Efficient Cargo Tank Operations – Inerting, Purging and Cooling Techniques

Cargo Tank Operations play a crucial role in ensuring safety and efficiency during the handling of cargo. This includes several key processes such as inerting by displacement, which reduces the risk of explosion during gassing up. Special procedures are necessary for ammonia tank inerting, which involves maintaining specific gas levels within the tank. Additionally, the operation relies on effective compressors and reliquefaction systems to manage the pressures involved.

Proper preparation for gassing up and cooling down is essential to facilitate smooth purging operations. Techniques like cooling with vapor return ashore and cooling without vapor return further optimize the overall efficiency and safety of cargo tank management.

General Information

Inerting cargo tanks and Pipelines in Marine Terminals: Key Considerations for Handling Liquefied Gaspipe work systems is undertaken primarily to ensure a non-flammable condition in the subsequent gassing up with the vapor of the cargo to be loaded. For this purpose a reduction in the oxygen concentration to 5 % by volume is generally judged to be adequate, although lower values are usually obtainable and preferred.

For some of the more active chemical gases, VCM or butadiene, oxygen levels as low as 0,1 % may be required to avoid chemical reaction with the incoming gassing-up vapor. This level will be difficult to achieve using shipboard inert gas plant.

Reminder:

• Use air for ammonia cargoes.
• Nitrogen for VCM/Butadiene/PO, PO-EO mixes, Ethylene.
• Inert gas may be used for hydrocarbons gases.

Inerting by Displacement

This is generally considered to be the most efficient method of inerting tanks. It relies on stratification in the tank as a result of the difference in vapor densities between the gas entering the tanks and the gas already in the tank. The heavier gas is introduced beneath the lighter gas, and at a low velocity to minimize the turbulence. If perfect stratification could be achieved with no mixing at the interface, then one tank volume of the inert gas would completely displace the gas already in the tank. In practice some mixing does occur and it will be necessary to use more than one tank volume of Air and Inert Gas Dryersinert gas. This may vary from 1,25 to 4 times the tank volume, depending upon the relative densities and tank and piping configurations. There is lithe density difference between air and inert gas; inert gas from a combustion generator is slightly heavier than air while nitrogen is slightly lighter. These small density differences make inerting by displacement alone very difficult to achieve, and usually the process becomes partly displacement, partly dilution.

Before introducing nitrogen a purge with dry air is usual. The relative densities of nitrogen and air are similar which makes the change over from air to nitrogen, and the clearance of all air from the tank difficult. In order to improve the separation during this process the dry air in the tank should be as cold as possible, and the nitrogen as warm as possible. The nitrogen must be introduced through the upper distribution line with displacement taking place in the direction top to bottom. This process must not be hurried. Nitrogen gas is expensive, and the need to re-inert following an unsuccessful first attempt is costly and time consuming. Tanks can be inerted in parallel or series, and with flow within the tanks or either top to bottom or bottom to top, depending on the relative densities of the two gases.

Inerting by Dilution. In the dilution method the incoming gas mixes with the vapor already in the tank. This can be done in several ways depending on the type of the vessel.

Repeated Pressurisation (not on fully refrigerated vessels). Dilution can be achieved by a process of repeated pressurization of the tank with inert gas using a compressor, followed by a release of the compressed contents to atmosphere. Each repetition will bring the tank contents nearer to the oxygen concentration level of the injected inert gas. Thus to bring the tank contents to a level of 5 % oxygen within a reasonable number of repetitions, an inert gas quality better than 5 % oxygen content is required.

Quicker results will be achieved by more numerous repetitions each at a lower pressurization level, than by repetition using the higher pressurization levels, of which the tank and compressor may be capable.

Repeated Vacuum (not on fully refrigerated vessels). Inerting by successive dilutions may be carried out by repeatedly drawing a vacuum on the tank by the compressors, and then breaking the vacuum using inert gas. If, for instance a 50 % vacuum can be drawn then on each vacuum cycle half the oxygen content of the tank will be withdrawn. Some of the withdrawn oxygen will, of course, be replaced by the oxygen content of the subsequent vacuum breaking inert gas but, if the quality of inert gas is good, this method is probably the most economical in the use of minimum inert gas quantity in order to achieve the desired inerting level in the tank. The overall time taken may be longer than with pressurization because of the reduction in capacity of the compressor on vacuum, and the limitation of the rate of vacuum breaking output capacity of the inert gas generator.

