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

In the ship's cargo oil transportation system, marine cast iron screw cargo oil pumps, as core equipment for transporting high-viscosity cargo oil, hold a crucial position due to their low-pulsation delivery, high stability, and cost advantages. These pumps are designed based on the positive displacement principle of screw meshing, specifically for the loading, unloading, and barge operations of cargo oil tanks on oil tankers and bulk carriers. They exhibit unique technical advantages, especially in the transportation of viscous media such as crude oil and heavy fuel oil. The following provides an in-depth analysis from the dimensions of working principles, performance advantages and disadvantages, market dynamics, and maintenance key points.

I. Working Principles and Structural Design

Marine cast iron screw cargo oil pumps operate on the positive displacement principle of screw meshing, achieving efficient transportation of cargo oil through the meshing motion of the driving screw and the driven screw. When the screws rotate at a speed of 800 - 1800 rpm, the screw grooves push the cargo oil from the suction end to the discharge end. The pressure increases from 0.05 MPa at the inlet to 0.6 - 1.5 MPa at the outlet, with a flow rate ranging from 10 - 120 m³/h, meeting the cargo oil loading and unloading requirements of ships of different tonnages.

Core Features of Structural Design

Screw Pair Design: A twin-screw or triple-screw structure is adopted (with a lead 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 1500 cSt heavy crude 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 explosion-proof shell design meets the ATEX Zone 0 explosion-proof requirements, and the measured explosion-proof grade reaches Ex d IIB T4.

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 standards of IMO MEPC.307(73) Resolution.

II. Core Performance Advantages

1. Stability in Transporting High-Viscosity Cargo Oil

Viscosity Adaptability: It can stably transport cargo oils with a viscosity of 50 - 2500 cSt. When transporting 2000 cSt thick oil at 50°C, the volumetric efficiency reaches 88 - 93%, which is 12% higher than that of gear pumps.

Low Pulsation Characteristics: The outlet pressure pulsation rate is < 1%. During the cargo oil loading and unloading of a VLCC, the pressure fluctuation is ≤ 0.02 MPa, ensuring the stability of the loading and unloading system.

Self-Priming Performance: No priming device is required, with a self-priming height of up to 6 m. A stable oil pressure can be established within 20 seconds after startup, making it suitable for low-level operations in cargo oil tanks.

2. Wear-Resistance, Corrosion-Resistance, and Explosion-Proof Design

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

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

Explosion-Proof Certification: The fully enclosed shell design, along with the motor and accessories, has passed explosion-proof certification, making it suitable for hazardous environments in cargo oil tank areas.

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 DN200 cast iron screw pump is 3500 - 5500 US dollars, while that of a stainless-steel pump is 7000 - 9000 US dollars.

Easy Maintenance: With a simple structure (screws + bearings + seals), the maintenance man-hours are 35% less than those of gear pumps. An annual maintenance cost savings of 8000 US dollars can be achieved for a container ship.

Accessory Compatibility: Screws, seals, and other accessories are compatible with most ship cargo oil 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 cargo oils with a pH < 5 or emulsified oils with a water content exceeding 1.5%.

Poor Low-Temperature Adaptability: When the viscosity of cargo oil > 2500 cSt, preheating is required. For polar routes, an electric tracing system (power 3 - 5 kW) is needed.

Restrictions in High-Speed Conditions: When the rotation speed > 2000 rpm, the noise > 82 dB, making it unsuitable for high-speed cargo oil barge operations.

2. Efficiency and Weight Shortcomings

Efficiency Decay with Viscosity Change: When the viscosity of cargo oil < 30 cSt, the efficiency decreases by 12%, and the pump type needs to be switched.

Heavy Equipment: A DN200 cast iron screw pump weighs 130 kg, which is 160% heavier than an aluminum alloy pump.

Cavitation Sensitivity: The NPSH requirement is ≥ 3.2 m. When the liquid level in the cargo oil tank is low, cavitation is likely to occur.

3. Installation and Maintenance Requirements

High Alignment Accuracy Requirement: The alignment error of the screw shaft needs to be < 0.03 mm.

Dependence on Oil Quality: The solid content of cargo oil needs to be ≤ 0.05%, and the contamination level needs to be ≤ ISO 4406 17/15/12 grade.

Regular Disassembly and Maintenance: Disassembly and cleaning are required every 6000 hours to prevent sealing failure.