Pipes are everywhere. Countless miles of them, an essential but low-profile part of the infrastructure of human society. Multitudes of those pipes are associated with water and sewage, a sector that calls up large quantities of them each year.
Water and sewage pipes have variously been made from metals like lead (the word ‘plumbing’ is derived from the Latin word for this), iron and copper, plus concrete, clay, ceramic, plastic and even wood. Today, plastics are bearing a growing share of the load. Often these are unreinforced thermoplastics such as polyvinyl chloride (PVC), polypropylene (PP) and high density polyethylene (HDPE), but fibre reinforced plastics (FRP) play an important role where superior mechanical and corrosion resistance properties are required.
Reinforced plastic pipes have significant advantages. Mechanically, they are stronger and stiffer, for a given size and wall thickness, than unreinforced equivalents. They therefore need fewer supports and resist greater loads from surrounding soil, backfill, surface traffic etc. They can be pushed and drawn into position more forcefully during installation and may be buried deeper. They can be made strong enough to resist seismic loads, structural settlement and high internal surge pressures. A low coefficient of thermal expansion limits pipe movement in environments characterised by high temperature variations. FRP pipes are not damaged by frost.
In contrast to steel, a traditional pipe material, composite pipes do not corrode, taint water, suffer thermal stress or require cathodic protection; nor do they have to be surveyed periodically throughout life. Composites, appropriately formulated, can withstand even the highly acidic and sulphurous sewage environment. Smooth interior pipe surfaces present minimal impedance to flow and burst strength is generally sufficient to enable pipes to withstand pressure from clearing jets when blockages do occur. Jet pressures required to clear smooth-bore composite pipes are less than those needed for concrete and metal pipes.
Composite pipes are thermally tolerant, being able to withstand temperatures of around 110°C and even up to 200°C in special cases. They can accommodate the continuous pressures used to drive liquids from one place to another, as well as those in forced sewer lines. Where high internal pressures have to be allowed for, hoop strength conferred by filament winding continuous unidirectional fibres can provide the necessary containment. Composites pipes can be highly durable in service, lasting anything up to 50 years, and require minimal maintenance. A future possibility is to include optical fibres in pipes as a means to monitor them for breakages, blockages and other serviceability issues.
The Saudi Arabia-headquartered Amiantit organisation has four decades of experience in the technology of pipes based on traditional materials as well as plastics, both reinforced and non-reinforced. The company has noted growing use of FRP pipes to replace or supplant traditional systems. Expounding further on their advantages, Amiantit contends that fibreglass pipes weigh only a quarter of what comparable ductile iron or steel pipes would weigh, and a tenth the weight of concrete. This, it says, makes them easy to handle and transport, even on steep slopes, thus facilitating safe installation. FRP pipes do not require welding when fitting or modifying them. Instead joints are adhesively bonded or laminated, the major joint elements being incorporated during pipe manufacture.
Amiantit has found that polyesters, appropriately formulated, can resist a range of corrosive or abrasive liquids including hot wastewater, sewage, seawater and industrial cooling water.
Superior mechanical strength and flexibility against overload peaks make FRP pipes a preferred option for irrigation lines, a prime application for the company. Smooth bores result in low friction losses, so minimising the pump energy needed to maintain a given flow rate. Slime builds up only slowly inside smooth-bore pipes, reducing cleaning frequency and costs. FRP pipe, along with proven FRP jointing systems, have made it possible to substantially diminish the perennial problem of wastage – up to 50% – that can occur in irrigation systems based on traditional materials due to leakage, evaporation and operational losses.
Most composite pipes used in water and sewage applications are of glass reinforced polyester (GRP), but within that commonality there is a diversity of features and properties. Much depends on the method of manufacture. Many commodity pipes are made by centrifugal casting, a technique in which chopped fibres and liquid resin are thrown together against the inside of a rotating cylindrical steel mould by centrifugal force until the resin has set. Alternatively, superior properties can be achieved by using continuous fibres, either filament winding these onto an advancing mandrel or helically winding them. All three methods lend themselves to production of pipes in long lengths.
Pipes and fittings are classified according to pipe thickness - 1250, 2500, 5000 and 10 000 Pa ratings are standard - and nominal pressure – for example, 1, 6, 10, 16, 20, 25 and 32 bar. They can be made in diameters of up to 3 or 4 m as standard, or larger as custom-made products. Finished pipe can be delivered as tailor-made spools.
For general purposes, short fibre-based commodity pipes produced by centrifugal casting are often both affordable and adequate. An advantage of this production method compared with filament winding is that high filler content can be achieved. Using sand, chalk or other filler to thicken the walls makes pipes stiffer since stiffness rises in proportion to thickness cubed. Producers like Hobas Engineering in Austria, Saudi Arabia’s Amiantit, Grootint GRP Systems in the Netherlands and Turkey’s Superlit Pipe Industries, seek market leadership with their own particular versions of the technology and are understandably reticent about their material formulations and processes. Even so, the underlying principles are clear.
