This article tells about Cargo Temperature Control, Pressure and also Cargo Ventilation System for Ships Carrying Liquefied Gases.
Pressure and Temperature Control
Unless the entire cargo system is designed to withstand the full gauge vapour pressure of the cargo under conditions of the upper ambient design temperatures, maintenance of the cargo tank pressure below the MARVS is to be provided by one or more of the following means, except as otherwise provided in this article.
- A system which regulates the pressure in the cargo tanks by the use of mechanical refrigeration.
- A system whereby the boil-off vapours are utilized as fuel for shipboard use and/or waste heat system subject to the provisions of “LNG Cargo as fuel”Use of Cargo as Fuel on Gas Tankers. This system may be used at all times, including while in port and while maneuvering, provided that a means of disposing of excess energy is provided, such as a steam dump system, that is satisfactory to the Society.
- A system allowing the product to warm up and increase in pressure. The insulation or cargo tank design pressure or both is to be adequate to provide for a suitable margin for the operating time and temperatures involved. The system is to be acceptable to the Society in each case.
- Other systems acceptable to the Society.
- In addition to the above means, the Society may permit certain cargoes to be controlled by venting cargo vapours to the atmosphere at sea. This may also be permitted in port with the permission of the Port Administration.
Note: For ships trading to territorial waters of the United States of America the relevant requirements for cargo temperature/pressure control of the US Coast Guard, given in 46 CFR (Code of Federal Register) Part 154, § 154.702/703 should be observed.
The systems required above are to be constructed, fitted and tested to the satisfaction of the Society. Materials used in their construction are to be suitable for use with the cargoes to be carried. For normal service, the upper ambient design temperatures shall be:
- Sea: 32 °C;
- Air: 45 °C.
For service in especially hot or cold zones these design temperatures are to be increased or reduced as appropriate by the Society.
For certain highly dangerous cargoes specified in “Special requirements”Special Requirements for LNG and LPG gas carriers, the cargo containment system is to be capable of withstanding the full vapour pressure of the cargo under conditions of the upper ambient design temperatures irrespective of any system provided for dealing with boil-off gas.
A refrigeration system shall consist of one or more units capable of maintaining the required cargo pressure/temperature under conditions of the upper ambient design temperatures. Unless an alternative means of controlling the cargo pressure/temperature is provided to the satisfaction of the Society, a standby unit (or units) affording spare capacity at least equal to the largest required single unit is to be provided.
A “stand-by unit” shall consist of a compressor with its driving motor, control system and any necessary fittings to permit operation independently of the normal service units. A stand-by heat exchanger is to be provided unless the normal heat exchanger for the unit has an excess capacity of at least 25 % of the largest required capacity. Separate piping systems are not required.
Where two or more refrigerated cargoes which may react chemically in a dangerous manner are carried simultaneously, special consideration is to be given to the refrigeration systems to avoid the possibility of mixing cargoes. For the carriage of such cargoes separate refrigeration systems each complete with a stand-by unit as specified above are to be provided for each cargo. However, where cooling is provided by an indirect or combined system and leakage in the heat exchangers cannot cause mixing of the cargoes under any envisaged condition, separate refrigeration units need not be fitted.
Where two or more refrigerated cargoes are not mutually soluble under the conditions of carriage, so that their vapour pressures would be additive on mixing, special consideration is to be given to the refrigeration systems to avoid the possibility of mixing cargoes.
Where cooling water is required in refrigeration systems, an adequate supply is to be provided by a pump or pumps used exclusively for this purpose. This pump or these pumps are to have at least two sea suction lines, where practicable leading from sea chests one port and one starboard. A spare pump of adequate capacity is to be provided which may be a pump used for other services so long as its use for cooling would not interfere with any other essential service.