Continuous Dilution. Inerting by dilution can be a continuous process. An increased flow of inert gas and hence better mixing and a reduction in overall time may be achieved by maintaining the tank under vacuum by passing the diluted efflux through the compressor. Care must be taken to ensure continued good quality inert gas under the increased outflow conditions of the inert gas generator. It is immaterial where the inert gas inlet or the tank outlet is located provided that good mixing is achieved. It is generally found more satisfactory to introduce the inert gas at high speed through the vapor line and exit through the liquid loading line. Where several tanks are to be inerted it may be possible to achieve a reduction in the total quantity of inert gas used, and in the overall time, but inerting two or more tanks in series. This procedure also provides a ready way of inerting pipe work and equipment at the same time.

Ammonia Tank Inerting

Inert gas from a combustion type generator must never be used in preparation for carrying ammonia because of the reaction of ammonia vapor with the carbon dioxide content of such inert gas to form carbamates. Normally, however, inerting prior to loading ammonia is not required because it is recognized that ammonia vapor, though flammable, is not readily ignited. Liquid ammonia must never be sprayed into a tank containing air as there is a risk of creating a static charge which would cause ignition, and the conditions for ammonia stress corrosion cracking. If the ship’s Flag Administration or the loading terminal require inerting prior to loading ammonia then nitrogen should be used.

Compressors and Reliquefaction

If the compressors are used to create vacuum in the tanks, they are to be connected on the suction side to the tank gas suction lines on deck, and on the discharge side to the gas discharge lines on deck. If condensable gas is drawn from the tanks this may be reliquefied and discharged ashore via the condesate and liquid cross over lines, or to the deck tank or another cargo tank.

When using the compressors care must be taken to avoid raising the level of non-condensable gases (nitrogen/inert gas) thus causing an increase in the temperature and pressure in the condenser, and overheating at the compressor outlet as this will stop the reliquefaction process.

Preparation for Gassing up and Cooling Down

If possible a quantity of the next, or compatible, cargo should be taken into a deck tank for the voyage to the loading port. This will enable the gassing up and cooling down process to be started during the ballast passage. The limited quantity of cargo may mean that only one or two tanks can be prepared, but this would be a useful contribution to minimizing port time.

Cargo Tank Purging (Gassing up)

General Information

Neither nitrogen nor CO₂, the main components of inert gas, can be condensed by the ship’s liquefaction plant because at cargo temperatures they are above their critical temperatures. Purging the inert gas out of the cargo tank with vapor of the cargo to be loaded is necessary so that the reliquefaction plant can operate continuously and efficiently. Similarly, on change of cargo without inerting, it may be necessary to purge out the vapor of the previous cargo with vapor of the cargo to be loaded.

Purging Operations

If a liquid product is received it will have to be vaporized in the vaporizers, heated by sea water. If two cargoes are to be carried and sufficient heat is obtained in the vaporizer from sea water, the two purges can be done simultaneously.

For purging all tanks with a common gas the ship’s two gas handling systems can be integrated by inserting spool pieces and removing line blinds. If two gases are to be used the systems must be segregated by removal of the spool pieces and insertion of line blinds.

Purging may be done with the tanks connected in parallel or in series, with the flow of gas in the tanks either from top to bottom or from bottom to top, using the tank upper and lower distribution lines and the gas suction line, as appropriate.

The compressors may be required to boost the pressure of the purge gas to the cargo tanks. Single or two stage compression may be used, but inter cooling is seldom necessary. If the source of the liquid is a deck tank and there is insufficient pressure differential between the deck tank and the evaporator the deck tank must be pressurized by the compressors.

Before purging with the new gas all tank, plant and equipment should be drained, gases expanded and liquids vaporized. The Officer with overall responsibility for cargo operations should check with his opposite number ashore if waste gas is allowed to be blown off at the mast. Allow the Best Practices and Strategies for Effective Cargo Tank Managementcargo tanks to expend to atmospheric pressure before purging. Blow off from the mast is strictly prohibited during a thunderstorm, or if a thunderstorm is imminent. Butadiene and VCM must not be vented to atmosphere.

