How to Order Butterfly Valves
Butterfly valves are quarter-turn rotary valves used for isolation and throttling in large-diameter piping. They are lighter, more compact, and less expensive than gate or ball valves of the same size, making them the standard choice for utility services, cooling water, HVAC, and, when specified as triple offset, for high-performance hydrocarbon and steam isolation.
The governing standard is API 609, which defines two categories: Category A (concentric, resilient seated) and Category B (high-performance double offset and triple offset, metal seated). Selecting the wrong category or omitting the seat material leads to leakage, premature failure, or rejection at inspection.
Butterfly Valve Types
| Type | Offset Geometry | Seat | Typical Service | API 609 Category |
|---|---|---|---|---|
| Concentric | Zero offset; disc pivot is centered | Resilient (EPDM, NBR, Viton) | Water, air, low-pressure utility | Category A |
| Double offset | Shaft offset from disc center + seat center | PTFE, RPTFE, or laminated | Medium-pressure process, steam, hydrocarbons | Category B |
| Triple offset | Double offset + conical seat geometry | Metal-to-metal (Stellite, Inconel overlay) | High-pressure, high-temperature, fire-safe, zero leakage | Category B |
Core Attributes
Every butterfly valve line item in an RFQ or purchase order must include all of the following attributes.
| # | Attribute | Values / Options | Notes |
|---|---|---|---|
| 1 | Design standard | API 609 Category A or Category B | Cat. A = concentric resilient seated; Cat. B = high-performance double/triple offset |
| 2 | Butterfly valve type | Concentric, double offset, triple offset | Triple offset provides bi-directional metal-to-metal zero-leakage shutoff |
| 3 | Body style | Wafer, lug, double flanged | Lug style allows downstream piping removal without depressurizing upstream; double flanged for high-pressure and large sizes |
| 4 | Size | NPS (typically 2” through 80”) | For wafer/lug types NPS 2”–48” is standard; double flanged extends to NPS 80” |
| 5 | Pressure class | 150, 300, 600 (triple offset up to 900) | Concentric valves are typically Class 150 only; double and triple offset cover Class 150–600+ |
| 6 | Body material | ASTM A216 WCB, A351 CF8M, A352 LCB, ductile iron ASTM A395 | Ductile iron is acceptable for Cat. A utility service; cast steel is required for hydrocarbon or high-pressure service |
| 7 | Disc material | Ductile iron (DI), CF8M (316 SS), duplex, Inconel overlay, Monel | Disc material must be compatible with the process fluid; Inconel/Stellite overlay is typical for triple offset |
| 8 | Seat material | EPDM, NBR, Viton, PTFE, RPTFE, laminated (graphite/SS), metal-to-metal | EPDM for water; Viton for hydrocarbons; metal-to-metal for triple offset fire-safe service |
| 9 | Shaft/stem material | SS 316, 17-4PH, Monel | 17-4PH is the standard for high-performance valves; Monel for sour or seawater service |
| 10 | Actuation | Lever (up to NPS 6), gear operator, pneumatic actuator (DA/SR), electric actuator | Lever is only practical for small sizes; specify DA (double acting) or SR (spring return) for pneumatic actuators |
Optional and Special Attributes
| Attribute | When Required | Details |
|---|---|---|
| API 607 fire-safe | Hydrocarbon service, refinery, offshore | Certifies the valve maintains shutoff integrity after fire exposure; requires fire-test certification |
| Fugitive emission (ISO 15848) | Environmental regulation, fugitive emission control | Specifies stem packing leakage class (Class A, B, or C) per ISO 15848-1 |
| Cryogenic service | LNG, ethylene, cryogenic applications | Extended bonnet or stem extension to protect packing from low temperatures |
| Bi-directional shutoff | Triple offset in pipeline or critical isolation | Confirms zero-leakage shutoff in both flow directions; standard for triple offset but must be stated explicitly |
| ATEX-rated actuator | Explosive atmospheres (Zone 1, Zone 2) | Actuator and accessories must carry ATEX or IECEx certification; specify the zone classification |
| Positioner | Control/throttling duty | Smart positioner (HART, Fieldbus) for modulating service; specify signal type and failure mode |
| Limit switches | Remote position indication | Mechanical or proximity type; specify voltage and contact rating |
| NACE MR0175 / ISO 15156 | Sour service (H2S-containing environments) | Restricts body, disc, shaft, and fastener materials to NACE-compliant grades and hardness limits |
| Certificates | Almost always required | EN 10204 Type 3.1 (MTR) minimum; Type 3.2 for critical services |
Example Line Items
Correct
6 pcs - Triple offset butterfly valve, API 609 Cat. B, NPS 16, Class 300, lug type, A216 WCB body, SS 316 disc with Stellite 6 seat ring, laminated graphite seal, gear operated, API 607 fire-safe, MTR EN 10204 3.1
This line item defines: quantity, valve type, design standard and category, size, pressure class, body style, body material, disc material, seat ring material, seal material, actuation, fire-safe certification, and certificate requirement.
Incorrect
6 - Butterfly valve 16 inch
This is missing: the API 609 category, valve type (concentric vs triple offset), body style, pressure class, body material, disc material, seat material, shaft material, actuation method, fire-safe requirement, and certification. The supplier cannot determine whether this is a low-cost wafer concentric valve or a high-performance triple offset valve, two products with an order-of-magnitude price difference.
Common Pitfalls
| Pitfall | Why It Matters |
|---|---|
| Not specifying the API 609 category | Category A and Category B define fundamentally different valve designs. A Cat. A concentric valve cannot replace a Cat. B triple offset valve in high-pressure hydrocarbon service |
| Omitting the body style | Wafer valves cannot be used where downstream piping must be removed for maintenance under pressure; lug or double flanged is required. Specifying the wrong style causes rework during installation |
| Confusing seat and seal materials | The seat is the sealing surface against the disc; the seal (or packing) is around the shaft. Both must be specified. EPDM seats are not suitable for hydrocarbon service; PTFE is not suitable above 260 deg C |
| Ordering a gear operator for a control application | Gear operators provide manual on/off operation. Throttling or modulating service requires a pneumatic or electric actuator with a positioner. Mixing these up means the valve cannot perform its control function |
| Missing fire-safe certification for hydrocarbon service | API 607 fire-safe testing is a project requirement in virtually all oil and gas facilities. A valve without this certification will be rejected at the inspection stage |
| Specifying ductile iron body for Class 300+ | Ductile iron bodies (A395) are limited to low-pressure utility service (typically Class 150). Cast steel (A216 WCB) or stainless steel (A351 CF8M) is required for Class 300 and above |
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