Figure 4.1 Cargo tank dome piping arrangement — Type 'C' tank

The types of isolation valve normally found on gas tankers are ball, globe, gate or butterfly valves. These valves are usually fitted with pneumatic or hydraulic actuators. Ball valves for liquefied gas service are provided with a means of internal pressure relief. This is usually a hole drilled between the ball cavity and the downstream side of the valve. Valves must be of the fail-safe type.

In the LNG trade, strainers are commonly provided at the manifold connections for loading and discharging. It is important not to bypass these strainers. Furthermore, they should be frequently checked and cleaned. The strainers are installed to protect cargo handling equipment from damage by foreign objects. Many strainers are de­signed for one-way flow only (see also Reference 2.33).

4.1.3 Emergency shut-down (ESD) systems

At a number of locations around the ship (bridge front, gangway, compressor room and cargo control room, emergency control station), pneumatic valves or electric push buttons are provided. When operated, these controls close remotely actuated valves and stop cargo pumps and compressors (where appropriate). This provides an emergency-stop facility for cargo handling. Such emergency shut-down (ESD) is also required to be automatic upon loss of electric control or valve actuator power. Furthermore, if a fire should occur at tank domes or cargo manifolds (where fusible elements are situated), the ESD system is automatically actuated. Individual tank filling valves are required to close automatically upon the actuation of an overfill sensor in the tank to which they are connected. ESD valves may be either pneumatically or hydraulically operated but in either case they must be fail-safe; in other words they must close automatically upon loss of actuating power.

A vital consideration, particulary during loading, is the possibility of surge pressure generation when the ship's ESD system is actuated. The situation varies from terminal to terminal and is a function of the loading rate, the length of the terminal pipeline, the rate of valve closure and the valve characteristic itself. The phenomenon of surge pressure generation is complex and its effects can be extreme, such as the rupture of hoses or hard arm joints. Precautions are, therefore, necessary to avoid damage and sometimes, loading jetties are fitted with surge pressure drums (see 5.3.2). Terminals should confirm ship's ESD valve closure times and adjust loading rates accordingly or place on board a means to allow the ship to actuate the terminal ESD system and so halt the flow of cargo before the ship's ESD valves start to close (see also 10.5). In this respect consultation between the ship and shore must always take place, to establish the parameters relevant to surge pressure generation and to agree upon a safe loading rate (see also 6.4).

4.1.4 Relief valves for cargo tanks and pipelines

The Gas Codes require at least two pressure relief valves of equal capacity to be fitted to any cargo tank of greater than 20 cubic metres capacity. Below this capacity, one valve is sufficient. The types of valves normally fitted are either spring-loaded or pilot-operated. Pilot-operated relief valves may be found on all tank-types while spring-loaded relief valves are usually only used on Type 'C' tanks. The use of pilot-operated relief valves on fully refrigerated tanks ensures accurate operation at the low-pressure conditions prevailing; their use on Type 'C' tanks allows variable relief settings to be achieved using the same valve. This may be done by changing the pilot-spring. Figure 4.2 shows a typical pilot-operated relief valve. Other types of pilot valve are available for adjustment of set pressure and blowdown pressure.

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