RTRP (GRE GRP GRV) Piping Specification & Performance Testing

The application of reinforced thermosetting resin pressure piping (RTRP) has grown considerably in chemical plants, oil and gas facilities, water and sewarage systems due its corrosion resistance, strength and durability. FRP, GRP, GRE and GRV are abbreviations for specific types of RTR pipes depending on the resin systems used in the manufacture of piping system.

RTRP is a composite material comprising of glass fiber reinforcements, thermosetting plastic resins and additives. RTRP composites obtain their strength from the fiberglass reinforcement and their corrosion resistance from the resin. A wide range of composites can be manufactured using a combination of glass types, resin types, orientation of glass fibers and use of additives. 

Glass and Resin Types

The commonly used glass types are:

• E-Glass or Electrical glass. Electrical glass is used for the structural layer because of its high strength and is used in the form of rovings, mats, fabrics or chopped fibers.
• C-Glass or Chemical glass. Chemical glass is the natural choice for the initial corrosion barrier surface because of its chemical resistance.
• ECR-Glass. ECR glass is similar to E-glass but without boron trioxide and fluorine and hence is considered environmental friendly. In comparison to E-glass, ECR glass has better chemical and thermal resistance and higher dielectric strength.

The commonly used resin systems are:

• Polyster Resin - Isophthalic resin (These pipes are referred to as GRP - Glass Reinforced Polyster Piping Systems). Isopthalic polysters are suitable for service temperature range of 50°C to 75°C
• Epoxy Resin (These pipes are referred to as GRE - Glass Reinforced Epoxy Piping Systems)
• Vinylester Resin (These pipes are referred to as GRV - Glass Reinforced Vinyester Piping Systems). Vinyl ester resins are suitable for service temperature range of 75°C to 100°C

RTRP piping is also commonly referred to as Fiberglass Reinforced Plastic (FRP) piping system. 

Additives in RTRP piping

Additives like fillers, catalyst, accelerators, inhibitors, aggregates and pigments are used together with glass fiber and resin to achieve the desired properties of the fiber glass product. Fillers are used to enhance a specific property of a laminate. For example, antimony trioxide is added to develop flame retardance or enhance self extinguishing properties. In the presence of chlorine or bromine in the resins, antimony compounds form antimony oxychlorides or oxybromides, which are effective snuffing compounds. Use of Antimony compounds does not enhance flame retardance in resins that do not contain chlorine or bromine. Catalyst is added to resin in presence of an accelerator determines the polymerization reaction at ambient temperature. Accelerator is a chemical compound used together with a catalyst to shorten the polymerization time. Inhibitor is added to the resin to reduce its reactivity at ambient temperature.

Advantage of using RTRP piping

Fiberglass piping systems can offer significant advantages some of which are listed below:

1. RTR piping provides corrosion resistance for a wide range of service including seawater, acids and other chemicals.
2. Fiberglass piping is much lighter than steel and hence does not require heavy lifting equipment for site erection and installation.
3. Because of smooth inner liner the coefficient of friction for fiberglass piping is low which lowers the frictional losses and energy requirements for pumping.
4. Fiberglass piping does not corrode from naturally occuring soil and ground water conditions. Hence use of buried fiberglass piping obviates the need for external corrosion protection as well as cathodic protection requirements.
5. RTR piping does not provide nourishment to marine life and hence fouling is not common. Fouling could occur in pipes lying stagnant for extended periods.

RTRP Piping Manufacturing Process

Fiberglass pipes and fittings are manufactured using one of the following processes:

Filament Winding

ASTM D 2996 covers machine made reinforced thermosetting resin pressure pipe (RTRP) up to 24 inches manufactured by filament winding process. In the filament winding process the glass fiber roving or roving tape is wetted with the resin and wound on a mandrel in a specific pattern unitl the required structural thickness is achieved. The recommended winding angle is 54º ± 2°. This winding angle provides optimum axial and hoop strength to RTR pipe. The method of manufacture results in higher glass fiber to resin ratio and lower wall thickness. Since the pipe is wound on the mandrel, its internal diameter is fixed and the outside diamter varies with the wall thickness of the pipe. Filament winding offers much higher tensile strength and a much uniform glass pattern than hand layup method. The strength of filament wound pipes is a function of fiber glass orientation, pretensioning of the glass roving and resin content. Note that for corrosion resistant applications which require resin rich (about 90% resin content) inner layer, fabrication normally starts with a hand lay-up process to construct the resin rich inner layer.

