Figure 4.24 Nitrogen bubbler level gauge

The use of such instruments in connection with the carriage of ethylene is not recommended. Ethylene has a strict specification for nitrogen contamination and the use of such equipment could damage the cargo.

Differential pressure gauges

Differential pressure gauges operate on pressure differences between liquid and vapour. The signal lines for the instrument are normally purged with inert gas. This type of gauge can only be used on ships when the cargo tank is situated completely above deck, thus such measuring equipment is more generally found in use on shore in terminal tanks. Figure 4.25 shows the principle of the differential pressure gauge.

Figure 4.25 Differential pressure level gauge

Capacitance gauges

Capacitance gauges measure the change in electrical capacitance between two probes as cargo liquid, rather than vapour, takes up the space between them. Figure 4.26 illustrates just such a device in which the two probes are enclosed within an open protective tube. This tube extends throughout the depth of the tank and provides a continuous indication of liquid content at all levels.

For single preset level indication, as for a high-level alarm or overfill shut-off, a short probe sensor may be fitted at the precise level required. The electrical circuits are, of course, intrinsically safe. The devices, having no moving parts, are usually reliable but must be kept free from dirt, rust, water and ice since such contaminants can cause inaccuracy.

Figure 4.26 Electrical capacitance level gauge

Ultrasonic gauges

Ultrasonic gauges operate on a similar principle to echo sounders. They measure either the ullage or the liquid depth by reflecting sound waves from the liquid surface. The tranceiver may be mounted either in the tank dome or at the tank bottom, depending on whether the ullage depth or liquid dip is to be measured.

Ultrasonic gauges fitted on gas carriers can be unreliable. There can be problems in obtaining satisfactory readings when loading tanks by the spray line. Other points of inaccuracy can develop as cargoes of differing types are loaded. Furthermore, readings can vary, depending on temperature and pressure changes.

Radar gauges

Another type of tank gauging equipment is that designed to operate on the principle of radar. Such equipment works at very high frequencies — approximately 11 giga­hertz (11 x 10⁹). Radar type liquid level gauges have now been specially developed for liquefied gases and their usage on gas tankers. The equipment provides measurements adequate to meet industry requirements .

All the above devices are classed as closed devices (see Appendix 2). This means that, when in use, no cargo liquids or vapours are released to the atmosphere during level measurement. The slip-tubes, which are described in the following paragraph, are classed as restricted devices and have some disadvantages over the equipment described previously.

Slip-tubes

As defined in the Gas Codes, slip-tubes constitute a restricted type of gauging device — so named because a small amount of cargo vapour or liquid is released to atmosphere during level measurement. Some terminals disallow the use of this type of equipment — depending on cargo type — because released gases can have harmful effects on personnel working nearby. These dangers are discussed in Chapter Nine and relate mostly to product toxicity.

Slip-tubes have an orifice at their upper end through which liquid or vapour can be released. The Gas Codes limit the size of this orifice to 1.5 millimetres in diameter, unless an excess flow valve is fitted. The lower end of the slip-tube is open to the cargo tank environment. The device slides up and down through a gland fitted in the tank dome. The observed differences between either liquid or vapour venting from the orifice gives an indication of when the liquid level has been reached and, by reading from the markings on the tube itself, the actual liquid level is read-off. Because of the considerable depth of many tanks, it is usual to find that a number of slip-tubes are fitted, with each individual unit covering a certain range of tank level measurements.

Slip-tubes represent a simple and direct method of measurement but, because of a certain amount of high-pressure spray released to the atmosphere, special precau­tions must be taken. These include the wearing of protective clothing. Furthermore, operational procedures should be established to direct the spray away from personnel. As described earlier in this section, the use of slip-tubes is limited to back­up of the closed devices (described above) and to Type 'C' tanks only.

4.9.2 Level alarm and automatic shut-down systems

With the exception of Type 'C' tanks having a capacity of less than 200 cubic metres, every cargo tank must be fitted with an independent high level sensor giving audible and visual alarms. The float, capacitance or ultrasonic sensors (as covered in 4.9.1) may be used for this purpose. The high-level alarm — or other independent sensor — is required to automatically stop the flow of cargo to the tank.

During cargo loading, there is a danger of generating a significant surge pressure if the valve stopping the flow closes too quickly against a high loading rate. (For further information on surge pressure see 4.1.3 and 10.5).

4.9.3 Pressure and temperature monitoring

The Gas Codes call for pressure monitoring throughout the cargo system. Appropriate positions include cargo tanks, pump and compressor discharge lines, liquid crossovers and vapour crossovers. In addition, pressure switches are fitted to various systems to protect personnel and equipment by operating alarms and shut-down systems.

