What Is a Flange Isolation Kit?

A flange isolation kit electrically isolates two flanged piping sections from each other. It prevents galvanic corrosion and preserves cathodic protection (CP) systems by breaking the electrical path across the bolted flange joint. The kit consists of an isolating gasket, bolt sleeves, and washers that eliminate metal-to-metal contact between the flanges and the bolting.

Kit Components

Component	Function	Material
Isolating gasket	Seals and electrically isolates the flange faces	G10/G11 laminate, PTFE, or phenolic with elastomer seal
Bolt sleeves	Insulate stud bolts from bolt holes	G10/G11 fiberglass or mylar
Isolation washers	Insulate bolt/nut from flange face	G10/G11, PTFE, or phenolic
Steel backup washers	Distribute bolt load over isolation washer	Zinc-plated carbon steel
(Optional) O-ring	Additional gasket seal for sour/hazardous service	Viton, Buna-N, or PTFE

When Isolation Kits Are Required

Flange isolation kits are mandatory in these scenarios:

• Cathodic protection breaks: Where buried or subsea pipelines connect to above-ground piping or equipment—the CP current must not drain through the facility
• Dissimilar metals: Where carbon steel piping connects to stainless steel, copper-nickel, or other alloy equipment (galvanic corrosion risk)
• Pipeline metering stations: Isolating custody transfer metering skids
• Underground-to-aboveground transitions: Pipeline entry points to process facilities
• Marine and offshore: Corrosion protection of risers and platform piping

Pro Tip

After installation, verify electrical isolation with a multimeter. Resistance across the joint should exceed 1 megaohm. If resistance is low, a metallic bridge exists—likely caused by a missing sleeve, misaligned washer, or debris between the flange faces. Re-inspect and correct before commissioning.

Gasket Types for Isolation Kits

Type	Description	Temperature Limit	Service
Type E	G10 laminate + elastomer	200degC (392degF)	Standard
Type F	G10 laminate with PTFE core	260degC (500degF)	Sour service, H2S
Type D	Phenolic laminate + Buna-N	150degC (302degF)	Water, mild chemicals
Type O	Full PTFE with O-ring seal	230degC (446degF)	Chemical, corrosive

For sour gas (H2S) service per NACE MR0175, Type F kits with PTFE sealing elements and high-dielectric-strength gaskets are standard. The kit must maintain isolation integrity under the pressure and temperature conditions of the service.

Common Mistake

Installing isolation kits without steel backup washers. The bolt torque can crush the softer isolation washers, creating bolt-to-flange contact that defeats the isolation. Steel backup washers distribute the load and protect the insulating components.

Standards and Specifications

Standard	Scope
NACE SP0286	Electrical isolation of cathodically protected pipelines
EN ISO 26623	Flange isolation gaskets
ASME B16.5 / B16.47	Flange dimensions the kit must fit
NACE MR0175 / ISO 15156	Material requirements for sour service

Isolation kits are sized to match the NPS and pressure class of the flange pair. For a complete reference on flange isolation, see the flange isolation kits guide.

Read the full guide to flanges