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Piping System of pressure vessels on gas tankers

This article contains important information about piping system of pressure vessels on natural gas carriers.

General

With regard to this article, reference is made to the Unified Requirement G3 of the International Association of Classification Societies (IACS).

Process pressure vessels and piping systems are to comply also with the relevant Rules for Machinery Installations (Part 1, Vol. III) Sec. 8, and Rules for Machinery Installations (Part 1, Vol. III) Sec. 11.

The requirements for type C independent tanks in “Cargo containment system of gas vessel”Cargo containment, may also apply to process pressure vessels. The words “pressure vessels” as used in “Cargo containment system of gas vessel”Cargo containment cover both independent tanks type C and process pressure vessels.

Cargo and process piping

General

The requirements of text below apply to product and process piping including vapour piping and vent lines of safety valves or similar piping. Instrument piping not containing cargo is exempt from these requirements.

Provision is to be made by the use of:

  • offsets;
  • loops;
  • bends;
  • mechanical expansion joints such as bellows;
  • slip joints and ball joints;

or similar suitable means to protect the piping, piping system components and cargo tanks from excessive stresses due to thermal movement and from movements of the tank and the hull structure. Where mechanical expansion joints are used in piping they are to be held to a minimum and, where located outside of cargo tanks, shall be of the bellows type.

Piping system of gas carrier
Cargo piping system of gas carrier

Low temperature piping is to be thermally isolated from the adjacent hull structure, where necessary, to prevent the temperature of the hull from falling below the design temperature of the hull material. Where liquid piping is dismantled regularly, or where liquid leakage may be anticipated, such as at shore connections and at pump seals, protection for the hull beneath is to be provided.

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

Suitable means are to be provided to relieve the pressure and remove liquid contents from cargo loading and discharging crossover headers and cargo hoses to the cargo tanks or other suitable location, prior to disconnecting the cargo hoses.

Piping systems or components which may be isolated in a liquid full condition are to be provided with relief valves. Piping systems capable of being isolated, and the volume of which does not exceed 0,05 m3, need not be provided with relief valves.

Relief valves discharging liquid cargo from the cargo piping system are to discharge into the cargo tanks; alternatively they may discharge to the cargo vent mast if means are provided to detect and dispose of any liquid cargo which may flow into the vent system. Relief valves on cargo pumps are to discharge to the pump suction.

Scantlings based on internal pressure

Subject to the conditions stated in “Permissible stress” the wall thickness of pipes shall not be less than:

t=t0+b+c1a/100 [mm],

t0=p·D20·K·e+p [mm].

Where:

  • t – minimum thickness;
  • t0 – theoretical thickness;
  • p – design pressure [bar] referred to in “Design Pressure”;
  • D – outside diameter [mm];
  • K – allowable stress [N/mm2] referred to in “Permissible stress”;
  • e – efficiency factor where e is 1,0 for seamless pipes and for longitudinally or spirally welded pipes, delivered by manufacturers approved for making welded pipes, which are considered equivalent to seamless pipes when non-destructive testing on welds is carried out in accordance with the Rules. In other cases an efficiency factor less than 1,0 may be required by the Society depending on the manufacturing process;
  • b – allowance for bending [mm]. The value of “b” is to be chosen so that the calculated stress in the bend, due to internal pressure only, does not exceed the allowable stress. Where such justification is not given, “b” is to be:

b=b·t02,5·r [mm],

With:

  • r – mean radius of the bend;
  • c – corrosion allowance [mm]. If corrosion or erosion is expected, the wall thickness of the piping is to be increased over that required by other design requirements. This allowance is to be consistent with the expected life of the piping;
  • a – negative manufacturing tolerance for wall thickness in [%].

Design Pressure

The design pressure p in the formula for t0 in “Scantlings based on internal pressure” is the maximum gauge pressure to which the system may be subjected in service.

The greater of the following design conditions is to be used for piping, piping systems and components as appropriate:

  • for vapour piping systems or components which may be separated from their relief valves and which may contain some liquid, the saturated vapour pressure at 45 °C, or higher or lower if agreed upon by the Society (see “Design vapour pressure”Cargo containment system of gas vessel);
  • or 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 Society (see “Design vapour pressure”Cargo containment system of gas vessel), assuming an initial condition of saturated vapour in the system at the system operating pressure and temperature;
  • or the MARVS of the cargo tanks and cargo processing systems;
  • or the pressure setting of the associated pump or compressor discharge relief valve;
  • or the maximum total discharge or loading head of the cargo piping system;
  • or the relief valve setting on a piping system.

