Coating enclosed tanks and vessels

Over the years the companies business has gradually moved toward coating larger areas. The advent of Belzona® 1391 and more recently Belzona® 1591 has opened up many potential applications within process equipment requiring application within confined tanks and vessels. Large applications themselves need careful planning but working in closed vessels create further pitfalls we all need to be aware of. Recent technical queries indicate that not everyone is familiar with some of the special problems tank coating creates.

Firstly health and safety at work regulations require special procedures to be followed when personnel are entering an enclosed vessel and such procedures should be implemented and carefully followed to ensure the safety of all personnel involved. (See UK Technical Information circular 28/98 for further reading).

Coating large areas requires careful planning to ensure overcoat times for the surface preparation and subsequent coats are complied with. Enclosed vessels make this planning more critical. Access may be limited and coating the floor and roof of a tank at the same time is normally not possible. It is common practice and normally necessary to split the application into 2 or more sections. After coating the 1st section it is allowed to cure before blasting the next section. Raising and holding the substrate temperature well above the ambient air temperature will help hold the blast and reduce substrate oxidation.

The greatest problem liable to be experienced in coating an enclosed vessel is condensation and moisture. People working within the vessel will raise the moisture content (humidity) within the tank, reducing the overcoat time of the prepared substrate and potentially leading to condensation on the substrate during application. To overcome such problems it is important to warm the vessel to ensure the substrate temperature is above ambient and is maintained above ambient during the coating process.

Typically the substrate should be maintained a minimum 3-5°C higher than the ambient air temperature throughout the application to avoid condensation. Fluctuating air temperatures can complicate this procedure and in such situations the monitoring of temperatures and humidity to prevent application at or below the dew point is particularly important.

In addition a constant flow of warm air through the vessel will minimise the possibility of condensation occurring. (See also US Technical Information 3/22/96 covering use of dehumidification equipment and "Climate control" in the Application Know How area of the Belzona Electronic Library).

Vessel and tank applications are invariably more critical than other application areas. Once in service, inspection is limited. Since the coating is most likely present to prevent corrosion of the underlying metal substrate ANY breakdown in the coating, however small, can allow unseen corrosion to rapidly occur. The result can be a sudden leak in the tank wall as the corrosive contents eat their way through what may have started out as a pinhole in the coating.

Inspection of tank coatings is important and a high voltage spark tester is the ideal tool as it will not only accurately detect faults in the coating, but will identify unsuspected thin areas in the coating also. The wet sponge test is very much second best (but the only final inspection option on Belzona® 1321 and Belzona® 1391).

Ongoing inspection of the coating during application is important to avoid too many repairs being needed. To facilitate this and to ensure good application conditions adequate lighting must be provided. In the Oil & Gas and Petrochem industries vessels may be situated in hazardous areas and all equipment may well require to be intrinsically safe. Flame-proof generators, compressors, tools and lighting may be required and the provision and availability of such equipment needs considering at the initial planning stage.

All these aspects lead to the importance of an experienced contractor/application company being involved in such applications. Neglect of even one small aspect could lead to catastrophic results and the criticality of such applications should not be ignored. Careful planning should, however, eliminate most of the pitfalls but no short cuts or compromise of the Instructions for Use or Belzona Specification should be contemplated.

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Posted 11/08/02

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