8.5 Size of valves

 Pressure relief valves should 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:

• .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:

  
  where:

  Q	=	minimum required rate of discharge of air at standard conditions of 273 K and 1.013 bar.
  F	=	fire exposure factor for different cargo tank types:
  F	=	1.0 for tanks without insulation located on deck;
  F	=	0.5 for tanks above the deck when insulation is approved by the Administration. (Approval will be based on the use of an approved 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 open deck, the fire exposure factor should be determined on the basis of the surface areas above and below deck.
  G	=	gas factor with:
  T	=	temperature in kelvins (K) 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	=	constant based on relation of specific heats k, shown in Table 8.2; if k is not known, D = 0.606 should be used. The constant D may also be calculated by the following formula:
  Z	=	compressibility factor of the gas at relieving conditions; if not known, Z = 1.0 should be used.
  M	=	molecular mass of the product
  A	=	external surface area of the tank (m2) for different tank types: for body-of-revolution type tanks:
  A	=	external surface area; for other than body-of-revolution type tanks:
  A	=	external surface area less the projected bottom surface area; for tanks consisting of an array of pressure vessel tanks:
  	=	insulation on the ship's structure:
  A	=	external surface area of the hold less its projected area; insulation on the tank structure:
  A	=	external surface area of the array of pressure vessels excluding insulation, less the projected bottom area as shown in Figure 8.1.

Figure 8.1