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Use of Cargo as Fuel on Gas Tankers

LNG as fuel it’s popular, proven and profitable solution. This article talks about how to setup everything to use LNG as fuel for needs of the ship.

General

Methane (LNG) is the only cargo whose vapour or boil-off gas may be utilized in machinery spaces of Category A and in such spaces may be utilized only in boilers, inert gas generators, combustion engines and gas turbines.

These provisions do not preclude the use of gas fuel for auxiliary services in other locations, provided that such other services and locations should be subject to special consideration by the Administration.

Arrangement of machinery spaces of Category A

Spaces in which gas fuel is utilized are to be fitted with a mechanical ventilation system and are to be arranged in such a way as to prevent the formation of dead spaces. Such ventilation is to be particularly effective in the vicinity of electrical equipment and machinery or of other equipment and machinery which may generate sparks. Such a ventilation system is to be separated from those intended for other spaces.

Gas detectors are to be fitted in these spaces, particularly in the zones where air circulation is reduced. The gas detection system shall comply with the requirements of “Cargo Tank Instrumentation on Gas Tankers”Cargo Tank Instrumentation.

Electrical equipment located in the double wall pipe or duct specified in “Gas fuel supply” is to be of the intrinsically safe type.

Gas fuel supply

Gas fuel piping shall not pass through accommodation spaces, services spaces, or control stations. Gas fuel piping may pass through or extend into other spaces provided they fulfill one of the following:

  1. the gas fuel piping shall be a double wall piping system with the LNG fuel contained in the inner pipe. The space between the concentric pipes shall be pressurized with inert gas at pressure greater than the gas fuel pressure. Suitable alarms are to be provided to indicate a loss of inert gas pressure between the pipes;
  2. or the gas fuel piping shall be installed within a ventilated pipe or duct. The air space between the gas fuel piping and inner wall of this pipe or duct is to be equipped with mechanical exhaust ventilation having a capacity of at least 30 air changes per hour. The ventilation system is to be arranged to maintain a pressure less than the atmospheric pressure. The fan motors are to be placed outside the ventilated pipe or duct. The ventilation outlet is to be placed in a position where no flammable gas-air mixture may be ignited. The ventilation shall always be in operation when there is gas fuel in the piping. Continuous gas detection is to be provided to indicate leaks and to shut down the gas fuel supply to the machinery space in accordance with beyond. The master gas fuel valve required by beyond shall close automatically, if the required air flow is not established and maintained by the exhaust ventilation system.

The gas piping system and arrangement is to be designed in accordance with Rule for Machinery (Part 1, Vol. III) Sec. 11, C and Guidelines for the use of LNG as Fuel Sec. 2, F and Sec. 2, G.

Installation of gas piping

  1. Gas piping must be independent of other systems and may exclusively be used for the conveyance of gas. It is to be ensured by their arrangement that they are protected against external damages.
  2. The main gas line between the gas make up station and the machinery space shall be as short as possible. This applies also to the gas pipes to gas injection valves and burners. The gas piping has to be installed as high in the space as possible and at the greatest possible distance from the ship’s hull.

If a gas leak occurs, the gas fuel supply shall not be restored until the leak has been found and repaired. Instructions to this effect are to be placed in a prominent position in the machinery spaces.

The double wall piping system or the ventilated pipe or duct provided for the gas fuel piping shall terminate at the ventilation hood or casing required by beyond.

A ventilation hood or casing is to be provided for the areas occupied by flanges, valves, etc., and for the gas fuel piping, at the gas fuel utilization units, such:

  • as boilers;
  • diesel engines;
  • or gas turbines.

If this ventilation hood or casing is not served by the exhaust ventilation fan serving the ventilated pipe or duct as specified above, then it is to be equipped with an exhaust ventilation system and continuous gas detection is to be provided to indicate leaks and to shut down the gas fuel supply to the machinery space in accordance with this. The master gas fuel valve required by this shall close automatically if the required air flow is not established and maintained by the exhaust ventilation system. The ventilation hood or casing is to be installed or mounted to permit the ventilating air to sweep across the gas utilization unit and be exhausted at the tip of the ventilation hood or casing.

The ventilation inlet and discharge for the required ventilation systems is to be respectively from and to a safe location. Each gas utilization unit is to be provided with a set of three automatically operated valves:

  • two of these valves shall be in series in the gas fuel pipe to the consuming equipment;
  • the third valve shall be in a pipe that vents, to a safe location in the open air, that portion of the gas fuel piping that is between the two valves in series.

