This illustrated guide shows some common problems that can occur with polymer and elastomeric materials that are different from those that occur with metal seals and components.
The failure of polymer (plastic and elastomeric) components and its consequences can be as serious as the failure of metal equipment. The information presented describes some of the properties that affect polymer components of equipment used in industrial facilities. This information applies to some legacy O-rings, lined pipe, fiber reinforced plastic (FRP) and lined pipe. Examples of properties such as penetration, glass temperature, and viscoelasticity and their implications are discussed.
On January 28, 1986, the Challenger space shuttle disaster shocked the world. The explosion occurred because the O-ring did not seal properly.
The faults described in this article introduce some of the characteristics of non-metallic faults affecting equipment used in industrial applications. For each case, important polymer properties are discussed.
Elastomers have a glass transition temperature, which is defined as “the temperature at which an amorphous material, such as glass or polymer, changes from a brittle glassy state to a ductile state” [1].
Elastomers have compression set – “defined as the percentage of strain that an elastomer cannot recover after a fixed period of time at a given extrusion and temperature” [2]. According to the author, compression refers to the ability of rubber to return to its original shape. In many cases, the compression gain is offset by some expansion that occurs during use. However, as the example below shows, this is not always the case.
Fault 1: Low ambient temperature (36°F) prior to launch resulted in insufficient Viton O-rings on the Space Shuttle Challenger. As stated in various accident investigations: “At temperatures below 50°F, the Viton V747-75 O-ring is not flexible enough to track the opening of the test gap” [3]. The glass transition temperature causes the Challenger O-ring to fail to seal properly.
Problem 2: The seals shown in Figures 1 and 2 are primarily exposed to water and steam. The seals were assembled on site using ethylene propylene diene monomer (EPDM). However, they are testing fluoroelastomers (FKM) such as Viton) and perfluoroelastomer (FFKM) such as Kalrez O-rings. Although sizes vary, all O-rings shown in Figure 2 start out the same size:
What’s happened? The use of steam can be a problem for elastomers. For steam applications above 250°F, expansion and contraction deformations FKM and FFKM must be taken into account in packing design calculations. Different elastomers have certain advantages and disadvantages, even those that have high chemical resistance. Any changes require careful maintenance.
General notes on elastomers. In general, the use of elastomers at temperatures above 250°F and below 35°F is specialized and may require designer input.
It is important to determine the elastomeric composition used. Fourier transform infrared spectroscopy (FTIR) can distinguish between significantly different types of elastomers, such as EPDM, FKM and FFKM mentioned above. However, testing to distinguish one FKM compound from another can be challenging. O-rings made by different manufacturers may have different fillers, vulcanizations, and treatments. All this has a significant impact on compression set, chemical resistance and low-temperature characteristics.
Polymers have long, repeating molecular chains that allow certain liquids to penetrate them. Unlike metals, which have a crystalline structure, long molecules intertwine with each other like a strand of cooked spaghetti. Physically, very small molecules such as water/steam and gases can penetrate. Some molecules are small enough to fit through the gaps between individual chains.
Failure 3: Typically, documenting a failure analysis investigation begins with obtaining images of the parts. However, the flat, flexible, gasoline-smelling piece of plastic received on Friday had turned into a hard round pipe by Monday (the time the photo was taken). The component is reportedly a polyethylene (PE) pipe jacket used to protect electrical components below ground level at a gas station. The flat flexible plastic piece you received did not protect the cable. The penetration of gasoline caused physical, not chemical changes – the polyethylene pipe did not decompose. However, it is necessary to penetrate less softened pipes.
Fault 4. Many industrial facilities use Teflon-coated steel pipes for water treatment, acid treatment and where the presence of metal contaminants is excluded (for example, in the food industry). Teflon-coated pipes have vents that allow water seeping into the annular space between the steel and the lining to drain away. However, lined pipes have a shelf life after prolonged use.
Figure 4 shows a Teflon-lined pipe that has been used to supply HCl for over ten years. A large amount of steel corrosion products accumulates in the annular space between the liner and the steel pipe. The product pushed the lining inward, causing damage as shown in Figure 5. Corrosion of the steel continues until the pipe begins to leak.
In addition, creep occurs on the Teflon flange surface. Creep is defined as deformation (deformation) under constant load. As with metals, the creep of polymers increases with increasing temperature. However, unlike steel, creep occurs at room temperature. Most likely, as the cross-section of the flange surface decreases, the bolts of the steel pipe are overtightened until the ring crack appears, shown in the photo. Circular cracks further expose the steel pipe to HCl.
