Glass Reinforced Plastic (GRP) is widely used in the marine environment because of its many useful properties. Glass Reinforced Plastic is composed of fine glass fibres woven to form a flexible fabric. The weave pattern determines the strength and weight of the GRP. There are different weave patterns used for the production of GRP depending on the application. Different resins may be added to the woven fiberglass to give it added strength and allow it to be molded into numerous shapes.
Weave patterns for GRP
Advantages and Properties of Glass Reinforced Plastic piping
GRP pipes have a major advantage of corrosion resistance (a typical property of plastics), their strength and durability. This makes them ideal for conveying all kinds of water, chemicals and other cargoes. The major advantages of GRP are:
- Corrosion resistance, of both the wall in contact with the fluid carried by the pipe and the external wall. Therefore the need for extra protection such as coating or painting is minimal.
- Chemical resistance. GRP is minimally reactive, making it ideal for chemical pipelines and other areas where chemicals are used.
- Easy installation procedure
- Low weight
- The smooth internal walls avoids the formation of deposits.
- Requires minimal maintenance after installation
- GRP can be made fire resistant by the addition of special additives.
- Glass reinforced plastics do not conduct electricity, they also resist electric sparks.
- Long life.
Raw Materials Used in the production of GRP
Polyester resins and glass reinforcements are the main raw materials used in the manufacture of Glass Reinforced Plastics.
Polyester resins are the major resins employed in the manufacture of GRP, although other resins such as phynol, epoxy and vinylester are used when certain performance characteristics are required. Polyesters offer a good balance of electrical, mechanical and chemical resistance properties, at reasonably low cost. They are easy to handle and also have good dimensional stability. Polyesters are usually manufactured by reacting glycols and dibasic organic acids, both of which contain a double-bonded pair of carbon atoms. The polyester resin is a thermoset, when subjected to the right conditions it is capable of being cured from solid or liquid state. There are two main types of polyester resins used. Orthophthalic polyester resin is the standard resin widely used. Isophthalic polyester resin is the preferred material in the marine industry where superior water resistance or required or high corrosive products have to be conveyed.
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Glass fibre is the most widely used reinforcing material. Various components are mixed to produce a basic compound which is then melted at high temperature in a furnace to produce glass. The molten glass is drawn into thin filaments which are then processed into strands that are the basic reinforcements for the resins. Glass fibres are made from silica (SiO2) sand which melts at 1993K or 1720°C. SiO2 is heated up to 1720°C and then rapidly cooled to produce glass. Although glass fibre can be made from SiO2 alone, other materials can be added to improve or add certain properties. There are major grades of glass necessary to produce pipes used in the marine environment;
“C” glass grade: this grade has improved resistance to chemical attack and mostly used for making the inner liner of pipes.
“E” glass grade: strong and low in alkali content, good tensile and compressive strength, good electrical properties but resistance to impact is relatively poor. This grade is not totally inert in chemically corrosive environments so a protective liner is required to protect the integrity of the glass.
“E-CR” glass: this grade has long term resistance to chemicals. Although the composition doesn’t much from the E-glass, the resistance to acid corrosion is a lot better. This grade has Lloyd’s approval and are certified to meet the Boeing BMS-8-79 Specification.
Some additional raw materials may include catalysts for polyester resins, accelerators, inhibitors, thixotropic agents, silica.
Regulations associated with the use of GRP on ships
The International maritime Organization has specific guidelines for the use of plastics for pipes on ships. The guidelines provide acceptance standards, design requirements, and fire testing performance standards for plastics in piping systems independent of service or location. The guidelines refer to both thermosetting and thermoplastic plastics, with or without reinforcement.
Material Design Properties and Design Criteria for GRP Piping used for cargo transfer
The piping should have adequate strength to accommodate severe conditions of temperature, pressure, and the dynamic conditions of the environment. Pipes should also have a minimum wall thickness to ensure sufficient strength for handling, cargo transfer and other activities on board ships.
