Fundamental Characterization of PP Extrusion
Qualification Testing and Long-Term Durability of Plastic-Lined Metallic Pipe, Fittings and Flanges for Corrosive Applications Dr. Bryan E. Hauger, Bryan Hauger Consulting, Inc. Longmont, CO Abstract The potential for successful long-term application of plastic-lined metallic pipe, fittings and flanges in corrosive applications is influenced by a number of factors. One important factor is qualification testing that addresses hightemperature, low-temperature, thermal cycling and vacuum performance. Recently, clarification of the circumstances which lead to re-qualification testing has been balloted to an ASTM standard specification for such plastic-lined composite materials. This paper will document field failures which were potentially avoidable through appropriate qualification (or re-qualification) testing specifically related to dimensional measurements, liner ductility/strength and vacuum testing.
Introduction Many product details of plastic-lined metallic pipe, fittings, and flanges which serve a critical role in the safe transport of corrosive fluids are standardized 1 in ASTM F1545. The metallic housing primarily contributes strength and ease of assembly with ASTM F1545 standardizing dimensions and materials of construction for pipe, fittings and flanges. The plastic liner primarily contributes chemical resistance with ASTM F1545 establishing minimum requirements for several materials including Polypropylene, Poly (vinylidene fluoride) (PVDF) and, most importantly, Polytetrafluoroethylene (PTFE). Rapid and low-cost inspection of each spool and fitting by electrostatically or hydrostatically testing is mandated by ASTM F1545. ASTM F1545 also provides qualification testing requirements which seek to ensure that all compliant products have well understood hightemperature, low-temperature, thermal cycling and vacuum performance but are tested less frequently and are sometimes destructive. Recently, changes to the language of ASTM F1545 have been balloted to clarify a management of change philosophy which mandates requalification under some circumstances. The changes proposed to ASTM F1545 are supported by industry experiences of limited durability when changes to liners are not subjected to re-qualification testing. This paper will document potentially avoidable field failures and their relationship to qualification (or re-qualification) testing requirements in ASTM F1545. The cause of several fieldfailures will be discussed in the context of the mode of failure and application of appropriate diagnostic testing.
The History of Plastic-Lined Metallic Composite Systems in Corrosive Applications The utility of combining the chemical resistance of polymers with the strength of metallic materials has been understood for many years. Various documents provide an insight into the interaction between plastics and various chemical environments. First and foremost, it should be understood if the corrosive environment will interact with the polymer to produce polymer swelling or an irreversible chemical reaction. If so, the polymer is commonly referred to as “unsuitable” or “incompatible” with the corrosive environment. A standard practice is available which provides for short-term testing that is useful “in eliminating the most unsuitable materials”2. Strong recommendations against combining incompatible chemicals and plastics are often made by plastic suppliers. If a polymer / chemical pairing only results in permeation, it is useful to consider if the polymer contains more than one phase. Examples may include composite materials containing impermeable fillers or semi-crystalline plastics. In these situations, the concept of tortuosity is relevant in an attempt to understand the role of diffusion in chemicals passing around less permeable phases or materials3. The period immediately following World War II saw many rapid innovations in the field of plastic lined piping. The Dow Chemical Company patented4 Polyvinylidene Chloride in 1939, partnered with the Saran Lined Pipe company and was the first to offer a plastic lined piping system. At around the same time, the patent for PTFE was issued5 to Roy Plunkett and was later used by Resistoflex as a liner for rigid pipe and fittings. A wide variety of engineering thermoplastics were adapted as liners for piping systems. A plethora of products and manufacturers began to overwhelm the end-user community with disparate plastic-lined pipe offerings. Eventually, the industry came together and created consensus standards for plastic lined piping that were individually written for each plastic material. Originally approved as a new standard in 1995, ASTM F1545 has become a consensus standard encompassing a variety of plastic materials under one set of common performance requirements. This standard continues to evolve today, most recently, with clearer qualification testing requirements which seek to improve understanding and compliance in this critical safety product.
The Critical Importance of Inside Diameter Fit for Plastic-Lined Metallic Composite In a plastic-lined metallic component, localized liner buckling or global liner collapse are important performance considerations due to the relatively large difference between the liner ring stiffness and the outer shell ring stiffness. Permeation of the liner by volatile components may result in an instantaneous external pressure when housing venting systems are absent, clogged or insufficient in size. Differential external and internal pressure which creates an active collapse force can also be created by a variety of operating conditions. Considering the plastic liner as a wholly separate component, one might reasonably measure the pipe stiffness of the liner by test method6 ASTM D2412. As noted in ASTM D2412, “The EI of a pipe is a function of the material’s flexural modulus (E) and the wall thickness (t) of the pipe, since I = t3/12. As such it is a fixed value for any given set of material and dimensional parameters”. For a plastic material, it must also be acknowledged that the flexural modulus of the material is not a constant value but may vary significantly over the temperature range anticipated by the application environment. It is reasonable to first consider a close-fit liner in which the inside diameter of the metallic pipe will exactly match the outside diameter of the plastic liner. Aside from the practical complexities of manufacturing a close-fit liner system, there is an important material science limitation that can’t be overcome. Speaking generally, plastics have a coefficient of thermal expansion and contraction that is approximately 10 times larger than most metals. Many applications for plastic-lined metallic piping systems will require a temperature range from ambient to elevated temperatures of 350⁰F or higher. Over the projected service lifetime and depending on the specific application, one might reasonably anticipate many dozens, hundreds or even thousands of thermal cycles between ambient and application temperature. The amount of the thermal expansion / contraction will be proportional to the liner length. It naturally follows that as a plastic liner inside a long pipe spool will concentrate sizable and significant stresses at any point or plane of contact. Over time, repeated thermal cycling of close-fit liners qualified for ambient to slightly elevated temperature service will fail due to the mechanical stresses encountered. A preferred configuration for most applications will be an interference-fit liner in which the liner outside diameter, when not limited by a metallic host, would be larger than the inside diameter of the metallic housing in which it is placed. Let us also consider that this liner uses specialized installation techniques7 to obtain a tight fit against all interior mating surfaces equally. Such a liner would not exhibit differential movement within the metallic housing due to thermal expansion and contraction. Clearly, this tight-fit liner will not experience the same stress concentration effects of a close-fit liner. In this regard, one
should anticipate prolonged durability in applications that experience thermal cycling. Finally, if a plastic liner is loosely housed within the host pipe, then thermal expansion and contraction stresses are amplified by an additional complexity. A loose-fit liner will have an annular space between the liner and outer housing. Any free annular volume will serve as a reservoir for gas pressure increasing the potential that a pressure differential will buckle or collapse the liner. The potential for liner collapse can actually work in concert with liner thinning due to stress concentration resulting in significantly reduced service lifetime.
