What are the specific differences in requirements for stainless steel pipes among various types of

The functional differences of the ship's systems (such as medium type, pressure/temperature conditions, environmental risks, and safety priorities) directly determine the selection and technical requirements of stainless steel pipes. The core differences lie in three aspects: medium compatibility, operational tolerance, and safety compliance. Below, we will break down the specific requirements differences for stainless steel pipes for the 6 key systems of the ship one by one: 

I. Summary Table of Core Differences

First, present the key differences of each system in a table for a clear visual representation, and then provide detailed explanations:

Image 1 

II. Detailed Explanation of Specific Requirements for Each System

1. Ballast Water System: "Prioritize Resistance to Seawater Corrosion"

The ballast water system is used to regulate the draft and stability of the vessel. The medium is natural seawater (with a Cl⁻ concentration of approximately 1.8-3.5%), and it is one of the systems with the highest "corrosion resistance requirements" for stainless steel pipes. The core issue is pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC) caused by chloride ions.

Corrosion Resistance Requirements:

It must meet PREN ≥ 24 (corrosion equivalent for pitting), so 304 steel is prohibited (PREN ≈ 18-20), and 316L (PREN ≈ 24-26) or duplex steel 2205 (PREN ≈ 32-34, with stronger SCC resistance) are preferred;

The inner and outer surfaces must undergo acid pickling and passivation treatment (removing oxide scale and welding slag to form a dense passivation film), avoiding salt residue at the welds that could cause crevice corrosion;

It must pass the neutral salt spray test (ASTM B117), requiring no pitting or rust within 500 hours.

Process and Installation Requirements:

Welded pipes (TIG welding) can be used, but the welds must undergo 100% penetrant testing (PT) and 20% ultrasonic testing (UT) to prevent welding defects that could lead to seawater leakage;

The pipe slope must be ≥ 1‰ to avoid seawater stagnation (stagnation would exacerbate local corrosion);

When connecting to the hull structure, insulating gaskets (such as polytetrafluoroethylene) must be added to prevent "contact corrosion between dissimilar metals" (between stainless steel and carbon steel hulls forming a galvanic cell).

2. Fuel / Lubricating Oil System: "High Pressure Sealing + Resistance to Oil Corrosion"

This system is the core of the vessel's power (such as main engines, generator fuel supply), with the medium being diesel / lubricating oil (containing sulfur and organic acids), and the working conditions are characterized by high pressure, medium-high temperature, and vibration. The core risk is high-pressure leakage leading to fire, so "strength + sealing" are the core requirements.

Strength and Pressure Resistance Requirements:

It must use seamless stainless steel pipes (welded pipes have insufficient pressure resistance), with tensile strength ≥ 515 MPa and yield strength ≥ 205 MPa (316 steel standard);

The test pressure is twice the design pressure (far exceeding 1.5 times for other systems), if the design pressure is 3 MPa, the test pressure must reach 6 MPa, and it must be maintained for 60 minutes without leakage or deformation;

The high-temperature section (such as the fuel injection pipe for the main engine, temperature ≥ 120°C) requires a high-temperature steel grade 310S (able to withstand 1100°C, with strong resistance to high-temperature creep).

Anti-Oil Corrosion and Cleanliness Requirements:

The inner wall must be degreased (removing rolling oil and dust) to avoid oil contamination when mixed with fuel;

The material must be resistant to corrosion by sulfur and organic acids in the fuel, and the Cr-Ni-Mo alloy system of 316 steel can effectively resist chemical erosion by oil media;

The welds must undergo 100% radiographic testing (RT), with a grade reaching ASTM E446 Level II (free of incomplete fusion, pores, etc.).

3. Drinking Water System: "Hygiene and Safety Without Compromise"

This system provides drinking water for crew and passengers, with the medium being fresh water (conforming to GB 5749 or IMO MSC.160 (78) standards), and the core requirement is hygiene compliance (no harmful substances leaching) + prevention of bacterial growth, belonging to the "food grade" category.

Hygiene and Material Requirements:

It must use low-carbon stainless steel (304L, 316L, with a carbon content ≤ 0.03%), avoiding the formation of carbides that could lead to heavy metal leaching;

The inner wall must be mirror-polished, with a roughness Ra ≤ 0.8 μm (ordinary systems with Ra ≤ 3.2 μm are acceptable), reducing bacterial adhesion and dirt accumulation; It is necessary to obtain health certification, such as NSF 61 (USA) and GB/T 17219 (China). After the immersion test, the Cr, Ni, and Mo dissolved in the water should be ≤ 0.01mg/L.

Process and installation requirements:

Welding should adopt internal leveling TIG welding (to avoid the weld seam protruding and forming a hygiene dead corner), and after welding, a "passivation + sterile cleaning" process should be carried out;

It is prohibited to use welding wires or flux containing lead or mercury;

The pipeline system needs to be disinfected regularly (such as with sodium hypochlorite cleaning), so the steel pipes need to be resistant to the corrosion of weak oxidizing agents.

