Flange Isolation Kit Types & Materials

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. Required for cathodic protection systems and dissimilar metal connections.

What Is a Flange Isolation Kit?

A flange isolation kit is a set of dielectric components installed between two bolted flanges to break the electrical continuity across the joint. Without isolation, electric current flows freely through the metal-to-metal contact at the flange faces, bolt holes, and bolt/nut bearing surfaces. This current flow enables two destructive mechanisms: galvanic corrosion at dissimilar metal junctions and cathodic protection current drain where protected pipeline sections connect to unprotected structures.

Every flanged connection in a piping system is a potential path for stray currents. When a buried pipeline protected by cathodic protection (CP) connects to above-ground equipment, the protective current intended for the pipeline can drain through the plant, rendering the CP system ineffective. Isolation kits solve this by inserting non-conductive barriers at every point where metal touches metal in the joint.

Flange isolation kits interrupt electrical continuity between flanges, preventing electrochemical reactions that cause corrosion. Each kit contains four component types working together to achieve a complete electrical barrier.

Why Electrical Isolation Matters: Cathodic Protection Basics

Cathodic protection works by shifting the electrochemical potential of a steel structure to a more negative value (typically -850 mV vs. Cu/CuSO4 reference electrode), suppressing the anodic dissolution reaction that causes corrosion. Two CP methods exist: sacrificial anode systems (zinc or magnesium anodes corrode in place of the pipeline) and impressed current systems (an external DC power source forces protective current onto the steel).

Both methods require the protected structure to be electrically isolated from unprotected metalwork. If a cathodically protected buried pipeline connects to an above-ground plant without isolation, the protective current drains through the plant steelwork to ground. The result: the pipeline loses protection and corrodes, while the CP system wastes energy protecting structures that do not need it.

Pro Tip

Isolation kits are not just for cathodic protection. Any time two dissimilar metals are bolted together in the presence of an electrolyte (process fluid, condensation, seawater), a galvanic cell forms. The more active metal (typically carbon steel) corrodes at an accelerated rate. An isolation kit breaks the electrical circuit and stops the corrosion.

Components of a Flange Isolation Kit

Each kit contains four component types that work together to achieve complete electrical isolation across the bolted joint.

Component	Function	Common Materials
Insulating gasket	Prevents metal-to-metal contact between flange faces and provides a pressure seal	G10/G11 laminate, phenolic, PTFE, neoprene-faced phenolic
Insulating sleeves	Isolates stud bolts from flange bolt holes	G10/G11, polyethylene, polypropylene, PTFE, Mylar
Insulating washers	Provides electrical barrier under bolt heads and nuts	G10/G11, phenolic, PTFE, non-asbestos
Steel backup washers	Distributes bolt load and protects insulating washers from crushing	Galvanized carbon steel or stainless steel

The insulating gasket is the primary barrier between the flange faces. It must seal against the process fluid while maintaining dielectric properties under compression. The insulating sleeves are tubes that slide over each stud bolt, filling the annular gap between the bolt and the flange bore. The insulating washers sit under the bolt head and nut, preventing electrical contact at those bearing surfaces. The steel backup washers sit between the insulating washer and the flange face (or between the insulating washer and the nut/bolt head) to distribute the clamping load and prevent the softer insulating washer from being crushed or extruded.

Installation Critical

All four components must be present and correctly positioned. A missing sleeve, a cracked washer, or a displaced gasket creates a single-point metallic bridge that defeats the entire isolation. Each component addresses a specific current path through the joint.

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
Metering and custody transfer stations	Isolates metering skids from pipeline CP systems
Underground-to-aboveground transitions	Prevents CP current from draining into plant steelwork
Corrosive environments	Protects against electrochemical corrosion in chemical plants and offshore facilities
Marine and offshore risers	Isolates subsea pipework from topside equipment

Types of Flange Isolation Kits

Four kit types are designated by the gasket style and the flange face configuration they fit:

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

Type F (FF/RF Flanges)

Type F kits fit flat-face and raised-face flanges, the most common configurations in industrial piping. The full-face gasket extends to the bolt circle with holes for each bolt, covering the entire flange face. The gasket OD is smaller than the flange OD. Special band protectors can be added to prevent foreign material accumulation between the flange faces.