Grootint uses an automatic, electronically controlled centrifugal casting system. Pipe is formed layer by layer, the material qualities, mould rotation speed and internal temperatures being closely regulated to achieve the designed result. Glass fibre, polyester resin and silica sand are fed into the rotating mould, starting with the external surface of the pipe and working inwards until the required wall thickness is obtained. The resin is specially formulated so as to polymerise only after filling has taken place. Reinforcing glass filaments, pre-cut to the required length, are introduced from the head of a feeder arm which moves back and forth inside the mould distributing the fibres within the layers so as to provide the designed circumferential and axial resistances. Sand, a low-cost material, is used as a filler to bulk out the matrix, especially in pipes that will be buried.
The mould is rotated slowly to start with but, once all the raw material has been introduced, spinning speed is increased to ensure adequate compression while cure takes place. The Dutch company‘s ability to vary the quantity, proportions and orientation of the materials in the pipe layers gives it the flexibility to optimise pipes for a variety of applications, both pressurised and unpressurised. Resins are formulated according not only to the characteristics of the material a pipe is designed to carry, but also the environment in which it will be installed. Thus the highest specifications are called for where corrosive or abrasive liquid (water run-off from agricultural land, seawater, sewage etc), is carried at elevated temperature and the pipe runs through a corrosive environment, whether highly acidic/alkaline soil below ground or chemical fumes above ground.
Amiantit’s commodity pipes have internal diameters of up to 4 m and lengths from 6 m to 24 m. They are widely used to transport drinking-quality water, seawater, desalinated water and sewage in potable water, firefighting, irrigation, cooling and other applications. The company has dubbed its proprietary centrifugal casting process ’C-Tech.’ Hobas Engineering relies exclusively on centrifugal casting to produce its range of composite pipes. Technicians begin by carrying out quality control checks on incoming raw materials and then introducing body resin, liner resin, plus sand and chalk filler/bulking materials into tanks beside the feeder. At the start of a new production cycle, liquid resin is pumped to the feeder and catalyst is added. Roving stored on spools is chopped to the right length by speed-controlled cutters located in the feeder head. Sand and/or chalk filler is propelled by a frequency-controlled screw conveyor to an outlet in the feeder head. In an automated process, the feeder arm deposits pre-determined quantities of the materials, layer by layer, into the rotating mould.
Final application of a resin-rich coating can be used to achieve a smooth bore. In some cases, more usually with custom pipe, a dual resin system is used, the resin for the liner being a different formulation to that used for the bulk of the pipe. For example, a vinyl ester layer can be used as a corrosion liner, with polyester or epoxy forming the bulk of the product.
Close control of raw material delivery quantities is critical to achieving known and consistent quality. This is maintained by frequency-controlled pump drives and precision mass flow meters at the resin pump station, along with equivalent precision metering systems for the filler materials, the glass roving and the catalyst and accelerator needed for curing. All the important quantities and process parameters are recorded and logged as part of the quality control procedure. Product is sampled and tested periodically so that compliance with manufacturing standards can be checked.
As the raw ingredients are injected into the mould, the latter is spun up to high speed so that the materials are pressed against its inside wall at 50-70 G. This high centrifugal force produces a densely compacted pipe wall and totally de-aerated structure. Polymerisation is initiated by spraying hot water onto the outside of the mould. Styrene is captured and extracted for safe disposal. After the curing process, cold water is sprayed onto the mould to facilitate rapid extraction of the moulded pipe. This is then trimmed and bevelled and, as the final operation, a coupling is mounted to one end of each pipe.
Hobas is pleased with its system saying that it is readily adaptable to different products, giving freedom to design pipe to meet specific project requirements. It minimises manufacturing costs by permitting 24 hours production per day. The computer-controlled process is said to yield a product of consistent quality with physical properties uniform within close tolerances. Pipe diameters of up to 2.7 m are produced in standard lengths of up to 6 m. Pipes are produced for potable water lines, gravity sewers, water and sewage treatment plants, hydro-electric plants and outfalls, to name just a few applications.
An issue with commodity pipes, according to Alfred Newberry, an expert with FEMech Engineering in the USA, is that most are subject to a reduction in mechanical properties over time, this ageing effect being more pronounced the higher the pipe operating temperature. This is because, due to the short fibres used, properties are resin dominated. Being a polymer, resin is subject to creep and consequent property loss.
Where higher properties are required, improvement can be achieved by using continuous unidirectional reinforcing fibres. Winding these around a rotating mandrel provides high hoop strength while winding at an angle can enhance strength both circumferentially and axially. Centrifugal casting users wishing to step up in terms of mechanical properties usually make their first step the addition of a process that winds continuous fibre onto an advancing mandrel, typically at an angle of about 55°.
