Butterfly Valve: Lug, Wafer & Offset

Quick Reference

Butterfly valves use a rotating disc for quarter-turn isolation/regulation. Lighter and cheaper than gate or ball valves for large sizes. Types: concentric (rubber seat, low pressure), double offset (reduced wear), triple offset (metal seat, high temp/pressure). Spec: API 609. Applications: water, HVAC, chemical, low-pressure oil & gas.

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For flanged valve connections, download the ASME B16.5 Flange Chart or ASME B16.5 Bolt Chart.

Butterfly Valve

What Is a Butterfly Valve

Definition: A butterfly valve is a quarter-turn device that uses a rotating disc to isolate or regulate flow. Lighter and cheaper than gate or ball valves for large sizes, butterfly valves are available in concentric (rubber seat, low pressure), double offset, and triple offset (metal seat, high pressure/temperature) designs.

A butterfly valve uses a rotating disc mounted on a shaft inside the valve body. Turn the shaft 90° and the disc swings from fully open (parallel to flow) to fully closed (perpendicular). That quarter-turn action makes butterfly valves fast to operate (comparable to ball valves) but with one key difference: the disc stays in the flow path even when fully open, so there’s always some pressure drop.

triple eccentric butterfly valve

Why butterfly valves get specified so often: they’re significantly lighter and cheaper than gate or ball valves in large sizes (above 8-10 inches, the weight and cost difference becomes dramatic). A 24” Class 150 triple-offset butterfly valve weighs roughly 40-60% less than an equivalent gate valve and costs considerably less.

The trade-off is that butterfly valves cannot match gate valves for zero pressure drop, and concentric designs are limited to low-pressure, non-critical service. But with the introduction of double and triple offset designs, butterfly valves now compete directly with gate and globe valves in oil & gas and process applications.

Butterfly valves are classified by:

• Design: concentric, double offset (eccentric), triple offset (eccentric)
• End connection: wafer, lug (semi or full), flanged, double flanged
• Seat: soft (EPDM, PTFE, Buna, Viton) or metal-to-metal (SS316, Stellite, Inconel)
• Body/disc material: cast iron through to super duplex and nickel alloys
• Actuation: manual (lever, gear, worm gear), pneumatic, electric, hydraulic

Butterfly Valve Parts

Part	Function	Typical Materials
Body	Outer casing; connects to piping via flanges or wafer/lug ends	Cast iron, ductile iron, carbon steel, stainless steel, PVC
Disc	Rotating element that blocks or allows flow	Carbon steel, SS316, duplex, nickel alloy; must be compatible with process fluid
Stem (shaft)	Connects disc to actuator; transmits torque	SS 17-4PH, SS316, Monel
Seat	Sealing surface the disc closes against	Soft: EPDM, PTFE, Viton, NBR. Metal: SS316, Stellite, Inconel
Actuator	Rotates the disc: manual (lever/gear) or automated (pneumatic/electric/hydraulic)	-
Bushings	Reduce stem friction and wear	PTFE, bronze, graphite
Packing	Prevents stem leakage	PTFE, graphite, Viton
End connections	Wafer, lug, flanged, or butt-weld	Matched to body material

For valves above about 8-10 inches, manual lever operation becomes impractical because the torque is too high. These valves use gearboxes (worm gear) or actuators. On large butterfly valves (24”+), always verify actuator sizing with the valve manufacturer; operating torque can double under differential pressure.

How a Butterfly Valve Works

The disc sits at the pipe centerline and rotates 90° between open and closed. In the open position the disc is parallel to flow; in the closed position it’s perpendicular, blocking the bore. Intermediate positions allow throttling, though butterfly valves don’t throttle as precisely as globe or control valves. At low opening angles (10-30%), dynamic torque is high and cavitation can occur.

Unlike a ball valve, the disc remains in the flow stream at all times. Even fully open, it creates some pressure drop and turbulence. This is the main functional disadvantage compared to full-bore ball or gate valves.

(Source: GEMU Group Youtube Channel)

Pressure Drop

Because the disc stays in the flow path, butterfly valves have a higher pressure drop than full-bore gate or ball valves. The magnitude depends on disc position (fully open = minimal, partially closed = significant), flow rate, and fluid properties.

