Section 6 Material selection and testing

6.1 Materials

6.1.1 Piping systems

The choice and testing of materials used in piping systems shall comply with the requirements of Pt 11, Ch 6 Materials of Construction and Quality Control , taking into account the minimum cargo design temperature. However, some relaxation may be permitted in the quality of material of open ended vent piping providing the temperature of the cargo at the pressure relief valve setting is not colder than –55°C and provided no liquid discharge to the vent piping can occur. Similar relaxations may be permitted under the same temperature conditions to open ended piping inside cargo tanks, excluding discharge piping and all piping inside membrane and semi membrane tanks.
Materials having a melting point below 925°C shall not be used for piping outside the cargo tanks except for short lengths of pipes attached to the cargo tanks, in which case fire-resisting insulation shall be provided.

6.1.2 Cargo piping insulation system

Cargo piping systems shall be provided with a thermal insulation system as required to minimise heat leak into the cargo during transfer operations and to protect personnel from direct contact with cold surfaces.
Where applicable, due to location or environmental conditions, insulation materials should have suitable properties of resistance to fire and flame spread and should be adequately protected against penetration of water vapour and mechanical damage.
Where the cargo piping system is of a material susceptible to stress corrosion cracking in the presence of a salt laden atmosphere, adequate measures to avoid this occurring should be taken by considering material selection, protection of exposure to salty water and/or readiness for inspection.

6.2 Testing requirements

6.2.1 Type testing of piping components

Valves
1. Reference is made to the SIGTTO publication The Selection and Testing of Valves for LNG Applications. Each type of piping component shall be subject to the following type tests:
  Each type of piping component intended to be used at a working temperature below –55°C shall be subject to the following type tests:
  1. Each size and type of valve shall be subjected to seat tightness testing over the full range of operating pressures for bi-directional flow and temperatures, at intervals, up to the rated design pressure of the valve. Allowable leakage rates shall be to the requirements of LR. During the testing satisfactory operation of the valve shall be verified.
  2. The flow or capacity shall be certified to a recognised Standard for each size and type of valve.
  3. Pressurised components shall be pressure tested to at least 1,5 times the rated pressure.
  4. For emergency shutdown valves, with materials having melting temperatures lower than 925°C, the type testing shall include a fire test to a standard acceptable to the Administration. Reference is made to API Std 607 Fire Test for Soft Seated Quarter Turn Valves.
Expansion bellows
1. The following type tests shall be performed on each type of expansion bellows intended for use on cargo piping outside the cargo tank and where required by the Recognised Organisation, on those installed within the cargo tanks:
  1. Elements of the bellows, not pre-compressed, shall be pressure tested at not less than five times the design pressure without bursting. The duration of the test shall not be less than five minutes.
  2. A pressure test shall be performed on a type expansion joint, complete with all the accessories such as flanges, stays and articulations, at the minimum design temperature and twice the design pressure at the extreme displacement conditions recommended by the manufacturer without permanent deformation.
  3. A cyclic test (thermal movements) shall be performed on a complete expansion joint, which shall withstand at least as many cycles under the conditions of pressure, temperature, axial movement, rotational movement and transverse movement as it will encounter in actual service. Testing at ambient temperature is permitted when this testing is at least as severe as testing at the service temperature.
  4. A cyclic fatigue test (deformation of the ship unit) shall be performed on a complete expansion joint, without internal pressure, by simulating the bellows movement corresponding to a compensated pipe length, for at least 2 000 000 cycles at a frequency not higher than 5 Hz. This test is only required when, due to the piping arrangement, deformation loads from the ship unit are actually experienced.

6.2.2 System testing requirements

The requirements of this Section apply to piping inside and outside the cargo tanks.
After assembly, all cargo and process piping shall be subjected to a strength test with a suitable fluid. The test pressure is to at least 1,5 times the design pressure (1,25 times the design pressure where the test fluid is compressible) for liquid lines and 1,5 times the maximum system working pressure (1,25 times the maximum system working pressure where the test fluid is compressible) for vapour lines. When piping systems or parts of systems are completely manufactured and equipped with all fittings, the test may be conducted prior to installation onboard the ship unit. Joints welded onboard shall be tested to at least 1,5 times the design pressure.
After assembly onboard, each cargo and process piping system shall be subjected to a leak test using air, or other suitable medium to a pressure depending on the leak detection method applied.
In double wall gas fuel piping systems the outer pipe or duct shall also be pressure tested to show that it can withstand the expected maximum pressure at gas pipe rupture.
All piping systems, including valves, fittings and associated equipment for handling cargo or vapours, shall be tested under normal operating conditions not later than at the first loading operation, in accordance with recognised Standards.