Plastic Liner Materials and their Manufacturing Implications ASTM F1545 provides material requirements for several different plastics allowed as liner materials. A selection of the highly chemically-resistant plastics utilized in ASTM F1545 is provided in Table 1 along with their tensile properties and additional ASTM standards which provide further reference material for each. Liner Material ETFE PFA PVDF PVDFcopoly PP Type I PP Type II PP Type 30% Glass
Min. Strength at Break, psi 6500 3800 4500
Min. Elong. at Break, % 275 300 10
D31598 D33079 D322210
4000
300
D557511
4000* 3000*
10* 10*
D410112 D410112
2500*
2*
D410112
ASTM
D489413 D489514 Table 1. Summary of properties for selected materials from ASTM F1545. Materials marked with * indicate the marked property is measured at yield rather than break. PTFE
3000
250
It is instructive to consider ASTM D31598 as an example. ASTM 3159 is titled “Standard Specification for Modified ETFE-Fluoropolymer Molding and Extrusion Materials” and “covers melt processible molding and extrusion materials of modified ETFE-fluoropolymer”. The standard provides a cell classification system for “five types of modified ETFE- fluoropolymer supplied in pellet form classified according to their specific gravity. The resins of each type are divided into one to three grades according to their melt flow rate”. Natural Type I materials have a value between 1.69 to 1.76 for specific gravity and are qualified for use as a lining material in ASTM F1545 provided that they have a minimum tensile strength at break of 6500 psi and a minimum elongation at break greater than 275 per cent. Inspection of the additional material specific
ASTM standards relevant to ASTM F1545 indicate that most of them are plastic materials capable of extrusion processing. PTFE is not processed in traditional elevated temperature profile extrusion. In fact, both ASTM D4894 and D4895 indicate in their scope “The usual methods of processing thermoplastics generally are not applicable to these materials because of their viscoelastic properties”. ASTM D4894 is for “polytetrafluoroethylene (PTFE) that have never been preformed or molded and are normally processed by methods similar to those used in powder metallurgy or ceramics, or by special extrusion processes”. Similarly, ASTM D4895 covers PTFE ““fine-powder” resins or “coagulated-dispersion powder” resins. The conversion of these resins to finished products normally involves a process called “paste extrusion,” and sometimes involves formative processes such as calendering. A volatile liquid is present as a processing aid during these formative stages of conversion, and is subsequently removed during the finishing stages of conversion”. The significant difference in the plastics processing methods for PTFE liners when compared to all of the other plastic materials provided in ASTM F1545 will now be discussed in further detail as it is relevant to the failure modes observed for such PTFE lined composite piping materials.
Manufacturing Process for Plastic-Lined Metallic Composites Each of the previously discussed plastic-lined metallic components (e.g. loose-fit, close-fit and interference-fit) are manufactured by several processes. Additionally, different manufacturing processes are adapted to short pipe lengths (3 feet or less) or fittings than are used in manufacturing of long pipe spools (ie. 20 foot). Fittings and short lengths of pipe are similar in that the consequences of thermal expansion / contraction stresses are limited but are differentiated by the complexity of the final liner shape. Methods of manufacturing tight-fit plastic-lined fittings and short lengths of pipe might include such varied processes as injection molding with removable core sections and rotational molding using granulated powders. Short lengths of loose-fit plastic lined pipes can also be manufactured by simply sliding a prefabricated plastic liner into the metallic pipe. At some length, the processes discussed above to produce a tight-fit lined pipe spool become ineffective and are no longer used. However, other manufacturing methods are available depending on the specific properties of the plastic material. For plastic materials, an extruded tube can be obtained that has a useful recoverable strain. In this situation, it is reasonable to consider using this “elastic memory” effect. A plastic liner with a slightly larger outside diameter than the inside of the host pipe is pulled through a die and stretched axially resulting in a circumferential reduction. The stretched liner is then inserted into the metallic housing and the axial force
removed. Releasing this force allows the liner to return to its original dimensions resulting in expansion of the liner against the metallic housing. Alternatively, a heated die can be used to reduce the outside diameter of the plastic liner to fit inside the metallic host pipe. In both cases, the completed assembly is heated in an oven to expand the liner against the housing and reduce any residual stresses from the insertion process.