The pressure drop is estimated using the valve’s flow coefficient (Cv):

ΔP = Q² / Cv²

Where Q is the flow rate in US GPM and Cv is the flow coefficient at the given opening position. Valve manufacturers publish Cv values for each size and opening angle.

In practice, the pressure drop of a fully open butterfly valve is small enough that it rarely governs valve selection for on/off service. Where it does matter is throttling applications: operating a butterfly valve at 10-30% open creates substantial pressure drop and turbulence that can cause cavitation and vibration problems.

Applicable Standards

Standard	Coverage
API 609	The primary butterfly valve specification. Covers design, materials, FTF dimensions, P-T ratings, and testing for wafer, lug, and double-flanged types. Category A = concentric/soft seat; Category B = double/triple offset, higher pressure
ASME B16.34	Pressure-temperature ratings, shell material categories, testing. Triple-offset butterfly valves are often designed to B16.34
ASME B16.10	Face-to-face dimensions, critical for interchangeability
ASME B16.5	Flange dimensions and ratings (up to 24”)
ASME B16.47	Large-diameter flange dimensions (26”+)
MSS SP-67	Butterfly valve requirements (design, dimensions, testing), widely used alongside API 609
MSS SP-68	High-pressure butterfly valves with offset design
ISO 5211	Part-turn actuator attachments; standardizes the actuator mounting interface

Types of Butterfly Valves

By Design: Concentric vs. Eccentric Butterfly Valves

Concentric butterfly valve (left), double offset butterfly valve (center), triple offset butterfly valve (right)

Comparison of butterfly valve offset designs

Concentric (Zero Offset) Butterfly Valves

The stem passes through the center of the disc, and the disc is centered in the bore. The disc rubs against the resilient seat (rubber or PTFE) through the full 90° rotation, which limits concentric butterfly valves to low-pressure, low-temperature service where seat wear is acceptable.

Typical applications: water treatment and distribution, wastewater, fire protection systems, HVAC, natural gas supply, pumping stations, and aeration systems. The concentric butterfly valve dominates the water industry because it’s cheap, reliable, and bubble-tight with a soft seat.

Double Offset (High-Performance) Butterfly Valves

Two offsets: the shaft is moved behind the disc plane (first offset) and to one side of the pipe centerline (second offset). This “cam” action lifts the disc off the seat immediately on opening, so the disc only contacts the seat at the moment of closure. The result is dramatically reduced seat wear and lower operating torque compared to concentric designs.

Double offset butterfly valves compete directly with gate valves in large sizes: they’re lighter, cheaper, and for underground water mains they reduce excavation work. They handle higher pressures and temperatures than concentric designs, and work with both soft and semi-metallic seats.

Triple Offset Butterfly Valves

The third offset is the conical geometry of the seating surface. This means the disc contacts the metal seat only at the final point of closure, with no friction during rotation at all. The result is a metal-to-metal seal capable of bubble-tight shutoff at temperatures up to 700°C and pressures up to Class 2500.

Triple offset butterfly valves (TOV) have gained significant market share in oil & gas and process industries. They compete with gate and ball valves in critical isolation service but weigh 40-60% less. Materials range from stainless steel and duplex to super duplex and nickel alloys.

By End Connection: Lug vs. Wafer Butterfly Valves

Lug Butterfly Valves

The body has threaded lugs (bolt holes) so the valve bolts directly to each flange independently. This allows removal of downstream piping without affecting the upstream side; the lug valve can hold pressure on one side with the other flange removed. Required for dead-end service.

Wafer Butterfly Valves

The valve body sits between two flanges and is held in place by the through-bolts connecting the flanges. More compact and cheaper than lug type, but cannot be used for dead-end service. Both flanges must be in place and bolted for the valve to seal.

Common Mistake

Using wafer-type butterfly valves as end-of-line isolation. Wafer valves need pressure on both sides to seal properly. For dead-end service or where one side may be depressurized, use lug-type with full-rated studs.

