An API 6D gate valve is a bi-directional isolation valve for pipeline and piping service in which a wedge, slab or expanding gate moves perpendicular to the flow path to open or close the bore. It is specified under API Specification 6D, the pipeline valve standard maintained by the American Petroleum Institute, and is technically equivalent to ISO 14313.
The gate travels at 90° to the direction of flow between two seats. In the fully open position the gate is retracted into the bonnet and the flow passage is unobstructed; in the fully closed position the gate is pressed against both seats. This geometry gives the valve a very low pressure drop and a short, low-torque stroke, which is why gate valves are used for on/off isolation rather than for regulating or throttling flow. A partially open gate exposes the seat and gate sealing faces to high-velocity erosion, so API 6D gate valves are intended to operate fully open or fully closed only.
The body is normally a two-piece or three-piece bolted construction with a body cavity between the two seats. That cavity is the basis for the double block and bleed function described below, and it is fitted with a drain or vent connection so that seat tightness can be monitored in service and verified during the factory seat test.
Inside the body, the sealing faces of both the gate and the seats carry a hard facing to resist abrasion and wire drawing. Common overlays are 13% Cr stainless (UNS S41000, formerly 1Cr13), cobalt-based Stellite 6 (UNS R30006) and, for less severe duty, a stainless steel overlay. The choice between these is driven by the sand content, the pressure differential and whether the service is sour.
API 6D Gate Valve Specification
Body material: Carbon steel, Stainless steel, Alloy steel ,Monel, Duplex, Inconel
Size range(s):2"～72"
Pressure rating: Class150～Class 2500
End connection: RF, RTJ, BW
Working temperature: -46℃～+200℃
Mode of operation: Hand wheel, gear box, Electric , Pneumatic , etc.
Technical specification
Design & manufacturing standard as per API 6D
Face to face standard as per ASME B16.10 / API 6D
Fanged ends standard as per ASME B16.5 / MSS-SP-44
Testing & inspection standard as per API598 / API 6D
Size and Pressure Class Range
API 6D covers valve sizes from NPS 1/2 to NPS 60 in the pressure classes defined by ASME B16.34. The table gives the nominal ASME class with the corresponding cold working pressure for ASTM A216 WCB, which is the reference material for carbon steel pipeline valves.
| ASME Class | Cold working pressure (38 °C) | Typical size range | Typical service |
|---|---|---|---|
| Class 150 | 19.6 bar | NPS 2–60 | Low-pressure gathering, water injection |
| Class 300 | 51.1 bar | NPS 2–48 | General pipeline isolation |
| Class 600 | 102.1 bar | NPS 1/2–48 | Mainline crude and gas transmission |
| Class 900 | 153.2 bar | NPS 1/2–36 | High-pressure transmission, compressor station |
| Class 1500 | 255.3 bar | NPS 1/2–24 | High-pressure wellpad and manifold |
| Class 2500 | 425.5 bar | NPS 1/2–18 | Ultra-high-pressure and sour service |
Pressure ratings are temperature-dependent. Above 38 °C the allowable pressure falls along the ASME B16.34 curve for the specific material group, so a valve ordered as Class 600 for a 150 °C service is not rated at 102 bar. Always quote the design temperature with the class, not the class alone.
Design Features Required by API 6D
These are the characteristics that separate a genuine API 6D pipeline valve from a general-purpose gate valve of the same size and class. Each one is a point a buyer can and should verify in the data book.
| Feature | Requirement | Verification |
|---|---|---|
| Full-bore / piggable | Port diameter ≥ nominal pipe ID so a pig passes without damage to pig or valve | Bore dimension on drawing and dimensional report |
| Double block and bleed (DBB) | Both seats isolate independently; cavity drain allows bleed-down between them | Bidirectional seat test result per seat |
| Double isolation and bleed | DIB-1 or DIB-2 configuration where specified | Seat arrangement drawing |
| Body cavity relief (BCR) | Relief of trapped-fluid thermal expansion, self-relieving or external | BCR feature on body drawing |
| Fire-safe design | Metal-to-metal secondary seal, graphite packing, tested to API 607 / 6FA | Fire test certificate |
| Anti-static device | Electrical continuity gate–stem–body, resistance under 10 Ω | Anti-static test record |
| Anti-blowout stem | Positive mechanical shoulder prevents stem ejection on packing failure | Stem design review / drawing |
| Emergency sealant injection | Fittings at stem and both seats for sealant injection in service | Fitting layout on GA drawing |
| Low-emission packing | ISO 15848-1 / -2 when specified by the project | Type test certificate |
| Material traceability | PMI on all pressure-containing parts, MTR to EN 10204 3.1 | PMI report and MTR set |
Body, Trim and Seal Materials
Material selection for API 6D is made against ASME B16.34 for pressure-containing parts, with additional requirements for trim, seals and bolting. The single most common ordering error is to specify a low temperature that the chosen body material cannot meet.
