Why don’t my PTFE gaskets seal FRP, plastic and other light load flanges?
This month’s “Sealing Sense” was prepared by FSA member Charlie Miskell
Manufacturers and distributors of PTFE gaskets are being asked more frequently why PTFE gaskets do not seal fiberglass reinforced plastic (FRP), plastic and other light load flanges. The answer is simple, but the solution is complex. In a majority of cases, the answer is that too little load is applied to the gasket. Unfortunately, and as is the case in many such instances, the gasket is usually considered the problem (see “Sealing Sense,” January 2008). However, when other contributing factors are considered, the gasket carries little blame. This problem must be considered at two stages. First is the original design of the equipment, and the second is how to deal with the equipment during installation and maintenance.
Originally, FRP and plastic piping evolved in the process industry due to their low cost and excellent chemical resistance. However, neither material approaches the strength of metallic piping. Therefore, flange strength limits the amount of bolt torque that can be applied, which then limits the gasket stress. Primarily, the gasket materials used then and now are elastomers, which have good recovery properties and seal at relatively low loads, often with as little as 15 percent compression. However, due to the limited chemical compatibility of each elastomer, the selection of the proper elastomer must be made carefully. When this problem was compounded with the different elastomers often being the same color, improper gasket installations were all too frequent.
Current Gasket Concerns
In today’s chemical process plants, more exotic chemicals used at higher temperatures and pressures sometimes demand a shift from elastomers to a more chemically inert gasket. PTFE is the logical next choice as it is chemically compatible with all common aggressive chemicals within the pH range of 0 to 14—except molten alkali metals and elemental fluorine (especially at higher temperatures and pressures).
As with metallic piping, most FRP and plastic piping manufacturers follow ASME standards. ASME’s updated Reinforced Thermoset Plastic Corrosion-Resistant Equipment Standard RTP-1-2011 for vessels operating at pressures not exceeding 15 psig teaches the proper practices for design. Non-mandatory Appendix NM-9 Installation of RTP Vessels gives guidelines for gaskets, fasteners, torque, lubrication and other factors related to the success of a bolted flange connection. While not mandatory, most in the industry use these guidelines when discussing gaskets for equipment.
While some gasket designs seal at lower loads, most PTFE gasket manufacturers set minimum loads (from 2,500 psi to 3,000 psi) for their materials. However, the lower application pressures and temperatures typical of FRP and plastic piping may lower such requirements. Figure NM9-3 from Standard RTP-1-2011 provides the recommended torque and bolt-up procedure for standard RTP flanges and manways.
Table 1 shows the resultant gasket stress when the suggested torque (Figure NM9-3) is used. Obviously, the resulting gasket loads are well below the minimum threshold necessary to seal with a PTFE-based gasket.
Table 2 shows additional specific RTP details that may affect the load delivered to a gasket material in these flanges. The third column (added for this article) indicates some suggested improvements needed for better PTFE gasket results.
However, at this time, no suitable answer is available for this dilemma. If the vessel or piping will use PTFE gaskets instead of elastomers, these gaskets have little chance of working unless the recommended guidelines are changed. This is the main reason that end users experience gasket problems. There are two stages of this issue. In defense of the equipment manufacturers, they are able to increase the flange load capability of equipment to allow torques that provide the required stress. However, they have to make these changes in the design stages before quoting the equipment. Most manufacturers will make the appropriate adjustments to lessen this complaint.