Internal pressure: The internal pressure of piping systems should not be less than the maximum working pressure expected under working conditions or the highest set pressure of fitted safety or pressure relieve valves. The nominal internal pressure for a pipe should be calculated by dividing the short-term hydrostatic test failure by a factor of safety of 4.[i]
External pressure: In the design of these piping systems, the external pressure should not be less than the sum of maximum potential head of liquid outside the pipe. The nominal external pressure should be calculated by the collapse test pressure by a factor of safety of 3.[ii]
Axial Strength: the sum of all longitudinal stresses due to weight, pressure or dynamic loads should not be more than the allowable stress in the longitudinal direction. In this case where Glass Reinforced Plastic pipes are used, the sum of the longitudinal stresses should not surpass half of the nominal circumferential stress derived from the nominal internal pressure determined by dividing the short-term hydrostatic test failure by a factor of safety of 4.[iii]
Temperature: the piping system should be able to serve perfectly over the range of temperatures it will experience. The maximum working temperature should be at least 20°C less than the minimum heat distortion temperature of the resin. The minimum heat distortion temperature shouldn’t be lower than 80°C. Where pipes are to be used for low-temperature services, special attention should be given to the material properties.[iv]
Impact Resistance: the minimum resistance to impact of the piping should be to the satisfaction of the administration.
Ageing: When selecting design materials for pipes, the manufacturer should ensure that the environmental effects will not degrade the physical, mechanical or chemical properties of the pipes below the values to meet these guidelines. Satisfactory ageing tests should be performed on materials before selection.
Erosion Resistance: The effect of erosion should be considered when designing pipes. Adequate measures (such as increased wall thickness, use of special liners etc.) should be taken to ensure the erosion resistance of the pipes.
Compatibility: the materials used in producing the pipes should be compatible with the fluids the pipes are used to convey or immersed in.[v]
“The manufacturer should have a quality system that meets ISO 9001, “Quality Systems – Model for quality assurance in design/development, production, installation and servicing”, or equivalent.”[vi] The quality system during manufacture should bases on requirements and standards necessary to produce pipes with the most suitable physical and mechanicals properties.
GRP Pipes and Metal Pipes Comparison
Because of the materials used in the manufacture of glass reinforced plastics, they have good strength and light weight. GRP pipes have smooth, non-absorbent internal walls to allow for smooth flow of the cargoes they convey. They are also inert in nature and very durable. Some other advantages of GRP pipes include high tensile strength, corrosion resistance, low density, scaling resistance, flexibility and easy installation, long life and minimal maintenance.
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Metal pipes are more expensive to manufacture, they react to changes in temperature and corrode quite easily. Metals are also heavier and have a more tedious installation process. Galvanized steel pipes have a high possibility of lead contamination. Over time, the physical and mechanical properties of metals degrade. GRP piping is most suitable for cargo transfer as it has the most advantages.
The Reinforced Plastics Handbook by John Murphy http://books.google.co.uk/books?id=jo7KwTldSNQC&printsec=frontcover#v=onepage&q&f=false
Crystic Composites Handbook
December 2005, Scott Bader Company LTD http://www.scottbader.com/uploads/files/3381_crystic-handbook-dec-05.pdf
Resolution A.753(18), Guidelines For the Application of Plastic Pipes on Ships
http://www.netcomposites.com/guide/polyester-resins/8 25 August 2014
http://plumcots.com/grp-pipes-metal-pipes-comparison.php 25 August 2014
[i] Resolution A.753(18), Guidelines For the Application of Plastic Pipes on Ships, page 345, paragraph 2.1.2 http://www.imo.org/blast/blastDataHelper.asp?data_id=22601&filename=A753%2818%29.pdf
[ii] Resolution A.753(18), Guidelines For the Application of Plastic Pipes on Ships, page 345, paragraph 2.1.3
[iii] Resolution A.753(18), Guidelines For the Application of Plastic Pipes on Ships, page 345, paragraph 2.1.4
[iv] Resolution A.753(18), Page 345, paragraph 2.1.5
[v] Resolution A.753(18), Page 346
[vi] Resolution A.753(18), Chapter 3, Material Approval and Quality Control During Manufacture
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