These valves are to be arranged so that failure of the necessary forced draught, loss of flame on boiler burners, abnormal pressure in the gas fuel supply line, or failure of the valve control actuating medium will cause the two gas fuel valves which are in series to close automatically and the vent valve to open automatically.

Alternatively, the function of one of the valves in series and the vent valve can be incorporated into one valve body so arranged that, when one of the above conditions occurs, flow to the gas utilization unit will be blocked and the vent opened. The three shut-off valves are to be arranged for manual reset.

It must be possible to operate the shut-off devices locally. They must close automatically under the following service conditions:

  1. Whenever the gas pressure varies by more than 10 % or, in the case of supercharged engines, if the differential pressure between gas and charging air is no longer constant.
  2. If fault situations as described in “Requirements for gas fired main boilers” and “Special requirements for diesel/gas engines and gas turbines” arise.

A master gas fuel valve that can be closed from within the machinery space is to be provided within the cargo area. The valve is to be arranged so as to close automatically if leakage of gas is detected, or loss of ventilation for the duct or casing or loss of pressurization of the double wall gas fuel piping occurs.

High-pressure gas piping, which according to “Gas fuel supply” is installed in a ventilated pipe or duct, is to be subdivided into sections between master gas fuel valve and gas utilization unit by quick-closing valves, if a major volume of gas may penetrate into the duct or pipe in the event of pipe burst. The number of quick-closing valves is to be agreed with the Society, taking into account the gas pressure and the volume of the gas piping. Under the same criteria the quick-closing valves must close automatically like the master gas fuel valves mentioned in above.

Gas fuel piping in machinery spaces shall comply with “Piping System of pressure vessels on gas tankers”Cargo and process piping as far as found applicable. The piping shall as far as practicable, have welded joints. Those parts of the gas fuel piping, which are not enclosed in a ventilated pipe or duct according to “Gas fuel supply” and are on the open deck outside the cargo area shall have full penetration butt welded joints and shall be fully radiographed.

Provisions are to be made for inerting and gas-freeing that portion of the gas fuel piping system located in the machinery space.

Gas detection systems provided in accordance with the requirements of “Gas fuel supply” are to comply with “Cargo Tank Instrumentation on Gas Tankers”Gas detection requirements as applicable; they shall activate the alarm at 30 % of the lower flammable limit and shut down the master gas fuel valve referred to above before the gas concentration reaches 60 % of the a lower flammable limit.

Gas detectors are to be arranged at all points where gas must be expected to gather.

Gas make-up plant and related storage tanks

All equipment (heaters, compressors, filters, etc.) for making-up the gas for its use as fuel, and the related storage tanks shall be located in the cargo area in accordance with paragraph “Arrangement of the gas carrier ships”Segregation of the cargo area . If the equipment is in an enclosed space the space shall be ventilated according to “Mechanical Ventilation in The Cargo Area on Liquefied gas tankers”Mechanical Ventilation and be equipped with a fixed fire extinguishing system according to “Fire protection and Fire extinction on Liquefied Gas Carriers”Cargo compressor and pump rooms and with a gas detection system according to “Cargo Tank Instrumentation on Gas Tankers”Gas detection requirements as applicable.

Marmara Ereğlisi LNG Storage Facility
LNG tanker at LNG Storage Facility
Source: wikipedia.org

The compressors are to be capable of being remotely stopped from a position which is always and easily accessible and also from the engine room. In addition, the compressors are to be capable of automatically stopping when the suction pressure reaches a certain value depending on the set pressure of the vacuum relief valves of the cargo tanks. The automatic shut-down device of the compressors are to have a manual resetting.

Volumetric compressors are to be fitted with pressure relief valves discharging into the suction line of the compressor. The size of the pressure relief valves is to be determined in such a way that, with the delivery valve kept closed, the maximum pressure does not exceed by more than 10 % the maximum working pressure. The requirements of “Piping System of pressure vessels on gas tankers”Cargo system valving requirements apply to these compressors.

If the heating medium for the gas fuel evaporator or heater is returned to spaces outside the cargo area it shall first go through a degassing tank. The degassing tank is to be located in the cargo area. Provisions are to be made to detect and alarm the presence of gas in the tank. The vent outlet is to be in a safe position and fitted with a flame screen.

