Flange Isolation Kits: Types, Materials & Selection

Quick Reference

A flange isolation kit prevents galvanic corrosion by electrically isolating flanged joints. Components: insulating gasket (type F, E, D, or O), sleeves for each bolt, insulating washers, and steel backup washers. Essential for cathodic protection systems and dissimilar metal connections.

Flange Isolation Kit

Components and Function

Flange isolation kits interrupt electrical continuity between flanges, preventing electrochemical reactions that cause corrosion. Each kit contains four component types:

Component	Function	Materials
Insulating gasket	Prevents metal-to-metal contact between flange faces	Neoprene, PTFE, phenolic, G-10
Insulating sleeves	Isolates bolts from flange bolt holes	Polyethylene, polypropylene, PTFE, Mylar
Insulating washers	Provides electrical barrier under bolt heads/nuts	Phenolic, PTFE, non-asbestos, G-11
Steel backup washers	Distributes load, prevents insulating washer damage	Galvanized or stainless steel

Applications

Application	Purpose
Cathodic protection systems	Isolates pipeline sections for effective CP system operation
Dissimilar metal connections	Prevents galvanic corrosion between anodic and cathodic metals
Corrosive environments	Protects against electrochemical corrosion in chemical plants and offshore facilities

Pro Tip

After installing a flange isolation kit, always verify electrical isolation with a millivolt drop test or resistance measurement across the joint. A reading below 1 megohm indicates a compromised isolator - often from a bolt touching the flange or a damaged sleeve. Don’t assume installation is correct just because the kit is in place.

Installation Critical

Proper installation is essential-components must be correctly positioned and bolts tightened to recommended torque. Degraded insulating properties require regular inspection and replacement.

Specifications

Flange isolation kits prevent electrolytic corrosion of flanges and flanged joints caused by static currents. Kit materials are selected for chemical stability, low water absorption, and dielectric properties.

Availability: ASME B16.5, B16.47, and API flanges from 1/2” to 80”. Standard sizes up to 24”; larger sizes available on request.

Types of Flange Isolation Kits

Four kit types are designated by the gasket style and intended application:

Type	Flange Face	Application	Gasket Style
F	FF / RF	General service, basic isolation	Full-faced gasket covering entire flange face
E	RF	Elevated P/T, oil & gas, chemical	Ring gasket with enhanced sealing
D	RTJ	High P/T, offshore, marine, underground	Gasket fits RTJ groove
O	Orifice	Flow measurement systems	Ring or full-face with extra sealing elements

Selection Criteria

Factor	Consideration
Operating conditions	Temperature, pressure, corrosive environment
Electrical isolation	Level needed for galvanic protection
Material compatibility	Match to flange, pipeline, and fluid
Flange face type	FF, RF, or RTJ determines kit type

Field Note

When installing isolation kits, apply a thin film of anti-seize compound to the bolt threads but keep it off the insulating sleeves and washers - contamination can create conductive paths. Also, torque bolts in the correct star pattern; uneven loading damages the gasket and compromises both sealing and isolation.

Type F (FF/RF Flanges)

Type F kits fit flat-face and raised-face flanges-the most common configurations in industrial piping. The gasket OD is smaller than the flange face. Special band protectors can be added to prevent foreign material accumulation.

Gasket materials: 1/8” thick fabric-based phenolic (plain or nitrile-coated), non-asbestos fibers with high dielectric capacity.

Benefits: Versatile for oil & gas, chemical, and water treatment; full-faced design simplifies installation and alignment.

Type E (RF Flanges)

Type E kits fit raised face flanges only, with the gasket covering the raised face portion. This targeted design ensures compression over the critical sealing area and keeps foreign material outside the joint.

Gasket materials: Phenolic, neoprene-faced phenolic, and high-temperature materials.

Benefits: Precise alignment on RF flanges; effective for oil & gas, chemical, and water treatment where elevated P/T requires enhanced sealing.

Type D (RTJ Flanges)

Type D kits are designed for ring-type joint (RTJ) flanges used in high-pressure and high-temperature applications. The gasket fits within the RTJ groove, providing both sealing and electrical isolation.

Applications: Oil & gas, petrochemical, power generation where high P/T conditions require RTJ flanges. Effective against galvanic corrosion in systems with dissimilar metals.

Common Mistake

Installing isolation kits without the steel backup washers - the insulating washers alone can’t withstand bolt torque and will crush, crack, or creep over time. This causes loss of bolt tension and eventual leakage. Always include the backup washers; they’re part of the kit for a reason.

Type O (Orifice Flanges)

Type O kits are designed for orifice flanges used in flow measurement systems. The gasket features extra sealing elements (PTFE, Nitrile, or Viton) on both sides and is available in ring (E-style) or full-face (F-style) designs.

Gasket materials: Phenolic, G-10. Sleeve materials: Polyethylene, phenolic.

Applications: Oil & gas extraction, refining, chemical processing-anywhere accurate flow measurement requires electrical isolation to maintain meter accuracy and prevent galvanic corrosion.

Flange Isolation Kit Materials

Material selection depends on dielectric strength, temperature limits, water absorption, and chemical compatibility with the process fluid.

Insulating Gaskets

Gasket Material	Dielectric Strength	Maximum Temperature	Water Absorption
Neoprene faced Phenolic	500	175 F	0.45%
Plain Phenolic	500	225 F	1.10%
Garlock 3400	630	700 F
Klinger C4401	300	750 F
G10 (NEMA)	UL94 VO	FR	0.01%
Insul-Seal	500	175 F	0.50%
JM940 Red Devil	2400	700 F
Phenolic RTJ Type D	500	225 F	1.00%
Teflon	600	450 F	0.01%

Insulating Sleeves

Sleeve Material	Dielectric Strength	Maximum Temperature	Water Absorption
Phenolic	500	250 F	2.00%
Mylar	4000	350 F	0.22%
NEMA G10	UL94VO	FR	0.01%
Minlon	450	300 F	0.90%

Insulating Washers

Washers Material	Dielectric Strength	Maximum Temperature	Water Absorption
Phenolic	500	225 F	1.10%
Non-Asbestos	300	750 F
NEMA G11	UL94VO	FR	0.01%
Teflon	600	450 F	0.01%

Steel backup washers (galvanized or stainless) distribute load and protect insulating washers from damage during bolt tightening.

Related Articles

Flanges

• What Is a Flange for Pipes - Overview of flange types, specifications, and applications
• Flange Face and Finish Types - FF, RF, RTJ face types and surface finishes
• ASTM Materials for Forged Flanges - Material specifications for forged flanges

Corrosion

• Steel Corrosion - Types of corrosion and prevention methods