FLEXSEAL PRO 350 —
the gasket chosen for the face, not the medium
Worn flanges, old brass, limited bolt load and fittings opened too many times are not medium problems. They are contact problems. FLEXSEAL gives the joint more compressibility so the gasket can follow the face instead of only loading the high spots.
Selection rule: FLEXSEAL is selected when the face itself becomes the problem.
Use this when: worn faces, low bolt-load joints, old brass fittings, uneven mating surfaces and service work where higher compressibility is the real advantage.
Do not use this when: do not use it to solve a chemical compatibility problem or as a substitute for the approval required by the medium.
Move to the next material when: medium, approval or high-temperature duty becomes the controlling factor; then move to BLUESEAL or GRAPHITESEAL as appropriate.
What it is
FLEXSEAL PRO 350
FLEXSEAL PRO 350 is a high-compressibility gasket material based on synthetic fibres, NBR binder and special fillers. The formulation is engineered for two service conditions: low bolt load applications where joint geometry or access limits the torque available, and worn or uneven flange faces where a rigid gasket bridges surface irregularities rather than sealing against them.
It can be bent and shaped during installation without cracking or losing integrity — relevant in tight spaces where a standard rigid sheet may fracture if handled under constraint. Anti-stick treatment on both sides means it lifts cleanly at the next service interval without bonding to the face.
FLEXSEAL PRO 350 is also laboratory tested for hydrogen sealing performance by DONIT, with a specific leak rate of 0.009 mg/s·m at 2.0 mm thickness. This makes it a possible option for selected hydrogen and low-emission gas applications where face condition and sealing tightness both matter. For reference: BLUESEAL ULTRA 350 leak rate is 0.02 mg/s·m and GREENSEAL PRO 180 is 0.04 mg/s·m at 2.0 mm — both per Kinetics Line technical data. As with any H₂ application, suitability should be confirmed against the specific operating conditions before specifying.
The number that defines it: 25% compressibility. Most standard fibre gaskets sit between 9% and 12%. FLEXSEAL compresses more than twice as much under standard bolt load — which is what allows it to conform to surface irregularities that a stiffer gasket bridges. Higher compressibility means more contact with an imperfect face. More contact usually means a better chance of sealing on an imperfect face.
Compressibility across the range
Compressibility is the percentage of thickness lost under a standard bolt load test. Higher compressibility means the material conforms more readily to surface irregularities. Lower compressibility means the material holds more consistent thickness under load — the better answer for high-pressure, high-temperature flanges with good face condition.
Source: ASTM F36 / ASTM F36J compressibility test. FLEXSEAL and BLUESEAL values from Kinetics Line technical datasheets.
Where the face is the problem
A standard gasket on a worn face does what it always does: it seats on the high spots and leaves the low spots unsealed. Tightening harder may help temporarily — but at some point the fitting is being damaged to chase a seal that a stiffer material cannot reliably provide on that surface.
FLEXSEAL conforms to the face it finds. It is more likely to achieve full contact across an uneven surface than a standard fibre gasket at the same bolt load. This is the reason it is usually the first material considered when the face condition is the deciding variable — not when the medium or temperature is the challenge.
Heating engineers — older systems
Retrofit and maintenance work on heating systems where flanges and unions have been opened multiple times. FLEXSEAL is usually the first material assessed where face condition is uncertain and a standard gasket is a likely gamble.
Industrial MRO
Maintenance on older industrial pipework where flanges are not always in good condition and downtime for face dressing is not practical. Higher compressibility gives more margin against surface variation.
Pump and equipment service
Pump housings and equipment flanges after years of service. Low bolt load flanges where the answer is not increased torque but a gasket material that seals at the bolt load available.
Restricted access joints
Applications where a torque wrench cannot be positioned correctly and bolt load is whatever the access allows. FLEXSEAL gives more tolerance on the lower end of the bolt load range.
Approvals
Despite being selected primarily for face-condition reasons, FLEXSEAL PRO 350 carries a full set of approvals covering gas service, water service, emissions-controlled environments and food contact.
DVGW
DIN 30653 HTB · DIN 3535-6German gas approval — covers gas service where joint geometry or bolt load makes a stiffer gasket impractical. FLEXSEAL carries this alongside its face-condition positioning.
WRAS
Water Regulations Advisory Scheme — UKUK potable water approval. Required for gasket materials in contact with drinking water in UK installations.
TZW W270
Technologiezentrum Wasser — GermanyGerman drinking water approval, separate from DVGW gas certification. Covers water service in German-market installations.
TA-Luft
Technische Anleitung Luft — GermanyEmissions-controlled industrial environments where fugitive emissions are regulated. Relevant for TA-Luft specified industrial flanges.
EU 1935/2004
Food Contact Materials — EUEuropean food contact compliance. Relevant for food processing and beverage industry applications within the material's operating range.
H₂ Sealing
Laboratory tested — DONITSpecific leak rate 0.009 mg/s·m at 2.0 mm thickness. Not a certification — a laboratory-measured performance value. Confirm suitability against actual operating conditions before specifying for H₂ service.
FLEXSEAL vs BLUESEAL — the distinction
BLUESEAL is selected because of the medium. FLEXSEAL is selected because of the face.
If the application is demanding — high temperature, gas, chemicals — but the flange is clean and flat with adequate bolt load, BLUESEAL ULTRA 350 is usually the first material considered. If the face is worn, scored, or the bolt load is limited — and the medium is within FLEXSEAL's approval range — FLEXSEAL is usually the first material considered. They address different problems. The medium decides for BLUESEAL. The face decides for FLEXSEAL.
Field example: Old pump flange, face lightly scored, limited bolt access — FLEXSEAL. New clean gas flange, full bolt load, good face condition — BLUESEAL. The medium may be the same. The face condition decides which material is assessed first.
When to use it — and when not to
Where FLEXSEAL makes sense
- The mating face is worn, scored or shows surface variation
- The flange has been opened many times and shows surface wear
- Bolt load is limited by joint geometry or access constraints
- Old brass or cast iron fittings with minor surface irregularities
- DVGW gas service where a stiffer gasket is impractical on the joint
- TA-Luft emission-controlled environments
- Applications where the gasket needs to come off cleanly next service
When FLEXSEAL is not the first answer
- The face is clean, flat and in good condition — GREENSEAL PRO 180 or BLUESEAL ULTRA 350 is usually more appropriate
- Maximum residual stress retention at high sustained temperature — BLUESEAL ULTRA 350 or GRAPHITESEAL ULTRA 350 is the better answer
- The face damage is severe — cracked or deeply pitted faces require repair or replacement, not a different gasket
- Very high pressure flanges where material stiffness is required to prevent extrusion
High compressibility is not always the right answer. FLEXSEAL's 25% compressibility is well suited for worn faces and limited bolt load. On a clean, flat, high-pressure flange with full bolt load, a lower-compressibility aramid gasket like BLUESEAL ULTRA 350 is more likely to perform better over the service life — lower compressibility means more consistent thickness under high load and lower creep relaxation over time. The right material depends on the face condition and the application, not on the assumption that more compressibility is always better.
Practical notes
H₂ sealing — 0.009 mg/s·m specific leak rate at 2.0 mm
Kinetics Line technical data lists this as a measured performance value, not a certification. For TA-Luft emission-controlled environments and selected H₂ applications, this combination of high compressibility and low leak rate is the technical reason FLEXSEAL is considered. Suitability against actual operating conditions, project standard and documentation scope should be reviewed before specifying for H₂ service.
Can be bent and shaped on site
Unlike stiffer gasket materials, FLEXSEAL can be bent around tight radii during installation without cracking. In confined spaces where positioning a rigid sheet would be difficult, this matters. Standard rigid gasket sheet should not be forced into a position that requires bending.
Anti-stick both sides — lifts cleanly at next service
Both faces carry anti-stick treatment. At the next service interval, FLEXSEAL lifts away without leaving the bonded residue that untreated materials leave on the seating face. The face still requires inspection and cleaning before the replacement gasket is fitted, but the bulk removal step is significantly faster.
64% elastic recovery — check after first commissioning
On a worn or damaged face, it is good practice to check the joint after the first full operating cycle — particularly on heating systems that were cold when the gasket was fitted. Thermal expansion can reveal sealing gaps not apparent on initial tightening. FLEXSEAL's 64% elastic recovery helps maintain contact through thermal movement, but a visual check after first heat is advisable on any face that was not in ideal condition.
Available 0.5 to 3.0 mm — sheet and cut
Standard supply in 1500 × 1500 mm sheets, also 3000 and 4500 mm lengths, in 0.5, 1.0, 1.5, 2.0 and 3.0 mm nominal thickness. For most heating and plumbing applications, 1.5 or 2.0 mm is the standard starting point. Thicker gaskets provide more compressibility range on a heavily worn face but require correspondingly more bolt load to seat correctly.
Sizes and packaging
| BSP size | Dimensions (mm) | SKU 25 pcs | SKU 100 pcs |
|---|---|---|---|
| 3/8″ | 14.6 × 9 × 2 mm | B-2014GGA01C-25 | B-2014GGA01C-100 |
| 1/2″ | 18.5 × 11 × 2 mm | B-2014GGA02C-25 | B-2014GGA02C-100 |
| 3/4″ Normal | 24 × 18 × 2 mm | B-2014GGA03B-25 | B-2014GGA03B-100 |
| 3/4″ Special | 24 × 16 × 2 mm | B-2014GGA04B-25 | B-2014GGA04B-100 |
| 1″ Normal | 30 × 24 × 2 mm | B-2014GGA05B-25 | B-2014GGA05B-100 |
| 1″ Special | 30 × 21 × 2 mm | B-2014GGA06A-25 | B-2014GGA06A-100 |
| Box set — 300 pcs — 3/8″ to 1″, 6 sizes | B-2014BOX6 | ||
Custom dimensions and non-standard sizes available on request through B2B contact.
Most gaskets need a good face. FLEXSEAL PRO 350 is intended for the joints where the face is the problem.
Worn flanges. Old cast iron. Soft brass that has been opened too many times. Limited bolt load. Restricted access where full torque is not achievable. DVGW-documented gas service where joint geometry makes a stiffer material impractical.
It lifts cleanly next time. It conforms where a stiffer gasket bridges. It is more likely to achieve a seal on a face that a standard gasket would struggle with. That is where the 25% compressibility matters — as a material response to worn faces and limited bolt load, not as a general advantage.