Isolation & Insulation
Bolt Isolation Sleeves & Insulating Washers
Bolt Insulating Sleeves & Washers
These are the per-bolt insulating components that break the electrical path through a flanged joint: one sleeve to keep the stud clear of both flanges, plus insulating and steel backing washers under each nut. You fit them to keep a cathodic protection system doing its job, or to stop galvanic corrosion where two dissimilar metals bolt together.
These are the per-bolt components. Need the complete gasket, sleeve and washer set? Explore our Flange Isolation Kits.
How Bolt Isolation Works
Isolation happens one bolt at a time. Each stud gets a full-length sleeve that keeps it clear of both flange bores, so the bolt never bridges the two flanges. At each end, an insulating washer breaks the path under the nut, and a steel backing washer spreads the clamp load across it. That is what a per-bolt set does; fit every bolt in the joint the same way and the flange stops conducting. For the whole-joint picture, including how the gasket, sleeves and washers work together to hold a cathodic protection current or stop galvanic corrosion, see how a joint is isolated in our flange isolation guide.
One Per-Bolt Set
Per bolt: one full-length insulating sleeve, two insulating washers, and two steel backing washers. The sleeve covers the shank; a washer pair sits at each end.
Never Skip the Steel Washer
The insulating washer cannot take nut torque on its own. Without a steel washer spreading the load it crushes, creeps, and loses both the seal and the isolation. The steel washer always sits between the insulating washer and the nut.
Sleeve & Washer Materials
Material choice comes down to temperature first, then the bolt-hole clearance and the media. Mylar covers most ambient and warm service; move to Nomex once you are past 150 °C. Here is how we usually steer it.
Figures are typical service limits, not guarantees. Where a cell shows a dash, the limit depends on grade and wall thickness, so tell us the service and we will confirm.
The Sleeve and Washers
One insulating sleeve runs the whole length of each bolt, through both flanges and past the gasket, reaching at least halfway into the steel backing washer at each end. That leaves no bare shank inside a flange bore, which is the one thing that shorts a joint. Every bolt gets one.
One Sleeve, Full Length
The sleeve length is fixed by the joint (both flanges plus the gasket), so it is the washers that change with the job. A double washer set puts an insulating and a steel washer at each end and isolates both flanges; it is the default for a joint you are isolating properly. A single washer set leaves the insulating washer off one end where a drawing calls for one-sided isolation. The sleeve is full length either way.
Get the Fit Right
Match the sleeve and washers as a set. If the steel washer's inner diameter is undersized, it grabs and twists the sleeve as you tighten, which is sleeve shear. If the washer diameters do not match, the insulating washer squeezes out under load, which is extrusion. Mixing washers from two suppliers is how both happen, so we ship them matched. For field work with fewer loose parts, we can supply one-piece "top-hat" insulators that combine the sleeve and one washer into a single moulding.
Isolation is all-or-nothing. One un-sleeved bolt, one crushed washer, or a smear of conductive anti-seize across the gap shorts the whole flange, and the joint reads dead however careful you were with the rest. Our flange isolation guide covers why isolation fails and how to test and commission the joint.
Sizing & Specification
Sleeves are sized by the bolt, so a quick spec gets you an accurate quote. We supply metric and imperial sizes to suit standard flange bolting, and we match the wall to your bolt-hole clearance.
What We Need to Quote
- Bolt or stud diameter, metric or imperial
- Stud length, or the combined flange thickness the sleeve has to span
- Flange standard: ASME B16.5, AS 2129 (Table D/E), or AS 4087
- Service temperature, so we pick the right film or laminate
- Whether you want sleeves only, or sleeves with insulating and steel washers
Not sure of the bolt size? Send the flange standard and pressure class and we will work back to the bolting.
Common Questions
Do I Need a Sleeve on Every Bolt?
Yes. One bare bolt bridges the two flanges and shorts the isolation, so every bolt in the joint gets its own full-length sleeve. There is no partial version that works.
Do the Washers Go on Both Sides?
For proper isolation, yes: an insulating washer and a steel backing washer at each end of the bolt. A single-sided set is a cost option we supply where a drawing calls for it, but it gives less assurance than a double set.
Can I Buy Sleeves on Their Own, or Do I Need the Full Kit?
Both. If you already have a sound isolating gasket and only need to replace or add bolt insulation, sleeves and washers on their own are the right buy. If you are isolating a joint from scratch, you want the complete set. See our Flange Isolation Kits for the full assembly.
Can You Supply Washers Separately from Sleeves?
Yes. We can supply insulating washers, steel backing washers, and sleeves as separate line items, or matched as a set. Matched is safer, since mismatched washer diameters are a common cause of sleeve and washer failure.
How Do I Know the Isolation Worked?
Test the joint after bolt-up: on a dry, above-ground flange a 500 V megohmmeter reading above 1 MΩ is the usual pass, while buried or cathodically protected pipe needs an RF isolation tester rather than a plain ohmmeter. See our testing and commissioning steps for the full method.
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Need Bolt Isolation Sleeves?
Tell us the bolt size, flange standard, and service temperature. We'll sort the sleeves and washers, on their own or as part of a complete isolation kit.
Request a QuoteDisclaimer
This page is provided for general engineering reference only and does not constitute professional advice, specification, or guarantee of performance. Actual results depend on specific application conditions. Universal Gaskets Pty Ltd accepts no responsibility or liability for decisions made based on this information. For full terms, see our Terms & Conditions.