The refrigeration system may be arranged in one of the following ways:
- a direct system where evaporated cargo is compressed, condensed and returned to cargo tanks. For certain cargoes specified in “Special requirements”Special Requirements for LNG and LPG gas carriers this system shall not be used;
- an indirect system where cargo or evaporated cargo is cooled or condensed by refrigerant without being compressed;
- a combined system where evaporated cargo is compressed and condensed in a cargo/ refrigerant heat exchanger and returned to the cargo tanks. For certain cargoes specified in “Special requirements”Special Requirements for LNG and LPG gas carriers this system shall not be used (see also “Refrigeration system”Special Requirements for LNG and LPG gas carriers).
All primary and secondary refrigerants must be compatible with each other and with the cargo with which they may come into contact. The heat exchange may take place either remotely from the cargo tank or by cooling coils fitted inside or outside the cargo tank.
Cargo Vent Systems
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, interbarrier 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 is to be connected to a vent piping system designed so as to minimize the possibility of cargo vapour accumulating about the decks, or entering accommodation spaces, service spaces and control stations, and machinery spaces, or other spaces where it may create a dangerous condition. Pressure control systems specified by “Pressure and Temperature Control” are to be independent of the pressure relief valves.
Pressure relief systems
Each cargo tank with a volume exceeding 20 m3 is to be fitted with at least two pressure relief valves of approximately equal capacity, suitably designed and constructed for the prescribed service. For cargo tanks with a volume not exceeding 20 m3, a single relief valve may be fitted.
If cargo holds with independent tanks may be completely closed, overpressure/vacuum relief valves are to be provided for in order to avoid pressure variations of more than 0,15 bar above and below atmospheric pressure.
Interbarrier spaces are to be provided with pressure relief devices complying with recognized standards. The combined relieving capacity of the pressure relief devices for interbarrier spaces surrounding type A independent cargo tanks where the insulation is fitted to the cargo tanks may be determined by the following formula:
- Qsa – minimum required discharge rate of air at standard conditions of 273 K and 1,013 bar;
- Ac – design crack opening area [m2]
- δ – max. crack opening width [m];
- δ – 0,2 ∙ t [m];
- t – thickness of tank bottom plating [m];
– design crack length [m] equal to the diagonal of the largest plate panel of the tank bottom;
- h – max. liquid height above tank bottom plus 10 ∙ MARVS [m];
- ρ – densitiy of product liquid phase [kN/m3] at the set pressure of the interbarrier space relief device;
- ρv – density of product vapour phase [kN/m3] at the set pressure of the interbarrier space relief device and a temperature of 273 K.
In general, the setting of the pressure relief valves shall not be higher than the vapour pressure which has been used in the design of the tank. However, where two or more pressure relief valves are fitted, valves comprising not more than 50 % of the total relieving capacity may be set at a pressure up to five percent above MARVS.
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 is to be given to the construction and arrangement of pressure relief valves on cargo tanks subject to low ambient temperatures.
Valves are to be constructed of materials with a melting point above 925 °C. Consideration of lower melting point materials for internal parts and seals shall be given if their use provides significant improvement to the general operation of the valve.
Pressure relief valves are to be prototype tested to ensure that the valves have the capacity required. Each valve is to be tested to ensure that it opens at the prescribed pressure setting with an allowance not exceeding:
- ± 10 % for 0 to 1,5 bar;
- ± 6 % for 1,5 to 3,0 bar;
- ± 3 % for 3,0 bar and above.
Pressure relief valves are to be set and sealed by the Society and a record of this action, including the values of set pressure, is to be retained aboard the ship.
Prototype testing includes testing at design temperature.
In the case of cargo tanks permitted to have more than one relief valve setting, this may be accomplished by:
- installing two or more properly set and sealed valves and providing means as necessary for isolating the valves not in use from the cargo tank;
- and installing relief valves whose settings may be changed by the insertion of previously approved spacer pieces or alternative springs or by other similar means not requiring pressure testing to verify the new set pressure. All other valve adjustments are to be sealed.