Failure 5: High-density polyethylene (HDPE) liners are commonly used in the oil and gas industry to repair corroded steel water injection lines. However, there are specific regulatory requirements for liner pressure relief. Figures 6 and 7 show a failed liner. Damage to a single valve liner occurs when the annulus pressure exceeds the internal operating pressure – the liner fails due to penetration. For HDPE liners, the best way to prevent this failure is to avoid rapid depressurization of the pipe.
The strength of fiberglass parts decreases with repeated use. Several layers may delaminate and crack over time. API 15 HR “High Pressure Fiberglass Linear Pipe” contains a statement that a 20% change in pressure is the test and repair limit. Section 13.1.2.8 of Canadian Standard CSA Z662, Petroleum and Gas Pipeline Systems, specifies that pressure fluctuations must be maintained below 20% of the pipe manufacturer’s pressure rating. Otherwise, the design pressure may be reduced by up to 50%. When designing FRP and FRP with cladding, cyclic loads must be taken into account.
Fault 6: The bottom (6 o’clock) side of the fiberglass (FRP) pipe used to supply salt water is covered with high-density polyethylene. The failed part, the good part after failure, and the third component (representing the post-manufacturing component) were tested. In particular, the cross-section of the failed section was compared with the cross-section of a prefabricated pipe of the same size (see Figures 8 and 9). Note that the failed cross-section has extensive intralaminar cracks that are not present in the fabricated pipe. Delamination occurred in both new and failed pipes. Delamination is common in fiberglass with a high glass content; High glass content gives greater strength. The pipeline was subject to severe pressure fluctuations (more than 20%) and failed due to cyclic loading.
Figure 9. Here are two more cross-sections of finished fiberglass in a high-density polyethylene-lined fiberglass pipe.
During on-site installation, smaller sections of pipe are connected – these connections are critical. Typically, two pieces of pipe are butted together and the gap between the pipes is filled with “putty.” The joints are then wrapped in several layers of wide-width fiberglass reinforcement and impregnated with resin. The outer surface of the joint must have sufficient steel coating.
Non-metallic materials such as liners and fiberglass are viscoelastic. Although this characteristic is difficult to explain, its manifestations are common: damage usually occurs during installation, but leakage does not occur immediately. “Viscoelasticity is a property of a material that exhibits both viscous and elastic properties when deformed. Viscous materials (such as honey) resist shear flow and deform linearly over time when stress is applied. Elastic materials (such as steel) will deform immediately, but also quickly return to their original state after stress is removed. Viscoelastic materials have both properties and therefore exhibit time-varying deformation. Elasticity typically results from the stretching of bonds along crystalline planes in ordered solids, while viscosity results from the diffusion of atoms or molecules within an amorphous material ” [4].
Fiberglass and plastic components require special care during installation and handling. Otherwise, they may crack and damage may not become apparent until long after hydrostatic testing.
Most failures of fiberglass linings occur due to damage during installation [5]. Hydrostatic testing is necessary but does not detect minor damage that may occur during use.
Figure 10. Shown here are the inner (left) and outer (right) interfaces between fiberglass pipe segments.
Defect 7. Figure 10 shows the connection of two sections of fiberglass pipes. Figure 11 shows the cross section of the connection. The outer surface of the pipe was not sufficiently reinforced and sealed, and the pipe broke during transportation. Recommendations for reinforcement of joints are given in DIN 16966, CSA Z662 and ASME NM.2.
High-density polyethylene pipes are lightweight, corrosion-resistant, and are commonly used for gas and water pipes, including fire hoses on factory sites. Most failures on these lines are associated with damage received during excavation work [6]. However, slow crack growth (SCG) failure can also occur at relatively low stresses and minimal strains. According to reports, “SCG is a common failure mode in underground polyethylene (PE) pipelines with a design life of 50 years” [7].
Fault 8: SCG has formed in the fire hose after more than 20 years of use. Its fracture has the following characteristics:
SCG failure is characterized by a fracture pattern: it has minimal deformation and occurs due to multiple concentric rings. Once the SCG area increases to approximately 2 x 1.5 inches, the crack propagates rapidly and macroscopic features become less obvious (Figures 12-14). The line may experience load changes of more than 10% each week. Old HDPE joints have been reported to be more resistant to failure due to load fluctuations than old HDPE joints [8]. However, existing facilities should consider developing SCG as HDPE fire hoses age.
