BS4994 is the "specification for the design and construction of vessels and storage tank
Storage tank
A storage tank is a container, usually for holding liquids, sometimes for compressed gases . The term can be used for reservoirs , and for manufactured containers. The usage of the word tank for reservoirs is common or universal in Indian English, American English and moderately common in British...

s in reinforced plastic
Reinforced plastic
Reinforced plastics are a recent class of materials in which the low modulus and temperature limitations of plastics are overcome by reinforcing it with fibres of high modulus....

s". It specifies a code of practice for use by manufacturers of such containers.

Dual laminate construction, simple FRP with glass mats, or a combination of unidirectional filament winding are common.

DUAL LAMINATE : A thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 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
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...

ECTFE is a fluorocarbon-based polymer , a kind of plastic. It is marketed under the brand name Halar ECTFE by Solvay Solexis...

, CPVC, PVDF 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
FRP tanks and vessels
FRP is a modern composite material of construction for chemical plant equipment like tanks and vessels...

Further reading

— a case study of the design process of a cylindrical vessel, using the BS 4994 methodology
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