.
• Electrical heat tracing: Heat tape on casing and piping. Used where steam is not available. Must be thermostatically controlled.
• Jacketed piping: All suction and discharge piping is steam-jacketed to maintain temperature. Critical for asphalt service.
• Insulation: All heated piping and pump must be insulated to maintain temperature and protect personnel.

Temperature Limits

• Standard GNF pump: Up to 150°C (nitrile or FKM seals).
• High-temperature model: Up to 250°C (FFKM or PTFE seals, special bearings).
• Above 250°C: Requires special design (cooled bearing housing, special materials).

Thermal expansion: At high temperatures, the screws and liner expand at different rates. The pump must be designed with adequate running clearances for the operating temperature. Running a high-temperature pump cold can cause seizure. Clearances are set for operating temperature — at low temperature they are larger, which increases slip but prevents seizure.

Material Selection

For slurry oil (catalyst fines): Use hardened steel screws and cast iron liner (bronze wears too fast with abrasive particles). Use SiC vs SiC mechanical seal. Install a strainer at suction (1–2 mm mesh) to remove large particles, but accept that fine catalyst fines will pass through and cause gradual wear.

System Design

Pressure Relief

Mandatory for all positive displacement pumps:

• Internal relief valve: Built into the pump casing, recirculates fluid from discharge to suction. Set at 10–15% above operating pressure. Only for temporary overpressure (valve closed during startup).
• External relief valve: Separate valve in the discharge line, recirculating to suction tank. More reliable, adjustable, and can handle continuous bypass. Recommended for critical service.
• Important: Do NOT use a relief valve as a flow control device. Continuous bypass causes fluid heating and pump damage. Flow is controlled by speed (VFD) or by a bypass valve with cooler.

Suction Strainer

• Install a basket strainer at pump suction (1–3 mm mesh).
• Use a strainer with differential pressure indicator — alerts when strainer is clogging.
• For heavy oil, the strainer must be steam-jacketed to prevent solidification.
• Clean strainer regularly — a clogged strainer causes cavitation and pump damage.

Startup and Shutdown Procedure

1. Pre-heat: Open steam to jacketed casing and piping. Heat to operating temperature (verify with temperature gauge). This may take 30–60 minutes for a cold system.
2. Vent: Open vent valve on pump casing to release air. Close when fluid appears (no spattering — hot oil can cause burns).
3. Open valves: Fully open suction valve. Open discharge valve (screw pumps must NOT be started against closed discharge — unlike centrifugal pumps).
4. Start pump: Start motor. Verify flow (pressure gauge, flow meter).
5. Shutdown: Stop motor. Close discharge valve (to prevent backflow). Keep suction valve open and maintain heating (if pump will be restarted soon). If shutting down for extended period, flush pump with light oil or diesel to prevent solidification.

Common Problems and Solutions

Problem 1: Pump will not prime (no flow). Heavy oil is too viscous at current temperature, or air is trapped in the casing. Solution: verify casing and piping are at operating temperature (check temp gauge). Vent the pump thoroughly (open vent until steady fluid flow). Check suction valve is fully open. Verify suction tank has fluid and level is above pump. Check strainer is not clogged. If still no prime, use a priming pump or fill suction line with light oil.

Problem 2: Excessive noise and vibration. Cavitation from insufficient NPSH, or worn screws/liner. Solution: check suction pressure and temperature (increase temp to reduce viscosity). Verify suction strainer is clean. Check suction piping for restrictions. If noise persists after correcting suction, inspect screws and liner for wear (measure clearance). Replace worn components. Reduce pump speed if viscosity is higher than design.

Problem 3: Flow rate lower than expected. Internal slip from worn screws/liner, or speed too low for viscosity. Solution: verify pump speed (check motor RPM and gear ratio). Measure actual flow and compare to theoretical displacement — if volumetric efficiency is below 70%, screws/liner are worn. Check discharge pressure (higher pressure = more slip). For abrasive service, wear is expected — schedule liner/screw replacement.

Problem 4: Motor overload (high current). Fluid too viscous (cold), or pump speed too high, or discharge pressure too high. Solution: verify fluid temperature (increase heating). Check discharge pressure (relief valve may be passing, or downstream blockage). Verify pump speed matches viscosity (reduce speed for high viscosity). Check motor sizing — startup on cold fluid may require larger motor. Use VFD to start at low speed and ramp up.

Problem 5: Seal leakage. Mechanical seal worn or damaged from abrasive particles, or temperature exceeds seal rating. Solution: check seal flush plan (Plan 11 or 32 for abrasive service). Verify seal material rating (FKM max 150°C, FFKM for higher). For slurry oil, use SiC vs SiC faces. Replace seal at scheduled intervals (based on service hours). Ensure pump is properly vented (dry running destroys seals instantly).

Problem 6: Fluid solidifies in pump (after shutdown). Heating was turned off, or pump was not flushed. Solution: maintain steam tracing on pump and piping even during short shutdowns. For extended shutdown, flush pump with light oil or diesel (recirculate through pump to drain). Never restart a pump with solidified fluid — the motor will trip or the pump will seize. Pre-heat thoroughly before restart.

Problem 7: Rapid wear on slurry oil service. Catalyst fines (FCC equilibrium catalyst) are highly abrasive. Solution: use hardened steel screws and cast iron liner (not bronze). Install fine suction strainer (1 mm) and clean regularly. Use SiC vs SiC seal. Accept that slurry oil service requires more frequent maintenance (liner/screw replacement every 6–18 months depending on catalyst loading). Consider a lined slurry pump for very high catalyst content.

Problem 8: Thermal cracking at high temperature. Vacuum residue or asphalt held at high temperature for too long can thermally crack, forming coke particles that damage the pump. Solution: minimize residence time in the pump (don't bypass continuously). Maintain temperature within spec (don't overheat). Use jacketed casing with controlled steam pressure. For asphalt, keep temperature at 150–180°C (not above 200°C unless required). Flush pump after use.

Selection Summary

1. Define fluid: type, viscosity at operating temp, temperature, SG, solids content, abrasiveness
2. Confirm heating requirement: steam-jacketed casing and piping for fluids above 100 cP at ambient
3. Select GNF three-screw pump for heavy, viscous, pulse-free service
4. Choose flow rate (1–150 m³/h typical) and pressure (5–40 bar)
5. Select speed based on viscosity: 1500–3000 rpm (<100 cP), 750–1500 rpm (100–1000 cP), 200–750 rpm (1000–10000 cP)
6. Size motor for cold startup (2–3x operating power) or specify pre-heat interlock
7. Verify NPSH: flooded suction preferred, NPSHa - NPSHr ≥ 2–3 m at operating viscosity
8. Select materials: bronze liner for clean service, hardened steel/cast iron for abrasive (slurry oil)
9. Install pressure relief valve (mandatory) — external preferred for critical service
10. Install jacketed suction strainer with differential pressure indicator
11. Use SiC vs SiC mechanical seal for abrasive service; FFKM/PTFE for high temp
12. Insulate all heated piping and pump casing
13. Develop startup/shutdown procedure (pre-heat, vent, flush)
14. Plan preventive maintenance: seal replacement 1–2 years, liner/screw 2–5 years (sooner for abrasive)
15. For lighter refinery services (diesel, gasoline), use API 610 OH2 process pumps; for large-volume water service, use API 610 BB1 double-suction pumps

For refinery heavy fuel oil and residue pump selection, our API 610 OH2 process pump range and API 610 BB1 double-suction pump range cover lighter refinery services, while GNF screw pumps are specified for heavy viscous service. For pump sizing and heating system design, talk to our engineers with your fluid properties, flow sheet, and temperature requirements.