API 602 Explained — Compact Forged Steel Gate, Globe & Check Valve Standard

When small-bore isolation valves are needed for high-integrity refinery, petrochemical, or oil and gas service — where compact dimensions, superior mechanical properties, and reliable performance under cyclic pressure and thermal loading are essential — API 602 is the governing specification. Published by the American Petroleum Institute, API 602 defines the design, material, dimensional, and testing requirements for compact forged steel gate, globe, and check valves in sizes NPS ½ through NPS 4, covering pressure classes from Class 150 through Class 2500. This page provides a complete, engineering-level breakdown of API 602: its scope and application, design and material requirements, mandatory testing, its relationship with API 600 and ASME B16.34, and practical guidance for procurement and quality assurance teams.

For a complete overview of all major valve standards used in industrial applications, visit our Valve Standards pillar page.

What Is the API 602 Standard?

API 602 Standard Overview

API 602 — formally titled Gate, Globe, and Check Valves for Sizes DN 100 and Smaller for the Petroleum and Natural Gas Industries — is published by the American Petroleum Institute and establishes minimum requirements for compact, forged steel gate, globe, and check valves intended for use in petroleum refinery, petrochemical, and oil and gas processing service. The standard is specifically focused on small-bore valves manufactured from forgings rather than castings — a construction method that produces superior grain structure, higher strength-to-weight ratios, and improved resistance to pressure cycling and fatigue compared to equivalent cast products.

The most important distinction between API 602 and its companion standard API 600 lies in the material form and size range: API 600 governs cast steel gate valves from NPS ½ through NPS 24, with body and bonnet components manufactured from steel castings; API 602 governs forged steel gate, globe, and check valves from NPS ½ through NPS 4, where the superior mechanical properties of forgings are particularly valuable in compact, high-pressure configurations. In process plant projects, API 602 and API 600 are typically applied together as a paired specification covering small-bore and large-bore valves respectively, with the size transition boundary defined in the project valve specification.

Scope of API 602

Valve Types and Size Range

API 602 covers three fundamental valve types in the small-bore range, each serving a distinct functional role in process plant piping systems:

  • Gate valves: The primary isolation valve type covered by API 602. Both outside screw and yoke (OS&Y) and bolted bonnet configurations are addressed. Gate valves to API 602 provide full-bore isolation with minimal flow restriction when fully open, making them suitable for main process line block and bypass service.
  • Globe valves: Used for throttling and control applications where a gate valve’s on/off design is insufficient. API 602 globe valves in forged construction are commonly used in bypass circuits, pressure-reducing service, and applications requiring frequent operation.
  • Check valves: Non-return valves that prevent reverse flow, covered in both piston (lift check) and swing check configurations under the API 602 framework.

The standard covers valves from NPS ½ through NPS 4 (DN 15 through DN 100), across pressure classes 150, 300, 600, 800, 900, 1500, and 2500. Class 800 is a particularly common pressure rating for API 602 forged steel valves — it is a socket weld and threaded end connection rating that does not directly correspond to ASME flange classes but is widely used for small-bore forged valves in process plant service. End connection types addressed by the standard include socket weld, butt-welding, threaded (NPT), and flanged ends, giving API 602 broader end connection coverage than API 600.

Typical Application Areas

API 602 forged steel valves are ubiquitous in petroleum refinery and petrochemical plant environments. Their compact, high-integrity construction makes them the preferred choice for a wide range of small-bore applications:

  • Instrument root valves and isolation valves: The single largest application category for API 602 valves. Root valves isolate pressure transmitters, level gauges, flow meters, and other instruments from the process — service conditions that demand reliable sealing performance and high mechanical integrity in a compact, lightweight package.
  • Bypass and equalizing lines: Small-bore bypasses around larger control valves and main block valves, used for startup, pressure equalization, and maintenance purposes.
  • Drain and vent connections: Process drain valves and vent valves in high-pressure service where the forged construction provides superior resistance to local stress concentrations around drain boss connections.
  • High-pressure hydrocarbon service: Class 1500 and Class 2500 small-bore valves in high-pressure gas, hydrogen, and sour service applications where the homogeneous grain structure of forged steel provides better resistance to hydrogen embrittlement and stress corrosion cracking than cast equivalents.
  • Cryogenic service: API 602 includes provisions for extended body (cryogenic) gate and globe valves with elongated bonnets that keep the packing away from the cold process fluid, preventing packing freeze-up and ensuring reliable stem sealing at sub-zero temperatures.

