CHAPTER 6 FUEL CONTAINMENT SYSTEM
6.7 Regulations for Pressure Relief System
6.7.1 General
6.7.1.1 All fuel storage tanks shall be provided with a pressure relief system appropriate to the design of the fuel containment system and the fuel being carried. Fuel storage hold spaces, interbarrier spaces, tank connection spaces and tank cofferdams, 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 6.9 of the Guidelines shall be independent of the pressure relief systems.
6.7.1.2 Fuel storage tanks which may be subject to external pressures above their design pressure shall be fitted with vacuum protection systems.
6.7.2 Pressure relief systems for liquefied gas fuel tanks
6.7.2.1 If fuel release into the vacuum space of a vacuum insulated tank cannot be excluded, the vacuum space shall be protected by a pressure relief device which shall be connected to a vent system if the tanks are located below deck. On open deck a direct release into the atmosphere may be accepted by the Administration for tanks not exceeding the size of a 40 ft container if the released gas cannot enter safe areas.
6.7.2.2 Liquefied gas fuel tanks shall be fitted with a minimum of 2 pressure relief valves (PRVs) allowing for disconnection of one PRV in case of malfunction or leakage.
6.7.2.3 Interbarrier spaces shall be provided with pressure relief devices.(12) For membrane systems, the designer shall demonstrate adequate sizing of interbarrier space PRVs.
6.7.2.4 The setting of the PRVs shall not be higher than the vapour pressure that has been used in the design of the tank. 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.
6.7.2.5 The following temperature regulations apply to PRVs fitted to pressure relief systems:
.1 PRVs on fuel tanks with a design temperature below 0℃ 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; and
.4 sensing and exhaust lines on pilot operated relief valves shall be of suitably robust construction to prevent damage.
6.7.2.6 In the event of a failure of a fuel tank PRV a safe means of emergency isolation shall be available.
.1 procedures shall be provided and included in the operation manual (refer to chapter 18 of the Guidelines);
.2 the procedures shall allow only one of the installed PRVs for the liquefied gas fuel tanks to be isolated, physical interlocks shall be included to this effect; and
.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 at the PRV.
Note 12:
Refer to IACS Unified Interpretation GC9 entitled Guidance for sizing pressure relief systems for interbarrier spaces, 1988
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6.7.2.7 Each pressure relief valve installed on a liquefied gas fuel tank shall be connected to a venting system, which shall be:
.1 so constructed that the discharge will be unimpeded and normally be directed vertica lly upwards at the exit;
.2 arranged to minimize the possibility of water or snow entering the vent system; and
.3 arranged such that the height of vent exits shall normally not be less than B/3 or 6 m, whichever is the greater, above the weather deck and 6 m above working areas and walkways. However, vent mast height could be limited to lower value according to special consideration by the Administration.
6.7.2.8 The outlet from the pressure relief valves shall normally be located at least 10 m from the ne arest:
.1 air intake, air outlet or opening to accommodation, service and control spaces, or other non -hazardous area; and
.2 exhaust outlet from machinery installations.
CR Interpretation of 6.7.2.8
Proposals for alternative pressure relief valve outlet positions will be considered on a case-by-case basis and subject to the submission of an appropriate risk assessment. Such a risk assessment could be based on a ship specific gas dispersion analysis.
6.7.2.9 All other fuel gas vent outlets shall also be arranged in accordance with 6.7.2.7 and 6.7.2.8 above.
Means shall be provided to prevent liquid overflow from gas vent outlets, due to hydrostatic pressure from spaces to which they are connected.
6.7.2.10 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.
6.7.2.11 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.
6.7.2.12 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.
6.7.2.13 PRVs shall be connected to the highest part of the fuel tank. PRVs shall be positioned on the fuel tank so that they will remain in the vapour phase at the filling limit (FL) as given in 6.8 of the Guidelines, under conditions of 15° list and 0.015L trim, where L is defined in 2.2.25 of the Guidelines.
6.7.3 Sizing of pressure relieving system
6.7.3.1 Sizing of pressure relief valves
6.7.3.1.1 PRVs shall have a combined relieving capacity for each liquefied gas fuel tank to discharge the greater of the following, with not more than a 20% rise in liquefied gas fuel tank pressure above the MARVS:
.1 the maximum capacity of the liquefied gas fuel tank inerting system if the maximum attainable working pressure of the liquefied gas fuel tank inerting system exceeds the MARVS of the liquefied gas fuel tanks; or
.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 liquefied gas fuel 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 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); and
F = 0.1 for 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:
𝐺 =12.4 𝐿𝐷 √𝑍𝑇
𝑀𝐺
where:
T = temperature in 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:
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 liquefied gas fuel to be carried is to be determined and the highest value shall be used for PRV sizing.
A = external surface area of the tank (m2), as for different tank types, as shown in Figure 6-6 of the Guidelines.
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Figure 6-6
Cylindrical tanks with spherically dished, hemispherical or semi-ellipsoidal heads or spherical tanks
L excluded Prismatic tanks
≤ Lmin/10
excluded
Bilobe tanks
excluded
≤D/10
D
excluded Horizontal cylindrical
tanks arrangement
≤ D/10
≤ D/10 D
IACS Interpretation of 6.7.3.1.1.2 and Figure 6-6 For prismatic tanks:
.1 Lmin, for non-tapered tanks, is the smaller of the horizontal dimensions of the flat bottom of the tank. For tapered tanks, as would be used for the forward tank, Lmin is the smaller of the length and the average width.
.2 For prismatic tanks whose distance between the flat bottom of the tank and bottom of the hold space is equal to or less than Lmin/10:
A = external surface area – flat bottom surface area
.3 For prismatic tanks whose distance between the flat bottom of the tank and bottom of the hold space is greater than Lmin/10:
A = external surface area
6.7.3.1.2 For vacuum insulated tanks in fuel storage hold spaces and for tanks in fuel storage hold spaces separated from potential fire loads by coffer dams or surrounded by ship spaces with no fire load the following applies:
If the pressure relief valves have to be sized for fire loads the fire factors according may be reduced to the following values:
F = 0.5 to F = 0.25 F = 0.2 to F = 0.1 The minimum fire factor is F= 0.1
6.7.3.1.3 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).
6.7.3.2 Sizing of vent pipe system
6.7.3.2.1 Pressure losses upstream and downstream of the PRVs, shall be taken into account when determining their size to ensure the flow capacity required by 6.7.3.1 above.
6.7.3.2.2 Upstream pressure losses
.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 6.7.3.1 above;
.2 pilot-operated PRVs shall be unaffected by inlet pipe pressure losses when the pilot senses directly from the tank dome; and
.3 pressure losses in remotely sensed pilot lines shall be considered for flowing type p ilots.
6.7.3.2.3 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 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.
6.7.3.2.4 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.
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