Below is a good video of how the filament winding actually works in practice.

Centrifugal Casting

ASTM D 2997 covers covers machine-made glass-fiber-reinforced thermosetting-resin pressure pipe manufactured by the centrifugal casting process. In the centrifugal casting process the resin and glass fiber reinforcement are applied to the inside of a mould so the outside diameter remains constant and the inside diameter of pipe is dictated by the wall thickness of the reinforcement. This method of manufacture provides the most consistent mechanical properties and close tolerance on dimensions.

Below is a good video of how centrifugal casting process is used in manufacture of fiberglass pipes.

Hand Layup

Hand Layup method of construction provides a high resin to glass fiber ratio making it suitable for highly corrosive service. Hand layup fittings are typically used in critical applications

Typical Cross-section of RTRP Pipe

A typical RTRP pipe comprises of the following layers:

1. Inner surface of 0.25mm to 0.5mm smooth resin rich (about 90% resin content) laminate reinforced with surface veil to provide optimum corrosion resistance and a smooth finish with low friction factor. Veils are non-woven materials composed of uniformly distributed glass fiber strands. Veils made from C-glass will have good chemical and corrosion resistance whereas veils made from E-glass will have good insulation property. 
2. Before using structural strength layer some specifications may mandate another interior layer of chemical resistant resin rich (about 70%-80% resin content) liner ranging from thickness of 2.5mm upwards in the form of chopped strand mat which limits chemical permeation to the structural layer. The manufacturer recommendation shall be followed for type and thickness of the inner surface veil and the chemical resistant barrier as this will be dependant on the process conditions, fluid velocities, design life etc.
3. The next layers of woven roving and chopped strand mat are used to built the structural strength layer to the desired pipe wall thickness. 
4. The final exterior layer provides protection against weather, fumes, spillage and ultraviolet attack and increases the design life of the pipe.
5. Some specifications may prescribe an ultraviolet resistant polyurethane top coat after completion of hydrotest.

 

RTRP Piping Joining Systems

Fiberglass piping is available with a wide range of jointing systems which are briefly described below. The pressure rating of the joint shall be equal to or greater than the pressure rating of the piping.

Adhesive Bonded Joint or Cemented Joints

Adhesive bonded joints are available in the following types:

• Tapered bell and tapered spigot
• Tapered bell and straight spigot
• Straight bell and straight spigot

The adhesive shall be resistant to the service fluid and resist UV degradation. The adhesive cement kit is recommended and supplied by the manufacturer. Vendors procedures for jointing shall be strictly followed for getting a joint with good integrity. After application of adhesive to the joints, the joint shall be held in place until the adhesive is cured as per the manufacturers recommended application procedure. If the joints are to be cured by application of heat, thermostatically controlled heat blankets shall be used. Tapered bell and tapered spigot joints are preferred by many organizations. 

Mechanical Joints or Rubber Seal Joint

The rubber seal joints shall be designed to withstand the design end thrust pressure. These joints can be either of the screwed or socket type with a locking key and are sealed by means of an O-ring which is compatible with the service fluid. Manufacturers have their own proprietary design of mechanical joints. One such example below is figure of rubber seal lock joint design from manufacturer Wavistrong.

Laminated Joint (Butt and Wrap with Reinforced Overlays)

The laminated joint also referred to as butt and wrap joint is made up by aligning two straight end pipes and then wrapping the joint with multiple layers of resin impregnated glass fiber materials. For contact-molded pipes and fittings the joints shall be made in accordance with section 6.2 of ASTM D6041. They are more suitable for jointing large size pipes typically 16 inches and above. The joint is made by first installing a corrosion barrier similar to pipes and fittings. Below is an example of video on how a laminated joint is made.

Threaded Joint

Threaded pipes are covered in API Spec 15HR. Pipe threads shall be in accordance with API Spec 5B. Threaded fiberglass piping is generally used in high pressure line pipe and downhole tubing.

