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Fluoropolymer-Lined Piping & Components: Critical Success Factors in Chemical handling

Posted on 29 January, 2010 | Tags: Pipes and Components Linings for Corrosion Protection

Fluoropolymer-lined systems have proved to be economical and useful alternatives to metallic alloy piping in many applications such as corrosive solvents in Chloroalkali plants, Pesticides, Pharmaceuticals and paper & Pulp. This paper deals with information on the successful long termn use of lined piping for corrosion mitigation, product purity and non-stick applications

The chemical process industry faces a continued and growing need for the containment of corrosive or harmful products. Since Dr. Roy Plunckett of Dupont discovered the first generation of fluoropolymers in 1938 their exceptional chemical resistance and high temperature performance have found use as linings for vessels, piping, valves, hoses and expansion joints. Under the brand name "Teflon®".  Fluoropolymer-lined systems have proved to be cost effective alternatives to metallic alloy piping in many applications
 Such as corrosive solvents in Chloroalkali plants, Pesticides, Pharmaceuticals and paper & Pulp. Indeed, there are applications where fluoropolymers alone will stand up to the chemicals.  Dupont Company has the additional advantage of successfully using these materials in their own chemical plants for over 50 years.

Features of Fluoropolymer Lined Piping
Plastic-lined piping combines a structurally sound metallic shell with a corrosion-resistant plastic liner. Liners are inserted into a metallic pipe and flared over the flange faces. Pipes are connected to each other through a bolted flange connection. Gaskets are not required initially but are used if the joint is disconnected for some reason. Standard pipe sizes of 1" through 12" are available off the shelf. Larger sizes are typically available as special order.
The most common plastic liners are polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene (PP), Perfluoroalkoxyalkane copolymer (PFA), perfluoro (ethylene propylene) copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE). Liner selection is primarily dependent upon the nature & concentration of chemicals, Vacuum/ Pressure conditions and normal operating temperature. Although PTFE, PFA have a maximum use temperature rating of 500 F the actual use temperature could be lower depending on the chemical use. Pipes of upto 8' sizes can withstand full vacuum upto 450 F.
Metallic housing (pipe) has vents to prevent pressure buildup between the liner and the metal pipe resulting from permeation. Common venting systems used by plastic-lined pipe manufacturers are a series of 1.6 mm (1¦16 in) to 4.0 mm (5¦32 in) diameter holes in the metallic shell, or a helical groove system inside the housing that connects flange vents (Figure 1).  The venting system provides a pathway for permeating chemicals (vapor) to escape.
For plastic-lined pipe and fittings with vent holes, vent couplings, as shown in Figures 2 and 3, can be welded over the vent holes to provide a connection for a vent extender.

Critical success factors for Fluoropolymer Lined components:
The items listed below must be considered during the selection, design, installation and operation of Fluoropolymer-lined pipe in chemical service applications.
Liner Selection:- Although all materials are available as liners PTFE accounts for 95% of the fluorpolymers lining in the fully fluorinated class. PFA can be available as special order for some special applications; FEP and ETFE are rarely used. Generally, fully fluorinated polymers like PTFE; PFA are resistant to all known chemicals except few alkali metals Resin suppliers can provide additional compatibility data

• Selection of right manufacturer/supplier:
  Selection of right fabricator is the single most important factor for success. Fabricators use different manufacturing processes for the liners such isostatic molding, ram extrusion and paste extrusion. However, all have to qualify the final lined pipe product to the ASTM standard F 1545 which governs the product and which is universally accepted as the standard. Manufacturer is also required to have a quality assurance program of liner inspection A quality manufacturer should also have an engineering manual for its products describing product description, dimensions, installation procedures and startup assistance.

ASTM Qualification Standard for Lined Piping and Components:-
The ASTM F1545 specifications cover requirements for workmanship, dimensions, design, construction, working pressures and temperatures, test methods, identification markings, and the materials used in the construction of the final product. Additionally it requires the manufacturer to qualify each size to steam cycle -cold water test, thermal ageing test and vacuum rating test. These tests have to be certified by an independent testing facility. It is important for the user to ask the supplier for the certification of conformance to this qualification.

• Installation & Maintenance:
  The successful use of lined system also depends upon proper installation. Careful attention must be given to selection of right flange design, gaskets, applying torques using torque wrentch Torque values are provided by the manufacturer for each flange size.  Retorqueing is needed after 24 hours to counter the effect of creep of the flange face liner.

Permeation: Permeation is a special concern for fluoropolymers.  Although permeation cannot be eliminated its effects can be successfully managed.

• Permeation Theory
  Permeation is the molecular transport of a chemical through a barrier material by adsorbing and absorbing at the high concentration side, diffusing through the bulk of the material in the direction of lower concentration, and then desorbing from the low concentration side.
  
• Influencing Factors
  The table below shows the relationship between various factors and the permeation rate.  A quick look reveals that the end user has control over the temperature differential and the thickness of the liner.  Although permeation rate is an important consideration the break-though time is perhaps a more meaningful value, which the current literature data does report.  Special testing is needed and should be considered in critical applications.
  
• Design considerations:
  Conclusions
  Fluoropolymer-lined systems prove to be both useful and economical in many chemical applications. Such systems help protect the process equipment and minimize loss of the chemical process fluids, which protects the environment. Use of proper engineering designs that address permeation will further increase system integrity. These designs should acknowledge the major factors influencing permeation, namely: service conditions, process fluid chemistry, and liner material. Best practice dictates that these designs be tested prior to use in new applications.
  
  
  Contact: Narendra Thakkar
  Tel.: +91-22-6751 5000
  E-mail: Narendra.Thakkar@ind.dupont.com