How can I apply expansion joints to plastic and fiberglass reinforced piping?
Thermoplastic and fiberglass reinforced (FRP) piping is often used for corrosive or corrosive and abrasive fluids. They generally are applied where temperatures range from negative 70- to 300-deg F (negative 57- to 149-deg C). Chemical compatibility becomes a particularly important consideration in these applications, not only for the piping itself but also for the expansion joints. Both must function with the same resistance to attack or problems will arise.
Thermal and mechanical characteristics differ considerable from those for metal piping. The coefficient of thermal expansion (CTE) is 2 to more than 4 times that of steel. The movements and consequent thrust loads that one has to deal with are significantly greater. Another important design consideration is that the strength of these nonmetallic pipes can decrease rapidly with increased temperature.
Thermoplastic vs. FRP
The coefficients of thermal expansion for thermoplastic pipe typically are 3 to 8 times that of steel, while that for FRP pipe is roughly 2 times that of steel. This results in an expansion joint design that must deal with twice the thermal movement of steel in an unconstrained system. Mechanical considerations also are important. Since FRP is a composite there are two distinctive axial modulii of elasticity: compression and tensile. The axial compression modulus of elasticity varies from 3 to 10 percent that of steel.
The elastic modulus of thermoplastic pipe is considerably less than that of either FRP or steel. It also decreases rapidly as the temperature increases above 100-deg F (38-deg C). As a result, very short support spans are required for thermoplastic pipe at elevated temperatures. Again, something the expansion joint design must take into consideration.
Expansion Joint Design
Optimum expansion joint performance requires a determination of the complete range of thermal movements expected in the system. To do this a calculation must be made of the maximum thermal expansion and thermal contraction that will be encountered during operation. The expansion joint must be capable of absorbing the full range of thermal movement with an appropriate margin of safety.
Case Study
A Common Cooling Water Project with a capacity 1.32 million-gpm provided the main source of cooling water to a whole industrial complex. It had a network of large diameter FRP piping that distributes the cooling water. Nearly 100 large rubber expansion joints in sizes 60-in (DN 1500) to 144-in (DN 3600) were incorporated throughout the system. Each valve location required a dismantling joint.
Figure 1. Overview of FRP piping complex
Figure 2. Typical expansion joint installation
The expansion joints were designed to compress 2- to 3/8-in (60-mm) on itself, functioning as a dismantling joint, while also accommodating the large thermal movements in the system. These very large parts had to be designed for a test pressure of 200-psi (13.5-bar).
The higher sealing requirement was the primary design focus. The body of the expansion joint was reinforced to handle the high pressure. Since the limitation to previous designs had been leakage at the flange, a review of flange designs was undertaken. It was clear that different flange designs had different advantages and disadvantages. The goal was to take the best features from various designs and develop an optimal design.
Figure 3. Flange designs for FRP piping system
Advantages and disadvantages of the designs shown in Illustrations 6, 7, and 8 of Figure 3 were considered. Analyses resulted in the use of a flange design that incorporated the pressure point feature of the solid floating flange design shown in Illustration 6 with that of the full rubber design of Illustration 8. This new design is shown in Illustration-9 and incorporates a bulb in the retaining ring.
The bulb focuses a sealing force on a relatively small area beneath the bolt circle of the rubber flange. As a result the bolt torque is minimized and a higher-pressure rating can be achieved. The new design also does not require periodic retorquing of the bolts.
The new design also requires half the bolt torque that the flat-faced design required for the same test pressure. Pressure ratings that were previously un-obtainable now are obtainable. Flange leakage and the maintenance intensive task of periodic retorquing of the bolts also was no longer a concern.
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Fluid Sealing Association
Sealing Sense is produced by the Fluid Sealing Association as part of our commitment to industry consensus technical education for pump users, contractors, distributors, OEMs, and reps. As a source of technical information on sealing systems and devices, and in cooperation with the European Sealing Association, the FSA also supports development of harmonized standards in all areas of fluid sealing technology. The education is provided in the public interest to enable a balanced assessment of the most effective solutions to pump systems technology issues on rational Total Life Cycle Cost principles.
The Piping Systems Non-Metallic Expansion Joint division of the FSA is one of five with a specific product technology focus. As part of their mission they develop publications such as the Technical Handbook Non-Metallic Expansion Joints and Flexible Pipe Connectors and the Non-Metallic Piping Expansion Joint Installation Guide. The former provides construction, installation, and application details while the latter is a “hands-on” simplified guide for maintenance operators and engineers. Both are primers intended to complement manufacturer’s documents produced by the member companies. In addition, standards such as FSA-NMEJ-701-06 Non-Metallic Expansion Joint Hydrotesting and Vacuum Testing, FSA-NMEJ-702-98 Rubber Flanged Non-Metallic Expansion Joint Installation, Maintenance, and Storage, and FSA-NMEJ-703-99 Specifications of Elastomers Used in Piping Systems Non-Metallic Expansion Joints have been developed in response to important user issues.
The following members of the Piping Systems Non-Metallic Expansion Joint division sponsor this Sealing Sense:
- Bachmann Dampjoint, Inc.
- Crane Resistoflex
- Daikin America, Inc.
- DuPont Performance Elastomers L.L.C.
- 3M Dyneon
- Garlock Sealing Technologies
- General Rubber Corp.
- Holz Rubber Co., Inc.
- KE-Burgmann A/S
- Mercer Rubber Company
- The Metraflex Co.
- Proco Products, Inc.
- Solvay Solexis, Inc.
- Unaflex Incorporated