Piping and pressure vessels in the gas fuel conditioning system are to comply with “Piping System of pressure vessels on gas tankers”Piping system.

Requirements for gas fired main boilers

Each boiler is to have a separate uptake. A system suitable to ensure the forced draught in the boilers is to be provided. The particulars of such a system are to be to the satisfaction of the Society. Combustion chambers of boilers are to be of suitable form such as not to present pockets where gas may accumulate.

The burner system is to be of dual type, suitable to burn either oil fuel or gas fuel alone or oil and gas fuel simultaneously. Only oil fuel is to be used during maneuvering and port operations unless automatic transfer from gas to oil burning is provided in which case the burning of a combination of oil and gas or gas alone may be permitted provided the system is demonstrated to the satisfaction of the Society. It shall be possible to change over easily and quickly from gas fuel operation to oil fuel operation. Gas nozzles are to be fitted in such a way that gas fuel is ignited by the flame of the oil fuel burner.

A flame scanner is to be installed and arranged to assure that gas flow to the burner is cut off unless satisfactory ignition has been established and maintained. On the pipe of each gas burner a manually operated shut-off valve is to be fitted. An installation is to be provided for purging the gas supply piping to the burners by means of inert gas or steam, after the extinguishing of these burners.

For fuel oil and air supply to the pilot burner see Rule for Machinery Installations, Volume III Section 9, B.3.

The gas supply must be automatically stopped by the shut-off devices specified in this, if:

  1. the air supply is insufficient for complete combustion of the gas;
  2. the pilot burner for an operating burner extinguishes, unless the gas supply line to every individual burner is equipped with a quick- closing valve that automatically cuts off the gas;
  3. the pressure in the fuel oil admission line drops to an in admissibly low level.

Alarm devices are to be fitted in order to monitor a possible decrease in liquid fuel oil pressure or a possible failure of the related pumps.

Arrangements are to be made that, in case of flame failure of all operating burners for gas or oil or for a combination thereof, the combustion chambers of the boilers are automatically purged before relighting. Arrangements are to also be made to enable the boilers to be manually purged.

Special requirements for diesel/gas engines and gas turbines

As main engines only dual fuel engines will be approved, which are designed either for combined dual fuel operation by ignition-ray process or for fuel oil operation only. For single gas fuel engine, Requirement of Guidelines for the use of LNG as fuel Sec. 2, G.2.3 and Sec. 6, E to be taken into account.

The gas is to be conveyed close to the cylinder inlet valves through gas inlet valves, without prior mixing with combustion air, or else blown directly into the cylinders. Also, in case of sudden shut-off of the gas supply, the engines must be capable of continuous operation by fuel oil exclusively. Starting and reversing of the engines is admissible during operation by diesel fuel only.

For protection against the effects of misfiring, blow-off safety devices capable of blow-off without risk are to be in the exhaust gas manifold and in the charge air/air inlet duct. The exhaust-gas pipes from multi-engine plants are to be arranged such as to lead to the open air separately. Combination of these pipes is not admissible. In the starting-air lines, in front of each cylinder non-return valves and flame arresters are to be arranged. The crankcases of trunk piston engines are to be continuously vented. In the case of ventilation by induced draught fans the negative pressure in the crankcase must not exceed a water head of 25 mm. Vent lines have to extend to the open deck at a safe point. A gas detecting device is to be fitted in the vent line between crankcase and fan.

For safety equipment see Rule for Machinery Installations (Part 1, Vol. III)Sec. 2, F and additionally, Guidelines for the use of LNG as Fuel Section 5 should be taken into account.

The gas supply must be stopped automatically by the shut-off valves specified in this, if:

  • the pilot fuel injection pump or the supply of pilot fuel fails;
  • the engine speed drops below the lowest service speed;
  • the gas detector in the crankcase vent line indicates a gas concentration approaching the lower explosion limit.

Requirements for gas turbines are to be agreed with the Society in each individual case.

Testing of pipings

Before being put into operation, the gas piping system is to be subjected to a hydrostatic pressure test to 1,5 times the working pressure, but at least to a pressure of 5 bar, and to a pneumatic tightness test. Additionally, all shut off devices are to be pneumatically tested to tightness of their sealing faces, to 1,1 times the working pressure.

Safety, quick-closing or automatic stop valves, level indicators, temperature meters, pressure gauges, gas detection and warning systems are to be subjected to function test.

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