A more detailed liquid conductivity range with the following flow velocity limits is now recommended for plastic lined piping systems particularly those lined with PTFE:

External and Internal Grounding Issues for Plastic-Lined Pipe

Users of flanged steel piping systems with corrosion resistant liners have long been aware of the need to provide electrical continuity across flanged joints to permit grounding of these systems. Achieving a path to ground is desirable to prevent an electrical arc, which can ignite flammable vapors that may be present in the atmosphere. Steel piping systems lined with fully fluorinated polymers such as TEFLON® PTFE and PFA, partially fluorinated polymers such as Tefzel® ETFE and Kynar-Flex® PVDF, and thermoplastics, including polypropylene, are often joined in a series to create continuity to a ground source. An additional consideration when using steel pipes lined with fluoropolymer and other electrically insulating liners may be providing an electrical path from the fluid being conveyed to a grounding source. This is done to prevent static charge accumulation that can result in static discharges that cause pinholes in the liner wall. Grounding issues and options, both for external continuity between flanged pipe sections and relieving internal static build up, are discussed below. The reader should understand that any external charge that forms on the metallic pipe shell is the direct result of an induced charged when a charge build-up begins to form on the non conductive plastic liner. Thus any prevention program should focus on both the internal causes of charge build-up and the external effects of sparking when grounding. External Grounding Considerations and Recommendations Resistoflex and Dow PLPP had suggested two methods of providing low resistance continuity between flanged sections of lined steel pipe and fittings. These methods are (1) providing welded grounding lugs or studs several inches behind each flange on pipes and a single lug or stud on each fitting, or (2) use of bolts and nuts alone, or in conjunction with star washers behind one or more of the bolt assemblies joining the flanges together. Prior to 1998, Dow PLPP suggested that continuity was achieved via the bolts and nuts used to connect flanged joints alone. Testing was performed by Dow using then standard pipe sections, including a light "shop coat" external rust inhibiting coating and a combination of threaded flanges (fig 1) and two piece threaded hub and rotating flange assemblies (fig 2). These tests did prove that properly tensioned bolts resulted in an extremely low resistive connection capable of transferring static charge. As the bolt and nut assembly is tightened, the bolt head cuts through the Blue Shop Coat on the flange surface and results in good metal to metal contact. Many users of Dow PLPP product added the use of star washers on at least one bolt assembly, more fully assuring metal to metal contact between the bolt assembly and the flange. However, to provide for additional exterior corrosion protection of the external steel housings, Dow PLPP discontinued the use of the Blue Shop Coat, using instead a zinc rich epoxy coating, Intercure 200. Intercure 200 is applied at a much greater mill thickness than the earlier Blue Shop Coat, calling into question the continuity of between flanged sections of plastic lined pipe supplied with the new coating product. Additionally, the greater use of both two piece threaded hub and rotating flange assemblies (fig 2), and TEFLON® PTFE lined fittings with lap joint style rotating flanges further put into question continuity between flanged sections. Dow PLPP recreated earlier continuity testing and discovered that with the use of the Intercure 200 primer system, continuity was not maintained through properly tensioned bolts. To offer a lined steel pipe system which allowed electrical continuity across flanged joints, a threaded lug or stud, welded onto the steel housing of pipe sections after lining was necessary. Thus, if external grounding is a key consideration in safely operating a facility using plastic lined pipe, and Dow PLPP product was installed with the Intercure 200 coating (or Crane Resistoflex swaged pipe after Sept. 1998), relying only on bolt tensioning or bolt tensioning and star washers, continuity testing should be done to confirm continuity. A more secure and reliable method of creating continuity between flanged sections of lined steel pipe and fittings is to require that the lined steel pipe and fittings be supplied with a threaded lug or stud, located several inches behind each spool flange and one per fitting (fig 4). At one time, welding on pipe lined with plastics was not considered, due to potential liner damage caused by heat generation of welding. Crane Resistoflex Thermalok pipe (fig 3) provides for an interference fit between liner and housing, allowing welding to take place with the liner safely removed from the steel shell, at the time of manufacture or final spool fabrication. Fittings, being fully manufactured at the factory, required that grounding studs be applied prior to the liner being inserted into the shell. Thus, fittings were a custom ordered item, typically resulted in longer than normal deliveries. To overcome the lead time associated with grounding studs and fittings, Resistoflex developed procedures, utilizing a low resistance welding gun, allowing grounding studs to be applied to either fittings or finished spool fabrications without imparting heat damage to the internal liner. With this procedure available, we provide grounding lugs securely welded to TEFLON® PTFE lined pipes and fittings as a standard factory built product. Additionally, Resistoflex distributors and customers performing fabrication of spools to cut-length dimensions are able to install grounding lugs to pipes and fittings. This procedure can be used on both Thermalok or swaged pipes (swaged style lined steel pipe has a liner which is mechanically secured to the steel housing). Thus, utilizing grounding lugs on lined steel pipe and fittings provides a secure means of providing continuity across flanged joints, regardless of paint coating or flange connection design. What external grounding does not address, however, is potential charge generation buildup occurring inside the piping system, and providing a path to ground which does not harm the plastic liner. Internal Charge Generation Incidences of pinholes in TEFLON® PTFE pipes have been attributed to electrostatic discharge occurring on the inside of pipe. The flow of fluids with low-conductivity (typically less than 1000 pS/m) can create charge generation. Charge generation is dependent upon the following: Potential of the liner surface to accept or donate electrons. Velocity of the flow. Conductivity of the liner. Conductivity of the fluid. When the rate of charge generation is greater than the rate of charge relaxation, charge accumulation occurs. The charge accumulation can build to the point that it exceeds the breakdown voltage of the liner, creating a discharge. Fluoropolymer liners have very high breakdown voltage values (480 volts per mil for PTFE), so the eventual discharge energy is so high that a hole can be burnt through the liner at the time of discharge. Options for safeguarding against liner damage caused by internal static discharge include, but are not limited to: Changing (lowering) flow conditions to limit turbulence and hence charge generation. Additives to change the charge generation properties of the fluids being conveyed. The use of conductive liners. Use of grounding paddles to conduct the charge from the inside of the pipe to the outside. With regard to flow conditions and additives, these are typically considerations that require process design input for possible resolution. British Standard (BS) 5958 suggests the following flow velocity restrictions during pumping of flammable liquids through steel piping:

Liquid Conductivity BS 5958 Recommended Flow Velocity greater than 1000 pS/m No restriction 50 - 1000 pS/m less than 7 m/s less than 50 pS/m less than 1 m/s

A more detailed liquid conductivity range with the following flow velocity limits is now recommended for plastic lined piping systems particularly those lined with PTFE: Liquid Conductivity Recommended Flow Velocity For PTFE lined piping 1000 - 10000 pS/m less than 3m/s 50 - 1000 pS/m lless than 2 m/s less than 50 pS/m less than 1 m/s

These values may not be practical, however, due to productivity demands. Where higher flows are necessary, pinholes can be avoided by the prevention of charge accumulation. Liner conductivity: Typical fluoropolymer liners have high surface resistivities in the 1015 ohm/sq to 1016 ohm/sq range that will not dissipate charge faster than it accumulates. (Note: Due to the surface resistivity measuring procedure, ohm/sq is a unit that is not dependent on the surface area. Therefore, the number will be the same whether it is square meter, square inch, or square anything.) Many users inquire about conductive or anti-static PTFE liners (surface resistivities in the 104 ohm/sq to 106 ohm/sq range) as a solution to internal charge accumulation. Although this seems to be a magic bullet, closer examination reveals otherwise. The largest problem with a conductive liner is that to make a liner conductive, carbon must be added. Leaching of carbon out of the PTFE liner is inevitable, causing concerns about contamination and/or discoloration. If the potential contamination is not a concern, the fact still remains that the carbon will leach out of the liner. As it does, the conductive properties begin to disappear, resulting in a liner that no longer can carry an electrical charge to the housing. While no studies are available indicating how much time it takes for the liner to become insulating again, it is generally excepted that this will occur in under 2 years in the best conditions. Thus, providing a grounding path from the fluid to a ground source is typically handled through strategic placement of conductive components (grounding paddles, unlined components). Examination of the flow and electrical properties of the media is required to measure charge generation and recommend location of conductive component placement. This conductive component is then connected to the steel housing or to earth. When supplied with grounding studs or lugs, lined steel pipe can be joined in a series and ultimately ground to earth. Conductive components designed to remove an electrical charge from within the lined steel pipe can be joined to the housing, now ground to earth. Note: The selection of the type and location of the grounding component, as well at the grounding scheme is the responsibility of the end user. This may require the expertise of engineering consultants that specialize in the field. For additional information, please contact Crane Resistoflex Technical Resources at 828-724-4000. Other Resources: Engineering consultant technical articles Preventing Static-Electricity Fires, Chemical Engineering, December 21, 1961 and February 2, 1965 Electrostatics, Chemical Engineering, March 13, 1967 The Basic Mechanisms of Static Electrification, Science, December 7, 1945 Reducing Electrostatic Hazards, Chemical Engineering, July 1997 Avoiding Static Ignition Hazards in Chemical Operations by Laurence C. Britton, Center for Chemical Process Safety of the American Institute of Chemical Engineers, New York, NY., 1999, ISBN 0-8169-0800-1 Electrostatic Ignitions of Fires and Explosions by Thomas H. Pratt, Center for Chemical Process Safety of the American Institute of Chemical Engineers, New York, NY., 2000, ISBN 0-8169-9948-1 Handling Flammable Liquids by James C. Mulligan, Chilworth Technology, pp 48-56, July 2003, CEP Magazine. Chilworth Technology, Inc. Princeton, NJ

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