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Knowing when Enough is Enough
When high-strength expansion joints are needed, designers are increasingly turning to high-performance plastics such as polytetrafluoroethylene (PTFE). If reliable prediction of their service lives and fatigue limits are to be made, validated test methods are required. The article illustrates test methods for precise determination of the service life of pipe components.

Like anywhere else, infrastructure in the process industry is only as strong as its weakest link. In the process industry tiny components may be the deciding factors for safety, reliability and availability. This especially applies to pipe expansion joints - flexible elements connecting piping systems - which are sometimes used in extreme conditions. Expansion joints must accommodate vibrations or thermally induced changes in length and circumference and withstand even highly aggressive service fluids and high pressures. One material suitable for this type of application is polytetrafluoroethylene (PTFE), more commonly known under its brand name of Teflon.

Research Group Evaluates Test Methods PTFE is a linear semi-crystalline polymer made of fluorinated carbon. Besides other excellent features, the synthetic polymer is lightweight, one of the most thermally stable plastics available and has outstanding chemical resistance. Given this, PTFE is the perfect material for the bellows of expansion joints as it satisfies almost all requirements - especially those relevant for the chemical and process industries. Highly flexible and vibration-proof PTFE expansion joints can compensate for large movements and expansions. They resist adhesion of foreign matter and are non-flammable.

The expansion joints themselves comprise a bellows with several convolutions and loose-type flanges for fitting into piping systems. Types of bellows used comprise convoluted white, non-conductive PTFE bellows and convoluted black PTFE bellows. The latter are preferred by the chemical industry for plants with explosion hazards. Given the wide variety of different chemical processes, the quality of expansion joints must fulfil the highest demands.

Past experience has revealed that - in view of the differences in plastic types, functions and applications - not all test methods supply reliable predictions of service life. Following an incident, a separate research group called "AKChemie PTFE-Kompensatoren" (Chemistry Research Group PTFE Expansion Joints) was established in 2000 and given the task of validating the service life of pipe expansion joints. This research group was made up of Infraserv GmbH, today Interessengemeinschaft Regelwerke Technik (IGR) e. V., the German Community of Interest for Industrial Practice, and represented by experts from TÜV SÜD, and representatives from Bayer AG, BASF AG, Infracor GmbH and Dyneon GmbH.

The experts started by first examining the PTFE expansion joints available on the market and the defects known at this stage. The information and specifications concerning PTFE expansion joints available at this stage were still insufficient. Given this, the research group set itself the targets of developing detailed specifications with the relevant minimum requirements, identifying suitable test methods and using this as a basis for drawing up valid, technical specifications for PTFE expansion joints.

From Failure Sources to Specification Guideline
The examination of existing expansion joints revealed the following production-related key weaknesses: fluctuations in wall thickness, notches in the material and inhomogeneous dispersion of carbon black. To avoid these non-conformities in the future and ensure that the experience and understanding collected would be used to improve the design, the members of the research group invited component manufacturers to join them. Together, the stakeholders specified the identified failure sources and defined safety-related minimum requirements. In May 2004, the work of the research group resulted in „PTFE expansion joints, technical delivery conditions, dimensions‰, a first specification guideline for the chemical industry, and a detailed documented procedure for the quality control of finished components. Today, this guideline is part of the specification guidelines of the chemical industry and used as a basis for the design, manufacturing and delivery of PTFE expansion joints with metal back-up rings.
From Specification Guideline to Valid Test Methods
The parts designed in accordance with the specification guideline met the expectations of the experts. However, under extreme conditions, isolated deformations of expansion joints occurred. Some models burst in service. Consequently, in November 2008, the research group reconvened with the task of specifying which expansion joints can be used safely under which conditions and which test methods are suitable to ensure the prevention of failures in the future. Up to that stage, in the majority of cases the service life of PTFE expansion joints had been calculated on the basis of burst tests. However, comparative testing of expansion joints with defined defects and flawless expansion joints demonstrated that burst tests are unsuitable as they cannot be reproduced.
By contrast, internal-pressure creep rupture tests - an established test method for predicting the service life of piping systems - delivered robust results. Based on the assumption that pipe expansion joints primarily behave like pipes, experts can now successfully determine how components will perform over the course of time at a defined temperature and pressure. Extrapolation of the measured results enable sound statements to be made, including for longer periods.
Pressure-increase tests are also suitable for predicting service life. The pressure determined in the test provides guidance for identifying the test points that are needed for reliable determination of the creeprupture curve. Creep-rupture tests and pressure-increase tests thus enable reliable predictions of the service life of PTFE components to be made provided the actual operating conditions are taken into account.

Safety Through Experience
According to the Federal Environmental Protection Office, technical faults are to blame for 38 per cent of incidents in industrial facilities and thus account for the largest proportion. Problems occurring in process equipment or valves and corrosion damage top the list. The annual costs caused by the resulting damage to property are not to be underestimated. All good arguments for the owners/operators of chemical and process plants, suppliers and designers to rely on the well-founded expert knowledge set forth in the specification guidelines and standards of the "AK-Chemie PTFE-Kompensatoren" research group. To support plant owners/ operators, manufacturers of components and designers in the best possible manner, TÜV SÜD Chemie Service operates a host of testing facilities including a "long-term test rig". Some pipes in TÜV SÜD Chemie ServiceÊs long-term test rig have been subjected to continuous testing since 1956. These pipes prove that our service-life predictions are reliable and well-founded. The plastic experts also have an in-house testing laboratory with test stations, heating ovens and pressure equipment for testing a wide variety of different components and materials.