Manufacturing Process for PTFE-Lined Metallic Composites As mentioned previously, the extra-ordinary viscosity of molten PTFE material makes standard extrusion processing impractical. A “paste extrusion” process previously mentioned was patented15 in 1954. This patent discloses mixing colloidal size PTFE powder (as described in ASTM D4895) into liquid lubricant, usually a hydrocarbon and the resulting mix extruded into the desired form, in our case a tube. To be clear, the extrusion temperature is below the melting point of PTFE which remains a solid during extrusion. Following extrusion, the PTFE tube is heated; first driving off the lubricant, and then, at a higher temperature, sintering the PTFE. Additionally, an isostatic molding process16 has been disclosed for making tubular and other shapes from the free-flowing granular PTFE resins described in ASTM D4894. In this process, the resin is poured into the mold and compressed onto the inner core of the mold with hydraulic pressure applied to a rubber bladder. The compressed part is then removed from the mold, and sintered. Further techniques are necessary for the production of PTFE lined composite pipes from sintered PTFE tubes. A manufacturing technique that is applicable to the production of PTFE-lined pipe does not depend on the extrusion of the plastic. A process commonly referred to as “swaging” begins by creating a loose fit composite pipe by insertion of a slightly undersized sintered PTFE tube into a host pipe. The composite pipe is then passed through tapered reducing dies under significant force resulting in a reduction of both the inner and outer steel pipe diameter until the inside diameter is small enough to fit the liner. The process is controlled to provide a finished metal pipe outside diameter and wall thickness that conform to standardized metallic pipe dimension systems. A different technique is commonly applied to the manufacture of PTFE lined long pipe spools. A sintered tube of PTFE is heated to above 150⁰C - still significantly below the melting point13 (sources vary but commonly cite a melting temperature in excess of 310⁰C). While in this heated state, the PTFE is placed under axial tension resulting in axial elongation and diametrical reduction. The tube is then cooled while under stress effectively fixing the liner dimensions at room temperature. As described in US patent7, “If the initial size of the P.T.F.E
tubing is properly selected, then the tube of reduced perimeter or girth will be susceptible of easy insertion into the tubular member it is to line. The assembled tubes may now be placed in an oven and heated as in the previous example in order to return the P.T.F.E. tube towards its original size. Due allowance should be made for the decrease in axial length of the liner during the last step. It will be found that the liner will expand its girth so as to tightly engage the inner wall of the pipe or other tubular member with which it is positioned”. The elastic memory of the pre-stressed PTFE tube is used to create a liner that has been thermally locked in place. In both swaging and in the thermal locking technique, the manufacturing processes literally cause the liners to grip the inner wall of the metallic pipe. This grip in turn causes the plastic and steel to act as a single unit during thermal cycles. The amount of compression imparted to the plastic liner is easily demonstrated by measuring the liner “push-out” resistance of composite pipe manufactured using these manufacturing techniques to the same resistance obtained by use of other manufacturing processes.
Liner Retraction Failure Mode One of the ways that ASTM F1545 requirements and qualification testing guards against product failure is through dimensional requirements. Two such requirements attempt to guard against failure due to liner retraction. There is a critical dimensional requirement related to the minimum dimensions of the “radius or chamfer in the transition from pipe wall to flange or lap face”. Section 4.2.3 of the standard specifies that “a 1⁄8-in. minimum radius must be provided”. The standard indicates that the purpose of this minimum radius is “to reduce stress concentrations in the plastic liner as it is flared or molded over the flange face or stub end”. Dimensional requirements are also provided in an attempt to avoid the durability issues that arise from a loose fit liner. Section 5.4.1 of F1545 specifies that “the linings shall fit snugly inside the pipe and fitting housings. Any bulges or other obvious indications of poor contact with the housing shall be cause for rejection”. Of course, it is difficult to provide a measurement to define the tightness of fit for the broad variety of plastic-lining materials provided for in the standard. Therefore, the Steam-Cold Water Cycling Test provided in Section 6.3 subjects each product configuration and material to a minimum of 100 thermal cycles “to determine the ability of the lined components to withstand rapid temperature changes”. During and following this qualification testing, it is required that “there shall be no leakage” and “the liner shall exhibit no buckling or cracking”. Field failure related to poor liner restraint against the metallic host wall (such as in a loose fit system) or when the transition from the pipe wall is improperly severe is commonly observed as a liner retraction failure. Tension
in the plastic liner from thermal changes may overcome the restraining force holding the liner against the metallic host wall. The outcome of such a failure is for the liner retract inside the host as shown in Figure 1.
Figure 1. Plastic-lined composite pipe following a liner retraction failure mode. A similar failure mode is observed in the instance of a severe transition, or poor liner fit. Either of these conditions creates a point or, more commonly, a ring of stress concentration in the liner as it is subjected to a series of thermal expansion and contraction cycles. This stress arises from the well-known order of magnitude difference in the coefficient of thermal expansion for metals and plastics. Positive indication of this type of failure is a circumferential rupture of the liner at the point of maximum stress and liner retraction subsequent to failure.
Circumferential Cracking Failure Mode Previously, Table 1 indicated the tensile and elongation requirements for various plastic liner materials associated with ASTM F1545. For most of the plastic lining materials, many material grades and fabrication methods are available. As previously discussed, PTFE is not extrusion melt processed into liner systems but is manufactured in processes commonly involving sintering of a formed tube. Improper sintering processes or incomplete sintering of PTFE liners introduce another layer of complexity to change management. Minor imperfections in the PTFE liner may result is significant changes in tensile performance. The recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to ensure that all plastic materials meet tensile strength and elongation requirements. The balloted changes make it abundantly clear that any changes in liner supplier, materials, liner thickness, or fabrication methods will be cause for requalification testing of the plastic-lined composite metallic component. Inferior tensile strength and elongation performance can lead to a number of failure
modes. This type of failure mode is generally indicated by thinning of the liner and / or circumferential cracking as observed in Figure 2.
