Cast Iron Ejector Pumps: Analysis of Material Characteristics, Application Advantages, and Adaptability to Marine Conditions

I. Classification of Marine Cast Iron Materials and Their Adaptability to Ejector Pumps

1.Gray Cast Iron (HT)

Gray cast iron is one of the most commonly used materials for marine Ejector pumps, with typical grades such as HT200 and HT250. Carbon exists in the form of flake graphite, with a compressive strength of 200-300MPa and a tensile strength of 150-250MPa. It has excellent vibration damping performance, with a vibration attenuation capacity 10 times that of steel. In Ejector pumps, gray cast iron is often used to manufacture non-critical pressure-bearing components such as pump bodies and nozzles, suitable for low-pressure (≤1.6MPa) and non-corrosive or weakly corrosive medium transportation, such as ship bilge water drainage and freshwater circulation systems. With low costs and mature processing techniques, it is the preferred material for basic marine Ejector pumps and meets the material requirements for non-critical components specified by classification societies.

2.Ductile Cast Iron (QT)

Ductile cast iron improves mechanical properties significantly through spheroidization treatment, making graphite distribute in a spherical form. Common grades such as QT500-7 and QT600-3 have a tensile strength of 400-800MPa, 30% higher fatigue resistance than gray cast iron, and toughness close to steel. In marine Ejector pumps, ductile cast iron is suitable for medium-high pressure (≤3.5MPa) conditions, such as ship ballast water Ejector pumps and fire protection system Ejector pumps. For example, in ballast water systems, QT600-3 Ejector pumps can withstand a working pressure of 2.5MPa, and their service life in ballast water with a sand content ≤10% is 2-3 times longer than that of gray cast iron, meeting the pressure resistance requirements of the International Maritime Organization (IMO) for ballast water management systems.

3.Corrosion-Resistant Cast Iron

Corrosion-resistant cast iron enhances corrosion resistance by adding elements such as chromium, nickel, and silicon. For example, high-silicon cast iron (with about 14.5% silicon content) has a corrosion rate <0.05mm/year in seawater. In marine Ejector pumps, corrosion-resistant cast iron is mainly used to manufacture pump bodies and nozzles for transporting seawater and slightly acidic ballast water, such as ejectors for seawater desalination devices and ballast water Ejector pumps after electrolysis treatment. A 100,000-ton oil tanker using high-silicon cast iron Ejector pumps to transport seawater showed no obvious corrosion perforation within 5 years, complying with the corrosion resistance requirements (Norsok M-630 standard) for seawater system materials specified by DNV GL classification society.

4.Wear-Resistant Cast Iron 

Wear-resistant cast iron improves wear resistance through surface hardening treatment or alloying. For example, white cast iron can reach a hardness of HRC55-60 after heat treatment. In marine Ejector pumps, wear-resistant cast iron is suitable for transporting particle-containing media, such as sludge drainage Ejector pumps for port ships and mud Ejector pumps for sand mining vessels. In mud with a sand content of 15%, the wear rate of wear-resistant cast iron nozzles is 50% lower than that of gray cast iron, but the processing difficulty is high, mainly used for Ejector pump components in special working conditions.

II. Core Technical Advantages of Marine Cast Iron Ejector Pumps

1.No Moving Parts, Strong Anti-Clogging Ability

Marine cast iron Ejector pumps rely on the negative pressure generated by high-speed working fluids (such as seawater and freshwater) passing through nozzles to suction media, with no moving parts such as impellers or bearings inside. This feature makes them particularly suitable for transporting media containing impurities and fibers, such as ship bilge sewage (containing oil, sediment, and fiber waste), without worrying about clogging issues. A container ship using HT250 gray cast iron Ejector pumps to treat bilge water operated continuously for 8,000 hours without clogging when the solid impurity content was ≤5%, while centrifugal pumps required frequent impeller cleaning, reducing maintenance costs by 60%.

2.Simple Structure, Low Maintenance Cost

Ejector pumps are composed of static components such as nozzles, suction chambers, and diffusers, requiring no auxiliary systems such as lubrication or sealing. Maintenance only involves regular inspection of nozzle wear and pump body corrosion. Taking gray cast iron Ejector pumps as an example, their maintenance cycle can reach 12-18 months, and each maintenance only requires replacing the nozzle (cost about 1/5 of that for centrifugal pumps). A bulk carrier comparing the use of Ejector pumps and centrifugal pumps to treat ballast water saved approximately 32,000 USD in annual maintenance costs and reduced downtime by 70%, improving ship operational efficiency.