The changing of the set pressure under the provisions in list above and the corresponding resetting of the alarms referred to in “Pressure gauges”Cargo Tank Instrumentation on Gas Tankers, shall be carried out under the supervision of the master in accordance with procedures approved by BKI and specified in the ship’s operating manual. Changes in set pressures are to be recorded in the ship’s log and a sign posted in the cargo control room, if provided, and at each relief valve, stating the set pressure.
Stop valves or other means of blanking off pipes between tanks and pressure relief valves to facilitate maintenance are not to be fitted unless all the following arrangements are provided:
- suitable arrangements to prevent more than one pressure relief valve being out of service at the same time;
- a device which automatically and in a clearly visible way indicates which one of the pressure relief valves is out of service;
- and pressure relief valve capacities are such that if one valve is out of service the remaining valves are to have the combined relieving capacity required by “Size of valves”. However, this capacity may be provided by the combined capacity of all valves if a suitably maintained spare valve is carried on board.
Each pressure relief valve installed on a cargo tank is to be connected to a venting system, which is to be so constructed that the discharge of gas will be unimpeded and directed vertically upwards 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 to be not less than B/3 or 6 m whichever is greater, above the weather deck and 6 m above the working area and the fore and aft gangway, deck storage tanks and cargo liquid lines.
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, service and control station spaces, 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.
All other cargo vent exits not dealt with in other Sections are to be arranged in accordance with above.
If cargoes which react in a hazardous manner with each other are carried simultaneously, a separate pressure relief system is to be fitted for each cargo carried. In the vent piping system, means for draining liquid from places where it may accumulate are to be provided. The pressure relief valves and piping is to be so arranged that liquid can under no circumstances accumulate in or near the pressure relief valves.
Suitable protection screens are to be fitted on vent outlets to prevent the ingress of foreign objects. 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. 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 “Size of valves”.
The pressure drop in the vent line from the tank to the pressure relief valve inlet shall not exceed 3 percent of the valve set pressure. For unbalanced pressure relief valves the back pressure in the discharge line shall not exceed 10 percent of the gauge pressure at the relief valve inlet with the vent lines under fire exposure as referred to in this formula.
Pressure relief valves are to be positioned on the cargo tank so that they will remain in the vapour phase under conditions of 15° list and 0,015 Lc trim, where Lc is as defined in Sec. 1, C.23, at the maximum allowable filling limit (FL).
The adequacy of the vent system fitted on tanks loaded in accordance with “Filling Limits”Filling Limits for Cargo Tanks on Liquefied Gas Tankers is to be demonstrated using Guidelines developed by the Organization. A relevant certificate shall be permanently kept on board the vessel. For the purposes of this paragraph, vent system means:
- the tank outlet and the piping to the pressure relief valve;
- the pressure relief valve;
- the piping from the pressure relief valve to the location of discharge to the atmosphere and including any interconnections and piping which joins other tanks.
Additional pressure relieving system for liquid level control
Where required by “Filling Limits”Filling Limits for Cargo Tanks on Liquefied Gas Tankers, an additional pressure relieving system to prevent the tank from becoming liquid full at any time during relief under the fire conditions referred to in “Size of valves” is to be fitted to each tank. This pressure relieving system is to consist of:
- one or more relief valves set at a pressure corresponding to the gauge vapour pressure of the cargo at the reference temperature defined in “Filling Limits”Filling Limits for Cargo Tanks on Liquefied Gas Tankers;
- and an over-ride arrangement, whenever necessary, to prevent its normal operation. This arrangement shall include fusible elements designed to melt at temperatures between 98 °C and 104 °C and to cause relief valves specified above to become operable. The fusible elements shall be located, in particular, in the vicinity of the relief valves. The system shall become operable upon loss of system power if provided. The over-ride arrangement shall not be dependent on any source of ship’s power.