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. The large contact area provides good stability and centering.

Pressure/temperature range: Suitable for most general-service applications. For elevated pressure and temperature, Type E kits provide better sealing performance on RF flanges.

Type E (RF Flanges)

Type E kits fit raised-face flanges only, with the gasket covering the raised face portion. The ring gasket concentrates compression over the critical sealing area and keeps foreign material outside the joint. Elastomeric seal rings (Viton, Buna-N, or PTFE) can be incorporated into the gasket for enhanced sealing in sour or hazardous service.

Gasket materials: G10/G11 laminate, phenolic, neoprene-faced phenolic, and high-temperature materials.

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

Sour Service

For sour gas (H2S) service per NACE MR0175/ISO 15156, Type E kits with PTFE sealing elements and G10/G11 laminate gaskets are the standard choice. The kit must maintain isolation integrity under the full range of operating pressure and temperature conditions.

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. The groove geometry constrains the gasket, resulting in excellent containment under high pressure.

Gasket materials: Phenolic laminate with elastomer seals, G10/G11 laminate.

Applications: Oil & gas, petrochemical, power generation where high P/T conditions require RTJ flanges. Critical for subsea, offshore, and underground installations where dissimilar metals meet.

Type O (Orifice Flanges)

Orifice flanges used in flow measurement systems require Type O kits. 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, and chemical processing where accurate flow measurement requires electrical isolation to maintain meter accuracy and prevent galvanic corrosion.

Selection Criteria

Factor	Consideration
Flange face type	FF, RF, or RTJ determines kit type (F, E, or D)
Operating conditions	Temperature, pressure, corrosive environment
Electrical isolation	Level needed for galvanic protection or CP system
Material compatibility	Match to flange, pipeline, and process fluid
Service type	Sour service (NACE MR0175), chemical, subsea

Flange Isolation Kit Materials

Material selection depends on dielectric strength, temperature limits, water absorption, and chemical compatibility with the process fluid. Low water absorption is critical: absorbed moisture creates conductive paths that compromise isolation over time.

Gasket Material Comparison

Gasket Material	Dielectric Strength (V/mil)	Max. Temperature	Water Absorption	Best For
G10/G11 (NEMA)	UL94 VO rated	200 C (392 F)	0.01%	General service, sour gas, offshore
Plain Phenolic	500	107 C (225 F)	1.10%	Moderate service, economical
Neoprene-faced Phenolic	500	80 C (175 F)	0.45%	Low-temperature, water service
PTFE (Teflon)	600	232 C (450 F)	0.01%	Chemical service, corrosive fluids
Garlock 3400	630	371 C (700 F)	—	High-temperature service
Klinger C4401	300	399 C (750 F)	—	High-temperature service
JM940 Red Devil	2400	371 C (700 F)	—	High dielectric strength
Insul-Seal	500	80 C (175 F)	0.50%	General service
Phenolic RTJ Type D	500	107 C (225 F)	1.00%	RTJ flanges

Material Selection Guide

G10/G11 laminate is the preferred choice for most oil and gas applications. It combines excellent dielectric strength with very low water absorption (0.01%) and good mechanical properties. Phenolic materials are more economical but absorb more moisture over time, which can compromise isolation in buried or submerged installations.

Insulating Sleeves

Sleeve Material	Dielectric Strength (V/mil)	Max. Temperature	Water Absorption
Mylar	4000	177 C (350 F)	0.22%
NEMA G10	UL94VO	FR rated	0.01%
Phenolic	500	121 C (250 F)	2.00%
Minlon	450	149 C (300 F)	0.90%

Mylar sleeves offer the highest dielectric strength (4000 V/mil), making them the preferred choice for critical applications. G10 fiberglass sleeves provide the best combination of mechanical strength and low water absorption for demanding environments.

Insulating Washers

Washer Material	Dielectric Strength (V/mil)	Max. Temperature	Water Absorption
NEMA G11	UL94VO	FR rated	0.01%
PTFE	600	232 C (450 F)	0.01%
Phenolic	500	107 C (225 F)	1.10%
Non-Asbestos	300	399 C (750 F)	—

Steel Backup Washers

Steel backup washers (galvanized carbon steel or stainless steel) distribute bolt load and protect the softer insulating washers from damage during bolt tightening. They are not optional; omitting them is the most common installation mistake.

