BS4994 (formally:

British Standard British Standards (BS) are the standards produced by the BSI Group which is incorporated under a royal charter and which is formally designated as the national standards body (NSB) for the UK. The BSI Group produces British Standards under the a ...

4994:1987) is the "specification for the design and construction of vessels and

storage tank Storage tanks are containers that hold liquids, compressed gases (gas tank; or in U.S.A "pressure vessel", which is not typically labeled or regulated as a storage tank) or mediums used for the short- or long-term storage of heat or cold. The t ...

s in

reinforced plastic Fibre-reinforced plastic (FRP; also called fibre-reinforced polymer, or in American English ''fiber'') is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass (in fibreglass), carbon (in carbon-fibr ...

s". It specifies a code of practice for use by manufacturers of such containers. With the publication of BS EN 13121-3, BS 4994:1987 Specification for design and construction of vessels and tanks in reinforced plastics is declared obsolescent, which will still cover those tanks still in service as tanks made from GRP are generally accepted to have a long working life. Dual laminate construction, simple FRP with glass mats, or a combination of unidirectional filament winding are common. DUAL LAMINATE : A

thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...

lining material, preferably 3mm to 5mm thick sheet functions as a corrosion barrier. This thermoplastic liner is not considered to contribute mechanical strength. FRP which is constructed over this lining provides the strength requirements for materials to withstand design conditions like pressure, vacuum, hydrostatic load, etc. The choice of thermoplastic is based on the chemical corrosion requirement of the equipment. PP, PVC,

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...

, ECTFE,

CPVC Chlorinated polyvinyl chloride (CPVC) is a thermoplastic produced by chlorination of polyvinyl chloride (PVC) resin. CPVC is significantly more flexible than PVC, and can also withstand higher temperatures. Uses include hot and cold water deli ...

PVDF Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. PVDF is a specialty plastic used in applications requiring the highest pur ...

are used as common thermoplastic liners. FRP (GRP): Glass mats in the form of chopped strand mat and woven roving is most common in hand lay-up method. These mats are laid on the mold and impregnated with 'initiated' resins like polyester, epoxy, vinyl ester, bisphenol epoxy vinyl ester, etc. The choice of resin is based on the chemical corrosion requirement of the equipment. An earlier version of the specification was BS 4994:1973. NEW LOAD UNITS: It is to avoid the uncertainty associated with specifying the thickness alone, that BS4994 introduced the concept of "unit properties". It is property per unit width, per unit mass of reinforcement. For example, UNIT STRENGTH is defined as load in Newtons per millimeter (of laminate width) for a layer consisting of 1 kg of glass per square meter, i.e., the unit is N/mm per kg/m2 glass. This standard still remains as most popular reference material for FRP tanks and vessels

Further reading

See also