Cast Iron Screw Lubricating Oil Pumps: Performance Characteristics, Market Applications, and Marine Adaptability Analysis

I. Working Principle and Structural Design

Marine cast iron screw lubricating oil pumps operate based on the positive displacement principle of screw meshing. The efficient transportation of lubricating oil is achieved through the meshing motion of the driving screw and the driven screw. When the screws rotate at a speed of 1000 - 1800 rpm, the screw grooves push the lubricating oil from the suction end to the discharge end. The pressure increases from 0.05 MPa at the inlet to 0.8 - 1.5 MPa at the outlet, with a flow rate ranging from 5 - 80 m³/h, meeting the lubrication requirements of lubrication systems on ships of different tonnages.

Core Features of Structural Design

Screw Pair Design: A twin - screw or triple - screw structure is adopted (the lead is 3 - 5 times the diameter). The surface of the screws is quenched to a hardness of HRC30 - 35, with a wear rate of < 0.02 mm/year when transporting 320 cSt lubricating oil.

Pump Body Structure: The pump body is made of gray cast iron HT250, equipped with replaceable wear - resistant bushings (made of QT600 - 3). The double - layer shell design enhances the vibration resistance, and the measured vibration value is ≤ 2.0 mm/s (within the limit of ISO 10816 standard).

Sealing System: A dual - protection structure of mechanical seals and fluororubber O - rings is used, with a leakage rate ≤ 2 ml/h, complying with the anti - leakage requirements of IMO MEPC.176(58) Resolution.

II. Core Performance Advantages

1. Stability in Transporting High - Viscosity Media

Viscosity Adaptability: It can stably transport lubricating oils with a viscosity of 50 - 2000 cSt. When transporting 1000 cSt lubricating oil at 100°C, the volumetric efficiency reaches 88 - 92%, which is 8% higher than that of gear pumps.

Low Pulsation Characteristics: The outlet pressure pulsation rate is < 1%. In the lubrication system of a cruise ship's main engine, the pressure fluctuation is ≤ 0.03 MPa, ensuring the lubrication of precision components.

Self - Priming Performance: No priming device is required, with a self - priming height of up to 5 m. A stable oil pressure can be established within 20 seconds after startup, making it suitable for the quick start of the lubrication system.

2. Wear - Resistance, Corrosion - Resistance and Long - Life Design

Wear - Resistant Material: The fatigue strength of ductile iron screws reaches 380 MPa. A screw pair of an ocean - going cargo ship has been in continuous operation for 10 years without replacement.

Oil Resistance: Cast iron forms a passivation film in lubricating oil, with a corrosion rate of < 0.003 mm/year, which is better than ordinary carbon steel.

Contamination Resistance: It can tolerate solid particles ≤ 30μm in lubricating oil. A bulk carrier can still operate normally when the impurity content of the lubricating oil is 0.03%.

3. Cost and Maintenance Advantages

Low Initial Investment: The manufacturing cost is 25 - 40% lower than that of stainless steel screw pumps. The selling price of a DN150 cast iron screw pump is 2500 - 4000 US dollars, while that of a stainless - steel pump is 5000 - 7000 US dollars.

Easy Maintenance: With a simple structure (screws + bearings + seals), the maintenance man - hours are 30% less than those of gear pumps. An annual maintenance cost savings of 6000 US dollars can be achieved for a container ship.

Accessory Compatibility: Screws, seals and other accessories are compatible with most ship lubrication systems, reducing inventory costs by 30%.

III. Application Limitations

1. Medium and Working Condition Restrictions

Taboo for Strongly Corrosive Media: It is prohibited from transporting acidic lubricating oils (pH < 6) or emulsified oils with a water content exceeding 0.5%. Improper use on a chemical ship may lead to screw corrosion.

Poor Low - Temperature Adaptability: When the viscosity of lubricating oil > 1500 cSt, preheating is required. For polar routes, an electric tracing system (power 1 - 2 kW) is needed. A research ship once experienced startup jams due to lack of preheating.

