What Is a Double Flange Butterfly Valve?
A double flange butterfly valve is a quarter-turn valve whose body carries a flange on each side, so that it bolts between two pipe flanges and becomes a rigid, self-supporting part of the line. The disc rotates about a shaft mounted in the body and, in the closed position, presses against a seat around the bore to shut off the flow. Turning the disc through about 90 degrees opens the valve and, because the disc can be held at any intermediate angle, the same valve can also regulate flow.
The double flange pattern is one of the three common butterfly body styles, alongside the wafer type that is clamped between flanges by long through-bolts and the lugged type that carries threaded inserts. Because the valve has its own flanges at both ends, it can be removed from a line without disturbing the adjacent pipework when the mating flange bolts on one side are released, which is a practical advantage in maintenance.
Construction and Product Features
Body and disc: cast in ductile iron for ductility and pressure resistance, with a baked paint or epoxy finish that gives a smooth, corrosion-resistant surface. Cast steel and stainless steel bodies are available for more aggressive duty.
Seat: a vulcanised or clamped seat, most commonly in EPDM, which seals reliably over a wide range of line media and resists ageing and abrasion. NBR, PTFE and FKM seats are offered for oils, chemicals and higher temperatures.
Shaft and bearings: a stainless steel shaft running in self-lubricating or bronze-backed bearings, with O-rings or packing protecting the shaft entry from the medium.
Operator: a hand lever, a worm gearbox, or a pneumatic or electric actuator, all mounted on a standardised top flange.
In the fully open position the only obstruction to the flow is the thickness of the disc, so the pressure loss across the valve is very small. This is why the double flange butterfly valve has good flow control characteristics and is chosen where a gate or globe valve of the same bore would introduce a much larger head loss.
Typical Parameters
| Parameter | Options and typical range |
|---|---|
| Nominal size | DN50 to DN1200 (NPS 2 to NPS 48) |
| Pressure class | PN10, PN16, PN25; Class 150 |
| Driving mode | Manual lever or gearbox, pneumatic, electric |
| Body material | Ductile iron, cast steel, 304, 316, 316L |
| Disc material | Ductile iron with coating, 304, 316, 316L |
| Seat material | NBR, EPDM, PTFE, FKM |
| Media | Water, liquid, gas, oil, and mildly acid or alkaline corrosive media |
| End connection | Flanged to EN 1092-2, ASME B16.5 or ASME B16.47 |
| Design standards | API 609, EN 593, MSS SP-67 |
| Testing | API 598, ISO 5208 |
Temperature limits follow the seat material rather than the body: an EPDM seat covers the ordinary water and utility range, while PTFE and FKM seats extend the usable temperature for oils, chemicals and hot water. The pressure and temperature rating of the selected body class always governs the application.
Applications
Water supply and distribution: isolation and regulation on mains, trunk lines and pumping station manifolds.
Food and beverage: hygienic lines where a smooth, cleanable bore and an elastomeric seat suit the process.
Petroleum and chemical: product lines, utility headers and mildly corrosive service where cast steel or stainless steel bodies are specified.
Pharmaceutical: process water, solvent and CIP circuits requiring reliable closure and low turbulence.
Solid-liquid mixtures and particles: slurry, sludge and sediment-laden water lines, where the disc sweeps across the seat and clears the passage.
Heating, ventilation and air conditioning: chilled and hot water circuits requiring modulating control.
Selection, Installation and Maintenance
Selection begins with the medium and its temperature, because the seat material is chosen first and it sets the usable temperature range. Then check the pressure class and the flange drilling so that the valve matches the pipe flanges on both sides, since a double flange valve must align with two separate flanges. Finally decide the operator: a lever for occasional manual duty, a gearbox for larger sizes where the operating torque is high, and a pneumatic or electric actuator where the valve forms part of an automatic control system.
Installation notes: the disc swings across the bore, so the valve must be mounted with the shaft horizontal or vertical as required by the design and must not be installed with the disc obstructing a bend or a branch. Leave clearance on both sides for flange bolts and for actuator removal, and support large valves so that the weight is not carried by the flanges. Where the valve is used for throttling, keep it out of the very low opening range, where high-velocity flow attacks the seat edge.
Maintenance is limited because the valve has few wearing parts. Inspect the seat for swelling or cracking, renew the shaft seals if leakage appears at the gland, and check that the disc still moves freely through its full quarter turn. Lubricate the gearbox and the actuator linkage at the interval stated in the maintenance manual, and record the operating torque so that a rise caused by seat wear or bearing damage is noticed early.
FAQ
Q: What is the difference between a double flange and a wafer butterfly valve?
A double flange valve carries a flange on each end and bolts to two pipe flanges, so it supports itself and can be removed without disturbing the adjacent pipes. A wafer valve has no flanges of its own and is clamped between the pipe flanges by long through-bolts, which makes it lighter but harder to remove.
Q: Which seat material should be selected?
EPDM for water, waste water and general utility duty, NBR for oils and fuels, PTFE for chemical resistance and higher temperature, and FKM for hot water, oils and aggressive chemicals. The seat material sets the usable temperature range of the valve.
Q: Can a double flange butterfly valve regulate flow?
Yes, the disc can be held at intermediate positions to modulate flow, and the low resistance of the open bore gives good control characteristics. For continuous throttling, however, a valve with a characterised trim such as a V-port ball valve is usually more durable.
Q: What are the body and disc made of?
Ductile iron is the standard material for water and utility duty, usually with a baked paint or epoxy coating. Cast steel and stainless steel grades such as 304, 316 and 316L are used where the medium is corrosive or the pressure class is higher.
Q: How is the valve tested before delivery?
Shell and seat tests are carried out in accordance with API 598 or ISO 5208 to prove body strength and seat tightness. Flange dimensions and face-to-face lengths are checked against the ordering standard, such as EN 1092-2 for flanges.