The design pressure is not to be less than 10 bar gauge except for open ended pipe where it is not to be less than 5 bar gauge.

Permissible stress

For pipes the permissible stress to be considered in the formula for t in “Scantlings based on internal pressure” is the lower of the following values:

K=RmA or K=ReHB,

Where:

  • Rm – specified minimum tensile strength at room temperature [N/mm2];
  • ReH – specified minimum yield stress at room temperature [N/mm2]. If the stress-strain curve does not show a defined yield stress, the 0,2 % proof stress applies.

The values of A and B are to be shown on the International Certificate of Fitness for the Carriage of Liquefied Gases in Bulk and have values of at least:

A = 2,7 and B = 1,8.

The minimum thickness is to be in accordance with Rules for Machinery Installations (Part 1, Vol. III) Sec. 11, C.2.

Where necessary for mechanical strength to prevent damage, collapse, excessive sag or buckling of pipes due to superimposed loads from supports, ship deflection or other causes, the wall thickness is to be increased over that required “Scantlings based on internal pressure”, or, if this is impracticable or would cause excessive local stresses, these loads are to be reduced, protected against or eliminated by other design methods.

Flanges, valves and other fittings are to comply with recognized standards, taking into account the design pressure defined in “Scantlings based on internal pressure”. For bellows expansion joints used in vapour service, a lower minimum design pressure may be accepted.

For flanges not complying with a standard the dimensions of flanges and relative bolts are to be to the satisfaction of the Society.

Stress analysis

When the design temperature is -110 °C or lower, a complete stress analysis, taking into account all the stresses due to weight of pipes, including acceleration loads if significant, internal pressure, thermal contraction and loads induced by hog and sag of the ship for each branch of the piping system is to be submitted to the Society.

For temperatures above -110 °C, a stress analysis may be required by the Society in relation to such matters as to the design, or to the stiffness of the piping system and to the choice of materials. In any case, consideration is to be given to thermal stresses, even though calculations are not submitted. The analysis may be carried out according to a code of practice acceptable to the Society.

Materials

The choice and testing of materials used in piping systems are to comply with the requirements of “Materials of construction”Materials of construction LNG and LPG tanks taking into account the minimum design temperature. However, some relaxation may be permitted in the quality of the material of open ended vent piping, provided the temperature of the cargo at the pressure relief valve setting is -55 °C or greater and provided no liquid discharge to the vent piping can occur. Similar relaxation may be permitted under the same temperature conditions to open ended piping inside cargo tanks, excluding discharge piping and all piping inside of membrane and semi-membrane tanks.

Materials having a melting point below 925 °C are not to 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 is to be provided.

Type tests on piping components

Each type of piping component is to be subjected to type tests as follows:

a Valves: Each size and type of valves intended to be used at a working temperature below -55 °C is to be subjected to a tightness test to the minimum design temperature or lower, and to a pressure not lower than the design pressure of the valves. During the test the satisfactory repeated operation of the valve is to be ascertained.

Type testing is to include hydrostatic test of the valve body at a pressure equal to 1,5 times the design pressure, hand cryogenic testing consisting of valve operation or safety valve set pressure, and leakage verification. In addition, for valves other than safety valves, a seat and stem leakage test at a pressure equal to 1,1 times the design pressure.

b Expansion Bellows: The following type tests are to be performed on each type of expansion bellows intended for use on cargo piping outside the cargo tank and, where required, on those expansion bellows installed within the cargo tanks:

  1. A type element of the bellows, not pre-compressed, is to be pressure tested at not less than five times the design pressure without bursting. The duration of the test is to be not less than 5 min.
  2. A pressure test on a type expansion joint complete with all the accessories such as flanges, stays and articulations, at twice the design pressure at the extreme displacement conditions recommended by the manufacturer without permanent deformation. Depending on the materials used, the Society may require the test to be conducted at the minimum design temperature.
  3. A cyclic test (thermal movements) is to be performed on a complete expansion joint.
  4. A cyclic fatigue test (ship deformation) is to be performed on a complete expansion joint, without internal pressure, by simulating the bellows movement corresponding to a compensated pipe length, for at least 2×106 cycles at a frequency not higher than 5 cycles/second. This test is only required when, due to the piping arrangement, ship deformation loads are actually experienced.
  5. The Society may waive performance of the tests referred to in this paragraph provided that complete documentation is supplied to establish the suitability of the expansion joints to withstand the expected working conditions. When the maximum internal pressure exceeds 1,0 bar gauge this documentation is to include sufficient test data to substantiate the design method used, with particular reference to correlation between calculation and test results.

