BASIC INFORMATION about pipe

• The majority of ships’ pipes are made of mild steel.

• Flow rate, viscosity and pressure of fluid being carried

determine a pipe’s diameter.

• Pipes in areas of a ship where there is a risk of gas explosion

are earthed because fluid flow can build up a static electricity

charge. Bonding strips are used across flanged joints to

maintain conductivity.

• Pipes that pass through other compartments pose potential

subdivision issues, especially open-ended pipes.

• Pipes, especially open-ended ones, compromise the integrity

of the compartments they pass through.

• The water circulating in cooling pipes will corrode them over

time.

• Pipes passing through tanks containing liquid are exposed to

corrosive attack on both surfaces.

• Pipes carrying liquefied gas seldom suffer internal corrosion.

• Visual checks of the external surfaces of a pipe will not indicate

its condition because it could be internally corroded and have

a reduced wall thickness.

• Most abrasive corrosion and consequent internal thinning

happens where the pipe bends and at elbows.

• Liquid flowing quickly will be turbulent as a result of fluid

separation and cavitation. Flow turbulence in a pipe will

cause pitting. A pipe with the correct diameter for the job

will eliminate turbulence.

• Pipes can be joined by butt-welding, with flange connections or

mechanical joints. However, the number of flange connections

allowed in the cargo pipes of a chemical tanker is strictly

controlled by classification society rules.

• Good pipe alignment during assembly of a run prevents

‘locked-in’ stress.

• The use of expansion (mechanical) joints, such as dresser-type

joints, is restricted to locations where pipes move because of

thermal expansion or contraction, or ship bending. Classification

society rules prohibit their use for the connection of cargo piping

in chemical tankers. The most common expansion joints are

compression couplings or slip-on joints.

• A pressure test of 1.5 times design pressure is a strength test;

a test at the design pressure is a tightness test. Pressure testing

can show the small cracks and holes that will not be found by a

visual examination.

• Pipes are held in place by supports or clips that prevent

movement from shock loads and vibration. Pipe failure is

common when pipes are allowed to vibrate.

• Pipes carrying flammable liquids have as few joints as possible

and these are shielded to prevent leaks from coming into

contact with hot surfaces.

• Mechanical joints are not normally fitted on pipes carrying

flammable liquids.