8.4 Sizing of pressure relieving system
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Resolutions - Maritime Safety Committee - Resolution MSC.370(93) – Amendments to The International Code for The Construction and Equipment of Ships Carrying Liquefied Gases In Bulk (IGC Code) – (Adopted on 22 May 2014) - Annex - Amendments to The International Code for The Construction and Equipment of Ships Carrying Liquefied Gases In Bulk (IGC Code) - Chapter 8 – Vent Systems for Cargo Containment - 8.4 Sizing of pressure relieving system

8.4 Sizing of pressure relieving system

  8.4.1 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% rise in cargo tank pressure above the MARVS:

  8.4.1.1 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

  8.4.1.2 Vapours generated under fire exposure computed using the following formula:

  Q = FGA0.82 (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 as follows:

  • - 1 for tanks without insulation located on deck;

  • - 0.5 for tanks above the deck, when insulation is approved by the Administration. Approval will be based on the use of a fireproofing material, the thermal conductance

  • - 0.5 for uninsulated independent tanks installed in holds;

  • – 0.2 for insulated independent tanks in holds (or uninsulated independent tanks in insulated holds);

  • - 0.1 for insulated independent tanks in inerted holds (or uninsulated independent tanks in inerted, insulated holds);

  • - 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 according to formula:

  • with:

  • T = temperature in degrees Kelvin at relieving conditions, i.e. 120% of the pressure at which the pressure relief valve is set;
  • L = latent heat of the material being vaporized at relieving conditions, in kJ/kg;
  • D = a constant based on relation of specific heats k and is calculated as follows:

    • where:

    • k = ratio of specific heats at relieving conditions, and the value of which is between 1 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 shall be used; and
  • 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), as defined in 1.2.14, for different tank types, as shown in figure 8.1.

  8.4.1.3 The required mass flow of air at relieving conditions is given by the formula:

  Mair = Q ρair (kg/s),

 where:

density of air(ρair ) = 1.293 kg/m3 (air at 273.15 K, 0.1013 MPa).

  8.4.2 Sizing of vent pipe system

 Pressure losses upstream and downstream of the PRVs shall be taken into account when determining their size to ensure the flow capacity required by 8.4.1.

  8.4.3 Upstream pressure losses

  8.4.3.1 The pressure drop in the vent line from the tank to the PRV inlet shall not exceed 3% of the valve set pressure at the calculated flow rate, in accordance with 8.4.1.

  8.4.3.2 Pilot-operated PRVs shall be unaffected by inlet pipe pressure losses when the pilot senses directly from the tank dome.

  8.4.3.3 Pressure losses in remotely sensed pilot lines shall be considered for flowing type pilots.

  8.4.4 Downstream pressure losses

  8.4.4.1 Where common vent headers and vent masts are fitted, calculations shall include flow from all attached PRVs.

  8.4.4.2 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 interconnections that join other tanks, shall not exceed the following values:

  • .1 for unbalanced PRVs: 10% of MARVS;

  • .2 for balanced PRVs: 30% of MARVS; and

  • .3 for pilot operated PRVs: 50% of MARVS.

 Alternative values provided by the PRV manufacturer may be accepted.

  8.4.5 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.

Parent topic: Chapter 8 – Vent Systems for Cargo Containment
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.