| Material | ASTM grade | Service temperature | Typical use |
|---|---|---|---|
| Carbon steel | A216 WCB | −29 to +425 °C | Standard oil, gas and water pipeline |
| Low-temp carbon steel | A352 LCB / LCC | −46 to +345 °C | Cold climate, −46 °C design |
| 1.25Cr–0.5Mo | A217 WC6 | −29 to +595 °C | High-temperature and steam |
| 2.25Cr–1Mo | A217 WC9 | −29 to +595 °C | High-temperature hydrogen service |
| 304 stainless | A351 CF8 | −196 to +425 °C | Corrosive, cryogenic |
| 316 stainless | A351 CF8M | −196 to +425 °C | Corrosive, marine, chemical |
| Duplex | A995 4A (S31803) | −50 to +300 °C | Chloride stress, offshore |
| Super duplex | A995 6A (S32750) | −50 to +300 °C | Severe sour and seawater |
| Nickel alloy | Monel 400, Inconel 625 | Per alloy data sheet | Acid, high-chloride, severe sour |
Temperature and material must be quoted together. A −46 °C design temperature is not achievable in WCB - impact testing of standard carbon steel becomes unreliable below −29 °C, and low-temperature A352 LCB/LCC has to be specified instead. Equally, +200 °C is within the WCB range but is close to the limit for PTFE soft seats, so a metal-seated trim or a PEEK seat is normally required above roughly 180 °C. State the design temperature, the minimum ambient and the medium in the enquiry so the material and seat can be selected together.
Category A and Category B Service
API 6D divides valves into two material categories. Category A is standard service. Category B applies to sour service and imposes additional material and hardness requirements to resist sulphide stress cracking, in accordance with NACE MR0175 / ISO 15156.
Hardness control - carbon and low-alloy steel pressure parts are typically limited to 22 HRC maximum, with heat-affected zones of welds hardness-tested as well.
Bolting - must also satisfy the sour service hardness limit, and must be low-temperature qualified where ambient can fall below −40 °C.
Trim and seals - overlay and seal materials are restricted to those qualified under the same standard; Monel or Inconel 625 overlay is common where carbon steel body is retained.
Documentation - a Category B order requires NACE compliance certificates and hardness test records per heat, not just a general statement.
Standards, Testing and Inspection
Testing under API 6D follows the standard's own Section 10 rather than a separate inspection standard. The table summarises the tests that are normally witnessed and the acceptance basis for each.
| Test | Pressure | Duration (min) | Acceptance |
|---|---|---|---|
| Shell (hydrostatic) | 1.5 × 38 °C rating | NPS ≤4: 2 · NPS 6–10: 5 · NPS 12–18: 15 · NPS ≥20: 30 | No visible leakage, no permanent deformation |
| Seat (hydrostatic) | 1.1 × 38 °C rating | NPS ≤4: 2 · NPS 6–18: 5 · NPS ≥20: 10 | ISO 5208 Rate A (soft) / Rate D (metal) |
| Back seat | 1.1 × rating | Per agreement | No visible leakage, where back seat is specified |
| Low-pressure seat | 5–7 bar air | Per agreement | Bubble count per NPS, when specified |
| Anti-static | - | - | Resistance under 10 Ω |
| Operational / torque | At rated differential | 3 open–close cycles | Free operation, torque recorded |
| Fire test | API 607 / 6FA | 30 min burn | Leakage within post-fire limits |
| NDE | Per QSL level | - | MT/PT on castings; RT/UT where QSL requires |
Standards referenced by a typical API 6D gate valve order:
API 6D - design, materials, welding, testing, marking (25th Edition, 2021; ISO 14313 equivalent)
ASME B16.34 - pressure–temperature ratings, wall thickness, material groups
ASME B16.10 - face-to-face and end-to-end dimensions
ASME B16.5 - flanged ends NPS 1/2–24; MSS SP-44 - flanged ends NPS 26–60
ASME B16.25 - buttweld ends
ISO 5208 - industrial valve pressure testing, leakage rate classification
API 607 / API 6FA - fire testing of quarter-turn and pipeline valves
NACE MR0175 / ISO 15156 - sour service materials, Category B
ISO 15848-1 - fugitive emission qualification, when specified
EN 10204 3.1 / 3.2 - material test report type
Seat Leakage Acceptance: ISO 5208 Rate A and Rate D
Seat tightness is the clause most often misquoted in valve enquiries. Under API 6D the acceptance criterion is expressed as an ISO 5208 leakage rate, and it depends on the seat construction - not on the valve being described as "Class VI".
