2.5 General pipe design

  2.5.1 The requirements of this section apply to gas piping. The Administration may accept relaxation from these requirements for gas piping inside gas tanks and open-ended piping after special consideration, such as risk assessment.

  2.5.2 Gas piping should be protected against mechanical damage and the piping should be capable of assimilating thermal expansion without developing substantial tension.

  2.5.3 The piping system should be joined by welding with a minimum of flange connections. Gaskets should be protected against blow-out.

  2.5.4 The wall thickness of pipes should not be less than:

The minimum wall thickness should be in accordance with recognized standards.

  2.5.5 The greater of the following design conditions should be used for piping, piping system and components as appropriate:

• .1 for systems or components which may be separated from their relief valves and which contain only vapour at all times, the superheated vapour pressure at 45°C or higher or lower if agreed upon by the Administration (refer to IGC Code, paragraph 4.2.6.2), assuming an initial condition of saturated vapour in the system at the system operating pressure and temperature; or

• .2 the MARVS of the gas tanks and gas processing systems; or

• .3 the pressure setting of the associated pump or compressor discharge relief valve if of sufficient capacity; or

• .4 the maximum total discharge or loading head of the gas piping system; or

• .5 the relief valve setting on a pipeline system if of sufficient capacity; or

• .6 a pressure of 10 bar except for open-ended lines where it is not to be less than 5 bar.

  2.5.6 For pipes made of steel including stainless steel, the allowable stress to be considered in the formula of the strength thickness in 2.5.4 should be the lower of the following values:

For pipes made of materials other than steel, the allowable stress should be considered by the Administration.

  2.5.7 Where necessary for mechanical strength to prevent damage, collapse, excessive sag or buckling of pipe due to superimposed loads from supports, ship deflection or other causes, the wall thickness should be increased over that required by 2.5.4 or, if this is impractical or would cause excessive local stresses, these loads should be reduced, protected against or eliminated by other design methods.

  2.5.8 Gas piping systems should have sufficient constructive strength. For high pressure gas piping systems this should be confirmed by carrying out stress analysis and taking into account:

• .1 stresses due to the weight of the piping system;

• .2 acceleration loads when significant; and

• .3 internal pressure and loads induced by hog and sag of the ship.

  2.5.9 Flanges, valves, fittings, etc., should be in accordance with recognized standards taking into account the design pressure defined in 2.5.5. For bellows and expansion joints used in vapour service, a lower minimum design pressure than defined in 2.5.5 may be accepted.

  2.5.10 All valves and expansion joints used in high pressure gas systems should be of an approved type.

  2.5.11 The following types of connections may be considered for direct connection of pipe lengths (without flanges):

• .1 Butt welded joints with complete penetration at the root may be used in all applications. For design temperature below -10°C, butt welds should be either double welded or equivalent to a double welded butt joint. This may be accomplished by use of a backing ring, consumable insert or inert gas back-up on the first pass. For design pressures in excess of 10 bar and design temperatures -10°C or lower, backing rings should be removed.

• .2 Slip-on welded joints with sleeves and related welding, having dimensions satisfactory to the Administration, should only be used for open-ended lines with external diameter of 50 mm or less and design temperatures not lower than -55°C.

• .3 Screwed couplings should only be used for accessory lines and instrumentation lines with external diameters of 25 mm or less.

  2.5.12 Flanges should be of the welding neck, slip-on or socket welding type. For all piping (except open-ended lines), the following apply:

• .1 For design temperatures < -55°C only welding neck flanges should be used.

• .2 For design temperatures < -10°C slip-on flanges should not be used in nominal sizes above 100 mm and socket welding flanges should not be used in nominal sizes above 50 mm.

  2.5.13 Piping connections other than those mentioned above may be accepted upon consideration in each case.

  2.5.14 Postweld heat treatment should be required for all butt welds of pipes made with carbon, carbon-manganese and low-alloy steels. The Administration may waive the requirement for thermal stress relieving of pipes having wall thickness less than 10 mm in relation to the design temperature and pressure of the piping system concerned.

  2.5.15 When the design temperature is -110°C or lower, a complete stress analysis for each branch of the piping system should be submitted. This analysis should take into account all stresses due to weight of pipes with cargo (including acceleration if significant), internal pressure, thermal contraction and loads induced by movements of the ship. For temperatures above -110°C, a stress analysis may be required by the Administration. In any case, consideration should be given to thermal stresses, even if calculations need not be submitted. The analysis should be carried out according to a recognized code of practice.

  2.5.16 Gas pipes should not be located less than 760 mm from the ships side.

  2.5.17 Gas piping should not be led through other machinery spaces. Alternatively, double gas piping may be approved, provided the danger of mechanical damage is negligible, the gas piping has no discharge sources and the room is equipped with a gas alarm.

  2.5.18 An arrangement for purging gas bunkering lines and supply lines (only up to the double block and bleed valves if these are located close to the engine) with nitrogen should be provided.

  2.5.19 The gas piping system should be installed with sufficient flexibility. Arrangement for provision of the necessary flexibility should be demonstrated to maintain the integrity of the piping system in all foreseen service situations.

  2.5.20 Gas pipes should be colour marked based on a recognized standard^footnote.

  2.5.21 If the fuel gas contains heavier components that may condense in the system, knock out drums or equivalent means for safely removing the liquid should be fitted.

  2.5.22 All pipelines and components which may be isolated containing liquid gas should be provided with relief valves.

  2.5.23 Where tanks or piping are separated from the ship's structure by thermal isolation, provision should be made for electrically bonding to the ship's structure both the piping and the tanks. All gasketed pipe joints and hose connections should be electrically bonded.