Where to Install Isolation Kits

Isolation kits must be installed at specific locations in a piping system to maintain the integrity of cathodic protection and prevent galvanic corrosion. The following locations are typical:

Installation Point	Reason
Pipeline entry to plant/facility	Prevents CP current from draining into plant steelwork
Buried-to-aboveground transition	Isolates cathodically protected buried pipe from unprotected above-ground pipe
Dissimilar metal flanged joints	Breaks the galvanic circuit between carbon steel and alloy/stainless components
Metering stations	Isolates custody transfer metering from pipeline CP and stray currents
Pipeline crossings	Prevents CP interference between parallel or crossing pipelines
Platform risers (offshore)	Isolates subsea cathodically protected pipe from topside equipment
Storage tank connections	Isolates tank CP system from incoming/outgoing piping

Design Note

The location of isolation kits should be determined during the detailed engineering phase by the cathodic protection engineer. Incorrect placement can leave sections of pipeline unprotected or create stray current interference with adjacent structures.

Testing Flange Isolation Kits

Post-installation testing is mandatory to confirm that the isolation kit is functioning correctly. A kit that looks properly installed may still have a hidden metallic bridge from a misaligned sleeve, conductive debris, or a cracked washer.

Resistance Testing

The primary test method is measuring electrical resistance across the isolated joint using a digital multimeter:

1. Set the multimeter to the megaohm range
2. Place one probe on the upstream flange and the other on the downstream flange
3. Acceptable: Resistance above 1 megaohm indicates effective isolation
4. Unacceptable: Resistance below 1 megaohm indicates a metallic bridge

If resistance is low, inspect for:

• Bolt sleeves that have shifted or split, allowing the bolt to contact the flange bore
• Conductive debris (mill scale, weld spatter, dirt) between the flange faces
• Cracked or crushed insulating washers
• Missing steel backup washers (causing washer extrusion and metal contact)

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. Do not assume installation is correct just because the kit is in place.

Holiday Detection

For critical installations, holiday detection (spark testing) can be used to verify the integrity of insulating coatings on the gasket and sleeves before assembly. A high-voltage, low-amperage probe is passed over the insulating surface. Any spark indicates a pinhole or defect that could compromise isolation.

Ongoing Monitoring

Isolation kits should be re-tested periodically, especially in buried or submerged installations where moisture ingress can degrade dielectric properties over time. Annual testing is standard practice for cathodic protection systems per NACE SP0169.

Common Installation Mistakes

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.

The following mistakes account for the majority of isolation kit failures:

Mistake	Consequence	Prevention
Missing steel backup washers	Insulating washers crush under bolt torque, creating metal-to-metal contact	Always include backup washers; they distribute the bolt load
Misaligned bolt sleeves	Bolt contacts flange bore, creating a conductive bridge	Center each bolt carefully; verify sleeve extends full bore length
Conductive debris on flange faces	Foreign material bridges the insulating gasket	Clean both flange faces thoroughly before assembly
Anti-seize on insulating components	Metallic anti-seize creates conductive paths across insulating surfaces	Apply anti-seize to bolt threads only; keep off sleeves and washers
Uneven bolt torquing	Gasket distortion, uneven compression, potential leak and isolation loss	Torque in star pattern per ASME PCC-1
Wrong kit type for flange face	Gasket does not seat properly, fails to seal or isolate	Match kit type (F, E, D) to the flange face (FF, RF, RTJ)
Not testing after installation	Hidden metallic bridge goes undetected until CP system fails	Always measure resistance across the joint before commissioning
Reusing old isolation kits	Degraded dielectric properties from compression set, moisture absorption, or damage	Install new kits; never reuse gaskets, sleeves, or washers

Common Mistake

Installing isolation kits without the steel backup washers is a frequent cause of failure. The insulating washers alone cannot 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 are part of the kit for a reason.

Temperature and Pressure Limitations

Isolation kit performance is limited by the weakest component in the assembly. The maximum allowable working pressure follows the flange pressure-temperature rating per ASME B16.5 or B16.47, but the gasket material imposes additional temperature limits.