Restrictions in High - Speed Conditions: When the rotation speed > 2000 rpm, the noise > 80 dB, making it unsuitable for lubricating high - speed auxiliary engines.

2. Efficiency and Weight Shortcomings

Efficiency Decay with Viscosity Change: When the viscosity of lubricating oil < 30 cSt, the efficiency decreases by 10%. A high - speed passenger ship may need to switch pump types for low - viscosity lubrication.

Heavy Equipment: A DN150 cast iron screw pump weighs 90 kg, which is 110% heavier than an aluminum alloy pump. A luxury cruise ship may require a special base design due to the overweight.

Cavitation Sensitivity: The NPSH requirement is ≥ 2.8 m. When the liquid level in the lubricating oil tank is low, cavitation is likely to occur, resulting in increased screw wear on a cargo ship.

3. Installation and Maintenance Requirements

High Alignment Accuracy Requirement: The alignment error of the screw shaft needs to be < 0.03 mm. Improper installation on a ship may lead to abnormal screw wear.

Dependence on Oil Quality: The contamination level of lubricating oil needs to be ≤ ISO 4406 17/15/12 level. A bulk carrier may experience a 15% decrease in efficiency due to deteriorated oil quality.

Regular Disassembly and Maintenance: Disassembly and cleaning are required every 6000 hours. Failure to maintain in a timely manner on an oil tanker may lead to sealing failure and oil leakage.

IV. Analysis of Marine Market Applications

1. Global Market Share

According to DNV GL 2024 data, cast iron screw lubricating oil pumps account for 30% of the merchant ship lubrication system market, as detailed below: 

2. Regional Market Differences

Asian Market: It accounts for 48%. Merchant fleets in China, India and other countries widely use them due to cost and stability. For example, 35 out of 80 bulk carriers of a Chinese shipping company use cast iron screw pumps.

European Market: It accounts for 32%. Due to strict environmental protection requirements, cruise ships mostly use them (accounting for 60%), and ordinary cargo ships account for 25%.

Polar Routes: It accounts for 3%. An electric tracing system is required, increasing the transformation cost of a Russian LNG ship by 8%.

3. Comparison with Competing Products

4. Technological Development Trends

Application of Ceramic Coating: Coating the screw surface with a SiC ceramic layer increases wear resistance by 4 times, extending the service life of the pump body of a pilot cruise ship to 12 years.

Lightweight Design: Adopting hollow screws combined with resin sand casting reduces the weight by 18%. The weight of a new type of DN150 pump is reduced to 74 kg.

Intelligent Monitoring: Integrated with pressure and temperature sensors, a shipping company can achieve a fault warning accuracy rate of 94% through predictive maintenance.

V. Key Points for Selection and Maintenance

1. Key Selection Indicators

Flow Rate and Head: Select according to the power of the main engine. For a 5000 kW main engine, it is recommended that the flow rate ≥ 15 m³/h and the head ≥ 1.2 MPa.

Viscosity Adaptation: Select the screw lead according to the viscosity of the lubricating oil. When the viscosity > 500 cSt, a screw with a large lead should be selected.

Certification Requirements: It needs to pass the mechanical integrity certification of classification societies such as LR and ABS.

NPSH Margin: Reserve ≥ 3.2 m to avoid cavitation. Insufficient NPSH may lead to screw damage on a cargo ship.

2. Best Maintenance Practices

Oil Quality Monitoring: Detect the viscosity and acid value of lubricating oil monthly. Exceeding the acid value limit on an oil tanker may lead to screw corrosion.

Bearing Lubrication: Apply lithium - base lubricating grease (NLGI grade 2) every 2000 hours. Lack of oil may lead to bearing burnout on a cruise ship.

Seal Inspection: Check the lip of the mechanical seal every voyage. Replace it if the wear exceeds 0.3 mm. Aging of the seal may lead to oil leakage on a bulk carrier.

Screw Wear Detection: Measure the thickness with ultrasonic technology every year. Replace the screw if the tooth surface wear exceeds 0.2 mm. Failure to detect in time may lead to a sharp drop in efficiency of a container ship.