Figure 2. Plastic-lined composite pipe showing circumferential cracking failure more due to a severe radius from the pipe bore to the flare. Qualification of the minimum strength and elongation capabilities of the liner should allow the industry to avoid potentially catastrophic field failures. In addition to changes in the tensile strength and elongation of the plastic liner, changes in liner manufacturing process can also lead to changes in the vacuum performance of the plastic-lined metallic composite pipe or fitting.
Trilobal Collapse Failure Mode ASTM F1545 provides procedures related to establishing the vacuum performance of plastic-lined composite piping systems in Section 6.4. Requirements include many factors of interest to end users including the number and type of components, the temperature range for testing, and the procedure for testing pipe spools including minimum sample length. In fact, measures are taken in the reporting of the testing results to ensure a conservative rating for vacuum service. Firstly, the vacuum failure threshold is defined as 1 in. Hg below that where failure occurs and, more importantly, the vacuum rating is reported as no greater than 80% of the failure threshold value. Additionally, the duration of vacuum testing is prescribed as a minimum of 48 hours. Although it might seem certain applications would not require a vacuum capability, in fact, some level of minimum vacuum performance is almost always required in real-world applications. Manufacturers may provide a vacuum rating for their composite piping components along with whatever additional guidance they believe is important to ensure long-term durable service. While ASTM F1545 proscribes the conditions under which vacuum service rating can be established, examining the calculation related to vacuum service is instructive. Equation 1 provides17 the calculation of the critical pressure, PCr, for an unrestrained tube or pipe of uniform thickness under vacuum in which;
3
𝑃𝐶𝑟 = [2𝐸⁄(1 − 𝜇 2 )] (𝑡 ⁄𝑑 3 )
(1)
PCr equals the critical external buckling pressure for internal vacuum, E represents modulus of elasticity, µ is Poisson’s ratio, t measures the wall thickness of the liner, and d is the outside diameter of the liner. In this equation, the units for critical pressure are equivalent to the units used for the modulus of elasticity. It should be clear that PCr is proportional to the plastic material’s modulus of elasticity which, in turn, is well understood to be a function of temperature as well as affected by the chemical environment, as mentioned previously. The equation also indicates that the external pressure required to collapse a tube under vacuum is proportional to the liner thickness to the third power and inversely proportional to the outside diameter to the third power. The actual vacuum performance of an installed pipe liner should be extremely sensitive to any thickness changes or non-uniformities. Furthermore, any process changes likely to impact the plastic material’s modulus can have an important influence on actual vacuum performance. It should be noted that positive-fit liner manufacturing techniques commonly outperform tight fit or loose fit techniques. Field failures related to vacuum performance show an unusual trilobal collapse which is highly diagnostic. The visual appearance of such a failure is shown in Figure 3.
Figure 3. Plastic-lined composite pipe showing trilobal collapse failure mode consistent with vacuum. However, it is a more complex matter to establish the root cause of a vacuum-related collapse due to the interaction of the various factors (fit, liner uniformity, material modulus as a function of environment and temperature, external pressure applied, pressure duration, etc). The recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to
ensure that all components will meet the manufacture’s vacuum rating following changes to the liner material or method of manufacturing. The balloted changes make it abundantly clear that any changes in liner sourcing, liner materials, liner thickness, or fabrication methods will be cause for requalification testing of the plastic-lined composite metallic component.
Discussion and Conclusions Plastic-lined metallic composite pipe, fittings and systems are an important part of industry. Under even the most challenging corrosive environments, these composites work very well. End-users should expect a high durability product when plastic-lined composite materials are manufactured to an industry standard. As demonstrated in this paper and others, the occasional failure modes are relatively well-understood. Root cause failure analysis should serve to guide both manufacturers and end-users to responsible steps which can be taken to avoid future failures. A recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to ensure that all components will meet the manufacture’s vacuum rating following changes to the liner materials, liner thickness, or fabrication methods. This enhancement to ASTM F1545 should result in even greater acceptance of these critical composite piping systems for years into the future.
References 1.
2.
3.
4. 5. 6.
7.
