Section 8 Cargo Tank and Cargo Hold Vent Systems

801. General

All cargo tanks are to be provided with a pressure relief system appropriate to the design of the cargo containment system and the cargo being carried. Hold spaces and cargo piping which may be subject to pressures beyond their design capabilities are also to be provided with a suitable pressure relief system. The pressure relief system are to be connected to a vent piping system so designed as to minimize the possibility of cargo vapour accumulating on the decks, or entering accom- modation spaces, service spaces, control stations and machinery spaces, or other spaces where it may create a dangerous condition. Pressure control systems specified by Sec 7 are to be in- dependent of the pressure relief valves.

802.

Pressure relief systems

1. Each cargo tank is to be fitted with at least two pressure relief valves of approximately equal ca- pacity, suitably designed and constructed for the prescribed service.

2. The setting of the pressure relief valves is not to be higher than the design pressure of the cargo tanks considering the tolerance of pressure relief valves.

3. Pressure relief valves are to be connected to the highest part of the cargo tank above deck level.

Pressure relief valves on cargo tanks with a design temperature below 0°C are to be arranged to

prevent their becoming inoperative due to ice formation when they are closed. Due

consideration

are to be given to the construction and arrangement of pressure relief valves on cargo tanks subject to low ambient temperatures.

4. Pressure relief valves are to be prototype tested to ensure that

the valves have the capacity

required. Each valve are to be tested to ensure that it opens at the prescribed pressure setting with

an allowance not exceeding ± 3 %. Pressure relief valves are to be

set and sealed by a competent

authority acceptable to the Society and a record of this action, including the values of set pressure, are to be retained aboard the ship.

5. Stop valves or other means of blanking off pipes between tanks and pressure relief valves to facili- tate maintenance are not to be fitted unless all the following arrangements are provided:

(1) suitable arrangements to prevent more than one pressure relief valve being out of service at the same time;

(2) a device which automatically and in a clearly visible way indicates which one of the pressure relief valves is out of service; and

(3) pressure relief valve capacities such that if one have the combined relieving capacity required by

valve is out of service the remaining valves

805. However, this capacity may be provided

by the combined capacity of all valves, if a suitably maintained spare valve is carried on board.

6. Each pressure relief valve installed on a cargo tank are to be connected to a venting system which are to be so constructed that the discharge of gas will be unimpeded and directed vertically up- wards at the exit and so arranged as to minimize the possibility of water or snow entering the vent system. The height of vent exits is not to be less than B/3 or 6 m, whichever is the greater, above the weather deck and 6 m above the working area, the fore and aft gangway, deck storage tanks.

7. Cargo tank pressure relief valve vent exits are to be arranged at a distance at least equal to B or 25 m, whichever is less, from the nearest air intake or opening to accommodation spaces, service spaces and control stations, or other gas-safe spaces. For ships less than 90 m in length, smaller distances may be permitted by the Society. All other vent exits connected to the cargo containment system are to be arranged at a distance of at least 10 m from the nearest air intake or opening to accommodation spaces, service spaces and control stations, or other gas-safe spaces.

8. All other cargo vent exits not dealt with in other chapters are to be arranged in accordance with

Pars 6 and 7.

9. In the vent piping system, means for draining condensate from places where it may accumulate are

to be provided. The pressure relief valves and piping are to be so arranged der no circumstances accumulate in or near the pressure relief valves.

10. Suitable protection screens made of wire gauze of 13 mm × 13 mm mesh

that condensate can un-

or below are to be fit-

ted on vent outlets to prevent the ingress of foreign objects.

11. All vent piping is to be so designed and arranged that it will not be damaged by temperature variations to which it may be exposed, or by the ship’'s motions.

12. The back pressure in the vent lines from the pressure relief valves are to be taken into account in determining the flow capacity required by 805. The relevant documents including calculation sheets are to be submitted to the Society.

803.

Cargo hold vacuum protection systems

1. Cargo holds and capable protection.

2. Cargo holds

designed to withstand a maximum external pressure differential exceeding 0.025 MPa of withstanding the maximum external pressure differential need no vacuum relief

not designed to withstand a maximum external pressure differential are to be fitted

with the following. And calculation for vacuum relief valve capacity is to be submitted for the Society review and approval.

(1) Two independent pressure switches to sequentially alarm a low pressure situation, or

(2) Vacuum relief valves with a sufficient gas flow capacity, set to open at a pressure sufficiently below the external design differential pressure of the cargo hold; or

(3) Other vacuum relief systems acceptable to the Society.

3. The vacuum relief valves for a cargo hold are to admit an inert gas or air to the cargo hold and are to be arranged to minimize the possibility of the entrance of water or snow.

4. The vacuum protection system is to be capable of being tested to ensure that it operates at the prescribed pressure.

804.

Cargo hold overpressure protection systems

1. Cargo holds are to be protected from overpressure due to high inert gas pressure, accidental release of cargo in hold due to containment failure, variation in ambient pressure and temperature, etc.

For cargo tank failure, it is to be assumed that one cargo tank has failed in any cargo hold and the total volume of cargo from that tank is released in the cargo hold. Any deviation from the above will be of special consideration by this Society.

2. In general, the setting of the pressure relief valves for cargo holds space is not to be higher than

0.025 MPa.

805.

Size of valves

1. Pressure relief valves are to have a combined relieving capacity for each cargo tank to discharge cargo with not more than a 3% rise in cargo tank pressure above the MARVS:

2. Valves for cargo holds are to be sized to limit structure. Relief valve capacity calculation needs to requirements.

pressure rise within the design limit of the comply with the recognized standards, i.e API