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Figure 5.22 A prover loop




When using meters, it is necessary to be able to calibrate the installation. The most usual way to calibrate a flow meter for liquefied gas is to pass a quantity of liquid through a prover loop. The common arrangement for terminal metering is to have a prover loop of accurately known volume within which there is a sphere. Figure 5.22 shows a typical prover loop arrangement. The sphere in the prover is designed to have a very close fit and is moved through the system by the flowing liquid. Sensors installed at the inlet and the outlet of the prover give a signal as the sphere passes. In the time taken for the sphere to pass from inlet to outlet sensor, the volume measured by the flow meter is electronically recorded. By this means, because the volume of the prover loop is accurately known, a direct calibration of the meter is possible.

This type of prover cannot be used under refrigerated conditions because the sphere becomes hard at low temperatures. For refrigerated products, a piston prover must be used, with a metal piston with low temperature seal materials replacing the sphere.

In order to correctly use the flowmeter, the relationship between the K factor, defined as number of pulses per unit volume, and flow rate should be programmed into the flow computer servicing the meter. This calibration is continuously applied to the meter during cargo loading.


Vortex meter

The vortex meter is starting to be used for some liquid loading lines. The principle of this meter is exactly that which causes a flag to flap on a flag pole as it moves from side to side with the wind generated vortices.

By counting the vortices, the fluid flow rate may be determined quite accurately. This meter has two advantages over the turbine meter. Firstly, it contains no moving parts and therefore needs no prover; secondly, it is significantly cheaper. Although the use of this meter is still in its infancy, it promises to be of great value in LPG service. Evidence exists that uncertainties as low as those of turbine meters on LPG can be achieved with vortex meters, that is of the order of 0.3% on volume.

Ultrasonic meter

The measurement of vapour quantity should not be ignored. If the liquid is metered during ship loading, the vapour return - if used - must also be metered. Ultrasonic meters offer the best possibility here, mainly because they can cope with the wide range of flow rates typically found in vapour return lines. The ultrasonic meter measures the time of flight of a sound signal passing diagonally across the pipe. Both times, in the direction of the fluid flow and against, are required in order to determine flow rate. These meters also require no prover and give no obstruction to the flow. These are often not calibrated since their performance can be determined from their installation - admittedly with a worse uncertainty than could be expected with liquid metering. However, this does not have a major effect on the overall measurement uncertainty since the liquid mass is always much greater than the vapour mass.

Coriolis mass flow meter

The Coriolis mass flow meter has the advantages of a high accuracy and of a direct mass flow metering. The measurement principal is based on the controlled generation of Coriolis forces. These forces are always present in systems when both transitional (straight line) and rotational (revolving) movements occur simultaneously. This principal has been adapted so an oscillation replaces the rotational movements. Two parallel measuring pipes with fluid flowing through them, are made to oscillate in antiphase so they act like a tuning fork. When mass is flowing through, there is a phase shift between the inlet and the outlet. As the mass flow increases, the phase difference also increases. This phase shift is determined using electro-dynamic sensors at the inlet and outlet. The measurement principle operates independently of temperature, pressure, viscosity, conductivity or flow profile.


5.4.2 Pressure, temperature and level instrumentation

The same types of primary measurement devices as described in Chapter Four for ship-board instrumentation are found in terminals.

5.4.3 Gas detection systems

Automatic gas detection systems for monitoring possible leakage of flammable and toxic vapours are installed in terminals and at jetties. The principle of operation of these systems is similar to the ship systems already described.

The number and location of detector heads will depend on the prevailing wind velocity and direction. Their positioning will also depend on the density of the gas being monitored and on the more likely sources of release.

5.5 FIRE-FIGHTING

Fire-fighting facilities installed in terminals and on jetties depend on such factors as location of the terminal, type and size of storage, sizes of ships and types of products handled.

The most important function of a fire-fighting system is the protection of personnel. Secondary, but nonetheless important considerations, are to minimise loss of equip­ment and product.

The fire-fighting equipment and the fire-fighting strategy employed at a terminal will vary with the type of fire considered. Fires may be broadly categorised as follows:

• Minor fires at pump glands, pipe flanges and relief valves

• Fires from confined liquid pools

• Fires from unconfined spillages

• Fires in confined spaces

The guiding principle for fire control is that effective attack should be carried out as early as possible. In certain circumstances, however, an appropriate strategy might be to stop the spillage and protect the surroundings while allowing the fire to burn itself out.

In understanding how best to control a liquefied gas fire, the behaviour of gas spills, their characteristics and potential hazards must be understood — this subject is described in Chapter Two (2.10, 2.20 and Figure 2.19) and Chapter Ten (10.1 and 10.2).