Purging at Sea. Liquid can be taken directly from the deck tank (if fitted) through the tank sprays (with the exception of ammonia) at a carefully controlled rate to avoid cold liquid impinging on warm tank surfaces. In this case mixing tends to predominate and the mixed cargo/inert gas mixture can be taken into other tanks or vented up the vent riser.

Alternatively, liquid from the deck tank can be vaporized in the cargo vaporizer and the vapor introduced gradually into the top or bottom of the cargo tank, depending on the relative densities, to displace the existing inert gas or vapor to other tanks or to the vent riser. Only when the concentration of cargo vapor in the tanks has reached approximately 90 % should the reliquefaction plant be started and cool down of the system begin.

Due to the limited quantity available from the deck tank, it may not be possible to gas-up all the tanks, but it should be possible for the ship to arrive at the loading terminal with at least one tank gassed-up and partially cooled.

Purging Alongside. If the ship arrives at the loading terminal fully or partially inerted, gassing-up will be completed using cargo supplied from ashore. At certain terminals facilities exist to allow the operation to be carried out alongside but these tend to be the exception as venting hydrocarbon vapors alongside may present a hazard and is therefore prohibited by most terminals and port authorities.

Thus, before a vessel arrives alongside with tanks inerted, the following points must be considered:

• Is venting allowed alongside? If so, what is permissible?
• Is a vapor return facility available?
• Is liquid or vapor provided for purging?
• Will only one tank be purged and cooled down initially from the shore? How much liquid must be taken on board to purge and cool down the remaining tanks?

Before commencing purging operations alongside, the terminal will normally required to sample the tank atmosphere to check that the oxygen is less than 5 % for LPG gases (some terminals require as low as 2 %) or the much lower concentration required for chemical gases such as VCM.

Where no venting to atmosphere is permitted, a vapor return line must be provided and used throughout the purging operation. Either the ship’s cargo compressors or a jetty vapor blower can be used to handle the efflux. Some terminals while prohibiting the venting of cargo vapor, permit the efflux to atmosphere of inert gas. Thus if a displacement method of purging is used, the need for the vapor return flow to shore may be postponed until cargo vapors are detected in the mast vented efflux. This point may be considerably postponed if tanks are purged in series.

Read also: LNG IMO Tanks/Containment Systems

Where a terminal supplies a cargo liquid for purging, it is to be taken on board at a carefully controlled rate and passed through the ship’s vaporizer or allowed to vaporize in the tanks. If the supply is of vapor, this can be introduced into the tanks at the top or bottom depending on the vapor density.

Where a vessel arrives alongside with its tanks containing a cargo vapor which requires to be replaced with the vapor of a different cargo to be loaded, then the terminal will normally provide a vapor return line. The vapors taken ashore will be flared until the desired vapor quality is achieved, at which point cool down can begin.

If no facilities (return line) are available for the ship to purge alongside, it is common practice for the ship to prepare one cargo tank and to take sufficient liquid on board so that the vessel can leave the berth, purge and cool down the remaining cargo tanks using this liquid and then return ready for loading.

Liquid Supplied By a Deck Tank. When the source of liquid is a deck tank the compressors are to be used to raise the pressure in the deck tank. One of the following methods should be used:

• With high vaporization pressures, i. e. high sea water temperature, liquid with low boiling point, the main body of the purge gas from the vaporizer is let to the cargo tanks with the balance going to the compressor to pressurize the deck tank.
• With low vaporization pressure, i. e. low sea water temperatures, liquid with high boiling point the compressors may be used to pressurize both the deck tank and cargo tanks simultaneously. The main body of purge gas from the Vaporizer is led to the cargo tanks with the balance going to pressurize the deck tank.

Cargo Tank Coodown

Introduction

Before loading a Transportation of the Petroleum Gas and Amonia Cargoes on the Fully-Refrigerated LPG Shipsrefrigerated cargo, the tanks must be adequately cooled down in order to minimize thermal stresses and excessive tank pressure during loading. Cool down consists of introducing cargo liquid into a tank at a low and carefully controlled rate. The lower the cargo carriage temperature, the more important the cool down procedure becomes.