API 602 Design and Material Requirements

Design Requirements

API 602 establishes comprehensive design requirements specifically tailored to forged steel valve construction in the small-bore size range:

  • Forged body and bonnet construction: All pressure-containing components — body, bonnet, and end connections — must be manufactured from forgings produced in accordance with recognized ASTM forging material specifications. The forging process produces a continuous, oriented grain structure that provides superior mechanical properties, impact toughness, and fatigue resistance compared to castings of equivalent nominal composition.
  • Body and bonnet connection types: API 602 covers both integral bonnet designs (where the bonnet and body are machined from a single forging) and bolted bonnet designs. Integral bonnet construction eliminates the bonnet joint as a potential leak path and is commonly preferred for the highest-integrity applications. Bolted bonnet designs follow requirements similar to API 600 but scaled for the small-bore, compact form factor.
  • Stem design and blowout prevention: As with API 600, all API 602 valves must incorporate a stem design that prevents ejection of the stem from the valve body under pressure. This blowout prevention feature is mandatory regardless of valve size and is particularly important for small-bore high-pressure valves where the stem-to-body clearance is minimal.
  • Backseat design: Gate and globe valves must include a backseat feature that provides a secondary stem seal when the valve is fully open, allowing repacking under pressure. The backseat must be verified by factory test as part of the mandatory inspection sequence.
  • End connection dimensions: Socket weld and butt-welding end connections must comply with ASME B16.11 and ASME B16.25 respectively. Flanged end connections must comply with ASME B16.5, with face-to-face dimensions governed by ASME B16.10.
  • Compact and extended body designs: The standard covers both standard and extended body configurations. Extended body designs — with elongated bonnets — are specified for cryogenic, insulated, or buried service where the standard bonnet length would be insufficient.

Material Specifications and Mechanical Property Requirements

The choice of forged steel over cast steel is not merely a manufacturing preference — it carries specific material performance advantages that are particularly relevant for small-bore, high-integrity valve applications:

  • Carbon steel forgings: ASTM A105 is the most widely used material for API 602 gate and globe valves in general refinery service at temperatures up to approximately 425°C (800°F). Its fine grain structure and consistent mechanical properties make it the standard material for Class 800 and Class 1500 socket weld valves in non-corrosive hydrocarbon service.
  • Low-temperature carbon steel forgings: ASTM A350 Grade LF2 provides the impact toughness required for sub-zero service, with Charpy V-notch test requirements ensuring ductile behavior at minimum design metal temperatures down to -46°C (-50°F).
  • Chrome-moly alloy steel forgings: ASTM A182 Grade F11 (1¼Cr-½Mo) and F22 (2¼Cr-1Mo) are specified for high-temperature steam and hydrogen service where carbon steel creep resistance is insufficient.
  • Stainless steel forgings: ASTM A182 Grade F316 (316 stainless steel) and F316L are used for corrosive service, cryogenic applications, and where post-weld heat treatment is undesirable or impractical.

Comparing forged and cast materials of equivalent nominal composition — for example, ASTM A105 forging vs. ASTM A216 WCB casting — the forged product typically offers higher tensile and yield strength, superior Charpy impact toughness at low temperatures, finer grain size, and fewer internal defects (no porosity or shrinkage cavities inherent in the casting process). These advantages make forged construction the preferred choice for small-bore valves in demanding service conditions, even where cast steel would be technically permissible.

Full material traceability is required for all pressure-containing components. On European and international projects where PED 2014/68/EU compliance is required, material test reports must be provided in EN 10204 3.1 format to satisfy the directive’s material traceability requirements and support CE marking documentation.

API 602 Testing and Inspection Requirements

Mandatory Factory Test Requirements

API 602 mandates a comprehensive series of factory pressure tests that must be completed for every valve before shipment. The test sequence mirrors the framework established for cast steel valves under API 600, with test pressures and acceptance criteria adapted for the forged steel small-bore valve configurations covered by the standard:

  • Shell (hydrostatic body) test: The complete valve assembly — with closure element partially open — is pressurized hydrostatically at 1.5 times the maximum allowable working pressure at 38°C (100°F) for the specified pressure class and material. All external surfaces, including the body wall, bonnet joint, gland, and any auxiliary connections, are inspected for visible leakage. Zero external leakage is required for test acceptance. Test duration varies with valve size — typically 15 to 60 seconds minimum — and must be held consistently to qualify as a valid test.
  • Backseat test: With the valve driven fully open to engage the backseat, the body cavity is pressurized to the shell test pressure. Zero leakage past the backseat seal confirms the secondary stem seal is functional and capable of supporting in-service repacking without process isolation.
  • Seat leakage test: The valve is closed and pressure is applied from each end sequentially. For metal-seated gate and globe valves, the acceptable leakage rate is defined by reference to API 598 leakage tables for the applicable valve type and size. The test may be performed using water (hydrostatic) or air/nitrogen (pneumatic) at low pressure — typically 5.5 to 8 bar for the pneumatic option — with leakage measured by bubble counting or collection volume as appropriate.
  • Check valve reverse flow test: For check valves manufactured under API 602, a reverse pressure test confirms that the closure element seals effectively against reverse flow at the specified test pressure, with leakage within the API 598 allowable limits for the valve type and seat configuration.
  • Optional / supplementary tests: High-pressure seat leakage tests at full rated working pressure, low-temperature shell tests for cryogenic service valves, and pneumatic shell tests for dry gas service valves may be specified as supplementary requirements in the purchase order. These are not mandatory under the base API 602 standard but are commonly added by engineering specifications for critical service applications.

All test results must be documented on factory test certificates cross-referenced to the valve serial number, forming part of the permanent quality documentation package. Third-party inspection at mandatory hold or witness points should be specified in the inspection and test plan (ITP) where the project quality requirements demand independent test verification.

API 602 Compared with Related Standards

API 602 vs. API 600

API 602 and API 600 are companion standards that together provide comprehensive coverage of gate, globe, and check valves for refinery and process plant service — but they are designed for fundamentally different valve constructions and size ranges:

  • Material form: API 600 governs cast steel gate valves — bodies and bonnets manufactured from steel castings (ASTM A216 WCB and equivalent alloys). API 602 governs forged steel valves — bodies and end connections manufactured from steel forgings (ASTM A105, A182, A350, and equivalent alloys). The forged construction of API 602 valves delivers superior mechanical properties, particularly in the small-bore, high-pressure configurations covered by the standard.
  • Valve types: API 600 covers only gate valves. API 602 covers gate, globe, and check valves — a broader functional scope that reflects the range of small-bore valve types commonly used in instrument and process service applications.
  • Size range: API 600 covers NPS ½ through NPS 24. API 602 covers NPS ½ through NPS 4. For sizes NPS ½ through NPS 4, both standards may be applicable — the choice between API 600 cast and API 602 forged construction is determined by the project specification’s valve material philosophy.
  • End connections: API 600 primarily addresses flanged and butt-welding end connections. API 602 additionally covers socket weld and threaded end connections — including the Class 800 pressure rating specific to small-bore socket weld and threaded valves — which are essential for instrument root and small-bore bypass applications.
  • Project specification boundary: Most engineering company project specifications define a clear size boundary between API 602 and API 600 application — for example, “API 602 forged steel for NPS ½ through NPS 1½; API 600 cast steel for NPS 2 and above” (or alternatively NPS 2 and below for API 602). This boundary should always be explicitly stated in the project valve selection philosophy document.

API 602 and ASME B16.34

ASME B16.34 provides the pressure-temperature rating framework within which API 602 forged steel valves are pressure-rated. When specifying an API 602 valve, the engineer must confirm that the valve’s body material group and pressure class, cross-referenced against the ASME B16.34 P-T rating tables at the design temperature, yield a maximum allowable working pressure (MAWP) that meets or exceeds the maximum process operating pressure. This verification step is the same for API 602 as for API 600 and any other valve governed by ASME pressure class ratings.

The Class 800 rating specific to API 602 socket weld and threaded end valves is not a standard ASME B16.34 flange class — it is an intermediate rating defined specifically for small-bore forged valves and corresponds to a pressure level between Class 600 and Class 1500. The P-T ratings for Class 800 valves are tabulated separately in API 602 and are consistent with the material group methodology of ASME B16.34. Engineers should use the API 602 P-T tables rather than ASME B16.34 flange class tables when selecting or verifying Class 800 socket weld forged valves.

API 602 in Engineering Selection and Procurement Practice

How to Reference API 602 in a Purchase Specification

A complete API 602 purchase specification clause must clearly identify the governing standard, valve type and size range, pressure class, body material, end connection type, and all supplementary testing and documentation requirements. As with all valve standards, vague or incomplete specification clauses create gaps that suppliers may interpret to their commercial advantage — typically by proposing lower-grade materials or less stringent test requirements than the project actually requires.