Properties in a 55° filament wound pipe still, however, tend to be resin dominated - even if less so than for an average centrifugally cast pipe. Stress between layers puts the resin in shear, promoting creep. An answer to this is to increase the proportion of fibre in the composite. Glass fibres are not subject to creep and, at temperatures up to about 200°C, the practical upper limit for GRP pipe, they lose minimal strength and stiffness. Fibre content can be increased by close filament winding or by double winding such that two windings cross each other biaxially (cross-filament winding).
Logically, an alternative approach would be to adopt a stronger, more durable resin. Producers could ‘trade up’ from an orthophthalic polyester to an isophthalic polyester or vinyl ester or, better still from the mechanical point of view, to an epoxy. Amiantit, for example, does exactly this, using a precision filament winding process to produce glass reinforced epoxy pipes in which continuous E-glass fibres wound at 55° are held in an amine-cured epoxy. Because epoxy has superior shear properties, creep is less pronounced and much slower than with polyester, so that durability is increased. Another Amiantit Group company, Bonstrand Ltd, produces filament-wound glass-reinforced epoxy (GRE) pipes able to withstand high external loads and used, in particular, for seawater applications on ships.
Most of Amiantit’s filament-wound pipes are, however, glass/polyester products within the company’s Flowtite™ brand. (Amiantit inherited the continuous filament winding technology first developed by Norway’s Flowtite Technology AS, a company which it acquired in 2001.) Flowtite pipes of 80 mm to 4 m in diameter and lengths up to 18 m are designed to withstand pressures ranging from one to 32 bar. They are serving globally in pressure/gravity systems such as raw water pumping mains, drinking water supply pipelines, sewage force mains and in water treatment plants. They are also used for slip lining/re-lining rehabilitation projects, in desalination plants, power plants, fire hydrant lines and in many industrial applications.
Burgeoning demand, up to 20% annual increases having been noted in the three years from 2005, has led Amiantit to expand its Flowtite production capacity with new lines in a number of countries.
Italian company Sarplast Iniziative Industriali filament winds glass/polyester and glass/vinyl ester pipes using dual helix winding machines to produce pipes of diameter 25 mm to 3 m. By adjusting the relative speeds of mandrel advance and movement of the glass distribution head, fibre can be wound at any angle between 45° and 88° to the pipe axis. Cross winding gives the flexibility, says the company, to cater for a wide range of axial and hoop strength requirements.
Wall thickness is built up with repeated winding passes. On large diameter pipes, silica sand can be added to increase wall thickness and hence pipe stiffness. Stiffnesses of 1250, 2500, 5000 and 10 000 Pa are available as standard, though higher values can be provided on request. Standard pressure classes are 4, 6, 10, 16 and 20 bars though, again, intermediate or higher ratings can be provided on request. Pipes are produced with integral bell and spigot lock joints.
Sarplast GRP pipes are used for the transmission of raw or potable water, in low and high pressure civil and industrial sewer systems, in irrigation and in sea water intakes and outfalls. Large diameter pipes find application in cooling water systems for power plants, in water wells and in marine platforms.
Dubai-based Future Pipe Industries uses filament winding to produce glass/polyester and glass/vinyl ester pipes (Fiberstrong™); together with glass/epoxy (Fibermar™) pipes. These are used extensively in water applications, both above and below ground, and for ship piping in the case of Fibermar.
Sekisui Chemical of Japan developed its own continuous winding composite pipe technology. Initially focused on the Japanese market, it has also become active in China, where it operates several helical winding plants originally delivered by Sarplast.
Other substantial producers of filament-wound pipes include Iran’s Farassan and Superlit Pipe Industries Inc of Turkey.
Commodity pipe produced in bulk is affordable and, especially in its filament wound and helically wound forms, increasingly capable in diameters up to about 4 m. For larger diameters and to meet more rigorous mechanical or hydraulic requirements, many producers also offer custom pipes - essentially ‘specials’ engineered and fabricated to meet specific customer requirements. Custom pipes have higher design factors, resulting in greater tolerance to upset conditions and longer service lives. They can also incorporate enhancements that it would be difficult to include in mass-produced pipes available off the shelf. An example would be laminate stiffeners, fabricated into the product to bear loads that might otherwise cause the pipe to buckle and collapse.
According to Ben Bogner, market development specialist with AOC Resins, composite pipe has a market share of about 15-20% in water and sewage applications, this penetration having been gained mainly at the expense of ductile iron and steel pipes. The reputation of composite pipes, battered in earlier decades by issues of deflection, strain corrosion, joint leakage and pipe collapse, has largely recovered. Numerous tests and engineering studies have underpinned the establishment of effective international standards such as ISO 14692, along with regional standards published principally in North America and the European Union. These, along with improved installation practices and a growing track record for durability, have encouraged engineers and consultants to specify with more confidence. Despite some remaining diffidence among specifiers, FRP pipes appear set to maintain their conquering ways.