Material	Max. Temperature	Max. Pressure	Typical Service
G10/G11 laminate	200 C (392 F)	Per flange rating	Oil & gas, offshore, sour service
Phenolic	107 C (225 F)	Per flange rating	Water treatment, moderate chemical
PTFE	232 C (450 F)	Per flange rating (watch cold flow)	Chemical, corrosive service
Garlock 3400	371 C (700 F)	Per flange rating	High-temperature refinery service
Non-asbestos fiber	399 C (750 F)	Per flange rating	High-temperature utility service

PTFE Cold Flow

PTFE gaskets are excellent for chemical resistance and dielectric properties, but they are susceptible to cold flow (creep) under sustained bolt load, especially at elevated temperatures. This can cause bolt relaxation and leakage over time. For PTFE gaskets, specify filled PTFE or use retainer rings to limit extrusion, and plan for bolt re-torquing after thermal cycling.

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

Standards and Specifications

Standard	Scope
NACE SP0286	Electrical isolation of cathodically protected pipelines; the primary standard for isolation kit selection and testing
ASTM F2836	Standard practice for gaskets for use in connection with flange isolation kits
NACE SP0169	Control of external corrosion on underground or submerged metallic piping systems (CP design)
NACE MR0175 / ISO 15156	Material requirements for sour service (H2S environments)
ASME B16.5	Pipe flanges and flanged fittings (dimensions that isolation kits must fit)
ASME B16.47	Large diameter steel flanges (NPS 26 through NPS 60)
ASME PCC-1	Guidelines for pressure boundary bolted flange joint assembly (torque procedures)
EN ISO 26623	Flange isolation gaskets (European standard)

NACE SP0286

NACE SP0286 is the most widely referenced standard for flange isolation. It covers design, selection, installation, and testing of electrical isolation devices for cathodically protected pipelines. The standard requires that isolation kits maintain a minimum resistance of 1 megaohm across the joint and specifies testing procedures for both new installations and in-service joints.

Frequently Asked Questions

What is a flange isolation kit and what does it do?

A flange isolation kit is a set of components (insulating gasket, bolt sleeves, insulating washers, and steel backup washers) installed at a bolted flange joint to electrically isolate the two flanges from each other. It prevents galvanic corrosion and preserves cathodic protection systems by breaking the electrical path across the joint. Without isolation, current flows through the metal-to-metal contact at the flange faces, bolt holes, and bolt/nut bearing surfaces.

When is a flange isolation kit required?

Flange isolation kits are required wherever cathodic protection systems must be preserved (buried or subsea pipelines connecting to above-ground equipment), at dissimilar metal connections (e.g., carbon steel to stainless steel), at pipeline metering and custody transfer stations, at underground-to-aboveground transitions, and in offshore and marine environments where corrosion protection of risers and platform piping is critical.

What is the difference between Type E, Type F, and Type D isolation kits?

Type F kits use a full-face gasket for flat-face and raised-face flanges and are suitable for general service. Type E kits use a ring gasket sized to cover only the raised face of RF flanges, providing better sealing at elevated pressure and temperature. Type D kits have a gasket that fits the RTJ groove for ring-type joint flanges, used in high-pressure and high-temperature service such as offshore and subsea installations.

How do you test a flange isolation kit after installation?

After installation, measure electrical resistance across the joint with a digital multimeter set to the megaohm range. A properly isolated joint should read at least 1 megaohm. Readings below 1 megaohm indicate a metallic bridge, typically caused by a missing or damaged sleeve, misaligned washer, or conductive debris between the flange faces. Testing should be performed before commissioning and repeated annually for cathodic protection systems.

What gasket material should I choose for a flange isolation kit?

G10/G11 fiberglass laminate is the most common choice for general and oil & gas service (up to 200 C) due to excellent dielectric strength and very low water absorption (0.01%). Phenolic is more economical for moderate temperatures (up to 107 C). PTFE handles chemical and corrosive service up to 232 C but is susceptible to cold flow under sustained load. For sour gas (H2S) service per NACE MR0175, use Type E or Type F kits with PTFE sealing elements and G10/G11 laminate gaskets.