Wednesday, 16 November 2011


Vessel/Tank Materials
Shop welded, field welded, and bolted storage tanks are customarily fabricated from mild quality carbon steel. Most common for welded tanks are A-36 structural steel and A-283 grade “C” structural quality carbon steel. Sheet gauge steels for bolted tanks are of commercial quality having a minimum tensile strength of 52,000 psi. A-612, A-515, and A-516 mild quality low carbon steels are used for fabricating the higher pressure storage products such as spheres and “bullets.”Various API and ASME Codes (listed in the References) to which the storage tank is fabricated, set forth the welding procedures, inspection procedures, testing requirements, and material selection. Some storage applications or service con-ditions (low temperature storage) require storage tanks to be fabricated from metals such as low alloy stainless steel, aluminum, or other specialty materials.

Older non-metallic tanks were customarily constructed from wood. Plastic materials have now replaced wood.  These materials have the advantage of being non-corroding, durable, low cost, and lightweight. Plastic materials used in the construction are polyvinyl chloride, polyethylene, polypropylene, and fiberglass-reinforced polyesters. The fiberglass- reinforced polyester (FRP) tanks are available in the larger sizes and are the most common. FRP tanks are suitable for outdoor as well as indoor applications. FRP tanks with special reinforced shells are designed for underground storage service. Above ground  tanks are primarily vertical, with or without top heads. Non-metallic tanks constructed of unreinforced plastics such as polyvinyl chloride or polyethylene materials are available in sizes up to about 6 ft in diameter by 11 ft high (2400 gallons). Horizontal underground FRP tanks will hold up to 12,000 gallons. Above ground vertical FRP tanks can store from 12,000 to 24,000 gallons, depending upon the shell construction. The temperature limits of plastic tanks are 40°F to 150°F. Color must be added to the outer liner for protection against ultraviolet radiation. The inner liner must be selected for compatibility with the chemical or product stored. Protection from mechanical abuse such as impact loads is a necessity. Good planning dictates that plastic storage should not be located next to flammable storage tanks. All closed plastic tanks should be equipped with pressure relief devices.
Protective Coatings:
Use of internal coatings is primarily to protect the inside surface of the tank against corrosionwhile also protecting the stored contents from contamination. Consideration must always be given to such factors as the type of product being stored, type of coating available, type of surface to be coated, surface preparation, compatibility of coatings, and number of coats required to obtain maximum protection. Many types of internal coatings are available. Due to the unlimited types and applications, only a few will be described as follows:
Coal Tar
Among the oldest and most reliable coatings. Extremely low permeability; protects surface by the mechanical exclusion of moisture and air; extremely water resistant; good resistance to weak mineral acids, alkalis, salts, brine solutions, and other aggressive chemicals.
Epoxy Resin Coatings 
Excellent adhesion, toughness, abrasion resistance, flexibility, durability, and good chemical and moisture resistance. Typical applications include linings for sour crude tanks, floating roof tanks, solvent storage tanks, drilling mud tanks, sour water, treated water, and pipelines.
Rubber Lining — Used as internal lining for storage tanks which are subjected to severe service such as elevated temperatures or for protection from extremely corrosive contents, such as concentrated chlorides and various acids such as chromic, sulfuric, hydrochloric, and phosphoric.
Galvanized — Galvanizing (zinc coating) is highly resistant to most types of corrosion. Bolted steel tanks are ideally suited for galvanizing since all component parts are galvanized by the hot-dip process after fabrication but before erection. Galvanized bolted tanks are recommended where the oil produced contains sulfur compounds and/or is associated with hydrogen sulfide gas. Galvanizing is also effective against corrosion in seacoast areas where atmospheric salt conditions accelerate
corrosion problems.
The basic requirements for external coatings are appearance and weather protection. Numerous types of external coatings are available, ranging from basic one-coat primers to primers with one or more topcoats. Environmental conditions usually dictate the extent of coating applied. Offshore and coastal installations require  more extensive coatings as compared to inland locations.

No comments: