Increased safety equipment

The use of Increased Safety Equipment is appropriate for electrically powered light fittings and motors. This equipment has a greater than normal separation between electrical conductors and between electric terminals. Starters are designed to mini­mise both arcing at contactors and to limit the temperature of components. Increased safety motors, with flameproof enclosures, are frequently used on deck on gas carriers. Here they may be found driving deepwell pumps or booster pumps. In such cases they must be protected by a suitable weatherproof covering.

4.9 INSTRUMENTATION

Instrumentation is an important part of gas tanker equipment and is required for the measurement of cargo level, pressure and temperature. It is also used for gas detection. Instrumentation must be carefully selected and well maintained.

4.9.1 Liquid level instrumentation

The Gas Codes and classification society rules require every cargo tank to be fitted with at least one liquid level gauge. Specific types of gauging system are required for certain cargoes as defined in Chapter 19 of the IGC Code. This information is summarised in Appendix 2.

The IMO classification for gauging systems is as follows:

• Indirect systems — these may be either weighing methods or flow meters

• Closed devices which do not penetrate the cargo tank — here ultrasonic devices or radio isotope sources may be used

• Closed devices which penetrate the cargo tank — such as float gauges and bubble tube indicators

• Restricted devices which penetrate the tank but which release small volumes of liquid or vapour to atmosphere when in use, such as fixed or slip-tube gauges. When not in use, the restricted device should be kept completely closed

In the LPG trade the most common types of level gauging are the last two described, while in the LNG trade, the closed devices are more usual.

Float gauges

The float gauge is widely used on all gas carriers. It consists of a float attached by a tape to an indicating device which can be arranged for local and remote readout. Figure 4.23 shows a typical float gauge installed in a tubular well. Alternatively guide wires may be fitted. Float gauges have gate valves for isolation so that the float can be serviced in a safe atmosphere.

Figure 4.23 Float level gauge

The float must be lifted from the liquid level when not in use; if left down, liquid sloshing, while at sea, will damage the tape-tensioning device. Float gauges cannot normally register a liquid level of less than ten centimetres from the tank bottom.

Nitrogen bubbler gauges

Nitrogen bubbler gauges measure the pressure necessary to displace liquid cargo from a small bore tube mounted vertically in a tank. A sufficient pressure of nitrogen is introduced into the tube to displace the liquid and to commence bubbling at the bottom. The pressure necessary to do this is measured and is a function of the liquid level and the liquid density. For cargoes of known density the level readout is obtained directly. By installing two such tubes, one alongside the other, and with their lower extremities a known vertical distance apart, the density of the cargo can also be determined. Figure 4.24 shows the principle of the bubbler gauge.

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