ASTM 1545- 97(2009), “Standard Specification for Plastic-Lined Ferrous Metal Pipe, Fittings, and Flanges” 2009 Annual Book of ASTM Standards, Volume 08.04, Plastic Piping Systems, ASTM International, West Conshohocken, PA. ASTM D543-06, “Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents” 2006 Annual Book of ASTM Standards, Volume 08.01, Plastics, ASTM International, West Conshohocken, PA. Chen, X.; Papathanasiou, T. D. “Permeability of Flakefilled Barrier Membranes: A Numerical Evaluation” ANTEC Proceedings, (2007). US Patent 2160903, “Polymeric Vinylidene Chloride”, Reilly, J. H., and Wiley, R.M., 1939. US Patent 2230654, "Tetrafluoroethylene Polymers", Plunkett, R. J., 1941. ASTM F2412 – 11, “Standard Test Method for Determination of External Loading Characteristics of Plastic Pipe by Parallel-Plate Loading” 2011 Annual Book of ASTM Standards, Volume 08.04, Plastic Piping Systems, ASTM International, West Conshohocken, PA. US Patent 3050786, “Methods of lining and jacketing tubular members with prestressed
polytetrafluoroethylene” St. John, A. N. T. and Titterton, W. E., 1962. 8. ASTM D3159 – 10, “Standard Specification for Modified ETFE-Fluoropolymer Molding and Extrusion Materials” 2010 Annual Book of ASTM Standards, Volume 08.01, Plastics, ASTM International, West Conshohocken, PA. 9. ASTM D3307 – 10, “Standard Specification for Perfluoroalkoxy (PFA)-Fluorocarbon Resin Molding and Extrusion Materials” 2010 Annual Book of ASTM Standards, Volume 08.02, Plastics, ASTM International, West Conshohocken, PA. 10. ASTM D3222 – 10, “Standard Specification for Unmodified Poly(Vinylidene Fluoride) (PVDF) Molding Extrusion and Coating Materials” 2010 Annual Book of ASTM Standards, Volume 08.02, Plastics, ASTM International, West Conshohocken, PA. 11. ASTM D5575 - 13, “Standard Classification System for Copolymers of Vinylidene Fluoride (VDF) with Other Fluorinated Monomers” 2013 Annual Book of ASTM Standards, Volume 08.03, Plastics, ASTM International, West Conshohocken, PA. 12. ASTM D4101 – 14, “Standard Specification for Polypropylene Injection and Extrusion Materials” 2014 Annual Book of ASTM Standards, Volume 08.02, Plastics, ASTM International, West Conshohocken, PA. 13. ASTM D4894 – 12, “Standard Specification for Polytetrafluoroethylene (PTFE) Granular Molding and Ram Extrusion Materials” 2012 Annual Book of ASTM Standards, Volume 08.02, Plastics, ASTM International, West Conshohocken, PA. 14. ASTM D4895 – 10, “Standard Specification for Polytetrafluoroethylene (PTFE) Resin Produced From Dispersion” 2010 Annual Book of ASTM Standards, Volume 08.02, Plastics, ASTM International, West Conshohocken, PA. 15. US Patent 2685707, “Extrusion of Tetrafluoroethylene Polymer”, Llewellyn, W.E. and Lontz, J.F, 1954. 16. NACE Middle East Corrosion Conference, “Generational Advancements in Fluoropolymer Lined Piping Systems”, 2014. 17. F. Seely, J. Smith, “Advanced Mechanics of Materials”, Chapter 21, Wiley, New York, 1952. Key Words: Plastic-lined, Composite Pipe, Failure.
Introduction Many product details of plastic-lined metallic pipe, fittings, and flanges which serve a critical role in the safe transport of corrosive fluids are standardized 1 in ASTM F1545. The metallic housing primarily contributes strength and ease of assembly with ASTM F1545 standardizing dimensions and materials of construction for pipe, fittings and flanges. The plastic liner primarily contributes chemical resistance with ASTM F1545 establishing minimum requirements for several materials including Polypropylene, Poly (vinylidene fluoride) (PVDF) and, most importantly, Polytetrafluoroethylene (PTFE). Rapid and low-cost inspection of each spool and fitting by electrostatically or hydrostatically testing is mandated by ASTM F1545. ASTM F1545 also provides qualification testing requirements which seek to ensure that all compliant products have well understood hightemperature, low-temperature, thermal cycling and vacuum performance but are tested less frequently and are sometimes destructive. Recently, changes to the language of ASTM F1545 have been balloted to clarify a management of change philosophy which mandates requalification under some circumstances. The changes proposed to ASTM F1545 are supported by industry experiences of limited durability when changes to liners are not subjected to re-qualification testing. This paper will document potentially avoidable field failures and their relationship to qualification (or re-qualification) testing requirements in ASTM F1545. The cause of several fieldfailures will be discussed in the context of the mode of failure and application of appropriate diagnostic testing.
The History of Plastic-Lined Metallic Composite Systems in Corrosive Applications The utility of combining the chemical resistance of polymers with the strength of metallic materials has been understood for many years. Various documents provide an insight into the interaction between plastics and various chemical environments. First and foremost, it should be understood if the corrosive environment will interact with the polymer to produce polymer swelling or an irreversible chemical reaction. If so, the polymer is commonly referred to as “unsuitable” or “incompatible” with the corrosive environment. A standard practice is available which provides for short-term testing that is useful “in eliminating the most unsuitable materials”2. Strong recommendations against combining incompatible chemicals and plastics are often made by plastic suppliers. If a polymer / chemical pairing only results in permeation, it is useful to consider if the polymer contains more than one phase. Examples may include composite materials containing impermeable fillers or semi-crystalline plastics. In these situations, the concept of tortuosity is relevant in an attempt to understand the role of diffusion in chemicals passing around less permeable phases or materials3. The period immediately following World War II saw many rapid innovations in the field of plastic lined piping. The Dow Chemical Company patented4 Polyvinylidene Chloride in 1939, partnered with the Saran Lined Pipe company and was the first to offer a plastic lined piping system. At around the same time, the patent for PTFE was issued5 to Roy Plunkett and was later used by Resistoflex as a liner for rigid pipe and fittings. A wide variety of engineering thermoplastics were adapted as liners for piping systems. A plethora of products and manufacturers began to overwhelm the end-user community with disparate plastic-lined pipe offerings. Eventually, the industry came together and created consensus standards for plastic lined piping that were individually written for each plastic material. Originally approved as a new standard in 1995, ASTM F1545 has become a consensus standard encompassing a variety of plastic materials under one set of common performance requirements. This standard continues to evolve today, most recently, with clearer qualification testing requirements which seek to improve understanding and compliance in this critical safety product.