Piping fabrication and joining details

The requirements of this paragraph apply to piping inside and outside the cargo tanks. Relaxations from these requirements may be accepted in accordance with recognized standards for piping inside cargo tanks and open ended piping.

The following direct connection of pipe lengths, without flanges, may be considered:

  1. Butt welded joints with complete penetration at the root may be used in all applications. For design temperatures below -10 °C, butt welds are to 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 of -10 °C or lower, backing rings are to be removed.
  2. Slip-on welded joints with sleeves and related welding, having dimensions in accordance with recognized standards are to be used only for open ended lines with external diameter of 50 mm or less and design temperatures not lower than -55 °C.
  3. Screwed couplings in accordance with recognized standards are only to be used for accessory lines and instrumentation lines with external diameters of 25 mm or less.

Flange connections

Flanges in flange connections are to be of the welding neck, slip-on or socket welding type.

Flanges are to be selected as to type, made, and tested in according with recognized standards. In particular, for all piping except open ended, the following restrictions apply:

  1. For design temperatures lower than -55 °C, only welding neck flanges are to be used.
  2. For design temperatures lower than -10 °C, slip-on flanges are not to be used in nominal sizes above 100 mm and socket welding flanges are not to be used in nominal sizes above 50 mm.

Piping connections, other than those mentioned here, may be accepted by the Society in each case.

Bellows and expansion joints are to be provided to allow for expansion of piping.

  1. If necessary, bellows are to be protected against icing.
  2. Slip joints are not to be used except within the cargo tanks.

Welding, postweld heat treatment and non-destructive testing:

a Welding is to be carried out in accordance with “Welding and non-destructive testing”Materials of construction LNG and LPG tanks.

b Postweld heat treatment are to be required for all butt welds of pipes made with carbon, carbon-manganese and low alloy steels. The Society 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.

c In addition to normal controls before and during the welding and to the visual inspection of the finished welds, as necessary for proving that the welding has been carried out correctly and according to the requirements of this paragraph, the following tests are required:

  1. 100 % radiographic inspection of butt welded joints for piping systems with design temperatures lower than -10 °C and with inside diameters of more than 75 mm or wall thicknesses greater than 10 mm. When such butt welded joints of piping sections are made by automatic welding procedures in the pipe fabrication shop, upon special approval the extent of radiographic inspection may be progressively reduced but in no case to less than 10 % of each joint. If defects are revealed the extent of examination shall be increased to 100 % and shall include inspection of previously accepted welds. This special approval can only be granted if well documented quality assurance procedures and records are available to enable the Society to assess the ability of the manufacturer to produce satisfactory welds consistently.
  2. For other butt welded joints not covered above of pipes, spot radiographic tests or other non-destructive tests are to be carried out at the discretion of the Society depending upon service, position and materials. In general at least 10 % of butt welded joints of pipes are to be radiographed.

Testing of piping

The requirements of this paragraph apply to piping inside and outside the cargo tanks. However, the Society may accept relaxations from these requirements for piping inside cargo tanks and open ended piping.

After assembly, all cargo and process piping are to be subjected to a hydrostatic test to at least 1,5 times the design pressure. When piping systems or parts of systems are completely manufactured and equipped with all fittings, the hydrostatic test may be conducted prior to installation aboard ship. Joints welded on board are to be hydrostatically tested to at least 1,5 times the design pressure. Where water can-not be tolerated and the piping cannot be dried prior to putting the system into service, proposals for alterna-tive testing fluids or testing means are to be submitted to the Society for approval.

After assembly on board, each cargo and process piping system is to be subjected to a leak test using air, halides, or other suitable medium to a pressure depending on the leak detection method applied.

All piping systems including valves, fittings and associated equipment for handling cargo or vapours are to be tested under normal operating conditions not later than at the first loading operation.

Cargo system valving requirements

Every cargo piping system and cargo tank is to be provided with the following valves, as applicable:

  • For cargo tanks with a MARVS not exceeding 0,7 bar gauge, all liquid and vapour connections, except safety relief valves and liquid level gauging devices, are to have shut-off valves located as close to the tank as practicable. These valves may be remotely controlled but are to be capable of local manual operation and provide full closure. One or more remotely controlled emergency shut-down valves are to be provided on the ship for shutting down liquid and vapour cargo transfer between ship and shore. Such valves may be arranged to suit the ship’s design and may be the same valve as required here and shall comply with the requirements of this.
  • For cargo tanks with a MARVS exceeding 0,7 bar, all liquid and vapour connections, except safety relief valves and liquid level gauging devices, are to be equipped with a manually operated stop valve and a remotely controlled emergency shut-down valve. These valves are to be located as close to the tank as practicable. Where the pipe size does not exceed 50 mm in diameter, excess flow valves may be used in lieu of the emergency shut-down valve. A single valve may be substituted for the two separate valves provided the valve complies with the requirements of this is capable of local manual operation and provides full closure of the line.
  • Cargo pumps and compressors shall be arranged to shut-down automatically if the emergency shut-down valves required by above of this list are closed by the emergency shut-down system required by this.

