Section 1 Vent Systems for Cargo Containment
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, July 2022 - Part 11 Production, Storage and Offloading of Liquefied Gases in Bulk - Chapter 8 Vent Systems for Cargo Containment - Section 1 Vent Systems for Cargo Containment

Section 1 Vent Systems for Cargo Containment

Parent topic: Chapter 8 Vent Systems for Cargo Containment

1.1 General

1.1.1 All cargo tanks shall be provided with a pressure relief system appropriate to the design of the cargo containment system and the cargo being carried. Hold space and interbarrier spaces, which may be subject to pressures beyond their design capabilities, shall also be provided with a suitable pressure relief system. Pressure control systems specified in Pt 11, Ch 7 Cargo Pressure/Temperature Control shall be independent of the pressure relief systems.

1.2 Pressure relief systems

1.2.1 Cargo tanks, including deck tanks, are to 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.

1.2.2 Interbarrier spaces shall be provided with pressure relief devices. Reference is made to IACS Unified Interpretation GC9 Guidance for sizing pressure relief systems for interbarrier spaces 1988. For membrane systems, the designer shall demonstrate adequate sizing of interbarrier space PRVs.

1.2.3 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 per cent of the total relieving capacity may be set at a pressure up to 5 per cent above MARVS to allow sequential lifting, minimising unnecessary release of vapour.

1.2.4 The following temperature requirements apply to PRVs fitted to pressure relief systems:
  1. PRVs on cargo tanks with a cargo design temperature below 0°C 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 fail-safe operation of the PRV is not compromised.
  4. Sensing and exhaust lines on pilot operated relief valves shall be of suitably robust construction to prevent damage.

1.2.5  Valve testing

PRVs shall be tested in accordance with a Recognised Standard or equivalent national standards. Reference is made to: ISO 21013-1 2008 – Cryogenic vessels – Pressure-relief accessories for cryogenic service – Part 1: Reclosable pressure-relief valves; and ISO 4126-1; 2004 Safety devices for protection against excessive pressure – Part 1 and part 4: Safety valves.
  1. 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.
    4. pressure containing parts are to be pressure tested to at least 1,5 times the design pressure.
  2. Each PRV shall be tested to ensure that:
    1. it opens at the prescribed pressure setting, with an allowance not exceeding ±10 per cent for 0 to 0,15 MPa, ±6 per cent for 0,15 to 0,3 MPa, ±3 per cent for 0,3 MPa and above.
    2. seat tightness is acceptable.
    3. pressure containing parts are to withstand at least 1,5 times the design pressure.

1.2.6  As soon as practicable prior to proceeding on gas trials, pressure relief valves are to be tested and installed in accordance with the manufacturer’s recommended procedures to the Surveyor’s satisfaction. Where valves are stored prior to installation on board, the storage arrangements are also to be in accordance with the manufacturer’s recommended procedures.

1.2.7 PRVs shall be set and sealed by the Administration or recognised organisation acting on its behalf and a record of this action, including the valves’ set pressure, shall be retained onboard the ship unit.

1.2.8 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.

1.2.9 Changing the set pressure under the provisions of Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.8, and the corresponding resetting of the alarms referred to in Pt 11, Ch 13, 1.4 Pressure monitoring 1.4.2, shall be carried out under the supervision of the Master in accordance with approved procedures and as specified in the operating manual of the ship unit. Changes in set pressure shall be recorded in the ship unit’s log and a sign shall be posted in the cargo control room if provided, in the main control area if separate from the cargo control room, and at each relief valve, stating the set pressure.

1.2.10 In the event of a failure of a cargo tank PRV a safe means of emergency isolation shall be available.
  1. Procedures are to be provided and included in the cargo operations manual (see Pt 11, Ch 18, 1.2 Cargo operations manuals).
  2. The procedures shall allow only one of the cargo tank’s 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 unit’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.
1.2.11 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 minimise 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.
  4. 6 m above working areas and walkways.

1.2.12 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 non-hazardous areas.

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 non-hazardous areas.

