What is Fiberglass Reinforced Pipe (FRP)?

Fiberglass reinforced pipes (FRP) are used in trenchless installation of pipelines because it is lightweight, temperature resistant, strong, and highly resistant to chemicals and corrosion. FRP basically consists of fiberglass and thermosetting resins. These pipes are easy to install and rarely need repair because they are low maintenance. In trenchless methods such as direct pipe, and microtunneling, FRP pipes are used because the jacking pressure required to drive the pipe is significantly reduced owing to its smooth outer surface.

Trenchlesspedia Explains Fiberglass Reinforced Pipe (FRP)

FRP pipes are manufactured by a winding process that reinforces thermosetting epoxy resins with continuous glass filaments. As the resins cure they undergo an irreversible chemical reaction giving it superior resistance to temperature variation, while the glass fibers give it superior mechanical strength. In microtunneling steel sleeves were used for the purpose of jacking requiring tons of force. FRP pipes are now extensively used for this purpose because the reduced force to hydraulically jack it makes it cost effective also allowing for longer lengths to be installed. FRP can also be used for trenchless rehabilitation methods like sliplining. FRP is resistant to corrosion and chemicals and makes it an ideal material for pipes that carry such effluents.

Their Properties, Specification, Codes, Joining, Supporting, and Applications (With PDF) – What Is Piping

The full form of FRP is fiberglass-reinforced plastic which is a composite material consisting of a polymer matrix reinforced with fibers. So, an FRP pipe is a pipe manufactured of FRP material by contact molding or filament winding method. Various types of resins like thermosetting polyester, epoxy, phenolic resin, etc are used to get specific FRP pipe properties in the final product. The most widely used reinforcement is the glass fiber “E-glass”. As a corrosion-resistant alternative to metallic piping, the FRP piping system has found worldwide application. By selecting FRP as the pipe material, the need for internal lining, external coating, and cathodic protection can easily be eliminated. FRP piping system is available in a wide range of sizes starting from 1 inch to 144 inches.

Applications of FRP Pipes

Due to its high durability, corrosion resistance, and moderate strength, the use of FRP pipes is increasing day by day. FRP piping systems are used in various industries like:

Potable Water and desalination industries
Chemical, Petrochemical, Oil & Gas industries.
Ducting and Vent piping
Irrigation and Sanitary services
Water distribution and transmission
Slurry piping
Power plants, etc.

Properties of FRP Piping

The main properties that make FRP Pipes an ideal selection for various industries worldwide are

Excellent corrosion resistance
Excellent strength-to-weight ratio. Note that, the strength-to-weight ratio of FRP pipes is higher than steel or other metallic pipes.
Lightweight which makes it easy for handling and transport.
Dimensional stability
Non-toxicity
Low coefficient of friction (>25% better than steel) that ensures excellent flow characteristics.
Good abrasion resistance
Suitable for both aboveground and buried piping
Resistance to biological attacks like bacteria
Non-conductive to electricity and
Low maintenance cost

Typical mechanical properties of the FRP piping system are provided in the table below:

Mechanical Properties of FRP PipeTypical RangeTensile Strength14 to 550 MpaTensile Modulus3.5 to 34.5 GpaFlexural Strength28-480 MpaFlexural Modulus6.9 to 34.5 GpaPoisson’s Ratio0.3Thermal Co-efficient14 to 54 mm/mm/0CSpecific gravity1.2 to 2.3Compressive Strength69-275 MpaTable 1: Range of Mechanical Properties for FRP Pipe

Joining of FRP Pipes

As the FRP pipe lengths are limited by transportation and handling, they are required to be joined. Also, various FRP Pipe fittings need to be joined as per the requirement. The joining system of the FRP pipe should be such that it does not leak for the intended service condition at the operating pressure. Depending on the specific joint configuration and design conditions, the FRP pipe joints may be restrained or unrestrained.

Unrestrained FRP Pipe Joints

Joints that can withstand the internal pressure but can not withstand the longitudinal tensile loads are known as Unrestrained FRP Pipe joints. Examples of such joints are Coupling joints, bell and spigot joints, mechanical coupling joints with elastomeric seals, flanged joints, butt joints with laminated overlay, etc.

Restrained FRP Pipe joints

Such pipe joints are capable of withstanding both internal pressure and longitudinal tensile loads. For these joints, supplemental restraining elements are added to restrict the longitudinal loads. Threaded joints, bell, and spigot joints with laminated overlay or adhesive bonds are examples of Restrained FRP pipe joints.

Note that FRP pipe joint tightness must be ensured following ASTM D4161.

