What Is API 602?
API 602 is an American Petroleum Institute standard that specifies design, materials, dimensions, inspection, and testing requirements for compact steel gate, globe, and check valves in sizes NPS 4 and smaller. It applies primarily to forged steel and barstock machined construction — manufacturing methods that provide superior mechanical integrity and pressure containment compared to cast alternatives at small bore sizes. API 602 is the governing standard for small-bore high-pressure process valves in refinery, petrochemical, and power generation service, and forms a key reference within the valve standards overview hub.
Key Takeaways
- API 602 covers small-bore steel gate, globe, and check valves — specifically NPS 4 and below, the size range where forged or barstock body construction is commercially practical and provides the superior mechanical properties required for high-pressure, high-temperature, and high-cycle process service.
- Applies primarily to forged or barstock construction — forging produces a refined, dense grain structure free of the casting defects (porosity, shrinkage, inclusions) that create stress concentration sites in cast bodies, making forged construction the mandatory choice for small-bore high-pressure valves above Class 600.
- Defines pressure ratings, dimensions, and testing requirements — API 602 references ASME B16.34 pressure-temperature ratings from Class 150 through Class 1500, specifies face-to-face and end-to-end dimensions per ASME B16.10, and mandates shell hydrostatic and seat leakage testing for every valve before shipment.
- Common in refinery and high-pressure process systems — instrument root valves, chemical injection isolation, sampling system block valves, high-pressure boiler auxiliary connections, and small-bore process line isolation are the primary API 602 valve positions in refinery and petrochemical plant piping systems.
How It Works
Scope and Small-Bore Application
API 602 defines its scope as compact steel gate, globe, and check valves in NPS 4 and smaller at Class 150 through Class 1500, with flanged, socket-weld, threaded, and butt-welding end connections, manufactured from forged steel or barstock. The Class 800 rating — which appears in API 602 but not in API 600 — is a commonly used pressure class for socket-weld and threaded end connections in small-bore process piping, providing a convenient intermediate rating between Class 600 (for flanged ends) and Class 1500 for threaded and socket-weld connections in instrument and auxiliary service. The small-bore focus of API 602 reflects the engineering reality that forged construction becomes progressively more economical relative to cast construction as nominal size decreases — at NPS 1/2 through NPS 2, forged steel bodies from bar stock or die forgings are cost-competitive with cast alternatives while providing measurably superior mechanical properties. At NPS 6 and above, the forging dies and process costs make forged construction less competitive than cast bodies, which is where API 600 cast gate valves take over. API 602 valves are the dominant valve specification for the large number of small-bore valve positions in a typical refinery or petrochemical plant — instrument manifold block valves, drain and vent valves, sample connection valves, and chemical injection isolation valves collectively represent a larger count than the large-bore mainline block valves specified to API 600. The broader valve standards framework contextualizing API 602 is addressed in the valve standards overview reference.
Pressure-Temperature and Testing Requirements
API 602 references ASME B16.34 for all pressure-temperature ratings — the allowable working pressure at any operating temperature for an API 602 valve is read from the ASME B16.34 pressure-temperature table for the valve’s body material group and Class designation. An API 602 Class 800 gate valve in ASTM A105 carbon steel forging is rated at approximately 138 bar (2,000 psi) at 38°C — the Class 800 rating is an intermediate API class not defined in ASME B16.34 directly, and API 602 provides the pressure-temperature rating table for Class 800 derived from the Class 600 and Class 900 ASME B16.34 tables for the applicable material groups. The complete ASME B16.34 pressure-temperature rating framework is addressed in the what is ASME B16.34 reference, and the broader pressure class explanation is addressed in the ASME pressure class explained reference. API 602’s mandatory production testing requirements align with API 598 — shell hydrostatic test at 1.5 times the rated pressure with no visible leakage acceptance criterion; seat leakage test at 1.1 times rated pressure with acceptance criteria per the applicable API 598 leakage class; and backseat test for gate and globe valves at 1.1 times rated pressure. The complete API 598 testing framework is addressed in the what is API 598 reference, with detailed production testing procedures addressed in the valve pressure testing procedure reference.
