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Rubber lined tanks of modern chemical tankers

In recent years a number of ships have been fitted with rubber lined tanks, for the transport of phosphoric acid, waste acids and hydrochloric acid.

The mild steel surfaces to be lined should be prepared by grinding away rough weld beads, surface defects, weld splatter etc. Then comes sandblasting to "bright metal" (Sa 2 1/2 - 3), priming with a rubber glue and an application of a contact rubber glue. Thereafter the uncured rubber is applied in wide sheets and pressed on by means of hand rollers. The rubber is usually 4-6 mm thick with reinforcements where mechanical wear may occur, such as hatches etc. The rubber is vulcanized by heating with steam or hot water for a period of 15-30 hours. Synthetic rubbers may have a curing accelerator added and will vulcanize at normal ambient temperature (20-30 degr. C) in a few weeks.

Natural rubber has been used in ships for phosphoric acid. But chloroprene (neoprene) synthetic rubber is a more usual choice on board. It has a much better resistance to sun radiation, oils and ozon, Chloroprene is, however, more expensive and is somewhat more difficult to apply. Chloroprene is resistant to strong acids (not sulphuric acid) and strong alkalies such as caustic soda. In some cases chloroprene lined tanks have been used for backhauls with fuel oils. This rubber will, however, not tolerate light hydrocarbons. Butyl rubber has a very good chemical resistance but is stiff and hard to apply.

Great caution must be exercised so that the rubber will not be subjected to mechanical damage due to falling objects, tools etc, Rubber lined tanks should have a minimum of fittings such as brackets, ladders, internal piping etc which can create weak spots.

Steel piping, including bends, can be rubber lined. The pipe diametres should be chosen somewhat liberally in order to keep liquid velocities relatively low. Membrane types of valves as well as pumps can be lined with hard rubber. As heavy wear may occur on these parts the use of stainless steel is, however, recommended. The advantage of having a pure rubber lined system with regard to a possible high contents of chlorides in the product will then, however, be lost,

Rubber linings are tested for pores and defects by means of a high voltage tester at 10 000 V (chloroprene) and 20 000 V (natural rubber). To check the progress of curing or aging a Shore Durometer is used. Shore tanks are sometimes lined with PVC (poly vinyl chloride) sheets glued into place. But this technique does not seem to be sufficiently developed for use on board ships.





Gaskets and packing

When PTFE (polytetrafluor ethylene, trade names: Teflon, Fluon, Hostaflon TF) came on to the market a few Years ago it solved many gasket problems. This material is resistant to all likely chemical cargoes and all common temperatures on board. The PTFE has some very typical properties: it is expensive, has a low frictional resistance and a low thermal conductivity, does not adhere to other materials and yields or "creeps" when under pressure.

PTFE as a gasket in flange connections should either be reinforced (asbestos or glass fibres), contained in a grove (male/female) or used as a relatively thin envelope around a core of more conventional gasket material. "Creep" can then be controlled. "Blue" asbestos generally gives better chemical resistance against acids (pH 1-4) than "white" asbestos. In "envelope" gaskets the cargo comes into contact with PTFE only, which is an advantage.

It is an advantage to use PTFE packings in pump and valve spindle gland boxes. But remember that the bottom clearance between spindle and housing may have to be less than usual as the packing will have a tendency to creep out this way.

Although PTFE will solve most problems one must remember that common, and cheaper, materials will often suffice. If an existing oil tanker is to carry strong solvents (e g aromatics, ketones etc) flange gaskets of asbestos- reinforced synthetic rubber will usually be acceptable. Flange gaskets of synthetic rubber-asbestos will normally stand up against strong solvents, alcohols, strong alkalies (caustic) and acids. If in doubt, check with the maker. Expansion glands in cargo piping and valve spindle glands, however, should be repacked with PTFE-asbestos packings.

Rubber gaskets are not suitable for oils or solvents. Rubber is suitable for phosphoric acid. Chloroprene rubber (Neoprene) is resistant to caustic soda and ammonia solutions.

Carbon fibre spindle packings are sometimes used in high speed pump glands. They are expensive but have the advantage of low friction, chemical inertness and high thermal conductivity.

Nitril and fluor (Viton) rubber have a very good chemical resistance, to strong solvents as well and are used particularly in O-rings for stem seals in valves.


Other Info pages

Cuprous alloys for shipbuilding of seagoing chemical tankers
Copper and its alloys corrode in many cargoes and may contaminate them, e g styrene, phenol, vinyl chloride, aniline, ammonia solutions etc. Check your cargo against information . Particularly aggressive are the ammonia compounds; they cause inter -crystalline corrosion of cuprous alloys very rapidly. The object in question disintergrates very soon. ....

Use of magnesium and alluminium alloys as sacrificial anodes
Alloys of magnesium and aluminium should never be used in the cargo tank area, due to their poor corrosion resistance in such environments. .....

Introducing rubber lined tanks for the transport of phosphoric acid, waste acids and hydrochloric acid
In recent years a number of ships have been fitted with rubber lined tanks, for the transport of phosphoric acid, waste acids and hydrochloric acid. .....

Tools for shipboard maintenance

Although grit blasting and the use of mechanically powered tools are not normally considered to fall within the definition of hot work, both these operations should only be permitted under controlled conditions. ......

Sea transport of various liquid chemicals at sea
Chemical tankers primarily transport organic and inorganic chemicals as well as vegetable oils and fats. The total global volume of chemicals is estimated at approximately 60 million metric tonnes per year. In addition, the transportation of vegetable oils, alcohols, molasses and lubricating oils amounts to 40 - 45 million tonnes per year. .....

stainless steel as shipbuilding materials for chemical tankers
Iron, steel Mild steel and high tensile steel is, and will continue to be the most important material in the building of chemical tankers and their cargo tanks.Steel is attacked by only a few products, mainly acids and, of course ballast and washing water. Steel itself contaminates very few products, one of them being high purity caustic soda. .... ....

Types of various chemical tankers at sea
modem chemical tanker is primarily designed to carry some of the several hundred hazardous products now covered by the IMO Bulk Chemical Codes. The following general types of chemical carriers have developed since the trade began: ....


Related Info:

Stainless steel as shipbuilding materials for chemical tankers

Practical tank cleaning methods for various noxious liquid cargo

Chemical spill via Annex II overboard line

Risk with toxic substances carried by chemical tankers

Cargo hose disconnection - Personal Safety on Chemical Tankers

Poisoning hazards and countermeasures

Static electricity -How they generate & required safety precautions

Voyage planning and related considerations



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