| Seat construction | Acceptance | Meaning in practice |
|---|---|---|
| Soft-seated (PTFE, PEEK, nylon) | ISO 5208 Rate A | No visible leakage during the test duration |
| Metal-seated (gate, wedge, slab) | ISO 5208 Rate D | Measurable allowance in mm³/s per NPS, unless otherwise agreed |
| Metal-seated, buyer-specified tighter | Rate C or D by agreement | Must be stated in the purchase order |
| Lubricated plug (reference) | ISO 5208 Rate A | Same basis as soft-seated |
Do not write "API 598 Class VI" or "FCI 70-2 Class VI" for an API 6D valve. Class VI is a control-valve seat classification from ANSI/FCI 70-2 and a different inspection standard, API 598. It does not define acceptance for a pipeline gate valve. The correct wording on the datasheet is seat test per API 6D Section 10.4, acceptance ISO 5208 Rate A for soft-seated and Rate D for metal-seated construction. If a buyer's template asks for Class VI, clarify in writing which basis is intended before quoting, because a metal-seated slab gate cannot normally meet a Class VI bubble-tight criterion.
API 6D vs API 600 vs API 602
These three standards all cover steel gate valves, which is why they are so often confused in an enquiry. They are not interchangeable, and quoting the wrong one changes both the valve and the price.
| Aspect | API 6D | API 600 | API 602 |
|---|---|---|---|
| Scope | Pipeline and piping valves | Bolted bonnet steel gate valves, process service | Compact forged steel gate valves |
| Typical size | NPS 1/2–60 | NPS 2–24 | NPS 1/2–4 |
| Typical class | 150–2500 | 150–2500 | Class 800, 1500, 2500 |
| Bore | Full bore, piggable | Standard bore, not piggable | Reduced / standard bore |
| Body style | Through-conduit, slab, expanding | Bolted bonnet, OS&Y rising stem | Forged, compact, often welded bonnet |
| DBB | Standard capability | Not a design requirement | Not a design requirement |
| Fire-safe | Required design feature | When specified | When specified |
| Primary use | Transmission pipeline, mainline isolation | Refinery, petrochemical, process plant | Small-bore utility, instrument, drain |
If the line will be pigged, or the valve is a mainline block valve on a transmission pipeline, the correct standard is API 6D. If the requirement is a general isolation valve inside a process plant and pigging is irrelevant, API 600 is normally the more economical and more readily available choice. An API 6D valve used where API 600 suffices costs more for capability that will never be used; the reverse mistake - an API 600 valve specified on a piggable mainline - cannot be corrected after installation.
Stem and Operator Configuration
Stem configuration has to be decided early, because it determines the bonnet height and the shipping envelope, and it cannot be changed on site.
| Option | Description | Where used |
|---|---|---|
| Rising stem (OS&Y) | Stem rises through the handwheel as the valve opens; position visible | Above-ground, accessible, smaller valves |
| Non-rising stem (NRS) | Stem rotates in place, gate travels on threads inside the body | Buried or space-restricted installation, large valves |
| Handwheel | Direct manual operation | Small sizes, low differential |
| Gearbox | Reduction gearing for high torque; handwheel on gearbox | NPS 8 and above, or high differential |
| Electric actuator | Motor-operated with limit switches and torque protection | Remote mainline block valve, SCADA |
| Pneumatic / hydraulic | Double-acting or spring-return cylinder | Fast closure, ESD service |
| Gas-over-oil | Pipeline gas as hydraulic power source | Remote pipeline locations without power |
API 6D pipeline mainline valves are frequently specified as through-conduit slab gate with a non-rising stem and a gearbox, because the valve may sit in a buried pit or a valve station with limited headroom, and because the gate has to seal equally in both directions. A rising-stem wedge gate is the correct choice for an above-ground process valve, which is the application the original description on this page referred to.
