Flange Insulation Kits: What They Are, Why They Matter, and How to Choose
Here’s a scenario that plays out more often than anyone in the pipeline business likes to admit: a perfectly good flanged joint corrodes from the inside out, not because the material was wrong or the gasket failed, but because two dissimilar metals were welded and/or bolted together and nobody thought about electrical isolation. Galvanic corrosion is real, it’s expensive, and it’s entirely preventable with the right hardware.
That’s where flange insulation kits come in. They electrically isolate one flanged section of pipe from another, breaking the galvanic circuit that drives corrosion. If you work with cathodic protection systems, cross-country pipelines, or mixed-metal piping systems, these kits should already be something you’ve considered for your application.
What a Flange Insulation Kit Actually Is
A flange insulation kit (sometimes called a flange isolation kit) is a set of components that fits between two mating flanges to create a complete electrical break across the joint. The kit electrically isolates the bolts, the gasket sealing surface, and the bolt holes so no metallic path exists between the two flanges.
A standard kit includes four components:
Insulating gasket: The centerpiece of the kit. It seals the flange faces just like a standard gasket but is made from non-conductive material. Common types include G-10/G-11 laminate (glass-reinforced epoxy), phenolic, and Type E (a composite of aramid fibers and nitrile binder). The gasket sits between the raised faces and provides both the seal and the primary electrical break.
Insulating sleeves: Full-length sleeves that slide over each stud bolt or cap screw, preventing metal-to-metal contact between the bolt and the flange bolt holes. Typically made from Mylar (polyester film), G-10 fiberglass, or Nomex, depending on the temperature rating required.
Insulating washers: Placed under each nut (and under the bolt head for cap screws) to prevent electrical contact between the fastener hardware and the flange face. Made from G-10 or similar non-conductive material rated for the service temperature and bolt load.
Steel backing washers (plated): Cadmium or zinc-plated steel washers placed between the insulating washer and the nut to distribute bolt load evenly and protect the insulating washer from damage during torquing. Not all kits include these, but they’re standard in higher-quality assemblies.
Why Flange Insulation Matters
The primary reason to install a flange insulation kit is to prevent galvanic corrosion driven by cathodic protection (CP) systems or by contact between dissimilar metals.
Cathodic protection isolation: Most buried or submerged steel pipelines use cathodic protection to prevent corrosion. CP works by imposing a small electrical current that makes the protected steel a cathode in an electrochemical cell. For CP to work efficiently, the protected section of pipeline needs to be electrically isolated from grounding points, station piping, and other metallic structures. Flange insulation kits create that isolation at flanged connections, which are natural break points in the system.
Dissimilar metal separation: When carbon steel flanges mate with stainless steel, copper alloy, or other dissimilar metals, the difference in electrochemical potential drives galvanic corrosion on the less noble metal (usually the carbon steel). An insulating kit breaks the electrical circuit. For more on how dissimilar metals interact in piping systems, check out our post on stainless steel handling, galling, and corrosion prevention.
Stray current protection: In industrial plants and near electrical substations, stray currents can travel through piping systems and accelerate corrosion at flanged joints. Insulation kits interrupt this path.
Selecting the Right Kit: Gasket Types Compared
Not all insulating gaskets are created equal. The right choice depends on your pressure class, temperature range, and the fluid in the line.
Insulating Gasket Types Compared
| Gasket Type | Max Temp | Pressure Rating | Best For |
| Type E (Aramid/Nitrile) | 400F | Up to 2500# | General service, oil and gas, water |
| G-10/G-11 (Glass Epoxy) | 285F | Up to 2500# | High dielectric strength, CP isolation |
| Phenolic | 250F | Up to 600# | Low-cost applications, moderate service |
| Type F (PTFE-Based) | 500F | Up to 2500# | Chemical service, high purity, FDA applications |
| Kammprofile with G-10 Core | 750F+ | Up to 2500# | High-temp, high-pressure critical service |
Type E is the most commonly specified insulating gasket for oil and gas applications. It handles the full range of ASME B16.5 pressure classes, provides a good seal on raised face flanges per ASME B16.20, and has solid dielectric properties for CP isolation work.
For higher temperatures or more aggressive chemical environments, kammprofile gaskets with insulating cores are gaining traction. They combine the sealing performance of a metal core gasket with the electrical isolation of a non-conductive layer.
Installation and Testing
Installing a flange insulation kit is straightforward, but there are a few things to get right:
Flange face condition matters. The raised face finish needs to be clean and within the standard surface finish range (125 to 250 AARH per ASME B16.5). Scratches, pitting, or corrosion on the flange face will compromise both the seal and the insulation integrity.
Sleeve fit is critical. Insulating sleeves must extend the full length of the bolt hole through both flanges. If the sleeve is too short, the bolt can contact the flange and bypass the insulation entirely. Verify that the sleeve length matches the combined flange thickness plus the gasket.
Test after installation. Use an ohmmeter or megohmmeter to verify electrical isolation across the joint after bolting. A properly installed kit should show megaohm-level resistance between the two flanges. Most CP engineers require a minimum of 1 megaohm across the joint per NACE SP0286 (now AMPP SP21486).
Don’t paint across the joint. Conductive coatings or paint bridging the insulating gasket can short-circuit the isolation. Keep the gap between flange faces clear of conductive material.
Common Specifications and Standards
Flange insulation kits don’t have a single dedicated ASME standard, but they reference and comply with several:
ASME B16.5 governs the flange dimensions the kit must fit. ASME B16.20 covers gasket dimensions for spiral wound and other metallic gaskets (some insulating gaskets follow these dimensions). NACE SP0286 / AMPP SP21486 provides guidelines for electrical isolation of cathodically protected pipelines. ASTM D-149 covers dielectric strength testing of insulating materials. ASTM F-1564 covers gasket performance testing relevant to insulating gaskets.
Kit sizing follows standard ASME B16.5 bolt patterns and flange dimensions, so you order by pipe size and pressure class, the same way you’d order any flange gasket.
When You Need a Kit (and When You Don’t)
Flange insulation kits are mandatory at certain points and optional at others. Required locations typically include: meter stations and custody transfer points on cathodically protected pipelines, transition points between dissimilar metals in any piping system, above-ground to below-ground transition points where CP systems are active, and plant battery limits where piping systems change ownership or protection zones.
You probably don’t need one at every flanged joint in a system. The CP design engineer specifies isolation points based on the cathodic protection design, soil resistivity surveys, and pipeline routing. Over-isolating can actually segment the pipeline into sections too small for effective CP coverage, so placement is strategic.
Get the Right Kit for Your Application
Flange insulation kits are a small investment that prevents a very expensive problem. Whether you’re building a new pipeline, retrofitting CP on an existing system, or separating dissimilar metals in a plant, the right kit needs to match your flange size, pressure class, temperature, and service environment.
Contact Texas Flange so we can get you a source on insulating gaskets and kits in any ASME B16.5 size and class.
Texas Flange & Fitting Supply | 281-484-8325 | texasflange.com