The Critical Importance of Inside Diameter Fit for Plastic-Lined Metallic Composite In a plastic-lined metallic component, localized liner buckling or global liner collapse are important performance considerations due to the relatively large difference between the liner ring stiffness and the outer shell ring stiffness. Permeation of the liner by volatile components may result in an instantaneous external pressure when housing venting systems are absent, clogged or insufficient in size. Differential external and internal pressure which creates an active collapse force can also be created by a variety of operating conditions. Considering the plastic liner as a wholly separate component, one might reasonably measure the pipe stiffness of the liner by test method6 ASTM D2412. As noted in ASTM D2412, “The EI of a pipe is a function of the material’s flexural modulus (E) and the wall thickness (t) of the pipe, since I = t3/12. As such it is a fixed value for any given set of material and dimensional parameters”. For a plastic material, it must also be acknowledged that the flexural modulus of the material is not a constant value but may vary significantly over the temperature range anticipated by the application environment. It is reasonable to first consider a close-fit liner in which the inside diameter of the metallic pipe will exactly match the outside diameter of the plastic liner. Aside from the practical complexities of manufacturing a close-fit liner system, there is an important material science limitation that can’t be overcome. Speaking generally, plastics have a coefficient of thermal expansion and contraction that is approximately 10 times larger than most metals. Many applications for plastic-lined metallic piping systems will require a temperature range from ambient to elevated temperatures of 350⁰F or higher. Over the projected service lifetime and depending on the specific application, one might reasonably anticipate many dozens, hundreds or even thousands of thermal cycles between ambient and application temperature. The amount of the thermal expansion / contraction will be proportional to the liner length. It naturally follows that as a plastic liner inside a long pipe spool will concentrate sizable and significant stresses at any point or plane of contact. Over time, repeated thermal cycling of close-fit liners qualified for ambient to slightly elevated temperature service will fail due to the mechanical stresses encountered. A preferred configuration for most applications will be an interference-fit liner in which the liner outside diameter, when not limited by a metallic host, would be larger than the inside diameter of the metallic housing in which it is placed. Let us also consider that this liner uses specialized installation techniques7 to obtain a tight fit against all interior mating surfaces equally. Such a liner would not exhibit differential movement within the metallic housing due to thermal expansion and contraction. Clearly, this tight-fit liner will not experience the same stress concentration effects of a close-fit liner. In this regard, one
should anticipate prolonged durability in applications that experience thermal cycling. Finally, if a plastic liner is loosely housed within the host pipe, then thermal expansion and contraction stresses are amplified by an additional complexity. A loose-fit liner will have an annular space between the liner and outer housing. Any free annular volume will serve as a reservoir for gas pressure increasing the potential that a pressure differential will buckle or collapse the liner. The potential for liner collapse can actually work in concert with liner thinning due to stress concentration resulting in significantly reduced service lifetime.
Plastic Liner Materials and their Manufacturing Implications ASTM F1545 provides material requirements for several different plastics allowed as liner materials. A selection of the highly chemically-resistant plastics utilized in ASTM F1545 is provided in Table 1 along with their tensile properties and additional ASTM standards which provide further reference material for each. Liner Material ETFE PFA PVDF PVDFcopoly PP Type I PP Type II PP Type 30% Glass
Min. Strength at Break, psi 6500 3800 4500
Min. Elong. at Break, % 275 300 10
D31598 D33079 D322210
4000
300
D557511
4000* 3000*
10* 10*
D410112 D410112
2500*
2*
D410112
ASTM
D489413 D489514 Table 1. Summary of properties for selected materials from ASTM F1545. Materials marked with * indicate the marked property is measured at yield rather than break. PTFE
3000
250
It is instructive to consider ASTM D31598 as an example. ASTM 3159 is titled “Standard Specification for Modified ETFE-Fluoropolymer Molding and Extrusion Materials” and “covers melt processible molding and extrusion materials of modified ETFE-fluoropolymer”. The standard provides a cell classification system for “five types of modified ETFE- fluoropolymer supplied in pellet form classified according to their specific gravity. The resins of each type are divided into one to three grades according to their melt flow rate”. Natural Type I materials have a value between 1.69 to 1.76 for specific gravity and are qualified for use as a lining material in ASTM F1545 provided that they have a minimum tensile strength at break of 6500 psi and a minimum elongation at break greater than 275 per cent. Inspection of the additional material specific
ASTM standards relevant to ASTM F1545 indicate that most of them are plastic materials capable of extrusion processing. PTFE is not processed in traditional elevated temperature profile extrusion. In fact, both ASTM D4894 and D4895 indicate in their scope “The usual methods of processing thermoplastics generally are not applicable to these materials because of their viscoelastic properties”. ASTM D4894 is for “polytetrafluoroethylene (PTFE) that have never been preformed or molded and are normally processed by methods similar to those used in powder metallurgy or ceramics, or by special extrusion processes”. Similarly, ASTM D4895 covers PTFE ““fine-powder” resins or “coagulated-dispersion powder” resins. The conversion of these resins to finished products normally involves a process called “paste extrusion,” and sometimes involves formative processes such as calendering. A volatile liquid is present as a processing aid during these formative stages of conversion, and is subsequently removed during the finishing stages of conversion”. The significant difference in the plastics processing methods for PTFE liners when compared to all of the other plastic materials provided in ASTM F1545 will now be discussed in further detail as it is relevant to the failure modes observed for such PTFE lined composite piping materials.