4. Main machine cooling system: "Anti-vibration fatigue is key"

This system is used to cool the main machine, generator, etc., and the medium is divided into fresh water (closed-loop) and seawater (open-loop). The working conditions are characterized by high-frequency vibration (vibration frequency of the main machine during operation is 10-1000Hz) + temperature fluctuation (30-80℃). The core risk is weld seam fatigue cracking caused by vibration.

Anti-fatigue and vibration requirements:

It is necessary to pass fatigue tests: 10⁷ cycles of load (simulating the full life cycle vibration of the main machine), without cracks;

Pipes connected to the main machine and pump body need to be equipped with flexible joints (such as metal bellows), to absorb vibration displacement and avoid stress concentration caused by rigid connection;

In cold sea areas (such as Arctic routes), it needs to pass the -40℃ low-temperature impact test, with impact energy Akv ≥ 34J (to prevent brittle fracture due to low temperature).

Medium compatibility requirements:

Fresh water closed-loop: 304 steel can be selected (with low Cl⁻ content), but corrosion inhibitors need to be added;

Seawater open-loop: 316L steel needs to be selected (PREN ≥ 24), with internal wall acid washing and passivation to prevent seawater corrosion;

The pipeline flow velocity should be ≤ 2m/s (to avoid "erosion corrosion" caused by high-speed water flow).

5. Exhaust system: "Resistant to high-temperature oxidation + Anti-thermal fatigue"

This system discharges the high-temperature exhaust gas from the main machine / boiler. The working conditions are characterized by ultra-high temperature (main machine exhaust temperature 400-800℃, boiler exhaust temperature up to 1100℃) + corrosive exhaust gas (containing SO₂, NOₓ), the core requirement is resistance to high temperature, high-temperature oxidation resistance, and anti-thermal fatigue.

High-temperature and oxidation resistance requirements:

It is necessary to select high-temperature stainless steel or superalloys: for the medium temperature range (400-600℃), 316L is used; for the high-temperature range (600-1100℃), 310S (Cr25% Ni20%, with an anti-high-temperature oxidation temperature of up to 1150℃) or Inconel 625 (nickel-based alloy, resistant to 1200℃ high temperature) is used;

It is necessary to pass high-temperature oxidation tests: continuously exposed to the design temperature for 1000 hours, with the oxide weight increase ≤ 0.1g/m² (to prevent oxide layer detachment and blockage of the pipeline).

Anti-thermal fatigue and fire resistance requirements:

The temperature fluctuation of the exhaust gas is large (such as the temperature difference between the start and stop of the main machine is up to 500℃), the steel pipes need to have good thermal fatigue performance to avoid cracking caused by thermal expansion and contraction;

The exhaust pipes in the engine room need to be covered with fire-resistant materials (such as ceramic fibers), in accordance with the IMO SOLAS convention "A-level fire-resistant separation" requirements, and the pipeline should not fail within 30 minutes during a fire.

6. Fire protection system: "High pressure + Fire-resistant dual guarantee"

This system is used for fire fighting, the medium is seawater or fresh water, the working conditions are characterized by high pressure (fire pump outlet pressure 1.0-2.5MPa) + high-temperature fire environment, the core requirement is high strength (resistance to high pressure) + fire-resistant (usable during a fire).

Strength and pressure resistance requirements:

Select seamless 316L steel, with tensile strength ≥ 515MPa, the pressure resistance test pressure is 1.5 times the design pressure, and maintain pressure for 30 minutes without leakage; The wall thickness of the pipeline needs to be increased by 10-20% compared to the ordinary system (for example, for a DN50 pipeline, the ordinary system has a wall thickness of 3.5mm, while the fire protection system requires 4.0mm), to withstand the structural stress during a fire.

Fire resistance and impact resistance requirements:

It needs to pass the fire resistance test (IMO FTP Code): exposed to a temperature of 840℃ for 30 minutes, the pipeline still maintains integrity (no rupture, leakage);

For pipelines installed in areas prone to impact such as decks and engine rooms, protective sleeves (such as stainless steel sleeves) need to be added to prevent deformation caused by impact. 

III. Summary of Root Causes of Differences and Selection Logic

The differences in requirements for stainless steel pipes among various systems are fundamentally determined by the chain of "medium characteristics → working condition risks → safety priority". When selecting, the following logic should be followed:

First, consider the medium: seawater / high Cl⁻ medium should prefer duplex steel / 316L (high PREN); sanitary-grade medium should choose 304L/316L (low carbon + polished); high-temperature medium should select 310S / nickel-based alloys.

Second, consider the working conditions: high-pressure systems should select seamless pipes (such as for fuel); high-frequency vibration systems should be equipped with flexible joints + fatigue resistance tests (such as for cooling); high-temperature systems should select high-temperature steel types + fireproof coating (such as for exhaust).

Finally, consider compliance: drinking water requires hygiene certification, fire protection requires SOLAS fire resistance certification, and all systems require classification society (CCS/DNV) material approval.

Through the above logic, the stainless steel pipe requirements for different ship systems can be accurately matched, avoiding "over-selecting (such as using 2205 for drinking water pipes)" or "insufficient selection (such as using 304 for ballast water pipes)" that leads to cost waste or safety hazards.