Cargo tank connections for gauging or measuring devices need not be equipped with excess flow or emergency shut-down valves provided that the devices are constructed so that the outward flow of tank contents cannot exceed that passed by a 1,5 mm diameter circular hole.

The requirements of above, providing relaxations for cargo tanks referred above (type C cargo tanks), do not apply to cargo tank connections for gauging or measuring devices of cargo tanks referred above.

One remote operated, emergency shut-down valve is to be provided at each cargo hose connection in use. Connections not used in transfer operations may be blinded with blank flanges in lieu of valves.

The control system for all required emergency shutdown valves shall be so arranged that all such valves may be operated by single controls situated in at least two remote locations on the ship. One of these locations shall be the control position required by “Cargo Tank Instrumentation”Cargo Tank Instrumentation on Gas Tankers or cargo control room. The control system shall also be provided with fusible elements designed to melt at temperatures between 98 °C and 104 °C which will cause the emergency shutdown valves to close in the event of fire.

Locations for such fusible elements shall include the tank domes and loading stations. Emergency shutdown valves shall be of the fail-closed (closed on loss of power) type and be capable of local manual closing operation. Emergency shutdown valves in liquid piping shall fully close under all service conditions within 30 s of actuation as measured from the time of manual or automatic initiation to final closure.

This is called the total shutdown time and is made up of a signal response time and a valve closure time.

The valve closure time should be such as to avoid surge pressures in pipelines. Information about the closing time of the valves and their operating characteristics shall be available on board and the valve closure time should be verifiable and reproducible. Such valves shall close in such a manner as to cut off the flow smoothly.

The closure time of 30 s for the emergency shutdown valve referred above should be measured from the time of manual or automatic initiation to final closure. This is called the total shutdown time and is made up of a signal response time and a valve closure time. The valve closure time should be such as to avoid surge pressure in pipelines. Such valves should close in such a manner as to cut off the flow smoothly.

Excess flow valves shall close automatically at the rated closing flow of vapour or liquid as specified by the manufacturer. The piping, including fittings, valves, and appurtenances protected by an excess flow valve, shall have a greater capacity than the rated closing flow of the excess flow valve. Excess flow valves may be designed with a by-pass not exceeding an area of 1,0 mm diameter circular opening to allow equalization of pressure, after an operating shut-down.

Ship’s cargo hoses

Liquid and vapour hoses used for cargo transfer are to be compatible with the cargo and suitable for the cargo temperature.

Hoses subject to tank pressure, or the discharge pressure of pumps or vapour compressors, are to be designed for a bursting pressure not less than five times the maximum pressure the hose will be subjected to during cargo transfer.

Cargo hoses supplied on board ships on or after 1 July 2002 shall be prototype tested, complete with end-fittings, at a normal ambient temperature with 200 pressure cycles from zero to at least twice the specified maximum working pressure. After this cycle pressure test has been carried out, the prototype test shall demonstrate a bursting pressure of at least 5 times its specified maximum working pressure at the extreme service temperature. Hoses used for prototype testing are not to be used for cargo service.

Thereafter, before being placed in service, each new length of cargo hose produced is to be hydrostatically tested at ambient temperature to a pressure not less than 1,5 times its specified maximum working pressure nor more than two-fifths its bursting pressure. The hose is to be stenciled or otherwise marked with its specified maximum working pressure, and if used in other than ambient temperature services, its maximum and/or minimum temperature. The specified maximum working pressure shall not be less than 10 bar gauge.

Cargo transfer methods

Where cargo transfer is by means of cargo pumps not accessible for repair with the tanks in service, at least two separate means are to be provided to transfer cargo from each cargo tank and the design is to be such that failure of one cargo pump, or means of transfer, will not prevent the cargo transfer by another pump or pumps, or other cargo transfer means. 

The procedure for transfer of cargo by gas pressurization must preclude lifting of the relief valves during such transfer. Gas pressurization may be accepted as a means of transfer of cargo for those tanks so designed that the design factor of safety is not reduced under the conditions prevailing during the cargo transfer operation.

Vapour return connections

Connections for vapour return lines to the shore installation are to be provided.

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