By Seat Type: Soft vs. Metal Seated

Rubber-lined (soft-seated) butterfly valves use EPDM, Viton, NBR, PTFE, or Buna-N seats. They provide bubble-tight shutoff and low operating torque, but are limited by the seat material’s temperature and chemical resistance.

Metal-seated butterfly valves (typically double or triple offset) use SS316, Stellite, or Inconel seats. They handle higher temperatures and abrasive media, but may not achieve zero-leakage shutoff. They’re designed to meet acceptable leakage rates per API 598 or FCI 70-2.

Double Offset vs. Triple Offset Butterfly Valve

Feature	Double Offset	Triple Offset
Offsets	Shaft behind disc + off-center	Shaft behind disc + off-center + conical seat geometry
Seat type	Soft (PTFE, rubber) or semi-metallic	Metal-to-metal (SS316, Stellite, Inconel)
Disc-seat contact	Cam action reduces friction, but disc still rubs seat briefly	Zero contact during rotation; only at final closure point
Max pressure	Typically up to Class 600	Up to Class 1500-2500
Max temperature	Limited by seat material (PTFE ~260°C)	Up to 700°C with metal seats
Shutoff class	Bubble-tight with soft seat	Bubble-tight with metal seat (zero-leakage)
Typical service	Water, HVAC, light chemical, moderate-pressure process	Steam, high-pressure oil & gas, critical shutdown, fired heater isolation
Seat life	Good, but soft seats wear and need replacement	Very long; no friction during cycling
Cost	Moderate	Higher (2-3× double offset for same size)

Pro Tip

For isolation service above Class 300 or temperatures above 400°F, always specify triple offset butterfly valves with metal seats. Double offset valves with resilient seats will degrade quickly under these conditions, and you’ll be replacing seats within a year.

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The images show the design difference between a double and a triple eccentric butterfly valve (respectively at the left and right side of the diagram).

Butterfly Valve Body Materials

The body/disc material is selected based on the process fluid, pressure, temperature, and corrosion environment. Here are the main material groups used for butterfly valve bodies (all are cast grades):

Material	Cast Grades	Typical Service	Key Consideration
Cast iron	ASTM A126	Water/wastewater, HVAC, low-pressure air	Cheapest option; limited to ~200°C and low pressures
Ductile iron	ASTM A536	Water distribution, gas supply, higher-pressure water	Stronger and more ductile than cast iron; dominates water industry
Carbon steel	ASTM A216 WCB, LCC	Oil & gas, power generation, general process	WCB for high-temp, LCC for low-temp; poor corrosion resistance without lining
Stainless steel	CF8 (304), CF8M (316), CF3M (316L)	Chemical, pharmaceutical, food & beverage, seawater	316/316L for chloride environments; 304 for general corrosion resistance
Duplex	A890 Gr. 4A (S31803/S32205)	Offshore, desalination, aggressive chemicals	Higher strength than austenitic SS; excellent chloride resistance
Super duplex	A890 Gr. 5A/6A (S32750/S32760)	Subsea, sour service, concentrated chloride environments	Best combination of strength + corrosion resistance in stainless steels
Nickel alloys	Inconel 625, Hastelloy C-276, Monel 400	Sour gas (H₂S), strong acids, high-temperature corrosion	For conditions where stainless and duplex steels fail; expensive
Titanium	Gr. 2 (pure), Gr. 5 (Ti-6Al-4V)	Chlorine gas, seawater, hot brines	Exceptional chloride resistance; very expensive; limited availability
Nickel aluminium bronze	C95800	Seawater systems, marine applications	Resists biofouling (unlike stainless steel); good for seawater valves
Bronze	C83600, C95400	Marine, desalination	Traditional seawater material; lower cost than NAB
Aluminium	-	Air handling, vacuum, dry bulk	Lightweight; low-pressure only
PVC/CPVC/PP/PE	-	Water treatment, chemical dosing, irrigation	Low cost; limited to low pressure and moderate temperatures

Carbon steel (WCB) with PTFE lining is the workhorse combination for most oil & gas butterfly valve applications; it gives acceptable corrosion resistance at a fraction of the cost of stainless or alloy steel bodies.