Figure 12. This photo shows where the T-branch intersects with the main pipe, creating the crack indicated by the red arrow.
Rice. 14. Here you can see close up the fracture surface of the T-shaped branch to the main T-shaped pipe. There are obvious cracks on the inner surface.
Intermediate Bulk Containers (IBCs) are suitable for storing and transporting small quantities of chemicals (Figure 15). They are so reliable that it is easy to forget that their failure can pose a significant danger. However, MDS failures can result in significant financial losses, some of which are examined by the authors. Most failures are caused by improper handling [9-11]. Although IBC appears simple to inspect, cracks in HDPE caused by improper handling are difficult to detect. For asset managers in companies that frequently handle bulk containers containing hazardous products, regular and thorough external and internal inspections are mandatory. in the United States.
Ultraviolet (UV) damage and aging are prevalent in polymers. This means we must carefully follow O-ring storage instructions and consider the impact on the life of external components such as open top tanks and pond linings. While we need to optimize (minimize) the maintenance budget, some inspection of external components is necessary, especially those exposed to sunlight (Figure 16).
Characteristics such as glass transition temperature, compression set, penetration, room temperature creep, viscoelasticity, slow crack propagation, etc. determine the performance characteristics of plastic and elastomeric parts. To ensure effective and efficient maintenance of critical components, these properties must be taken into account, and polymers must be aware of these properties.
The authors would like to thank insightful clients and colleagues for sharing their findings with the industry.
1. Lewis Sr., Richard J., Hawley’s Concise Dictionary of Chemistry, 12th edition, Thomas Press International, London, UK, 1992.
2. Internet source: https://promo.parker.com/promotionsite/oring-ehandbook/us/en/ehome/laboratory-compression-set.
3. Lach, Cynthia L., Effect of Temperature and O-Ring Surface Treatment on the Sealing Ability of Viton V747-75. NASA Technical Paper 3391, 1993, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940013602.pdf.
5. Best Practices for Canadian Oil and Gas Producers (CAPP), “Using Reinforced Composite (Non-Metallic) Pipeline,” April 2017.
6. Maupin J. and Mamun M. Failure, Risk and Hazard Analysis of Plastic Pipe, DOT Project No. 194, 2009.
7. Xiangpeng Luo, Jianfeng Shi and Jingyan Zheng, Mechanisms of Slow Crack Growth in Polyethylene: Finite Element Methods, 2015 ASME Pressure Vessels and Piping Conference, Boston, MA, 2015.
8. Oliphant, K., Conrad, M., and Bryce, W., Fatigue of Plastic Water Pipe: Technical Review and Recommendations for Fatigue Design of PE4710 Pipe, Technical Report on behalf of the Plastic Pipe Association, May 2012.
9. CBA/SIA Guidelines for the Storage of Liquids in Intermediate Bulk Containers, ICB Issue 2, October 2018 Online: www.chemical.org.uk/wp-content/uploads/2018/11/ibc-guidance-issue-2- 2018-1.pdf.
10. Beale, Christopher J., Way, Charter, Causes of IBC Leaks in Chemical Plants – An Analysis of Operating Experience, Seminar Series No. 154, IChemE, Rugby, UK, 2008, online: https://www.icheme. org/media/9737/xx-paper-42.pdf.
11. Madden, D., Caring for IBC Totes: Five Tips to Make Them Last, posted in Bulk Containers, IBC Totes, Sustainability, posted on blog.containerexchanger.com, September 15, 2018.
Ana Benz is Chief Engineer at IRISNDT (5311 86th Street, Edmonton, Alberta, Canada T6E 5T8; Phone: 780-577-4481; Email: [email protected]). She worked as a corrosion, failure and inspection specialist for 24 years. Her experience includes conducting inspections using advanced inspection techniques and organizing plant inspection programs. Mercedes-Benz serves the chemical processing industry, petrochemical plants, fertilizer plants and nickel plants worldwide, as well as oil and gas production plants. She received a degree in materials engineering from Universidad Simon Bolivar in Venezuela and a master’s degree in materials engineering from the University of British Columbia. She holds several Canadian General Standards Board (CGSB) non-destructive testing certifications, as well as API 510 certification and CWB Group Level 3 certification. Benz was a member of the NACE Edmonton Executive Branch for 15 years and previously served in various positions with the Edmonton Branch Canadian Welding Society.
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Post time: Nov-18-2023