Flanged Joint

RTR flanges are fabricated using hand layup, filament winding or compression moulding process. When flanged joints are used the fiberglass flanges shall have bolting dimensions to match ASME flanges. Fiberglass flanges are normally designed with flat face. Hence it is necessary to use spacer or filler rings for RTR flanges mating with metallic flanges which have a raised face to prevent transfer of additional bending moment on the flange. RTR flanges also require the use of flat washers on the bolts and nuts to prevent the bolt damaging the flange surface while applying torque. Gaskets should be used as per manufacturers recommendation. Normally recommended gaskets are 3mm thick synthetic rubber such as neoprene, chlorprene, buty rubber or Viton-A full-face gasket with a hardness of 50 to 60 Shore A. The gasket material must be compatible with the service fluid.

RTRP Piping Dimensional Tolerances

Tolerance on Wall Thickness

As per ASTM D2996, the minimum wall thickness of pipe furnished under this specification shall not at any point be less than 87.5 % of the nominal wall thickness published in the manufacturer’s catalog. Dimensions, tolerances and surface finishes shall be measured in accordance with ASTM D3567. In general dimensional tolerances should follow the applicable standard as listed below:

ASTM D2996 - Filament-Wound Fiberglass Pipe

ASTM D2997 - Centrifugally Cast Fiberglass Pipe

ASTM D4024 - Machine made Fiberglass Flanges

ASTM D5421 - Contact-Molded Fiberglass Flanges

ASTM D5685 - Fiberglass Pressure Pipe Fittings

ASTM D6041 - Contact-Molded Fiberglass Corrosion Resistant Pipe and Fittings

RTRP Piping Performance Requirements

RTRP piping exibihit a broad range of mechanical properties depending on the type of resin, type of glass, glass orientation and manufacturing process. Hence test methods are required to record the mechanical properties over a moderate time range and then statistically extrapolate the data to establish long term design values. These are referred to as regression curves. Figure below summarizes the major qualification tests for RTRP piping.Typical RTRP Perfomance Qualification Tests

Hydrostatic Design Stress

ASTM D2992 includes two procedures, Procedure A (cyclic) and Procedure B (static) for determing the hydrostatic design basis (HDB) or pressure design basis (PDB) of fiberglass piping. HDB refers to the estimated long-term hydrostatic strength of RTRP piping in the hoop (circumferential) direction. As per ASTM D2992, hydrostatic design basis (HDB) refers to hoop stress developed for fiberglass pipe by this practice and multiplied by a service design factor to obtain an Hydrostatic Design Stress (HDS). This strength in the wall of the pipe is equal to the circumferential stress due to internal hydrostatic pressure that will fail the pipe when extrapolated to 150 x 10⁶ pressure cycles (Procedure A) or to 100,000 hours under continuously applied pressure (Procedure B). It is recommended that the thickness of the inner liner and outer layer should not be considered in determining the HDB.

Pipe Stiffness

Pipe stifness testing is carried in accordance with ASTM D2412. At 10 percent deflection there should be no visual indication of cracking or structrual damage.

Beam Strength Test

Beam strength test shall be carried out in accordance with ASTM D2925. Many Company specifications allow the tests to be carried out using specimens with unrestrained end caps and simply supported at their centers of gravity. The apparent elastic modulus of pipe, calculated using the total maximum measured deflection shall not be less than 6900 MPa (1,000,000 psi). The maximum measured deflection includes deflection due to weight of pipe, the weight of water and creep during the test.

Hydrostatic Strength

This testing includes test of the complete piping system. All components shall be tested in accordance with ASTM D1599. The test speciment shall be considered to have failed in case of a leak, weep or rupure.

Joint Integrity Test

All restrained gasketed joints shall be tested in accordance with ASTM D4161.

Flange Pressure Rating

Flanges shall be pressure rated in accordance with ASTM D4024.

RTRP Piping Handling and Transportation

After fabrication of fiberglass piping, the ends of the pipes and fittings must be protected with end protectors for storage, handling and transport. The pipes and components must be handled once at a time. Wide slings of textile webbing on spreader bar must be used to handle pipes. Do not use wire rope or chain to handle fiberglass pipes and fittings.

Other General Requirements

Fiberglass fittings should be manufactured with the same resin and fiberglass combination to provide mechanical properties, pressure rating and corrosion resistance as the pipe.

It is recommended to not mix pipes, fitting, flanges and adhesives from different manufacturers in assembly of a fiberglass piping system.

RTRP Piping Applicable Standards

Table below provides a comprehensive list of standards applicable to design, manufacture, inspection and testing of fiberglass piping.