CHAPTER 8 | VENT SYSTEMS
8.2 PRESSURE RELIEF SYSTEMS
[For ships constructed from 01-Jul-2016, Applicable from 01-Jan-2016]
8.2.1 Cargo tanks, including deck tanks, shall be fitted with a minimum of two pressure relief valves (PRVs), each being of equal size within manufacturer’s tolerances and suitably designed and constructed for the prescribed service.
8.2.2 Inter barrier spaces shall be provided with pressure relief devices. For membrane systems, the designer shall demonstrate adequate sizing of inter barrier space PRVs.
8.2.3 The setting of the PRVs shall not be higher than the vapour pressure that has been used in the design of the tank. Where two or more PRVs are fitted, valves comprising not more than 50% of the total relieving capacity may be set at a pressure up to 5% above MARVS to allow sequential lifting, minimizing unnecessary release of vapour.
8.2.4 The following temperature requirements apply to PRVs fitted to pressure relief systems:
1. PRVs on cargo tanks with a design temperature below 0oC shall be designed and arranged to prevent their becoming inoperative due to ice formation;
2. The effects of ice formation due to ambient temperatures shall be considered in the construction and arrangement of PRVs;
3. PRVs shall be constructed of materials with a melting point above 925°C. Lower melting point materials for internal parts and seals may be accepted, provided that failsafe operation of the PRV is not compromised; and
4. Sensing and exhaust lines on pilot operated relief valves shall be of suitably robust construction to prevent damage.
8.2.5 Valve testing
22.214.171.124 PRVs shall be type tested. Type tests shall include:
1. Verification of relieving capacity;
2. Cryogenic testing when operating at design temperatures colder than 55° C;
3. Seat tightness testing; and
4. Pressure containing parts are pressure tested to at least 1.5 times the design pressure.
PRVs shall be tested in accordance with recognized standards.
126.96.36.199 Each PRV shall be tested to ensure that:
1. It opens at the prescribed pressure setting, with an allowance not exceeding ± 10% for 0 to 0.15 MPa, ± 6% for 0.15 to 0.3 MPa, ± 3% for 0.3 MPa and above;
2. Seat tightness is acceptable; and
3. Pressure containing parts will withstand at least 1.5 times the design pressure.
8.2.6 PRVs shall be set and sealed by the Administration or recognized organization acting on its behalf, and a record of this action, including the valves’ set pressure, shall be retained on board the ship.
8.2.7 Cargo tanks may be permitted to have more than one relief valve set pressure in the following cases:
1. Installing two or more properly set and sealed PRVs and providing means, as necessary, for isolating the valves not in use from the cargo tank; or
2. Installing relief valves whose settings may be changed by the use of a previously approved device not requiring pressure testing to verify the new set pressure. All other valve adjustments shall be sealed.
8.2.8 Changing the set pressure under the provisions of 8.2.7 and the corresponding resetting of the alarms referred to in 13.4.2 shall be carried out under the supervision of the master in accordance with approved procedures and as specified in the ship’s operating manual. Changes in set pressure shall be recorded in the ship’s log and a sign shall be posted in the cargo control room, if provided, and at each relief valve, stating the set pressure.
8.2.9 In the event of a failure of a cargo tank installed PRV, a safe means of emergency isolation shall be available:
1. Procedures shall be provided and included in the cargo operation manual (see 18.2).
2. The procedures shall allow only one of the cargo tank installed PRVs to be isolated.
3. Isolation of the PRV shall be carried out under the supervision of the master. This action shall be recorded in the ship’s log and a sign posted in the cargo control room, if provided, and at the PRV.
4. The tank shall not be loaded until the full relieving capacity is restored.
8.2.10 Each PRV installed on a cargo tank shall be connected to a venting system, which shall be:
1. So constructed that the discharge will be unimpeded and directed vertically upwards at the exit;
2. Arranged to minimize the possibility of water or snow entering the vent system;
3. Arranged such that the height of vent exits shall not be less than B/3 or 6 m, whichever is the greater, above the weather deck ; and
4. 6 m above working areas and walkways.
188.8.131.52 Cargo PRV vent exits shall be arranged at a distance at least equal to B or 25 m, whichever is less, from the nearest air intake, outlet or opening to accommodation spaces, service spaces and control stations, or other nonhazardous areas. For ships less than 90 m in length, smaller distances may be permitted.
184.108.40.206 All other vent outlets connected to the cargo containment system shall be arranged at a distance of at least 10 m from the nearest air intake, outlet or opening to accommodation spaces, service spaces and control stations, or other nonhazardous areas.
8.2.12 All other cargo vent outlets not dealt with in other chapters shall be arranged in accordance with 8.2.10, 220.127.116.11 and 18.104.22.168. Means shall be provided to prevent liquid overflow from vent mast outlets, due to hydrostatic pressure from spaces to which they are connected.
8.2.13 If cargoes that react in a dangerous manner with each other are carried simultaneously, a separate pressure relief system shall be fitted for each one.
8.2.14 In the vent piping system, means for draining liquid from places where it may accumulate shall be provided. The PRVs and piping shall be arranged so that liquid can, under no circumstances, accumulate in or near the PRVs.
8.2.15 Suitable protection screens of not more than 13 mm square mesh shall be fitted on vent outlets to prevent the ingress of extraneous objects without adversely affecting the flow. Other requirements for protection screens apply when carrying specific cargoes (see 17.9 and 17.21).
8.2.16 All vent piping shall be designed and arranged not to be damaged by the temperature variations to which it may be exposed, forces due to flow or the ship’s motions.
8.2.17 PRVs shall be connected to the highest part of the cargo tank above deck level. PRVs shall be positioned on the cargo tank so that they will remain in the vapour phase at the filling limit (FL) as defined in chapter 15, under conditions of 15° list and 0.015L trim, where L is defined in 1.2.31.
8.2.18 The adequacy of the vent system fitted on tanks loaded in accordance with 15.5.2 shall be demonstrated, taking into account the recommendations developed by the Organization. A relevant certificate shall be permanently kept on board the ship. For the purposes of this paragraph, vent system means:
1. The tank outlet and the piping to the PRV;
2. The PRV; and
3. The piping from the PRVs to the location of discharge to the atmosphere, including any interconnections and piping that joins other tanks.
Read More: IGC CODE 1983