Keeps the seal tight over time
RPTFE slowly creeps under constant load and does not spring back like rubber. The O-ring takes up that relaxation, so the contact pressure on the seat holds.
Triple Offset Seat Seals
Seat Seals for Triple Offset Butterfly Valves
RPTFE seat seals with an O-ring energiser, made for tight shut-off in both flow directions on triple offset butterfly valves.
Size rangeNPS 2 – 28
JacketGlass or carbon filled PTFE
EnergiserSilicone O-ring as standard
How it works
How a Triple Offset Valve Seals
Everything in a triple offset valve is arranged so the seal never rubs on the seat. Each offset plays its part.
First offset
The shaft sits behind the sealing plane, so the seal runs all the way round the disc without a break.
Second offset
The shaft is set to one side of the pipe centreline, so the disc swings away from the seat as it opens.
Third offset
The seat is a cone tilted to the pipe centreline. This is the offset that stops the seal rubbing on the way in and out.
Closing
The seal touches the seat only at the very end of the stroke, with no sliding beforehand.
Good to know: there is no mechanical stop. The seat itself stops the disc, so the actuator's closing torque decides how hard the seal is pressed home.
Seal design
Inside the ISMAT Seat Seal
A machined RPTFE jacket does the sealing. An O-ring inside it works as a spring and keeps the jacket pressed against the seat.
RPTFE jacket
Glass or carbon filled PTFE. Its outer edge is the part that seals against the body seat.
O-ring energiser
Sits inside the jacket. See the O-ring options.
Retaining ring
Part of the valve. It clamps the seal to the disc.
Disc
Carries the seal and turns with the shaft.
Body seat
The conical seat machined into the valve body.
Some valve designs carry the seal in the body instead of on the disc. The seal is made to suit whichever arrangement the valve uses.
Why the jacket is energised
Follows temperature changes
The jacket, disc and body grow and shrink at different rates. The O-ring keeps the jacket in contact as the gaps open and close.
Takes up tolerances
Small dimensional differences between the disc, retainer and seat are absorbed by the O-ring instead of leaving a gap.
Protects the O-ring
The jacket sits between the line fluid and the rubber, so the O-ring is picked mainly for temperature and compression set.
Uses line pressure
When the open side of the jacket faces the pressure, line pressure adds to the O-ring's load. The higher the pressure, the tighter the seal.
Easy to replace
The seal is a separate part, so it can be changed at overhaul without replacing the disc or the body seat.
Seal comparison
Energised RPTFE or Laminated Metal Seal?
Most triple offset valves leave the factory with a laminated metal and graphite seal ring. An energised RPTFE seal is the better choice when the service temperature allows it.
| Feature | Energised RPTFE sealISMAT | Laminated seal ringMetal and graphite |
|---|---|---|
| Construction | Glass or carbon filled PTFE jacket with an elastomer O-ring energiser | Alternating layers of metal and flexible graphite |
| How it seals | The polymer face moulds itself to the seat, so it seals at low contact pressure | Metal-to-metal contact, so even loading and burr-free machining are critical |
| Upper temperature | +200 °C with silicone, +230 °C with FFKM | Well above any polymer, from cryogenic to high-temperature service |
| Fire safety | The valve needs a metal secondary seat to pass a fire test | All-metal, so fire-safe by design |
| Typical services | Water, chemicals, utilities and clean gases | Steam, hot hydrocarbons and fire-safe isolation |
Jacket grades
RPTFE Jacket Materials
Four filled PTFE grades from the NOVUM range. The fillers make PTFE stiffer and harder wearing without losing its chemical resistance.
RPTFE · 15% glass fibreGeneral purpose
NOVUM GF 01
The usual starting point for water, utilities and mild chemicals.
- Highest tensile strength and elongation of the four grades
- Flexible enough to fit and seat easily
RPTFE · 25% glass fibreCreep-resistant
NOVUM GF 02
For larger seals and higher seating loads.
- More glass than GF 01, so it creeps less under load
- Wears better than virgin PTFE
RPTFE · 25% carbonGlass-free
NOVUM CF 02
The grade to use when the fluid attacks glass, such as strong alkalis or hydrofluoric acid.
- Deforms less under load than glass filled grades
- Gentler on the mating seat surface than glass
RPTFE · 10% glass + 15% carbonTwo fillers
NOVUM GC 01
For large rings that stay under load for long periods.
- Holds its shape well under constant load
- Good wear resistance from the carbon filler
Energisers
O-Ring Energiser Options
Silicone is fitted as standard. The other three are there for the services silicone cannot handle.
Standard · Silicone
CERULEAN S 01
−80 °C to +200 °CThe widest temperature range of the four, and it stays flexible in the cold.
- Compression set 25% max. after 22 h at 175 °C
Option · FKM
VERTEX FC 01
−20 °C to +204 °CFor oils, fuels and other hydrocarbons that could reach the O-ring.
- Compression set 20% max. after 22 h at 200 °C
Option · EPDM
CERULEAN EP 03
−55 °C to +120 °CFor hot water, steam exposure and outdoor valves that see weather and ozone.
- Compression set 25% max. after 22 h at 70 °C
Option · FFKM
VERTEX F 02
−10 °C to +230 °CFor aggressive chemicals, and the highest upper temperature of the four.
- Resists acids, bases and solvents
The O-ring sets the limit. The jacket can take more heat than any of these O-rings, so the finished seal is only good for the O-ring's temperature range.
Technical data
Jacket Material Properties
Specification values from ISMAT technical data sheets.
| Property | NOVUM GF 0115% glass | NOVUM GF 0225% glass | NOVUM CF 0225% carbon | NOVUM GC 0110% glass + 15% carbon |
|---|---|---|---|---|
| Density ASTM D792, g/cc | 2.19 min. | 2.15–2.25 | 1.95–2.15 | 2.13–2.23 |
| Tensile strength ASTM D412, MPa min. | 15 | 10 | 10 | 10 |
| Elongation at break ASTM D412, % min. | 210 | 100 | 80 | 100 |
| Operating temperature °C | −200 to +260 | −200 to +260 | −200 to +250 | −200 to +260 |
| API 6A sour fluid immersion | Qualified | Qualified | Qualified | Qualified |
Operating temperature: jacket, energiser and finished seal
RPTFE jacket
GF 01 · GF 02 · GC 01
CF 02
O-ring energiser
Silicone · standard
FKM
EPDM
FFKM
Finished seal
GF 01 + silicone
Real limits also depend on seal design, pressure and the line fluid.
Selection guide
Choosing the Jacket and Energiser
A starting point for the services these seals see most often.
| Service | Jacket | Energiser |
|---|---|---|
| Water, seawater and HVAC Ambient to moderate temperature | GF 01 or GF 02 | Silicone or EPDM |
| Hot water and steam exposure Up to +120 °C | GF 02, CF 02 or GC 01 | EPDM |
| Oils, fuels and hydrocarbons | GF 02 or GC 01 | FKM |
| Strong alkalis and hydrofluoric acid Media that attack glass | CF 02 | FFKM |
| Aggressive acids and solvents | GF 01 or GF 02 | FFKM |
| Cold outdoor service Down to −80 °C | GF 01 | Silicone |
Check media against the ISMAT Chemical Compatibility Guide and confirm by testing in your application.
Applications
Where These Seals Are Used
Industries that use triple offset valves with RPTFE seat seals.
Water and desalination
Intake, filtration, pumping and distribution valves, including seawater lines.
HVAC and district energy
Chilled and hot water isolation in large buildings, plant rooms and district networks.
Chemical and petrochemical
Process lines where the fluid attacks rubber or metal but leaves PTFE alone.
Oil and gas utilities
Cooling water, lube oil, fuel oil and utility systems on plants and terminals.
Power plant auxiliaries
Condenser cooling water, condensate and auxiliary water circuits.
Marine and offshore
Ballast, bilge, engine cooling and seawater systems on ships and platforms.
Design guidance
Getting a Tight Seal First Time
Most seat leaks on triple offset valves trace back to geometry, clamping or seating torque rather than material. These points are worth checking early.
Machine to the seat cone
The third offset makes the seat a section of an inclined cone. The seal is elliptical and the angle of its face changes around the ring, so a seal turned to a simple taper will not seat evenly.
Clamp firmly, not rigidly
The retaining ring should hold the seal securely while letting it settle onto the seat. Overtightening distorts the jacket and squeezes the O-ring beyond its design compression.
Close the path behind the seal
A perfect sealing face is no use if fluid leaks round the back of the seal, between it and the disc. Keep the clamping faces flat and clean, and seal them with the jacket heel or a gasket.
Set torque for a polymer seal
A seating torque worked out for a laminated metal ring can overload an RPTFE face and make it creep faster. Check the actuator setting when changing to a soft seal.
Protect the seat finish
Light machining marks do no harm, but a scratch running across the contact band is a leak path. Specify the body seat finish and protect it during assembly.
Inspect PTFE at a stated temperature
PTFE passes through a crystalline transition near 19 °C that changes its dimensions, which matters on large rings. Measure parts at a controlled 23 °C.
FAQ
Triple Offset Seat Seals: Common Questions
Can an RPTFE seal replace a laminated seal ring in an existing valve?
Sometimes. It depends on whether the disc and retaining ring can hold an energised seal, whether the service temperature stays within the O-ring's range, and whether the valve has to remain fire-safe. Send the valve drawing and the operating conditions and we will check it.
Which O-ring is fitted inside the jacket?
Silicone, which works from −80 °C to +200 °C. FKM, EPDM or FFKM is fitted instead when the temperature or the line fluid calls for a different compound.
Is a triple offset valve with an RPTFE seat seal fire-safe?
Fire safety belongs to the complete valve and is proven by fire type-testing to API 607 or ISO 10497. RPTFE and elastomers do not survive a fire, so soft-seated valves that pass these tests have a metal secondary seat that takes over once the soft seal has gone.
Which seat leakage standard applies?
Triple offset valves are usually seat-tested to API 598, where a soft-seated valve must show no visible leakage in either direction. Some buyers specify ISO 5208 Rate A instead, or ANSI/FCI 70-2 Class VI for control valves. Tell us which one your valve is tested to.
ADVANCE with ISMAT
Discuss Your Butterfly Valve Seal
Send your valve details and our engineers will recommend a jacket grade, an energiser and the seal profile.
Share with your enquiry
- Valve size, pressure class and design
- Seal drawing or sample, with cone angle if known
- Medium, pressure and minimum and maximum temperature
- Seat leakage test standard
- Quantity, and whether for new valves or overhaul
Disclaimer: The information on this page is general technical guidance and is not a service-life, pressure or temperature rating. Values are specification limits from ISMAT technical data sheets and do not by themselves indicate the performance of a finished part; evaluate parts under actual service conditions, particularly where failure may result in injury or damage.