Manufacturing Process for Plastic-Lined Metallic Composites Each of the previously discussed plastic-lined metallic components (e.g. loose-fit, close-fit and interference-fit) are manufactured by several processes. Additionally, different manufacturing processes are adapted to short pipe lengths (3 feet or less) or fittings than are used in manufacturing of long pipe spools (ie. 20 foot). Fittings and short lengths of pipe are similar in that the consequences of thermal expansion / contraction stresses are limited but are differentiated by the complexity of the final liner shape. Methods of manufacturing tight-fit plastic-lined fittings and short lengths of pipe might include such varied processes as injection molding with removable core sections and rotational molding using granulated powders. Short lengths of loose-fit plastic lined pipes can also be manufactured by simply sliding a prefabricated plastic liner into the metallic pipe. At some length, the processes discussed above to produce a tight-fit lined pipe spool become ineffective and are no longer used. However, other manufacturing methods are available depending on the specific properties of the plastic material. For plastic materials, an extruded tube can be obtained that has a useful recoverable strain. In this situation, it is reasonable to consider using this “elastic memory” effect. A plastic liner with a slightly larger outside diameter than the inside of the host pipe is pulled through a die and stretched axially resulting in a circumferential reduction. The stretched liner is then inserted into the metallic housing and the axial force
removed. Releasing this force allows the liner to return to its original dimensions resulting in expansion of the liner against the metallic housing. Alternatively, a heated die can be used to reduce the outside diameter of the plastic liner to fit inside the metallic host pipe. In both cases, the completed assembly is heated in an oven to expand the liner against the housing and reduce any residual stresses from the insertion process.
Manufacturing Process for PTFE-Lined Metallic Composites As mentioned previously, the extra-ordinary viscosity of molten PTFE material makes standard extrusion processing impractical. A “paste extrusion” process previously mentioned was patented15 in 1954. This patent discloses mixing colloidal size PTFE powder (as described in ASTM D4895) into liquid lubricant, usually a hydrocarbon and the resulting mix extruded into the desired form, in our case a tube. To be clear, the extrusion temperature is below the melting point of PTFE which remains a solid during extrusion. Following extrusion, the PTFE tube is heated; first driving off the lubricant, and then, at a higher temperature, sintering the PTFE. Additionally, an isostatic molding process16 has been disclosed for making tubular and other shapes from the free-flowing granular PTFE resins described in ASTM D4894. In this process, the resin is poured into the mold and compressed onto the inner core of the mold with hydraulic pressure applied to a rubber bladder. The compressed part is then removed from the mold, and sintered. Further techniques are necessary for the production of PTFE lined composite pipes from sintered PTFE tubes. A manufacturing technique that is applicable to the production of PTFE-lined pipe does not depend on the extrusion of the plastic. A process commonly referred to as “swaging” begins by creating a loose fit composite pipe by insertion of a slightly undersized sintered PTFE tube into a host pipe. The composite pipe is then passed through tapered reducing dies under significant force resulting in a reduction of both the inner and outer steel pipe diameter until the inside diameter is small enough to fit the liner. The process is controlled to provide a finished metal pipe outside diameter and wall thickness that conform to standardized metallic pipe dimension systems. A different technique is commonly applied to the manufacture of PTFE lined long pipe spools. A sintered tube of PTFE is heated to above 150⁰C - still significantly below the melting point13 (sources vary but commonly cite a melting temperature in excess of 310⁰C). While in this heated state, the PTFE is placed under axial tension resulting in axial elongation and diametrical reduction. The tube is then cooled while under stress effectively fixing the liner dimensions at room temperature. As described in US patent7, “If the initial size of the P.T.F.E
tubing is properly selected, then the tube of reduced perimeter or girth will be susceptible of easy insertion into the tubular member it is to line. The assembled tubes may now be placed in an oven and heated as in the previous example in order to return the P.T.F.E. tube towards its original size. Due allowance should be made for the decrease in axial length of the liner during the last step. It will be found that the liner will expand its girth so as to tightly engage the inner wall of the pipe or other tubular member with which it is positioned”. The elastic memory of the pre-stressed PTFE tube is used to create a liner that has been thermally locked in place. In both swaging and in the thermal locking technique, the manufacturing processes literally cause the liners to grip the inner wall of the metallic pipe. This grip in turn causes the plastic and steel to act as a single unit during thermal cycles. The amount of compression imparted to the plastic liner is easily demonstrated by measuring the liner “push-out” resistance of composite pipe manufactured using these manufacturing techniques to the same resistance obtained by use of other manufacturing processes.
Liner Retraction Failure Mode One of the ways that ASTM F1545 requirements and qualification testing guards against product failure is through dimensional requirements. Two such requirements attempt to guard against failure due to liner retraction. There is a critical dimensional requirement related to the minimum dimensions of the “radius or chamfer in the transition from pipe wall to flange or lap face”. Section 4.2.3 of the standard specifies that “a 1⁄8-in. minimum radius must be provided”. The standard indicates that the purpose of this minimum radius is “to reduce stress concentrations in the plastic liner as it is flared or molded over the flange face or stub end”. Dimensional requirements are also provided in an attempt to avoid the durability issues that arise from a loose fit liner. Section 5.4.1 of F1545 specifies that “the linings shall fit snugly inside the pipe and fitting housings. Any bulges or other obvious indications of poor contact with the housing shall be cause for rejection”. Of course, it is difficult to provide a measurement to define the tightness of fit for the broad variety of plastic-lining materials provided for in the standard. Therefore, the Steam-Cold Water Cycling Test provided in Section 6.3 subjects each product configuration and material to a minimum of 100 thermal cycles “to determine the ability of the lined components to withstand rapid temperature changes”. During and following this qualification testing, it is required that “there shall be no leakage” and “the liner shall exhibit no buckling or cracking”. Field failure related to poor liner restraint against the metallic host wall (such as in a loose fit system) or when the transition from the pipe wall is improperly severe is commonly observed as a liner retraction failure. Tension
in the plastic liner from thermal changes may overcome the restraining force holding the liner against the metallic host wall. The outcome of such a failure is for the liner retract inside the host as shown in Figure 1.
