The Key Components of the Concentric Butterfly Valve
Written by
Allen Zhang · Senior Application Engineer, LAUX VALVE
Understanding the key components of a concentric butterfly valve is essential for proper selection and maintenance. The concentric butterfly valve, also known as a centric or resilient-seated butterfly valve, features a symmetrical design where the stem passes through the center of the disc and the disc is centered within the seat bore. This article provides a comprehensive overview of each major component.
Valve Body
The body is the main pressure-containing component, typically manufactured from cast iron (ASTM A126), ductile iron (ASTM A536), carbon steel (ASTM A216 WCB), or stainless steel (ASTM A351 CF8M). The body type may be wafer, lug, or double-flanged, depending on the installation requirements. Wafer bodies are the most compact and economical, while lug bodies allow end-of-line service and double-flanged bodies provide the most robust connection.
- 1
Insert resilient seat
Cartridge or molded seat is press-fit into the body bore; ensures full 360° contact with disc edge later.
- 2
Install upper & lower bushings
Bronze, PTFE-impregnated or stainless steel bushings support the stem with low friction.
- 3
Position disc and pin to stem
Disc is centered in the seat bore; pinless tongue-and-groove or through-pin connection transfers torque from stem to disc.
- 4
Tighten gland packing
Stem packing (PTFE V-rings or graphite) is compressed to seal the stem-to-body interface; preload is set by torque spec.
- 5
Mount actuator on ISO 5211 pad
Direct mount via ISO 5211 F-flange means no adapter bracket. Verify open/closed travel stops before commissioning.
Disc
The disc is the flow-controlling element that rotates 90° between the open and closed positions. In a concentric butterfly valve, the disc is centered in the bore and makes uniform contact with the seat around its full circumference. Common disc materials include ductile iron, stainless steel (CF8, CF8M), aluminum bronze, and special alloys for corrosive service. The disc profile is typically a lens or flat shape, optimized for low torque and good flow characteristics.
Seat
The seat provides the sealing interface between the disc and body. In resilient-seated designs, the seat is molded from elastomers such as EPDM, NBR, FKM, or PTFE. The seat may be a cartridge type (replaceable without removing the valve), bonded type, or press-fit type. The seat material determines the valve's temperature range, chemical compatibility, and sealing performance.
Stem (Shaft)
The stem transmits the torque from the actuator to the disc. Most concentric butterfly valves use a one-piece or two-piece stem design. The stem passes through the disc and is supported by bearings in the body. Common stem materials include stainless steel grades 410, 416, 420, 431, and 316. The stem-to-disc connection may use pins, keys, or a square/hex drive.
Actuator
The actuator provides the force to rotate the disc. Options include manual lever (for smaller sizes), gear operator (for larger sizes or higher pressures), pneumatic actuator, and electric actuator. The ISO 5211 mounting pad on the valve body allows direct mounting of actuators without additional brackets, simplifying installation and reducing costs.
Material Selection Summary
| Service | Body | Disc | Stem | Seat |
|---|---|---|---|---|
| Potable water | Ductile iron + epoxy | Ductile iron NBR-coated | SS 416 | EPDM |
| Salt / sea water | Aluminum bronze C95800 | Al-bronze or duplex 2205 | Duplex 2205 | EPDM peroxide-cured |
| Hot HVAC water (≤ 120 °C) | Cast iron or DI | Ni-resist | SS 416 | EPDM |
| Hydrocarbons / fuel | Carbon steel WCB | SS 316 / CF8M | SS 316 | NBR or FKM |
| Strong acids / chemical | PTFE-lined DI | PTFE-encapsulated SS | Hastelloy C-276 | PTFE |
Frequently asked questions
What's the difference between concentric, double-offset and triple-offset butterfly valves?
In a concentric valve the stem axis intersects both the disc center and the seat plane — disc and seat are in contact through the entire 90° rotation. Double-offset moves the stem off the seat plane and disc center, reducing seat wear. Triple-offset adds a conical seating angle, eliminating sliding contact entirely — required for fire-safe and high-temp service.
How is the stem prevented from blow-out under pressure?
Concentric butterfly valves use an integral anti-blowout shoulder on the stem combined with a retaining ring or thrust washer at the gland end. The shoulder lands against a recess in the body, transferring axial pressure load to the casting rather than the packing — designed and tested per API 609 / EN 593.
Can I replace just the seat without removing the valve from the pipeline?
Only with cartridge-type seats. Bonded and press-fit seats require returning the valve to the workshop. When specifying, ask explicitly for a 'replaceable cartridge seat' if you anticipate field-level seat changes — typical for water-treatment plants with chlorine dosing where seats degrade fastest.
What is the typical packing material for the stem seal?
Three options dominate: PTFE V-rings (cleanest, food/pharma), graphite for steam and high-temp service, and aramid-fiber-impregnated PTFE for abrasive duty. All are field-adjustable; PTFE is the default for water service.
Are the bushings load-bearing or just guides?
Both. Upper and lower stem bushings carry the radial reaction load created by differential pressure across the closed disc, plus side loads from misaligned actuator torque. Bushing fit and lubrication directly affect operating torque, especially after years of service.