1.2.13 All other cargo vent outlets not dealt with in other chapters shall be arranged in accordance with Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.11 and Pt 11, Ch 8, 1.2 Pressure relief systems 1.2.12. Means shall be provided to prevent liquid overflow from vent mast outlets, due to hydrostatic pressure from spaces to which they are connected.

1.2.14 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.

1.2.15 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.

1.2.16 Suitable protection screens of not more than 13 mm square mesh shall be fitted on vent outlets to prevent the ingress of foreign objects without adversely affecting the flow. Protective screens when storing pentane are also to comply with Pt 11, Ch 17, 1.2 Flame screens on vent outlets.

1.2.17 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 motions of the ship unit.

1.2.18 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 Pt 11, Ch 15 Filling Limits for Cargo Tanks, under conditions of 15° list and 0,015L trim, where L is defined in Pt 11, Ch 1, 1.3 Definitions.

1.2.19 The adequacy of the vent system fitted on tanks loaded in accordance with Pt 11, Ch 15, 1.5 Maximum loading limit 1.5.2, is to be demonstrated using the Guidelines for the Evaluation of the Adequacy of Type C Tank Vent Systems, IMO Resolution A.829(19). A relevant certificate shall be permanently kept onboard the ship unit. For the purposes of this paragraph, vent system means:
  1. the tank outlet and the piping to the PRV.
  2. the PRV.
  3. the piping from the PRVs to the location of discharge to the atmosphere, including any interconnections and piping that joins other tanks.

1.3 Vacuum protection systems

1.3.1 Cargo tanks not designed to withstand a maximum external pressure differential 0,025 MPa, or tanks that cannot withstand the maximum external pressure differential that can be attained at maximum discharge rates with no vapour return into the cargo tanks, or by operation of a cargo refrigeration system, or by thermal oxidation, shall be fitted with:
  1. two independent pressure switches to sequentially alarm and subsequently stop all suction of cargo liquid or vapour from the cargo tank and refrigeration equipment, if fitted, by suitable means at a pressure sufficiently below the maximum external designed pressure differential of the cargo tank; or
  2. vacuum relief valves with a gas flow capacity at least equal to the maximum cargo discharge rate per cargo tank, set to open at a pressure sufficiently below the external design differential pressure of the cargo tank.

1.3.2 Subject to the requirements of Pt 11, Ch 17 Special Requirements, the vacuum relief valves shall admit an inert gas, cargo vapour or air to the cargo tank and shall be arranged to minimise the possibility of the entrance of water or snow see also Pt 11, Ch 8, 1.3 Vacuum protection systems 1.3.3. If cargo vapour is admitted it shall be from a source other than the cargo vapour lines.

1.3.3  Vacuum relief valves are not to admit air to the cargo tanks except where satisfactory controls, low pressure alarms and automatic devices for stopping cargo pumps and compressors, etc. are fitted and adjusted such that the pressure in the tanks cannot fall below a predetermined minimum safe level. Details are to be submitted for consideration.

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

1.4 Sizing of pressure relieving system

1.4.1  Sizing of pressure relief valves

PRVs shall have a combined relieving capacity for each cargo tank to discharge the greater of the following, with not more than a 20 per cent rise in cargo tank pressure above the MARVS:

  1. the maximum capacity of the cargo tank inerting system if the maximum attainable working pressure of the cargo tank inerting system exceeds the MARVS of the cargo tanks; or
  2. vapours generated under fire exposure computed using the following formula:
    Q = FGA 0,82 (m3/s)

    where

    Q = minimum required rate of discharge of air at standard conditions of 273,15 Kelvin (K) and 0,1013 MPa
    F = fire exposure factor for different cargo types
    F = 1,0 for tanks without insulation located on deck
    F = 0,5 for tanks above the deck when insulation is approved by LR. (Approval will be based on the use of a fireproofing material, the thermal conductance of insulation, and its stability under fire exposure)
    F = 0,5 for uninsulated independent tanks installed in holds
    F = 0,2 for insulated independent tanks in holds (or uninsulated independent tanks in insulated holds)
    F = 0,1 for insulated independent tanks in inerted holds (or uninsulated independent tanks in inerted, insulated holds)
    F = 0,1 for membrane and semi membrane tanks

