Gasket material can improve
cathodic protection reliability

Electrical isolating flange gaskets are used throughout the oil and gas and hydrocarbon transmission/process industries to cathodically isolate various metal components at bolted end-connections.

Electrical flange isolation can be used in conjunction with cathodic protection, used to isolate dissimilar metals (galvanic corrosion) or used to mitigate the corrosive effects of electrolysis.

Typically, this is accomplished by using non-metallic materials, which possess sufficient dielectric strength to electrically isolate flanges and bolts from one another by reducing the potential for stray current conducting through the piping system (beyond the flanged end-connection).

However, the various materials used in constructing these gaskets as well as the accompanying insulating sleeves and washers often are not suitable for the intended application. Problems generally associated with conventional insulating materials are related mainly to poor mechanical properties inherent in the materials themselves. Consequently, compression of the conventional gasket material can result in stress fractures, extrusion, non-sealing deformation and/or breakage.

Many conventional insulating materials, which must possess a relatively high dielectric strength, are brittle in nature and do not perform as well in a compressed state. Most notable are the phenolic-based materials. Reinforcing fibers are often used and can add significantly to compressive and tensile strength of the materials. However, the base resin still remains relatively brittle and unable to perform adequately under load. Once the gasket structure is damaged, hoop strength, sealing capacity and resistance to fluid/gas migration under system pressure is compromised.

Problems most often arise both in the installation phase or during the course of long-term operation. Problems can manifest themselves early on and are mostly related to sealing failure (leakage) or shorting wherein the connection is no longer electrically insulated and cathodic protection is lost. Since conventional gasket materials are often composite laminates, they also are susceptible to radial fluid and/or gas migration through the layers of material, particularly when the media is made up of relatively low molecular weight hydro-carbon molecules such as natural gas or methane. Radial fluid migration can contribute to undermining the gasket’s structural integrity and insulating capabilities.

An existing material, which is currently being used by many pipe line companies, is glass reinforced epoxy (GRE) material. When used as a gasket material, the structural/mechanical properties are suitable particularly in compression and resistance to radial fluid migration. When compared to standard paper or linen grade materials, GRE typically possesses 4-8 times the compressive strength, roughly equal dielectric strength with only a slight increase in cost.

Furthermore, since epoxy resins are stronger and denser than most other readily available resin compounds, fluid absorption can be reduced from .1 to .01 (H20 media) when compared to other materials. This means much better resistance to radial fluid migration. This can be particularly important for buried pipe lines, which are immersed in high ambient moisture environments. Additionally, the higher density of the resin material and the reinforcing fibrous matrix makes this material much more resistant to gas permeability (wicking). The same GRE material also is used as sleeve material for the stud bolts, providing the same mechanical benefits and offering much greater resistance to thread pinch of the sleeve between the stud bolts and bolt holes in the flanges.

Transcontinental Gas Pipeline has used this gasket design for several years. All of the newly constructed electrically isolated end-connections are being assembled in a fabrication facility and are then butt welded in place as a unit in the field. The prefabricated connections are assembled using approximately 35 ksi stud stress and are then tested for electrical continuity. Sizes range from 4-in. ANSI 600 class to 42-in. ANSI 600 class.

Flanges are assembled using hydraulic tensioning equipment. All of the prefabricated end-connections are hydrostatic pressure tested to 2,250 psi and then tested for electrical isolation after the test media has been removed from the vessel. The internal gap of the assembly also is filled with splash zone type epoxy compound and the external gap is filled with wax. All other external surface areas are coated with a minimum of 16 mil DFT of cold tar epoxy.

The system has worked well to date with a noticeable reduction in the gasket failure rate. Transcontinental also is using the new gasket design in field retrofit applications with excellent results. Cost of gasket failure either in terms of leaks, or shorted flanges can be extremely high when considering the cost of system shutdowns, excavation of buried pipe lines/flowlines and gathering lines as well as lost production expenses. It is anticipated that future operating expenses will be reduced on cathodically protected lines by maintaining proper electrical isolation of the system.