The following fire-fighting mediums may be found in a terminal's fire control system:

water, foam, dry chemical powder, smothering gas (carbon dioxide) and vaporising liquids (such as halon). A brief discussion on each follows and in each case the reader is referred to the corresponding section of the ship's fire-fighting procedures (see 10.3) since many observations are similar.


5.5.1 Water

Although inappropriate for direct application to a liquid gas fire, water is an essential element in a terminal's fire-fighting system. Water is readily available in unlimited quantities and may be used in a variety of ways.

Water is an excellent cooling medium. It is widely used to protect exposed plant and storage from fire and heat radiation. It may be used in the form of jets, sprays, fixed deluge systems or water-curtain radiation screens. Water is, however, a heat source for refrigerated product spills, promoting evaporation of spilled product (see 10.3.2).

Fixed deluge systems, designed to provide a layer of water over exposed surfaces, are customary for storage tanks and plant in potential fire areas. This provides a screen against fire radiation. Application rates vary from two to ten or more litres per minute per square metre.

Sprays from fixed monitors or hand-held hoses can provide essential radiation pro­tection for personnel in their approach to shut valves. Such spray shields are also benefical in an approach to a jet fire where an attack using dry chemicals to extinguish the flame is envisaged.

Of course, water is effective in cases where other combustible materials such as wood, insulation and paint have been ignited.

A special application of a water spray is to induce air movement into a vapour cloud and thereby deflect it from a source of ignition or to encourage its dilution (see also 10.3.2 and Reference 2.21 and 2.29).

5.5.2 Foam

Medium and high-expansion foam applied in large quantities to the surface of a confined, liquid pool fire will largely suppress radiation beaming its way from the flame into the liquid below. This will reduce vaporisation and, thus, the intensity of the fire will decrease. The rate of foam application should be sufficient to maintain a foam depth of one to two metres. High-expansion foams of 500:1 expansion ratio are the most effective for this purpose, but such foam blankets are very likely to be affected by high wind conditions (see also 10.3.2).

Application of such foam to unignited LNG pools can reduce the distance traveled by flammable vapour. This is achieved by using the foam to warm the generated vapour so increasing its buoyancy.

When used for the abovementioned application, foam is normally applied by fixed monitors controlled by remote means and suitably located around tank bund areas.

5.5.3 Dry chemical powders

Dry chemical powders such as sodium bicarbonate, potassium bicarbonate and urea potassium bicarbonate are effective in extinguishing small LNG and LPG fires. They attack the flame by absorbing the free radicals in the combustion process. Just a few seconds application are required for mixing with the flame before the chemical begins to affect the fire. The time taken to extinguish the fire is a function of the burning rate and the application rate of the chemical.


Dry chemicals may be applied from fixed, mobile or portable equipment. A well-trained operator using a hand line or monitor nozzle discharging at the rate of 23 kg/sec can extinguish a fire of about 100 square metres in area — greater areas require more operators or a number of fixed systems. Required application rates for successful extinguishment are very dependent upon wind speed and direction.

The presence of objects such as steel supports may cause problems by shielding parts of the fire from the chemical. Because powders have a negligible cooling effect, they can leave hot spots which, subsequently, are able to produce re-ignition after the initial extinguishment. For this latter reason, special attention should be given to eliminating the source of spillage when using dry chemical powders. Dry chemicals are particularly effective in dealing with fires at sumps, tank vents and leaking flanges. Fixed and portable dry chemical extinguisher systems provide an important defence against liquefied gas fires on many jetties.

5.5.4 Carbon dioxide (CO2) systems

CO2 injection systems act by reducing the oxygen content of the atmosphere to a level at which combustion cannot continue. They are thus only effective in enclosed spaces. It should be appreciated that such equipment should only be used for fire-fighting and not for inerting: this is due to the risk that static electricity may be generated.

Although substantial quantities of CO2 may be required to extinguish a fire, its action can be rapid and effective. It is essential that all personnel are evacuated from the space before CO2 is introduced because of the rapidity with which an asphyxiating atmosphere develops.

CO2 extinguishers are of little value on open jetties, except for the local extinguishment of electrical fires in junction boxes and similar equipment.

5.5.5 Halon replacements

Halon is stored in bottle banks as a liquefied gas under pressure. When released in the presence of a fire, the gas reacts with the combustion process and extinguishes the flames. For many years fixed total flooding systems using halon have been used for this purpose but halon can now no longer be designed into new projects and is to be phased-out from existing installations under the provisions of an international treaty. This is because it has a high Ozone Depletion Potential and is, thus, a danger to the environment. There has been considerable research into halon substitutes and there are now replacement becoming commercially available.

It may be possible for existing halon installations to be maintained in use for several years to come as a total ban on this CFC is not expected to be in place until the year 2005. At present, rules may vary from country to country but ships so fitted may experience difficulty in refilling a halon system after use, or when in need of maintenance or topping-up.