3.Adaptability to Ship Vibration Environment

Cast iron has excellent vibration damping performance, with a damping coefficient of 0.02 for gray cast iron, 5 times that of steel. In the vibration environment generated by the operation of ship main engines and auxiliary engines, cast iron Ejector pumps are less likely to suffer from component loosening or fatigue damage due to vibration. A cruise ship's fire protection Ejector pump made of QT500-7 ductile cast iron operated continuously for 5 years without structural cracks under the vibration condition of the main engine at 1800rpm, while aluminum alloy Ejector pumps in similar conditions had experienced connecting flange cracks, reflecting the advantages of cast iron materials in ship vibration environments.

4.Outstanding Cost Economy

The cost of gray cast iron is only 1/4-1/3 that of stainless steel, and ductile cast iron is 1/2-2/3 that of stainless steel. Taking a DN100 规格 (DN100 specification) Ejector pump as an example, the gray cast iron Ejector pump is priced at about 2,500 USD, while the stainless steel Ejector pump costs more than 7,000 USD. In non-critical ship systems (such as bilge drainage and deck washing), using cast iron Ejector pumps can significantly reduce procurement costs. An offshore workboat using all gray cast iron Ejector pumps saved 120,000 USD in equipment investment compared to stainless steel pumps, making it suitable for cost-sensitive small and medium-sized ships.

III. Application Limitations of Marine Cast Iron Ejector Pumps

1.Low Efficiency, High Energy Consumption

The working efficiency of Ejector pumps is usually 30%-50%, far lower than that of centrifugal pumps (60%-80%) and screw pumps (70%-85%). In large-flow and high-lift conditions, the energy consumption disadvantage of cast iron Ejector pumps is more obvious. A cargo ship using HT250 Ejector pumps to transport ballast water (flow rate 200m³/h, head 30m) consumed approximately 18,000 more liters of fuel per year than centrifugal pumps with the same parameters, increasing operating costs by about 15,000 USD. Therefore, Ejector pumps are more suitable for small-flow and low-lift scenarios, such as bilge water drainage (flow rate ≤50m³/h).

2.Limited Corrosion Resistance, Requiring Protection in Seawater Environment

The corrosion rate of ordinary gray cast iron and ductile cast iron in seawater is 0.05-0.1mm/year, requiring anti-corrosion treatment (such as coating with epoxy resin) for long-term use. A container ship that did not perform anti-corrosion treatment on gray cast iron Ejector pumps had a pump body wall thickness reduction of 1.2mm after 2 years, exceeding the safety limit (allowable reduction ≤1mm), forcing replacement. Although corrosion-resistant cast iron improves corrosion resistance, it increases costs by 30%-50% and has high processing difficulty, limiting its widespread application.

3.Limited Head and Flow Range

The head of cast iron Ejector pumps is usually ≤50m, and the flow rate is ≤100m³/h, unable to meet high-pressure and large-flow requirements. In ship fire protection systems, Ejector pumps are only suitable for small ships (fire flow ≤30m³/h), while large ships need to use centrifugal pumps or screw pumps. In addition, the performance of Ejector pumps is highly affected by the pressure of working fluids. When the pressure of working fluids fluctuates, the flow stability is poor, making them unsuitable for scenarios with high flow accuracy requirements (such as metering transportation).

4.Large Weight, Limited Installation

The density of cast iron is 7.85g/cm³, resulting in a large weight of cast iron Ejector pumps. A DN100 gray cast iron Ejector pump weighs about 55kg, while an aluminum alloy Ejector pump only weighs 25kg. When installing on ship superstructures or deck equipment, the large weight may affect the ship's center of gravity configuration, requiring additional reinforcement and support, increasing installation costs. A yacht gave up using cast iron Ejector pumps due to weight restrictions and switched to aluminum alloy materials, which increased costs but met the ship's stability requirements.

IV. Key Points for Ejector Pump Selection

Bilge drainage and freshwater systems: Prioritize gray cast iron (HT200/HT250) for low costs and meeting non-corrosive medium requirements.

Ballast water and seawater systems: Use corrosion-resistant cast iron (such as high-silicon cast iron) or ductile cast iron (QT500-7) with anti-corrosion coatings.

Particle-containing media: Select wear-resistant cast iron or ductile cast iron, and the nozzle part can be strengthened by tungsten carbide surfacing.