The Gas Codes also require at least two devices for indicating cargo tank tem­peratures. One is placed at the bottom of the tank and the second near the tank top, but below the highest allowable liquid level. Ships' officers should be aware of the lowest temperatures to which the cargo tanks can be exposed and these values should be marked on the temperature gauges — especially those at the cargo manifold.

Where cargo is carried in tanks requiring a secondary barrier at a temperature of below -55° C, the Gas Codes call for temperature-indicating devices within the insulation or

on the hull structure adjacent to the containment system. The thermo-couples should be set to provide adequate warning prior to the lowest temperature for the hull steel being approached.

The Gas Codes also recommends more thermometers to be fitted to at least one tank in order to monitor the cargo system during cool-down and warm-up operations. This is to avoid undue thermal stress.

4.9.4 Gas detection systems

The provision of gas detection systems on board gas carriers is of importance. The Gas Codes require gas carriers to have a fixed gas detection system with audible and visual alarms. These must be fitted in the wheelhouse, in the cargo control room and at the gas detector readout location. Detector heads are normally provided in the following spaces:—

• Cargo compressor room

• Electric motor room

• Cargo control room (unless classified as gas-safe)

• Enclosed spaces such as hold spaces and interbarrier spaces (excepting hold spaces containing Type 'C' cargo tanks)

• Airlocks

• Burner platform vent hoods and engine room gas supply pipelines (LNG ships only)

The detector heads should be sited having regard to the density of cargo vapours. This means that for heavier-than-air vapour, the detector heads should be sited at a low level and for lighter-than-air vapours, at high level — see Table 2.5. The sensing unit for the gas detection system is normally located in the cargo control room or the wheelhouse. Provision should be made for regular testing of the equipment: span gas of a certified mixture for calibration purposes should be readily available and permanently piped, if possible.

Sampling and analysing from each detector head is done sequentially. The Gas Codes call for sampling intervals from any one space generally not exceeding 30 minutes. Alarms should be activated when the vapour concentration reaches 30 per cent of the lower flammable limit.

In addition to the fixed gas detection system, every ship must have at least two sets of portable gas detection equipment adapted to the cargoes listed in the Fitness Certificate. Means for measuring oxygen levels in inert atmospheres are also required.

Gas carrier crews should be familiar with gas detection equipment and its operating principles. Manufacturer's instructions should always be followed. Chapter Nine deals with the principles and uses of gas detection equipment.

4.9.5 LNG custody transfer systems

LNG ships are generally fitted with custody transfer equipment which comprises a calibrated package of cargo measurement equipment. To meet the requirements for custody transfer, the cargo tanks are calibrated by an independent measurer and high

accuracy level, temperature and vapour pressure measuring equipment is installed. This is often supported by data logging and cargo calculation facilities. Such systems are usually approved by local customs authorities.

The need for such equipment has developed from a practice in the LNG trade of relying on shipboard measurement of cargo to determine the quantity of product transferred between seller and buyer. Accuracy is important in these circumstances since the quantities determined are also used as a basis for import duties and fiscal accounting.

Such a system normally includes the following equipment:—

• Level gauges — either float, ultrasonic or capacitance, all with remote readout

• Temperature sensors — frequently of the platinum resistance type, and

• Pressure gauges or sensors

Typical accuracies for LNG measurement equipment are as follows:—

• Level — ±7.5 millimetres at a specified temperature over the full tank height

• Temperature — in the range -150°C to -170°C, ± 0.2°C in the range -10°C to -150°C, ± 0.3°C in the range +80°C to -200°C, ±1.0°C

• Pressure — ±0.0015 bar (which will apply within the MARVS of the tank)

Some LNG ships are fitted with a means of determining cargo density. However, more usually its value is derived from the analysis of samples carried out in the terminal.

4.9.6 Integrated systems

Some gas carriers relay read-outs from cargo instrumentation to an on-line computing system. This allows the ship immediate access to cargo quantities and cargo tank conditions at any stage of loading or discharging. To permit this system to function a shipboard method of density determination is required, but such determinations should not be confused with density values which may be noted on the Bill of Lading used for custody transfer.

4.9.7 Calibration

Instrumentation, as sophisticated as it can be, is only accurate if properly calibrated. Calibration can be done on board by the crew, using calibration instruments, or it can be done by service engineers carrying their own calibration instruments. Calibration instruments must be calibrated at regular intervals in specialised facilities and carry a calibration certificate.

The ISM Code, in Chapter 9 of SOLAS (Reference 1.4), recommends that each ship carries a calibration procedure and that confirmation of compliance with that procedure is available on board.

Chapter 5

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