Tanks & pipes

Production techniques
Machine-made GRP pipes derive mainly from two generic processes:
- Filament winding
- Centrifugal casting
Each has its own specific qualities and advantages
Tanks and vessels are often a combination of filament (helical) winding and hand lay-up.

Filament winding
In the filament winding process, a number of continuous glass fibre rovings, woven glass tapes or unidirectional glass fabrics are impregnated with a matrix resin. These wetted fibres are applied onto the outside of a rotating mandrel in a predetermined pattern and under controlled tension.

The filament winding technique can roughly be divided into two principle types:
- Continuous filament winding
- Helical winding

With continuous filament winding (also called tangential winding), the glass fibres are wound in a closed pattern, or an overlap onto the outside of a (continuously advancing) mandrel, adding chopped fibres, resin and optional additives and fillers. The winding angle and the amount of materials applied at each rotation determine the wall thickness and the wall construction.

With helical winding, repeated passes of wetted fibres around a rotating mandrel in a specific helical pattern, results in a multiple layered wall construction of continuous fibres (either woven or as a unidirectional roving). The angles can vary in theory between 0 and 90° (in practice they average between 45° and 73°), and can be adapted to specific strength requirements of the product.

Centrifugal casting
In the centrifugal casting process, glass fibres and/or mats are placed or applied at the inside of a hollow mandrel (steel tube). As the steel tube rotates at high speed, resin is injected wetting out the reinforcement and optional fillers and additives. These materials are compressed against the wall due to the centrifugal forces, thus forming a dense pipe wall. The main difference compared to filament winding is that high filler content can be achieved.

Hand Lay-up
Hand Lay Up, also called contact moulding, is a production technique suitable for low volume production of GRP components. The fibres are manually placed onto a mould surface and impregnated with resin, usually by using a hand roller. More layers are added and after curing, the composite part can be removed from the mould. The process is very flexible as it can produce very small parts, up to very large parts in a wide variety of shapes and properties. The cycle time per part is very long, and hence this production technique is used mainly for small series or for large complex sh

Resin Transfer Moulding
Resin injection, also called resin transfer moulding (RTM), produces strong fibre reinforced plastic parts with two smooth surfaces. Several layers of dry continuous strand mat, woven roving or cloth are placed in a closable mould. A liquid resin is then injected into the mould, which is subsequently cured. As an option, a pre-form can be used as a core material, enhancing the economics and efficiency of this production technique. The advantages of RTM are the possibility to manufacture complex, high performance structures with a good surface finish, design flexibility and the possibility to integrate more components into one part.

Dual Laminates
Dual-Laminates have been used in chemical plant such as towers, scrubbers, process vessels and tanks for over thirty years in highly corrosive applications, where chemicals such as chlorine and chlor-alkali products, strong acids, strong bases, organic compounds and other corrosive media are present.
Dual laminates consist of a thermoplastic inner liner protected by a fibreglass composite outer layer, thus combining the advantages of thermoplastic corrosion resistance with the high mechanical properties of GRP. Thermoplastic liner materials include most grades used for manufacturing thermoplastic pipe and equipment, such as polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC), polypropylene (PP), the fluoropolymer family and others. PVC and CPVC are usually bonded directly to the FRP laminate using a bonding resin, while other thermoplastic liners are typically manufactured with an embedded fabric or fibreglass backing. This fabric backing can provide a mechanical bond with the fibreglass structural composite but requires compatibility of the base materials.

Liner essentials
The inner corrosion-resistant liner consists mainly of resin, reinforced with a corrosion veil or veils, sometimes backed by a chopped strand fibreglass mat. The veil(s) may be either a corrosion grade fibreglass (C- or ECR-glass), or an organic veil such as polyester (Nexus), ECTFE (Halar) or graphite. An organic veil would be used in environments known to attack glass, such as sodium hydroxide, hydrofluoric acid, etc. After curing, the liner thickness can vary between 0.25 to 2.5 mm at 10% to 50% reinforcement for C/ECR-glass. The fibreglass chopped strand E-glass mat that backs up the veil, generally contains up to 30% +/- reinforcement. These are however just general guidelines; the final corrosion resistant liner may vary depending upon the corrosive properties of the fluid contained. The performance of the liner is highly dependent on the quality and compatibility of both the resin and the reinforcing material, and liner designs are not interchangeable. Each new combination of materials has to be examined carefully. To avoid confusion, the corrosion liner and the corrosion allowance should be specified. Some specifications include the corrosion liner in calculating required overall pipe wall thickness, but generally specifications require the liner be treated as a sacrificial corrosion allowance, and not to be used in any of the pipe structural calculations for pressure and vacuum handling capability.