FRP Pipe Fittings

Various FRP pipe fittings are available for proper piping layout needs. Common FRP pipe fittings are

Long radius and short radius FRP Pipe Elbows (22.5 Degrees, 30 Degrees, 45 Degrees, 60 Degrees, 90 Degrees, 180 Degrees)
Tee Connections (Equal Tee and Reducing Tee)
FRP Pipe flanges
Steel backing flanges
Flanged elbows
Smooth flow-reducing elbow
Special radius bend
Lateral connection
Cross connections
Wye Connections
Reducers (Eccentric and Concentric)
Flanged reducers

Codes and Standards for FRP Pipes

Frequently used Codes and Standards that govern the FRP piping details are

ISO 7370
ANSI/AWWA C950
AWWA M45
ISO 14692
AWWA C590
ISO 10467
BS 5480
ISO 10639
ASTM D2996, ASTM D2997, ASTM D5421, ASTM D4024, ASTM D5685

Specification of FRP Pipes

While ordering FRP pipes the following data need to be provided to the vendor:

Pipe Diameter
Design and Operating temperature of the service fluid.
Design, operating, Surge, vacuum, and test pressures.
Live loads in case of buried piping.
Maximum/minimum buried depth and trench Widths.
Details of Soil properties and trench preparation.

Supporting of FRP Piping System

FRP piping systems must be supported properly to avoid excessive sagging. Maximum acceptable sagging is the lower of 12.5 mm or 0.5% of span length. The manufacturer’s guidelines with respect to the supporting shall be followed. Usually, clamped supports with an elastomeric pad are used for support.

Fig. 1: FRP Pipe Supporting

Drawbacks of FRP Piping

The main drawbacks of FRP piping systems are

FRP pipe is not recommended for carrying fluid with temperatures more than 1000 C
Slight degradation from UV rays is found to occur which can be reduced by using pigments, dyes, UV stabilizers, fillers, etc in the resin system.

FRP vs GRP: Difference between FRP and GRP

FRP stands for fiber-reinforced plastic while GRP stands for Glass reinforced plastic. So, from the name, it is clear that there is a change in the reinforcing fiber. But, both FRP and GRP are normally used to indicate the same plastic piping products.

FRP vs Steel: Differences between FRP and Steel

As FRP Pipes have superior corrosion resistance capabilities and over the long term it is economic, Steel pipes are replaced by FRP pipes. So, in this section, it will be great to find the differences between FRP and Steel.

Steel pipes are isotropic while FRP pipes are anisotropic and the properties change with respect to direction.
FRP pipes are more flexible than Steel pipes due to the lower modulus of elasticity.
FRP piping systems are designed considering a higher factor of safety than steel piping. The usual factor of safety in the design of FRP pipes varies in the range of 5 to 10.

Other major differences between FRP and Steel pipes are provided in Table 2 below:

PropertyFRPCSRemarksDensity1850 kg/cu.m7800 kg/cu.mLoads on support are less in the case of FRP pipe as compared to Steel. Handling and transportation of FRP are easier than Steel pipes.Co-efficient of thermal expansion27 X 10-6 mm/mm 0C11 X 10-6 mm/mm 0CExpansion is almost 2.5 times of Carbon Steel Pipe. So more thermal growth in the case of the FRP Piping system.* This value may change from vendor to vendorModulusAxial= 12000 N/sq.mmElastic= 211365 N/sq.mmConsiderable difference in the strength of FRP & CS. Anchor loads are less in FRP Pipes as compared to steel pipes.Shear= 11400 N/sq.mmTensile Strength80-135 MPa456 MPaMechanical Strength is higher for Steel material as compared to FRP.Yield Strength70-135 MPa227 MPaThe yield strength of FRP is lesser than that of Steel.Allowable Stress4,000 PSI to 20,000 PSI20,000 PSIThe strength of the GRE varies drastically and hence proper vendor data is a must.Corrosion resistanceSuperiorInferiorThe corrosion resistance of carbon steel is much lower than that of FRP pipes.JointsThreaded or gluedWeldedFRP joints are to be checked for higher axial loads and pressureTable 2: FRP vs Steel

FRP vs HDPE: Differences between FRP and HDPE Pipes

The main differences between FRP and HDPE pipes are listed in Table 3 below:

FRP PipeHDPE PipeFRP is Orthotropic composite materialHDPE is isotropic material.The cost of FRP pipe is very highThe cost of HDPE pipes is considerably lower than FRP Pipes.Lower thermal expansion coefficientThe thermal expansion coefficients of HDPE pipes are extensively higher as compared to FRP pipes.FRP pipes have a comparatively higher temperature range than HDPE PipesLowe temperature rangeFabrication time is comparatively longerQuicker fabrication.The strength and Elastic modulus for FRP pipes are higher than HDPELower strength and elastic modulus.Easy installation at the siteCostly complex equipment is required for installation.Highly skilled professionals are required for site work of FRP piping systemsHDPE pipe works can be done by semi-skilled operators.Much lighter in weight due to lower all thickness even though the density of FRP is normally higher than HDPEHeavier due to higher wall thickness.A fire-retardant version of FRP pipes can be made.HDPE pipes are highly flammableTable 3: FRP vs HDPE

The initial cost of FRP pipes is normally higher than the metallic piping systems. But when comparing the total cost over the complete service life FRP Pipes come as a winner due to their long service life.

Stress Analysis of FRP Piping System

In piping stress analysis guides or flexibility specifications, FRP lines are considered critical irrespective of their sizes. So, a proper stress analysis must be performed to investigate the stresses, loads, displacements, supports, etc to decide if the FRP piping system will work smoothly throughout its design life. I have developed an online course explaining step-by-step procedures for FRP piping stress analysis. You can check it out here.