For manufacturing and installation of seawater service pipes on the ship, the following considerations are to be made :

(i) Selection of Material

• Selection of the material must be based on the ability to resist general and localised corrosion such as pitting, impingement attack and cavitation throughout the pipe.
• It must have the ability to resist stress corrosion & corrosion fatigue.
• It must be compatible with other materials in the system, such as valve body and casing.
• Rubber lining in pipes is very effective against corrosion.

The following material may be used:

(ii) Design Aspects

• Design aspects include's
  • Design Pressure - 0.2 MPa
  • Design Temperature 32℃
  • Pipe size 25mm to 400mm as per requirement.
  • Thickness example:
  • Nominal Pressure Standard: JIS 5K or JIS 10K.
• Flanges may be fixed type or loose type.
• Water velocity should not be less than 3m/s to avoid fouling & subsequent pitting.

(iii) Fabrication & Installation

• Tested & approved fitting to be used
• Follow the welding procedure to avoid welding defects.
• NDT can be done to ensure no flaws in fabrication.

(iv) Design Approval.

• Proper drawings and plans must be sent to the company to get class approval.

Necessary Requirements for Approval

• Manufacturer's Test certificate for material, stating the maximum permissible design stress at maximum permissible service temperature.
• Fire endurance standard to be achieved as per Class requirement.
• Provision for expansion where necessary must be provided.
• All pipes are to be hydraulically tested to a pressure of 1.5 times the working pressure to the surveyor's Satisfaction.
• All welding must be done in accordance with Classification Rules. Inspection and Testing (NDT) must be done as per Class Requirements.
• The material to be used must be suitable for the medium and service for which piping is intended meeting Class Rules.

Minimum calculated thickness,

$$t\:=\:t_0\:+\:b\:+\:c$$

where,

b = allowance for bending (mm)

c = corrosion allowance (mm)

to = thickness calculated by the below formula (mm)

$$=\:\frac{PD}{20Ke\:+\:P}$$

where,

P = design pressure (bar)

K = permissible stress (N/mm²)

D = outside diameter (mm)