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LPG tanker cargo tank cooling safety guideline

Requirement of tank cooling

Before loading a refrigerated cargo, the tanks must be adequately cooled down in order to minimise 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 deg c/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.

Fully refrigerated gas carrier underway
Fig:Fully refrigerated LPG carrier at sea

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 vapours 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 vapour is being handled in the ship’s reliquefaction plant, difficulties may be experienced with “incondensable” remaining from the inert gas. A close watch must be kept on compressor discharge temperatures, and the incondensable 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 vapour pressure. This can be done by vaporising liquid using the vaporiser and introducing vapour into the tank with a compressor. Alternatively, vapour can be provided from shore.

Cooling with vapour return ashore

Note: head office must always be advised prior to using a shore vapour 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.

Vaporised product is swept on by a compressor and discharged ashore via the vapour return line or condensed and re-injected into the tank. The vapour 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 vapour return

Cargo liquid is sprayed into the tanks via the spray line. The vaporised 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 pressurised with vapour via the vaporiser 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 vapour 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. As a precaution against cargo pumps freezing they must be rotated periodically.

Related Information:

  1. LPG cargo loading special guideline

  2. Tackling fire onboard LNG & LPG ships

  3. Detail guideline for Ballast operation at sea by LPG carrier

  4. Handling cargo related documents for LPG carrier

  5. Cargo sampling procedure for liquefied gas cargo

  6. Cargo measurement and calculation guideline for LPG carriers

  7. Handling Propylene oxide, Ethylene oxide mixtures

  8. Special characteristics of Vinyl Chloride Monomer & Butadiene

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