Main Components
Forged Body and Valve Types
The forged or barstock body is the defining construction characteristic of API 602 valves. Forging — the process of pressing or hammering hot steel into a die to produce a near-net-shape component — refines the steel grain structure, closes porosity, and aligns grain flow with the component geometry, producing a body with higher yield strength, higher impact toughness, and greater resistance to fatigue and stress corrosion cracking than an equivalent cast body of the same material composition. Barstock machining — producing the valve body entirely from a solid bar of steel by CNC turning and milling — provides the highest material density and most homogeneous grain structure of any production method, at the cost of higher machining time and material waste compared to forgings. API 602 permits both methods, with the selection determined by nominal size, pressure class, and production economics. Common API 602 body materials include ASTM A105 (carbon steel forgings, general service to 425°C), ASTM A182 F11 and F22 (alloy steel forgings, elevated temperature and hydrogen service), ASTM A182 F316 and F316L (stainless steel forgings, corrosive service), and ASTM A182 F51 (duplex stainless steel, sour service per NACE MR0175). API 602 covers three valve types — gate valves for isolation service, globe valves for throttling and regulation service, and check valves for backflow prevention. The design principles for each covered valve type are addressed in the what is a gate valve, what is a globe valve, and what is a check valve references.
Fire, Emission, and Certification
API 602 defines the core design and construction requirements for compact forged steel valves but does not inherently mandate fire-safe or fugitive emission certification — these are supplementary requirements invoked by project specifications for specific service environments. In refinery hydrocarbon service, API 602 gate and globe valves at positions in flammable fluid service are commonly required by project specification to carry fire-safe certification per what is API 607, confirming that the valve’s metal seats provide backup sealing after soft seat destruction in a fire event. The complete fire-safe certification qualification process is addressed in the fire-safe certification reference. For API 602 gate and globe valves in environmentally regulated installations where stem seal fugitive emissions must be limited, ISO 15848 fugitive emission testing and classification may be required — the emission class and thermal class designation must be specified by the purchaser as a supplementary requirement. The complete fugitive emission testing framework is addressed in the fugitive emission testing and what is ISO 15848 references. For material traceability, API 602 requires EN 10204 Type 3.1 inspection certificates for all pressure-containing parts — the complete EN 10204 material certificate framework is addressed in the what is EN 10204 3.1 reference. The complete documentation package for a compliant API 602 valve delivery is addressed in the valve certification documents reference, and verification procedures are addressed in the how to verify valve compliance reference.
Advantages
Strength, Compactness, and Standardization
API 602’s forged or barstock construction requirement provides measurably superior mechanical properties compared to cast alternatives at equivalent nominal size and pressure class — forged ASTM A105 carbon steel has a minimum yield strength of 250 MPa versus 205 MPa for cast ASTM A216 WCB, a 22% advantage that translates directly into thinner allowable wall sections at equivalent pressure class or higher allowable pressure at equivalent wall thickness. This structural superiority makes API 602 forged valves the standard specification for the highest-consequence small-bore valve positions — high-pressure hydrogen service valves, sour service isolation valves per NACE MR0175, and emergency block valves on critical process lines — where the consequences of valve body failure exceed the cost premium of forged over cast construction many times over. The compact face-to-face dimensions of API 602 valves per ASME B16.10 make them suitable for the space-constrained installations typical of small-bore instrument and auxiliary piping systems in process plants, where multiple valves must be installed within a limited pipe rack width. API 602’s global recognition in refinery and petrochemical project specifications means that forged steel valves from any qualified manufacturer worldwide are acceptable for equivalent service positions without re-qualification. For the comparison between API 602 small-bore compact valve requirements and API 600 large-bore cast gate valve requirements — including the size and pressure class criteria that determine which standard applies at each valve position — refer to the what is API 600 reference. For the comparison between API 602 process plant valve requirements and API 6D pipeline valve requirements — including the service environment criteria that determine which standard applies at the boundary between plant and pipeline service — refer to the what is API 6D reference.
