Figure 4.21 The membrane system for producing nitrogen
4.7.3 Pure nitrogen from the shore
The quality of inert gas produced by shipboard systems is usually inadequate for oxygen-critical cargoes — see strict in-tank oxygen requirements in Table 2.3(b). Bearing in mind the components in the inert gas, this may create restrictions on use if tanks have been previously gas-freed for inspection; and this is often necessary when a change in grades is involved. Under these circumstances, and prior to loading, it is normal for shipmasters to arrange for cargo tanks to be inerted with pure nitrogen, taken from the shore. This is usually delivered by road tanker or barge. As deliveries are in liquid form, where immediate inerting is required, a nitrogen vaporiser is needed.
4.8 ELECTRICAL EQUIPMENT IN GAS DANGEROUS SPACES
A common definition of area safety classification for electrical equipment in shore installations is as follows:
Zone 0: An area with a flammable mixture continuously present
Zone 1: An area where flammable mixtures are likely during normal operations
Zone 2: An area where flammable mixtures are unlikely during normal operations
Electrical installations on gas carriers are subject to the requirements of the classification society and the Gas Codes. Zones and spaces on ships are classified as either gas-safe or gas-dangerous, depending on the risk of cargo vapour being present. For example, accommodation and machinery spaces are gas-safe, while compressor rooms, cargo tank areas and holds are gas-dangerous. In gas-dangerous spaces, only electrical equipment of an approved standard may be used; this applies to both fixed and portable electrical equipment. There are several types of electrical equipment certified as being safe for use on gas carriers and these are described in the following sections.
Intrinsically safe equipment
Intrinsically safe equipment can be defined as an electrical circuit in which a spark or thermal effect (under normal operation or specified fault conditions) is incapable of causing the ignition of a given explosive mixture.
Limitation of such energy may be achieved by placing a barrier, as shown in Figure 4.22, in the electrical supply. This must be positioned in a safe area. Zener barriers are frequently used for this purpose and, in the circuit shown, the voltage is limited by the Zener diodes so that the maximum current flow to the hazardous area is restricted by the resistors. The uses of such intrinsically safe systems are normally limited to instrumentation and control circuitry in hazardous areas. Because of the very low energy levels to which they are restricted, intrinsically safe systems cannot be used in high-power circuits.
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Figure 4.22 Intrinsic safety using Zener barriers
Flameproof equipment
A flameproof enclosure is one which can withstand the pressure developed during an internal ignition of a flammable mixture. Furthermore, the design is such that any flames, occurring within the enclosure, are cooled to below ignition temperatures before reaching the surrounding atmosphere.
Therefore, the gap through which hot gases are allowed to escape is critical and great care must be taken in assembly and maintenance of flameproof equipment to ensure that these gaps are well maintained. No bolts must be omitted or tightened incorrectly, while the gap must not be reduced by painting, corrosion or other obstructions.
Pressurised or purged equipment
The pressurisation or purging of equipment is a technique used to ensure that an enclosure remains gas-free. In the case of pressurisation, an over-pressure of about 0.5 bar, relative to the surrounding atmosphere, must be maintained. In the case of a purged enclosure, a continuous supply of purging gas must be provided to the enclosure. Air or inert gas can be used.
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.
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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.
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