    For independent tanks partly protruding through the weather decks, the fire exposure factor shall be determined on the basis of the surface areas above and below deck

    G = gas factor
    G =

    with

    T = temperature in Kelvin at relieving conditions, i.e. 120 per cent of the pressure at which the pressure relief valve is set
    L = latent heat of the material being vaporised at relieving conditions, in kJ/kg
    D = a constant based on relation of specific heats k and is calculated as follows
    D =
    k = ratio of specific heats at relieving conditions, and the value of which is between 1,0 and 2,2. If k is not known, D = 0,606 shall be used.
    Z = compressibility factor of the gas at relieving conditions; if not known, Z = 1,0 shall be used.
    M = molecular mass of the product.

    The gas factor of each cargo to be carried shall be determined and the highest value shall be used for PRV sizing.

    A = external surface area of the tank (m2) for different tank types, as shown in Figure 8.1.1 Horizontal cylindrical tanks arrangement:
    For prismatic tanks Lmin, and the associated external surface area of the tank, A, are to be taken as follows:
    • Lmin, for non-tapered tanks, is the lesser of the longitudinal or transverse horizontal dimensions of the bottom of the tank. The bottom of the tank is to be regarded as the flat portion only not including the lower chamfered sides.
    • For tapered tanks, as would be used for the forward tank, Lmin is the lesser of the length and the average width of the bottom of the tank.
    • For prismatic tanks whose distance between the bottom of the tank and bottom of the hold space is equal to or less than Lmin/10:

      A = external surface area minus the bottom surface area of the tank.

    • For prismatic tanks whose distance between the bottom of the tank and bottom of the hold space is greater than Lmin/10:

      A = external surface area.

    Figure 8.1.1 Horizontal cylindrical tanks arrangement

  3. The required mass flow of air at relieving conditions is given by:
    Mair = Q ρair (kg/s)

    where:

    Density of air (ρair) = 1,293 kg/m3 (air at 273,15 K, 0,1013 MPa)

1.4.2  Sizing of vent pipe system

As in Pt 11, Ch 5, 1.2 System requirements 1.2.2.(d) and Pt 11, Ch 5, 2.3 Cargo transfer arrangements 2.3.4 the relief system is to be designed in accordance with API 521 Guide for Pressure-relieving and Depressuring Systems: Petroleum petrochemical and natural gas industries – Pressure-relieving and depressuring systems, taking into account the following.
  1. Pressure losses upstream and downstream of the PRVs, shall be taken into account when determining their size to ensure the flow capacity required by Pt 11, Ch 8, 1.4 Sizing of pressure relieving system 1.4.1.

1.4.3  Upstream pressure losses

The pressure drop in the vent line from the tank to the PRV inlet shall not exceed 3 per cent of the valve set pressure at the calculated flow rate, in accordance with Pt 11, Ch 8, 1.4 Sizing of pressure relieving system 1.4.1.
  1. Pilot-operated PRVs shall be unaffected by inlet pipe pressure losses when the pilot senses directly from the tank dome.
  2. Pressure losses in remotely sensed pilot lines shall be considered for flowing type pilots.
1.4.4  Downstream pressure losses
  1. Where common vent headers and vent masts are fitted, calculations shall include flow from all attached PRVs.
  2. The built-up back pressure in the vent piping from the PRV outlet to the location of discharge to the atmosphere, and including any vent pipe inter-connections that join other tanks, shall not exceed the following values:
    • For unbalanced PRVs: 10 per cent of MARVS;
    • For balanced PRVs: 30 per cent of MARVS;
    • For pilot operated PRVs: 50 per cent of MARVS.

    Alternative values provided by the PRV manufacturer may be accepted.

1.4.5 To ensure stable PRV operation, the blow-down shall not be less than the sum of the inlet pressure loss and 0,02 MARVS at the rated capacity.

Copyright 2022 Clasifications Register Group Limited, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasifications Register Group Limited, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasifications Register'. Clasifications Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasifications Register entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract.