Interference fit · low pressure
Sealing without pressure
The disc edge is slightly oversize for the seat, so the jacket is already squeezed before any pressure arrives. That squeeze keeps the valve tight when the differential pressure is low.
Double Offset Seat Seals
Seat Seals for Double Offset Butterfly Valves
Energised RPTFE seat seals for double offset butterfly valves that have to shut off tight and still turn with low torque.
Size rangeNPS 2 – 28
JacketGlass or carbon filled PTFE
EnergiserSilicone O-ring as standard
How it works
How a Double Offset Valve Seals
The two offsets make the disc swing like a cam instead of turning on its centre. Follow one closing stroke and you can see what the seat goes through.
First offset
The shaft sits behind the sealing face, not through the middle of the disc. Nothing passes through the seat, so the seal can run unbroken all the way round.
Second offset
The shaft is also set to one side of the pipe centreline. That makes the disc swing on an arc, and it lifts off the seat within the first few degrees of opening.
Closing
On the way back the disc stays clear of the seat until the last 10 to 15 degrees of travel. The edge only rubs for a short distance.
Seated
Finally the disc edge presses into the seat and makes a line of contact all the way round. That ring of contact is what stops the flow.
Good to know: unlike a triple offset valve, the seat here is a flat circle, not a tilted cone. The disc still wipes the seat a little at the end of each stroke, so the seat material and the finish on the disc edge matter.
Seal design
Inside the ISMAT Seat Seal
The seal is a machined RPTFE jacket with an O-ring inside it. It sits in a pocket in the body and a retainer holds it there.
RPTFE jacket
Glass or carbon filled PTFE. The lip of the jacket is what the disc edge presses into.
O-ring energiser
Sits inside the jacket and pushes the lip towards the disc, like a spring. See the O-ring options.
Seat retainer
Part of the valve, not the seal. It clamps the seal into its pocket.
Body
Has the pocket, machined around the bore, that the seal sits in.
Disc
Turns on the offset shaft. Its polished edge is the surface the seal works against.
A few valves carry the seal on the disc instead of in the body. We make the seal to suit either layout.
Two ways the seat seals
Pressure assisted · higher pressure
Sealing that tightens with load
Line pressure gets behind the jacket and pushes the lip harder onto the disc edge, so the seal gets tighter as pressure goes up. The two modes work together across the pressure range.
Why the jacket is energised
Takes up wear
The disc edge wipes the lip on every cycle and a little material wears away. The O-ring keeps pushing the jacket out, so contact stays without anyone adjusting the valve.
Holds against creep
Under constant load RPTFE slowly flows, and it does not spring back by itself. The O-ring's elasticity makes up what is lost.
Follows temperature
PTFE expands roughly ten times as much as steel, so the gaps around the seal change as the line heats and cools. The energiser keeps the lip in contact through it.
Changes at overhaul
The seal is a separate part. When it wears out you change it at overhaul and leave the body and disc alone.
Valve comparison
Concentric, Double Offset or Triple Offset?
The more a seat rubs, the less pressure and temperature it can take, and the offsets decide how much it rubs. Double offset sits in the middle: far less wear than a concentric valve, and a simpler seat than a triple offset one.
| Feature | Concentric | Double offset | Triple offset |
|---|---|---|---|
| Seat contact | The disc rubs on the seat for the whole stroke | Only at the end of the closing stroke | None until the point of closing |
| Typical seat | Rubber liner | PTFE, RPTFE or elastomer; metal for high temperature | Laminated metal and graphite, or solid metal |
| Pressure | Low | Medium: ASME Class 150 and 300, PN 10 to PN 40, and some designs to Class 600 | High |
| Operating torque | Higher, as the disc drags on the liner | Lower, as the seat is touched only at the end | Set by the closing load, with no rubbing |
| Best suited to | Water, air and low-pressure isolation | Water, chemicals and utilities where a soft seat gives bubble-tight shut-off | High-pressure, high-temperature and fire-safe duty |
We make seat seals for triple offset valves too. See triple offset seat seals.
Jacket materials
RPTFE Jacket Grades and Properties
We offer four filled PTFE grades from the NOVUM range. The filler makes the PTFE stiffer and harder wearing, which helps against the wiping at the end of each stroke, and PTFE keeps its chemical resistance.
| Grade | 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 |
| Data sheet | View GF 01 | View GF 02 | View CF 02 | View GC 01 |
These are specification values from our technical data sheets, not a rating for a finished seal.
Energisers
O-Ring Energiser Options
We fit silicone as standard. The other three are for services where silicone is not suitable.
Standard · Silicone
CERULEAN S 01
−80 °C to +200 °CThe widest range of the four, and it stays soft in the cold.
- Compression set 25% max. after 22 h at 175 °C
Option · FKM
VERTEX FC 01
−20 °C to +204 °CResists oils, fuels and other hydrocarbons that may get past the jacket.
- Compression set 20% max. after 22 h at 200 °C
Option · EPDM
CERULEAN EP 03
−55 °C to +120 °CResists weather, ozone and hot water.
- Compression set 25% max. after 22 h at 70 °C
Option · FFKM
VERTEX F 02
−10 °C to +230 °CMade for aggressive chemicals, and it has the highest upper limit of the four.
- Resists acids, bases and solvents
Judge the temperature by the O-ring. The jacket can take more heat and more cold than any of these compounds, so the O-ring decides the working range of the whole seal.
Selection guide
Choosing the Jacket and Energiser
A starting point for the services we see most often. Tell us your fluid and temperature and we will confirm the pairing.
| Service | Jacket | Energiser |
|---|---|---|
| Water, seawater and wastewater Ambient to moderate temperature | GF 01 or GF 02 | Silicone or EPDM |
| Chilled and hot water Up to +120 °C | GF 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 |
| Low-pressure steam Check the grade with us | GF 02 or GC 01 | FFKM |
| Cold outdoor service Down to −80 °C | GF 01 | Silicone |
Check your fluid in the ISMAT Chemical Compatibility Guide, and test in your own application before you commit.
Design guidance
Getting a Tight Seal First Time
When a double offset seat leaks, the cause is usually the interference, the clamping or the disc edge, not the material. These are worth checking early.
Set the interference carefully
Too little and the valve leaks at low pressure. Too much and the seating torque climbs and the lip wears early. The pocket and the disc edge diameter set it, so agree the figure with us before you machine.
Tighten the retainer evenly
Tighten the retainer screws in a cross pattern, a little at a time. An uneven clamp pinches one side of the jacket and can leave a gap on the other.
Finish the disc edge
A smooth, burr-free edge is easiest on PTFE. A scratch across the contact band wears the lip on every stroke and ends up as a leak path.
Fit it without marking the lip
A nick in the lip will leak. Take the sharp edges off the pocket and the retainer, fit the seal by hand and keep metal tools away from the sealing face.
Keep throttling moderate
Double offset valves throttle best between roughly 20% and 80% open. Hard throttling at high differential pressure sets up vibration and eats away at the seat edge.
Allow for temperature swings
Fast thermal cycling is hard on a soft seat. When you measure large rings, do it at a controlled 23 °C, because PTFE changes size around 19 °C.
FAQ
Double Offset Seat Seals: Common Questions
Can you make a seal to replace the seat in my existing valve?
Usually, yes. The new seal has to fit your seat pocket and retainer, so we work from the old seat, a sample or a drawing. Send us the valve size, the disc edge material and the line conditions, and we will confirm the jacket, the energiser and the profile.
Does the seal work in both flow directions?
It depends on the valve as much as the seal. Many double offset designs seal both ways, but some are built for one direction only, and the seating torque changes with the flow direction and with which side the retainer is on. Tell us how your valve is installed and we will check the seal against it. Size the actuator for the highest seating and unseating torque, with a margin.
Which leakage standard should the seat meet?
Most double offset valves are seat-tested to API 598 or ISO 5208 Rate A. European buyers often ask for EN 12266-1 Rate A, and control valve users for ANSI/FCI 70-2 Class VI. Put the standard on your enquiry so we judge the seal against the right test.
Is a double offset valve with an RPTFE seat fire-safe?
Not on its own. RPTFE and rubber do not survive a fire, so fire safety belongs to the whole valve and is proven by type-testing to API 607 or ISO 10497. A valve that passes carries a metal secondary seat behind the soft seal.
ADVANCE with ISMAT
Discuss Your Butterfly Valve Seal
Send us your valve details and our engineers will recommend a jacket grade, an energiser and a seal profile.
Share with your enquiry
- Valve size, pressure class and make
- Old seat, sample or seat pocket drawing
- Disc edge material and finish
- Medium, pressure and minimum and maximum temperature
- Flow direction and 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.