Figure 1. Plastic-lined composite pipe following a liner retraction failure mode. A similar failure mode is observed in the instance of a severe transition, or poor liner fit. Either of these conditions creates a point or, more commonly, a ring of stress concentration in the liner as it is subjected to a series of thermal expansion and contraction cycles. This stress arises from the well-known order of magnitude difference in the coefficient of thermal expansion for metals and plastics. Positive indication of this type of failure is a circumferential rupture of the liner at the point of maximum stress and liner retraction subsequent to failure.
Circumferential Cracking Failure Mode Previously, Table 1 indicated the tensile and elongation requirements for various plastic liner materials associated with ASTM F1545. For most of the plastic lining materials, many material grades and fabrication methods are available. As previously discussed, PTFE is not extrusion melt processed into liner systems but is manufactured in processes commonly involving sintering of a formed tube. Improper sintering processes or incomplete sintering of PTFE liners introduce another layer of complexity to change management. Minor imperfections in the PTFE liner may result is significant changes in tensile performance. The recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to ensure that all plastic materials meet tensile strength and elongation requirements. The balloted changes make it abundantly clear that any changes in liner supplier, materials, liner thickness, or fabrication methods will be cause for requalification testing of the plastic-lined composite metallic component. Inferior tensile strength and elongation performance can lead to a number of failure
modes. This type of failure mode is generally indicated by thinning of the liner and / or circumferential cracking as observed in Figure 2.
Figure 2. Plastic-lined composite pipe showing circumferential cracking failure more due to a severe radius from the pipe bore to the flare. Qualification of the minimum strength and elongation capabilities of the liner should allow the industry to avoid potentially catastrophic field failures. In addition to changes in the tensile strength and elongation of the plastic liner, changes in liner manufacturing process can also lead to changes in the vacuum performance of the plastic-lined metallic composite pipe or fitting.
Trilobal Collapse Failure Mode ASTM F1545 provides procedures related to establishing the vacuum performance of plastic-lined composite piping systems in Section 6.4. Requirements include many factors of interest to end users including the number and type of components, the temperature range for testing, and the procedure for testing pipe spools including minimum sample length. In fact, measures are taken in the reporting of the testing results to ensure a conservative rating for vacuum service. Firstly, the vacuum failure threshold is defined as 1 in. Hg below that where failure occurs and, more importantly, the vacuum rating is reported as no greater than 80% of the failure threshold value. Additionally, the duration of vacuum testing is prescribed as a minimum of 48 hours. Although it might seem certain applications would not require a vacuum capability, in fact, some level of minimum vacuum performance is almost always required in real-world applications. Manufacturers may provide a vacuum rating for their composite piping components along with whatever additional guidance they believe is important to ensure long-term durable service. While ASTM F1545 proscribes the conditions under which vacuum service rating can be established, examining the calculation related to vacuum service is instructive. Equation 1 provides17 the calculation of the critical pressure, PCr, for an unrestrained tube or pipe of uniform thickness under vacuum in which;
3
𝑃𝐶𝑟 = [2𝐸⁄(1 − 𝜇 2 )] (𝑡 ⁄𝑑 3 )
(1)
PCr equals the critical external buckling pressure for internal vacuum, E represents modulus of elasticity, µ is Poisson’s ratio, t measures the wall thickness of the liner, and d is the outside diameter of the liner. In this equation, the units for critical pressure are equivalent to the units used for the modulus of elasticity. It should be clear that PCr is proportional to the plastic material’s modulus of elasticity which, in turn, is well understood to be a function of temperature as well as affected by the chemical environment, as mentioned previously. The equation also indicates that the external pressure required to collapse a tube under vacuum is proportional to the liner thickness to the third power and inversely proportional to the outside diameter to the third power. The actual vacuum performance of an installed pipe liner should be extremely sensitive to any thickness changes or non-uniformities. Furthermore, any process changes likely to impact the plastic material’s modulus can have an important influence on actual vacuum performance. It should be noted that positive-fit liner manufacturing techniques commonly outperform tight fit or loose fit techniques. Field failures related to vacuum performance show an unusual trilobal collapse which is highly diagnostic. The visual appearance of such a failure is shown in Figure 3.
Figure 3. Plastic-lined composite pipe showing trilobal collapse failure mode consistent with vacuum. However, it is a more complex matter to establish the root cause of a vacuum-related collapse due to the interaction of the various factors (fit, liner uniformity, material modulus as a function of environment and temperature, external pressure applied, pressure duration, etc). The recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to
ensure that all components will meet the manufacture’s vacuum rating following changes to the liner material or method of manufacturing. The balloted changes make it abundantly clear that any changes in liner sourcing, liner materials, liner thickness, or fabrication methods will be cause for requalification testing of the plastic-lined composite metallic component.
Discussion and Conclusions Plastic-lined metallic composite pipe, fittings and systems are an important part of industry. Under even the most challenging corrosive environments, these composites work very well. End-users should expect a high durability product when plastic-lined composite materials are manufactured to an industry standard. As demonstrated in this paper and others, the occasional failure modes are relatively well-understood. Root cause failure analysis should serve to guide both manufacturers and end-users to responsible steps which can be taken to avoid future failures. A recent ballot to ASTM F1545 clarifies that an appropriate management of change process will be used to ensure that all components will meet the manufacture’s vacuum rating following changes to the liner materials, liner thickness, or fabrication methods. This enhancement to ASTM F1545 should result in even greater acceptance of these critical composite piping systems for years into the future.
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