Butterfly Valve Seat Materials

Soft Seat Materials

Soft-seated butterfly valves provide bubble-tight shutoff and require low operating torque. They are the standard choice for concentric butterfly valves and most double offset designs. The seat material must be compatible with the process fluid and within its temperature range. Exceeding the temperature limit destroys the seat.

Seat Material	Max Temp	Chemical Resistance	Typical Service
EPDM	~120°C	Water, dilute acids/alkalis, steam; poor with hydrocarbons	Water/wastewater, HVAC, steam condensate
NBR (Buna-N)	~100°C	Hydrocarbons, oils, fuels; poor with ozone and strong acids	Oil & gas, fuel handling, hydraulic oil
Viton (FKM)	~200°C	Hydrocarbons, acids, solvents; poor with ketones	High-temperature chemical service, oil & gas
PTFE	~260°C	Nearly universal chemical resistance	Chemical processing, pharmaceutical, food & beverage
Reinforced PTFE	~260°C	Same as PTFE with improved wear resistance	High-cycle applications where pure PTFE deforms under load

Metal Seat Materials

Metal-seated butterfly valves handle higher temperatures and abrasive media but don’t achieve the same zero-leakage shutoff as soft seats. Leakage is rated per API 598 or FCI 70-2. They’re standard on triple offset butterfly valves.

Seat Material	Typical Application
SS 316	General process, water treatment, food & beverage
Stellite overlay	Severe service: high wear, high temperature, power generation
Inconel 625	High-temperature corrosion, oxidizing environments
Hastelloy C-276	Aggressive acids (H₂SO₄, HCl), chlorine gas
Monel 400	Seawater, caustic soda, HF acid
Titanium	Chloride brines, seawater, wet chlorine
Aluminium bronze	Seawater systems

Seat Coatings and Treatments

Coating	Purpose
PTFE coating	Chemical resistance + low friction on metal seats
Electroless nickel	General corrosion protection, uniform coverage
Ceramic coating	Extreme hardness for abrasive slurry service (mining)
Chrome plating	Surface hardness and wear resistance

Field Note

On large butterfly valves (24”+), always verify the actuator sizing with the valve manufacturer. Operating torque can double when there’s differential pressure across the disc, and undersized actuators fail at the worst possible moment.

. Double offset and triple offset butterfly valve (metal seated)

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Frequently Asked Questions

What is the difference between concentric, double-offset, and triple-offset butterfly valves?

A concentric (zero-offset) butterfly valve has the shaft centered on the disc and the seat ; the disc rubs against the seat during the full rotation, limiting it to low-pressure, non-critical service. A double-offset design moves the shaft behind and to one side of the disc center, reducing friction and seat wear. A triple-offset design adds a conical seating geometry for metal-to-metal contact, enabling bubble-tight shutoff at high pressures (up to Class 2500) and high temperatures.

Can butterfly valves be used for throttling service?

Yes, with limitations. Double-offset and triple-offset butterfly valves can throttle flow reasonably well. However, at partially open positions (especially 10-30%), the disc experiences high dynamic torque and the flow creates turbulence and cavitation. For precise throttling, a control valve is preferred. For coarse flow control (e.g., cooling water regulation), butterfly valves with positioners work adequately and are much more economical than globe-type control valves.

What is a triple-offset butterfly valve used for?

Triple-offset (TOV) butterfly valves provide metal-to-metal sealing without the friction problems of concentric designs. They are used for high-temperature service (up to 700°C with metal seats), high-pressure isolation (up to Class 2500), and critical on/off applications where zero leakage is required ; e.g., fired heater isolation, steam systems, and offshore platform shutdown valves. They compete directly with gate and ball valves but weigh 40-60% less.

What is the maximum pressure rating for butterfly valves?

Concentric butterfly valves (API 609 Category A) are typically limited to Class 150-300. Double-offset butterfly valves (API 609 Category B) reach Class 600. Triple-offset butterfly valves are rated up to Class 1500-2500 depending on the manufacturer, with some designs certified for ASME B16.34 Class 2500 service. The pressure rating depends on body material, seat type, and temperature.