FLEXSEAL PRO 350 —
a high-compressibility gasket for controlled conformance
Its measured compressibility can then help a verified joint conform to serviceable surface variation at the available seating load. It does not repair a damaged, out-of-flat or otherwise unacceptable face.
Documentation scope: Approval, compliance and certification references apply to the documented material grade, supplier certificate or listing, and stated test conditions unless a product-specific certificate says otherwise. A cut gasket size is not automatically certified as a separate product just because it is made from a documented material. For regulated applications, confirm the current certificate or listing scope, exact grade, medium, temperature, pressure and market requirement before specifying.
Selection rule: first match the medium, temperature, pressure and required documentation. Then compare the exact grade's compressibility, leakage data and stress window with the verified joint.
Use this when: the face remains within its acceptance criteria, the assembly can achieve the required seating stress, and the joint assessment shows that a more conformable grade is useful.
Do not use this when: the face is damaged or out of tolerance, the specified assembly load cannot be achieved, or the required material documentation is missing.
Choose another route when: the governing material, equipment or joint procedure requires a different grade, face repair or redesigned sealing geometry.
What it is
FLEXSEAL PRO 350
Source boundary: the mapped technical-data set supports 25% compressibility, 64% recovery, the 0.009 mg/(s·m) DIN 3535-6 result and the listed service maxima. One source separately describes hydrogen/light-gas suitability; it does not identify 0.009 as an H2-specific result. Obtain the applicable report for hydrogen work. Temperature and pressure maxima are not simultaneous.
FLEXSEAL PRO 350 is a high-compressibility gasket material based on synthetic fibres, NBR binder and special fillers. The supplier positions this material family for defined low-load duties and flange irregularity. That description still requires a serviceable face, an assembly method that achieves the required gasket stress, and confirmation of the exact grade against the medium and operating envelope.
The flexible sheet can be handled in constrained access more readily than a stiffer grade, but it must not be creased, damaged or forced into position. Anti-stick treatment is specified on both faces to reduce adhesion; clean removal after service is not guaranteed.
FLEXSEAL PRO 350 is described in one mapped technical sheet as suitable for hydrogen and light-gas applications. The same sheet separately reports a DIN 3535-6 specific leak rate of 0.009 mg/(s·m) at 2 mm, but does not identify that number as a hydrogen-specific test. Treat hydrogen suitability and the leak-rate value as two source statements, not one combined test result; obtain the applicable test report and project acceptance before specification.
Published compressibility: 25% under ASTM F36J. The mapped aramid and cellulose comparison grades report 11% and 9% under the same method. That comparison helps explain this grade's intended role in low-load joints and serviceable flange variation. It does not make a damaged face acceptable or guarantee that a joint will seal.
Compressibility across the range
ASTM F36J compressibility is the measured thickness change under that test method. It helps compare how readily exact grades conform, but it does not by itself rank pressure capability, creep, leakage or service life.
Source: ASTM F36 / ASTM F36J compressibility test. FLEXSEAL and BLUESEAL values from Kinetics Line technical datasheets.
Where controlled conformance matters
A gasket cannot turn an unacceptable face into an acceptable joint. First measure and assess scoring, pitting, roughness and flatness against the equipment or repair criteria. If the face is outside those limits, repair or replace it; neither higher torque nor higher compressibility is an engineering acceptance method.
Where the face is serviceable and the complete material specification matches, FLEXSEAL's measured compressibility may improve contact at a defined seating load compared with a stiffer grade. The decision still depends on the exact gasket stress window, leakage data, joint geometry and assembly procedure; face condition never replaces medium compatibility.
Heating engineers — older systems
Retrofit and maintenance work where the joint has first been cleaned, measured and found serviceable. If the original specification permits alternatives, FLEXSEAL can be compared with stiffer grades using the exact seating-stress and leakage data; uncertainty about face condition is a reason to inspect, not to guess.
Industrial MRO
Maintenance on older industrial pipework where minor, assessable flange variation remains after cleaning and inspection. Cracks, deep pits, distortion or damage outside the joint procedure still require repair or replacement.
Pump and equipment service
Pump housings and equipment flanges with measured, serviceable surface variation. Joints with limited available load only where calculation or manufacturer data confirms that the selected grade still reaches its required seating stress.
Restricted access joints
Restricted access does not justify uncontrolled bolt load. Use suitable access tools or a documented assembly method that can achieve the required load; evaluate FLEXSEAL only after the joint can be assembled within a controlled seating range.
Approvals
FLEXSEAL PRO 350 enters selection only after medium, temperature, pressure and documentation requirements have been screened. Its supplier source lists several gas, water, emissions and food-contact references whose current certificate and exact scope must be verified.
DVGW
DIN 30653 HTB · DIN 3535-6Supplier-sheet German gas references. Obtain the current document and confirm the exact gas, temperature, pressure, thickness, joint and market scope before specification.
WRAS
Water Regulations Approval Scheme — UKUK potable-water documentation. Relevant where UK drinking-water work specifies WRAS-listed material documentation, within the WRAS listing scope and conditions.
TZW W270
Technologiezentrum Wasser — GermanySupplier-sheet TZW/W270 reference. Identify the actual report, certification route and current German drinking-water requirements; the label alone is not product conformity.
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 — EUSupplier-sheet EC 1935/2004 reference. Obtain the applicable declaration and verify migration, food type, contact duration, temperature and finished-product scope.
Hydrogen assessment
Independent laboratory test dataThe sheet reports 0.009 mg/(s·m) under DIN 3535-6 at 2 mm and separately describes hydrogen/light-gas suitability. It does not label 0.009 as an H₂-specific result. Obtain the relevant report and project approval before hydrogen use.
FLEXSEAL vs BLUESEAL — the distinction
The difference cannot be reduced to medium versus face. Both grades still require a complete service, documentation and joint assessment.
Do not divide the choice into 'medium for BLUESEAL' and 'face for FLEXSEAL'. Start every selection with medium, temperature, pressure and documentation. If more than one documented grade remains suitable, compare the exact joint stress, compressibility, relaxation and leakage data; FLEXSEAL may then be considered where controlled conformance is useful and the face is within acceptance.
Field example: A cleaned pump flange with measured, serviceable variation may justify comparing a higher-compressibility grade. A gas joint still requires the exact gas-service documentation and joint procedure. Neither age nor colour decides the material; the verified service and joint data do.
When to use it — and when not to
Where FLEXSEAL makes sense
- The mating face has measured surface variation but remains within its acceptance criteria
- The flange has been opened many times and shows surface wear
- Bolt load is limited by joint geometry or access constraints
- The flange or fitting material, geometry and allowable load are known
- DVGW gas service where a stiffer gasket is impractical on the joint
- TA-Luft emission-controlled environments
- Applications where anti-stick treatment on both faces is specified, without assuming clean removal
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% value supports its low-load and flange-irregularity role. On a clean industrial flange, compare the exact grade's minimum seating stress, maximum gasket stress, residual stress, leakage data and pressure-temperature envelope; compressibility alone cannot rank service life.
Practical notes
Hydrogen assessment — keep two source statements separate
The material sheet describes hydrogen/light-gas suitability and separately reports 0.009 mg/(s·m) under DIN 3535-6. One mapped source sheet lists TA-Luft. None of those statements replaces a hydrogen-specific test report, joint calculation or project acceptance.
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
Follow the equipment and site procedure for post-assembly inspection and any permitted first-cycle leak check. The published 64% ASTM F36 recovery is a short-duration material test, not proof of retained stress through thermal cycling. A face outside acceptance is repaired before assembly; a later visual check does not validate a marginal face.
Available 0.5 to 3.0 mm — sheet and cut
Standard supply is listed in 1500 × 1500 mm sheets, with 3000 and 4500 mm lengths and nominal thicknesses from 0.5 to 3.0 mm. Select the thickness required by the flange, equipment or validated joint design. Do not use a generic starting thickness or extra material to hide face damage; verify thickness-specific data, closure geometry and the allowable gasket-stress window.
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.
FLEXSEAL PRO 350 is a documented high-compressibility option, not a repair for a bad face or an escape from the required assembly load.
Use it only after the medium, temperature, pressure and documentation match, the face has passed its acceptance check, and the joint can achieve the grade's required seating stress.
Its 25% ASTM F36J compressibility and anti-stick treatment are useful material properties. Their value must be judged with the exact leakage, recovery, relaxation and joint data; they do not guarantee sealing or clean disassembly.