The rates at which cargo tanks can be cooled without creating undue thermal stresses depend on the design of the containment system, and are typically a maximum of 10 degc/hour.

Normally the cooling down has been included, or at least started, during gassing-up which commenced during the latter stages of the ballast voyage, or at the terminal if the ship arrived with the cargo tanks inerted.

The procedure is for cargo liquid from shore or from a deck storage tank to be gradually introduced into the tanks through the spray lines. The vapors produced by the rapid evaporation of this liquid may be taken ashore or handled in the ship’s reliquefaction plant and returned to the tanks for continued cooling. Additional liquid is introduced at a controlled rate depending on the tank pressure and temperatures resulting. If vapor is being handled in the ship’s reliquefaction plant, difficulties may be experienced with «incondensibles» remaining from the inert gas. A close watch must be kept on compressor discharge temperatures, and the incondensible gases vented from the top of the reliquefaction condenser as required.

As the cargo containment system cools down watch should be maintained to ensure that associated pressures are maintained within operational limits. Normally pressure control systems supplying air or inert gas will maintain these pressures but watch should be kept on them as the cool down proceeds.

Cool down must continue until liquid begins to form in the bottom of the tanks. This can be seen from the temperature sensors. At this stage, in the case of cool down of cargo tanks for fully refrigerated ammonia for example, the pool of liquid formed will be of approximately -34 °C while the top of the tank may still be at about – 14 °C, i. e. at temperature gradient of approximately 20 °C on cool down. The actual temperature gradient depends on the size of the cargo tanks, position of sprays etc.

Many of the difficulties that occur during the cool down operation result from inadequate purging by inert gas or from Inadequate drying. In the latter case, ice or hydrates may form and ice-up valves, pump shafts, etc. Methanol can be added as anti-freeze provided that cargo is not put off specification.

It is necessary always to maintain a pressure within the cargo tank at least equal to saturated vapor pressure. This can be done by vaporizing liquid using the vaporizer and introducing vapor into the tank with a compressor. Alternatively, vapor can be provided from shore.

Cooling with Vapor Return Ashore

Note: Head office must always be advised prior to using a shore vapor return as this may have important implications affecting the charter party or other commercial considerations.

Tanks are cooled by spraying liquid cargo, supplied from shore, a deck tank or another cargo tank, via the spray line. This need not be refrigerated as long as the tank pressure is kept at or below the saturation pressure of the intended liquid pressure after expansion. Flash gas and evaporated cargo should be led ashore directly or via a compressor. During this operation maintenance of an adequate pressure difference is essential as it can significantly affect the rate of cooling. This should not exceed 10 °C/hour.

Vaporized product is swept on by a compressor and discharged ashore via the vapor return line or condensed and re-injected into the tank. The vapor can also be transferred into the next tank to be cooled. As a precaution against cargo pumps freezing they must be rotated periodically.

Cooling Without Vapor Return

Cargo liquid is sprayed into the tanks via the spray line. The vaporized cargo is then reliquefied in the reliquefaction plant and returned to the tank as liquid. By the intermittent operation of the spray valves a homogeneous cool down is achievable.

Compressors must be controlled so that the tank temperature decreases slowly. This should not exceed 10 °C/hour. If the source of cargo liquid is a deck tank or cargo tank remainders, the available quantity will be limited.

Bear in mind that some of the remaining cargo may have been used for tank purging and is not, therefore, available for cooling down. In this case tanks must be cooled sequentially. The first tank must be pressurized with vapor via the vaporizer until the desired pressure is achieved, or all the cargo has been used. The reliquefaction plant is then to be operated, and the cargo recirculated to the tank as liquid. During intermittent stops in the spraying operation the compressors are to be set to discharge into the next tank. When the first tank has been cooled vapor will be stored in the next tank and cooling can commence by a similar process.

If these conditions are not achievable when sourcing from the deck tank or cargo tank residues, cool down will have to start from a reduced pressure and lower spray temperature. During this operation special attention will need to be paid to the temperature gradient, and care taken with intermittent opening and closing of the spray valves.