The total relieving capacity of the additional pressure relieving system at the pressure mentioned above is not to be less than:
- Q′ – minimum required equivalent rate of discharge of air at standard conditions of 273 K and 1,013 bar;
- G′ – gas factor:
- ρr – relative density of liquid phase of product at relieving conditions (ρr = 1,0 for fresh water);
- m – gradient of decrease of liquid phase enthalpy against increase of liquid phase relative density [kJ/kg] at relieving conditions. For set pressures not higher than 2,0 bar, the values in Table 1 may be used. For products not listed in the table and for higher set pressures, the value of m is to be calculated on the basis of the thermodynamic data of the product itself.
|Table 1. Factor m
|m=-d i/dρr [kJ/kg]
- i – enthalpy of liquid [kJ/kg];
- T ′ – temperature at relieving conditions [K], i.e. at the pressure at which the additional pressure relieving system is set.
F, A, r, D, Z and M are defined in this formula.
Compliance with “Additional pressure relieving system for liquid level control “ requires changing of the setting of the relief valves provided for in this section. This is to be accomplished in accordance with the provisions of this and this.
Relief valves mentioned under “Additional pressure relieving system for liquid level control “ above may be the same as the pressure relief valves mentioned in “Pressure relief systems”, provided the setting pressure and the relieving capacity are in compliance with the requirements of this section.
Vacuum protection systems
Cargo tanks designed to withstand a maximum external pressure differential exceeding 0,25 bar and capable of withstanding the maximum external pressure differential which can be attained at maximum discharge rates with no vapour return into the cargo tanks, or by operations of a cargo refrigeration system, need no vacuum relief protection.
Cargo tanks designed to withstand a maximum external pressure differential not exceeding 0,25 bar, or tanks which 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 sending boil-off vapour to the machinery spaces, are to be fitted with:
- two independent pressure switches to sequential 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 design pressure differential of the cargo tank;
- or 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;
- or other vacuum relief systems approved by the Society.
Subject to the requirements of “Special requirements”Special Requirements for LNG and LPG gas carriers, the vacuum relief valves shall admit an inert gas, cargo vapour or air to the cargo tank and are to be arranged to minimize the possibility of the entrance of water or snow. If cargo vapour is admitted, it shall be from a source other than the cargo vapour lines. Air shall not be admitted to the cargo tanks by the vacuum relief valves.
The vacuum protection system is to be capable of being tested to ensure that it operates at the prescribed pressure.
Size of valves
Pressure relief valves are to 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;
2 or vapours generated under fire exposure computed using the following formula:
- Q – minimum required equivalent rate of discharge of air at standard conditions of 273 K and 1,013 bar.
- F – fire exposure factor for different cargo tank types:
a) 1,0 for tanks without insulation located on deck;
b) 0,5 for tanks above the deck when insulation is approved by the Society. (Approval will be based on the use of an approved fire proofing material, the thermal conductance of insulation, and its stability under fire exposure.);
c) 0,5 for uninsulated independent tanks installed in holds;
d) 0,2 for insulated independent tanks in holds (or uninsulated independent tanks in insulated holds);
e) 0,1 for insulated independent tanks in inerted holds (or uninsulated independent tanks in inerted, insulated holds);
f) 0,1 for membrane tanks and semi membrane tanks.
For independent tanks partly protruding through the open deck the fire exposure factor is to be determined on the basis of the surface areas above and below deck.
G = gas factor:
- T – temperature [K]:
a) (273 + °C) at the relieving conditions, i.e. 120 % of the pressure at which the pressure relief valve is set:
- r – latent heat of the material being vaporized at relieving conditions [kJ/kg];
- D – constant based on relation of specific heats (k), shown in Table 2; if k is not known D = 0,606 shall 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 shall be used.
- M – molecular weight of the product.
- A – external surface area of the tank [m2] for different tank types:
a) for body of revolution type tanks:
- A – external surface area;
b) for other than bodies of revolution type tanks:
- A = external surface area less the projected bottom surface area;
c) for tanks consisting of an array of pressure vessel tanks:
1 insulation on the ship’s structure:
- A – external surface area of the hold less its;
2 insulation on the tank structure:
- A – external surface area of the array of pressure vessels excluding insulation, less the projected bottom area.
|Table 2. Constant D