Applications
Crude oil and product transmission lines - mainline block valves, sectionalisng valves, pig trap isolation.
Natural gas transmission - compressor station suction and discharge isolation, line break valve stations.
Pipeline pigging - full-bore valves at pig trap and scraper stations where gauge passage is mandatory.
Pump and compressor stations - discharge isolation where the pump is started against a closed valve to limit starting load.
Storage terminals and tank farms - tank isolation, manifold block valves with double block and bleed.
Water injection and gathering - Class 150–300, coated or lined bore on request.
Sour service gathering - Category B, NACE MR0175 compliant trim and body.
API 6D Gate Valve in stock
packaging
Send Your Valve Datasheet for a Quotation
Send the size, class, bore type, end connection, body and trim material, design temperature and service medium. We will confirm the API 6D configuration, the seat leakage basis and the delivery schedule, and return a quotation with the technical datasheet attached.
FAQ
What is the difference between API 6D and API 600 gate valves?
API 6D is a pipeline valve specification requiring a full-bore piggable passage, double block and bleed capability and fire-safe design. API 600 covers bolted bonnet steel gate valves for process plant service, typically NPS 2–24, with a standard bore that cannot pass a pig. API 6D is specified for transmission pipeline mainline isolation; API 600 is normally the economical choice inside a refinery or process plant.
What size and pressure class range does API 6D cover?
API 6D applies to valves from NPS 1/2 to NPS 60 (DN15 to DN1500) in pressure classes 150, 300, 600, 900, 1500 and 2500 as defined by ASME B16.34. Our standard production covers NPS 1/2–60, with NPS 1/2–24 available from stock in the more common classes.
What seat leakage rate applies to an API 6D gate valve?
Seat acceptance is to ISO 5208. Soft-seated valves must meet Rate A, which is no visible leakage for the duration of the test. Metal-seated valves must meet Rate D, a measurable allowance expressed in mm³/s per NPS, unless a tighter rate is agreed in the purchase order. The test pressure is 1.1 times the 38 °C pressure rating and the duration depends on size.
Is an API 6D gate valve fire-safe?
Fire-safe design is a requirement of API 6D: the valve must retain sealing capability after exposure to fire, using a metal-to-metal secondary seal and graphite packing. Qualification is demonstrated by testing to API 607 or API 6FA, normally a 30-minute burn. Fire test certificates should be requested with the data book.
Can an API 6D gate valve be used for throttling?
No. A gate valve is an isolation valve and must be operated fully open or fully closed. A partially open gate places the seat and gate faces in the high-velocity flow path, which causes erosion and wire drawing and destroys seat tightness. Flow regulation requires a globe valve, a control valve or a needle valve.
What does double block and bleed mean on a gate valve?
DBB means both seats isolate independently and the body cavity between them can be bled to atmosphere through a drain or vent. If either seat leaks, the leak appears at the cavity drain rather than passing downstream, so the isolation can be verified in service. API 6D gate valves are built with the body cavity and drain connection needed for this function.
When is Category B material required?
Category B applies to sour service, where the produced fluid contains H2S at a partial pressure that can cause sulphide stress cracking. Materials must comply with NACE MR0175 / ISO 15156, with hardness normally limited to 22 HRC for carbon and low-alloy steels and hardness-tested heat-affected zones on welds. The H2S partial pressure, pH and chloride content are needed to confirm the category.
What is the minimum temperature for a carbon steel API 6D gate valve?
Standard ASTM A216 WCB body material is suitable down to −29 °C. For a −46 °C design temperature, low-temperature carbon steel to ASTM A352 LCB or LCC must be specified, with impact testing. Stainless steel bodies extend the range considerably further, to −196 °C for CF8 and CF8M.
Which documents come with the valve?
The standard data book includes material test reports to EN 10204 3.1, hydrostatic shell and seat test certificates, dimensional inspection report, anti-static test record and functional operation record. Positive material identification reports, fire test certificates, NACE compliance certificates and third-party inspection reports are supplied when specified in the order.
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