Halon (as for CO2) is generally only suitable for fire-fighting in enclosed spaces. It has an advantage over CO2 inasmuch as its action is more rapid. Additionally, because only low concentrations are required, the necessary storage can be of smaller size. A further advantage over CO2 is that halon may be injected into a space before all personnel have evacuated. Generally, at the concentrations normally used for fire-fighting, from a toxicity viewpoint, personnel have some ten minutes in which to evacuate in an orderly manner.


5.5.6 Inspection, maintenance and training

Fires at liquefied gas terminals occur only rarely but can have severe consequences. Accordingly, and with respect to emergency situations, the interest of operations personnel must be kept high by suitable drills including the use of equipment. This should help to ensure that fire-fighting equipment is effective when needed. The following considerations are relevant:

— Before commencing any ship/shore cargo operation, check all fire-fighting equipment and ensure that monitors are aligned to cover appropriate areas.

— Inspect fire-fighting equipment monthly. This should include the weighing of containers where necessary and the actuation of fire monitors.

— The rotation of portable extinguishers between the terminal and the training area is also encouraged. This will help to ensure the equipment is operated and returned to the terminal with a new charge.

— The immediate reporting (and suitable maintenance) of any faulty fire-fighting equipment in the shortest possible time should be a responsibility of all concerned.

— The provision of clear plans showing the fire-fighting equipment provided and its proper location is recommended.

— The correct marking of all fire-fighting equipment showing its intended function and proper operation should be established.

— The training (and refresher training) of all personnel who may be involved in fire-fighting should be addressed. Training should be done at an adjacent training area where realistic fires can be arranged under controlled conditions.

— The provision of adequate emergency plans and procedures covering terminal personnel and local authorities is an increasingly important aspect. Particular reference should be made to Reference 2.6.


Chapter 6






The Ship/Shore Interface

This chapter discusses a central theme contained within this book. This is the safety of both ship and terminal during cargo transfer operations. The methods developed and the check-lists used to achieve this objective are fully described.

All operations when a ship is alongside must be pre-planned and jointly managed in such a way that both the ship and the shore are aware of their respective responsibilities, capabilities and limitations. Throughout the cargo transfer operation both ship and shore should work together according to mutually agreed procedures and responsibilities.

For those requiring further background on this subject a listing of additional sources can be found in References 2.32 and 2.39.

6.1 SUPERVISION AND CONTROL

Within the gas trade, the ship/shore interface plays a vital part in operations. It is an area where differing standards and safety cultures may coexist. A central theme of this book is to close gaps in design standards and operational practices which may exist on either side of the interface. In so doing, this book, by explanation and training content, hopes to achieve a better understanding of the principles involved so that ship and shore personnel can order their procedures to suit the requirements of each party and, by so doing, achieve better reliability and safety.

With respect to the equipment fitted on jetties, the ship/shore interface covers:—

• Moorings

• Fenders

• Breasting dolphins

• Hard arms and hoses

• Ship/shore gangways

• Emergency shut-down arrangements

• Ship/shore links, and

• Fire-fighting equipment capability

Liquefied gases are loaded and discharged at many terminals around the world by a wide variety of ship types and sizes. Operations range from the very large self-contained LNG projects to smaller LPG terminals handling many different products.


In the case of large LNG projects, dedicated ships trade continuously between purpose-built terminals for contract periods of up to 25 years. Each link of the chain — the loading terminal, the gas carriers and the receiving terminal — is designed as part of an integrated whole. In this trade the ships are designed to be compatible with the terminal and ship and shore personnel should be familiar with each other's equipment and responsibilities. Similar observations apply also to some LPG projects, particularly those which involve long-term contractual arrangements and, inevitably, the use of large ships and dedicated terminals.

In contrast, however, there are many LPG terminals which handle many 'spot' cargoes delivered by a very wide variety of ships and shipowners. Here, different gases are handled under a variety of conditions and ships are frequently required to load more than one product simultaneously. Furthermore, in such trades, the ships may need to change cargoes on successive voyages, with the extension of operations sometimes requiring very careful control.

In all gas trades it is essential that ship and terminal operators are:—

• Familiar with the basic characteristics of each other's facilities,

• Aware of the division of responsibilities, and

• Able to communicate effectively during the port call.

It is only in this way that safe, efficient and reliable operations can be assured.

These issues are re-emphasized in the International Safety Guide for Oil Tankers and Terminals (see Section 7.1 in Reference 2.4) and in this respect information of value can also be found in References 2.8 and 2.31.

The International Safety Guide for Oil Tankers and Terminals under the heading of 'Supervision and Control', states that:—


General

The responsibility for safe cargo handling operations is shared between the ship and the terminal and rests jointly with the shipmaster and the responsible terminal representative. The manner in which the responsibility is shared should, therefore, be agreed between them so as to ensure that all aspects of the operations are covered.


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