Flange Isolation Bolt Sleeves: PET, PTFE, Nomex and HDPE Compared
Material vs Material · 2026-09-16 · 5 min read
A flange isolation kit has four parts doing the work: the gasket between the flange faces, the washers under the nuts, the sleeve over each bolt, and the bolt itself. Three of those are specified carefully. The sleeve is the one that gets substituted to save money, and it is also the one that decides whether the isolation holds, because a bolt that touches metal anywhere along its length defeats the whole assembly. The material choice is worth making deliberately.
What the sleeve has to do
The sleeve is the dielectric break between the bolt shank and both flange faces. On a cathodically protected pipeline or a tank sitting in wet soil, the driving voltage is small and the current is small, so the sleeve is not really a high-voltage component. It is a corrosion-control component with a length requirement and a fit requirement.
- It has to run the full length between the flange faces, not just cover part of the shank.
- It has to sit between the bolt and the hole without being crushed or sheared during torque-up.
- It has to hold its dielectric properties in the service temperature and in whatever the environment puts on it.
- It has to be there in the right count, because a missing sleeve on one bolt in eight is a completed circuit.
The four materials in use
| Material | Typical service temperature | Strengths | Limits |
|---|---|---|---|
| PET / Mylar film | Minus 55 C to plus 105 C continuous, softening above that | Lowest material cost for the duty, cuts and forms cleanly to length, good dielectric strength for flange work, thin wall fits tight clearances | Temperature ceiling and chemical ceiling are the lowest of the four; not for hot or aggressive service |
| PTFE | Up to about 260 C | Near-universal chemical resistance and the highest temperature ceiling of the four | Costs several times the PET sleeve. On a kit with dozens of bolts the premium is the dominant line item |
| Nomex aramid paper | Higher than PET, grade dependent | Good temperature capability and mechanical toughness, established in electrical insulation practice | Costs more than PET and absorbs moisture, which changes dimensions and dielectric behaviour |
| HDPE | Below PET in practice | Cheap and available, easy to extrude | Soft, so it creeps under bolt load; the temperature ceiling is the lowest of the four and chemical resistance is ordinary |
The published numbers for our own PET sleeve material sit at a wall range of 0.05 mm to 0.25 mm, dielectric strength of 15 kV/mm or better, tensile strength of 50 MPa or better, continuous service from minus 55 C to plus 105 C, and references ASTM D880 for the film and IEC 60250 on the electrical side, with RoHS and REACH compliance.
Where the decision actually lands
| Duty | Sensible choice | Reason |
|---|---|---|
| Standard pipeline and tank flanges at ambient to moderate temperature | PET | The dielectric duty is modest, the bolt count is high, and PET delivers the requirement at a fraction of the material cost |
| High-temperature flanges, steam service, adjacent to heat tracing | PTFE | PET softens above its continuous limit and creeps under bolt load once it does |
| Chemical exposure, aggressive wash-down, acid or caustic splash | PTFE | PET has ordinary chemical resistance. PTFE resists nearly all industrial media |
| Buried or submerged joints with a long design life | PET, with PTFE where temperature or chemistry demands it | PET handles the wet duty fine; the deciding factor is temperature |
| Bolt holes with tight clearance or an unusual wall requirement | PET | Thin-wall PET fits a clearance that a heavier material will not |
| A kit that has to survive being specified in writing and audited | Whatever the drawing says, verified per lot | The material matters less than whether the sleeve matches the bolt callout on every flange |
The failure that has nothing to do with material
Most flange isolation failures we get asked about are not material failures. They are fit failures: a sleeve cut short so the stud contacts the flange at one end, a sleeve supplied to the bolt diameter but not the bolt length, or a kit shipped with the wrong count and one bolt assembled bare. Those fail identically whatever the sleeve is made from.
- Size to the bolt shank, not to the hole. A sleeve that rattles in the hole still isolates; a sleeve that is loose on the bolt does not.
- Cut to the full bolt length so the break runs head through nut, with no exposed shank at either end.
- State the bolt standard as well as the size, because an M20 in one standard is not the same shank as an M20 in another.
- Ship the sleeve counted and packed per flange, not as a length to be cut on site.
In the wider build this belongs to the Mylar (PET) bolt sleeve route, which gathers every part it covers.
Products for this job
These are the PET and mylar parts we cut to a bolt schedule or an insulation drawing. Tell us the duty and we will say which of the options suits it, including when the answer is neither.
Related reading
- Mylar Bolt Sleeve Size Chart: M6 to M36
- Mylar (PET) Tube for Transformer and Motor Insulation
- Mylar / PET Film for Capacitor Sleeves
- Heat Shrink vs Cold Shrink Tubing
Common questions
PET or PTFE for flange bolt sleeves?
PET for standard flange duty at moderate temperature, which is most pipeline and tank work, because it gives the required dielectric break at a fraction of the cost. PTFE where the flange runs hot, where chemicals reach the bolts, or where the design life justifies the premium.
What temperature can a PET bolt sleeve take?
Minus 55 C to plus 105 C continuous on our material, softening above that. Above the continuous limit PET creeps under bolt load, which is exactly the wrong failure for a bolted joint. Move to PTFE for higher temperature.
Are bolt sleeves a high-voltage component?
No. They provide a dielectric break against galvanic and stray current, where the driving voltage and current are small. The requirements that matter are full-length coverage, correct fit and material stability in the service environment rather than high dielectric strength.
How long should the sleeve be?
Long enough to break the metallic path across the full joint, so from the bolt head through the nut with no exposed shank at either flange face. Cut to the actual bolt length, not to a nominal one.
Why do flange isolation kits fail?
Fit, most of the time. A short sleeve, a sleeve sized to the hole instead of the shank, or a kit short of one sleeve. The material is rarely the cause. That is also why counted, cut-to-length supply prevents more failures than a material upgrade.
Does HDPE work as a bolt sleeve?
It is cheap and available, but it is soft and creeps under bolt load, and its temperature ceiling is the lowest of the common materials. It is a marginal choice for a joint that has to hold isolation for a design life.
Can a sleeve be supplied as a kit component?
Yes, and it is the better supply model. We cut PET sleeves to the bolt callout and pack them to the bolts-per-flange count, so assembly has full coverage and nothing is missing. A loose length to be cut on site is where the short-sleeve failure comes from.
What bolt sizes are covered?
M6, M8, M10, M12, M16, M20, M24, M30 and M36 as standard, with ANSI and DIN variants, cut to the bolt length. Send the bolt callout with the standard, for example M20 by 90 DIN 931, and we cut and pack to that.
Specifying this part?
Send the drawing for a manufacturability review.