Typical Applications
Refinery, Petrochemical, and Power Systems
API 602 forged steel gate, globe, and check valves are the standard specification for the large population of small-bore valve positions that exist throughout any refinery or petrochemical plant. In refinery auxiliary and instrument service, Class 800 socket-weld and threaded API 602 gate valves in ASTM A105 carbon steel provide isolation at instrument root connections, pressure gauge and transmitter connections, level gauge and control valve bypass installations, and drain and vent connections on process vessels and heat exchangers — positions that exist in hundreds per process unit and whose collective valve count far exceeds the mainline API 600 gate valve count. In refinery process line service, Class 300 through Class 1500 flanged API 602 gate and globe valves in alloy steel or stainless steel provide block valve and throttling valve service on small-bore process branch connections, sample system isolation, and chemical injection outlet valves where the small bore size and high pressure class combination requires forged rather than cast body construction. In power generation high-pressure boiler and steam systems, Class 800 through Class 1500 API 602 gate and globe valves in ASTM A182 F22 alloy steel provide reliable isolation and flow regulation on boiler drum connections, steam trap inlet and outlet connections, high-pressure feedwater auxiliary connections, and instrumentation root valves — applications where the combination of high pressure, elevated temperature, and requirement for reliable sealing through thermal cycling makes the forged API 602 specification the correct choice over commodity valves. In European refinery and petrochemical projects where both API 602 technical compliance and PED CE marking are required for market access, the integration of API 602 with EU regulatory conformity assessment requirements is addressed in the what is PED 2014/68/EU reference.
Frequently Asked Questions
What valve sizes does API 602 cover?
API 602 covers compact steel gate, globe, and check valves in NPS 4 and smaller — specifically NPS 1/2 through NPS 4 in forged steel and barstock construction. Within this size range, the standard covers pressure classes from Class 150 through Class 1500 for flanged end connections, and Class 800 for socket-weld and threaded end connections. At NPS 4, the crossover with API 600 cast gate valves depends on pressure class — API 602 forged construction is typically specified at NPS 4 Class 600 and above, while cast API 600 construction is often specified at NPS 6 and above across all pressure classes.
How does API 602 differ from API 600?
API 602 applies to compact forged steel gate, globe, and check valves in NPS 4 and below — where forged or barstock body construction provides superior mechanical properties and the compact size suits instrument and auxiliary piping service. API 600 applies to cast bolted-bonnet steel gate valves in NPS 2 through NPS 24 — where cast body construction is economically practical at larger bore sizes and the bolted bonnet provides in-place maintainability for the higher operating cycle frequency of larger process piping. The two standards are complementary: API 602 covers the small-bore forged valve positions and API 600 covers the large-bore cast gate valve positions in the same process plant piping system.
Does API 602 include testing requirements?
Yes — API 602 mandates shell hydrostatic testing at 1.5 times the rated pressure and seat leakage testing at 1.1 times rated pressure for every valve before shipment, with acceptance criteria per the applicable API 598 leakage class. Backseat testing at 1.1 times rated pressure is required for gate and globe valves to verify that the fully-open backseat position seals the stem for under-pressure repacking. All test results must be documented in test certificates delivered with the valve as part of the mandatory documentation package.
Are API 602 valves always forged?
API 602 applies primarily to forged steel and barstock machined construction — these manufacturing methods are specified because they provide superior mechanical properties (higher yield strength, higher impact toughness, denser grain structure) compared to cast alternatives at the small bore sizes covered by the standard. However, the exact material form and manufacturing method may vary depending on nominal size, pressure class, and specific project requirements. At Class 150 and 300 in NPS 2 through NPS 4, some API 602 compliant valves may use cast bodies — the critical requirement is that the body material meets the applicable ASTM specification’s mechanical property requirements, regardless of whether it was produced by casting or forging.
Conclusion
API 602 is the governing technical standard for compact forged steel gate, globe, and check valves in NPS 4 and smaller for refinery, petrochemical, power generation, and high-pressure process service — its forged or barstock construction requirement, ASME B16.34 pressure-temperature rating compliance, mandatory shell and seat production testing, and full material traceability documentation provide a rigorous quality assurance framework for the large population of small-bore valve positions in process plant piping systems. Correct application requires confirming the valve’s nominal size and pressure class are within API 602’s covered range, specifying the body material for the process fluid, temperature, and corrosion service class, confirming whether Class 800 (socket-weld and threaded) or Class 150 through Class 1500 (flanged) end connections are appropriate, and verifying that the complete documentation package is delivered with each valve. Engineers requiring a comprehensive framework that integrates API 602 within the full landscape of valve design, testing, dimensional, and certification standards should consult the valve standards overview hub as the governing reference for all compact steel valve standards navigation.