The following example illustrates a technically complete API 602 reference clause:

“Gate, globe, and check valves NPS ½ through NPS 1½ (or as specified on the valve datasheet) shall be compact forged steel valves designed, manufactured, and tested in accordance with API 602, latest edition. Body material shall be ASTM A105 (carbon steel forging) for standard service; ASTM A182 F316 for stainless steel service as noted on the valve datasheet. Pressure class and end connection type shall be as specified on the valve datasheet. Face-to-face dimensions for flanged end valves shall comply with ASME B16.10. Pressure-temperature ratings shall comply with ASME B16.34 for the applicable material group and pressure class at the design temperature. Factory testing shall be performed in accordance with API 602, with seat leakage acceptance criteria per API 598. Material Test Reports for all pressure-containing components shall be provided in accordance with EN 10204 Type 3.1.”

Procurement teams should additionally specify the required inspection level (manufacturer’s own inspection, third-party witness, or hold point), documentation package requirements, and any supplementary requirements for sour service (NACE MR0175/ISO 15156 hardness limits), cryogenic service (Charpy impact test requirements and extended bonnet specification), or fire-safe service (API 607 or API 6FA fire-safe test certification).

Design and QA Review Checklist

During engineering design review or vendor document review for API 602 forged steel valves, the following key items should be systematically verified:

  • API 602 edition referenced in the purchase order and manufacturer’s design documentation are consistent
  • Valve type (gate, globe, or check), size, pressure class, and end connection type match the datasheet requirements
  • Body material specification and ASTM grade confirmed as appropriate for the service conditions (temperature range, fluid compatibility, and sour service requirements where applicable)
  • ASME B16.34 P-T rating verified at design temperature for the stated material group and pressure class — or, for Class 800 valves, API 602 P-T tables used as the rating reference
  • Forged construction confirmed — manufacturer’s material test reports show ASTM forging specification (A105, A182, A350, etc.), not casting specification
  • Stem blowout prevention design confirmed in assembly drawing
  • Backseat design confirmed for gate and globe valve designs
  • End connection dimensions confirmed as compliant with ASME B16.11 (socket weld/threaded), ASME B16.25 (butt-weld), or ASME B16.5/B16.10 (flanged)
  • Factory test plan confirms shell test, backseat test, and seat leakage test with acceptance criteria referencing API 598
  • EN 10204 3.1 material test reports available for body, bonnet, and all other pressure-containing components, with heat number traceability confirmed
  • For sour service: hardness values on MTR confirmed within NACE MR0175/ISO 15156 limits for all pressure-containing and pressure-controlling components

Frequently Asked Questions

Does API 602 Require API 598 Testing?

Yes — API 602 explicitly references API 598 for the seat leakage test acceptance criteria applicable to gate, globe, and check valves manufactured under the standard. This means that specifying API 602 compliance automatically incorporates API 598 seat leakage requirements — the two standards work as an integrated design-and-test framework. The shell test and backseat test pressures and durations are defined within API 602 itself; the acceptable seat leakage rates by valve type and size are taken from the API 598 leakage tables. Purchase specifications should explicitly reference both standards — stating that factory testing shall be in accordance with API 602, with seat leakage acceptance criteria per API 598 — to make this relationship transparent to the manufacturer, the quality inspector, and the document reviewer.

Is There a Conflict Between API 602 and PED / CE Requirements?

There is no fundamental conflict between API 602 and the Pressure Equipment Directive (PED) 2014/68/EU — the two frameworks operate at different levels and are mutually compatible. API 602 governs the engineering design, material, and testing requirements of the valve as a product. The PED governs the regulatory compliance and market access requirements for pressure equipment placed on the EU market. A valve manufactured to API 602 can simultaneously comply with the PED, provided the manufacturer completes the required PED conformity assessment for the applicable risk category, issues a Declaration of Conformity, and affixes CE marking.

For small-bore forged steel valves in typical refinery service (Group 2 fluids, lower pressure), the PED risk category may be Category I or II — allowing relatively straightforward conformity assessment without extensive Notified Body involvement. For small-bore valves in Group 1 dangerous fluid service (flammable hydrocarbons, toxic or explosive fluids) at higher pressure classes (Class 1500, Class 2500), the PED risk category may be Category II or III, requiring Notified Body involvement in the conformity assessment. Engineers and procurement teams should determine the PED risk category for each valve type early in the project to confirm the required conformity assessment route and ensure the manufacturer can deliver CE-marked documentation alongside the API 602 compliance package.

Related Resources & Further Reading

Valve Standards Pillar Page

API 602 is one component of a comprehensive standards ecosystem that governs industrial valve engineering across all scales and service conditions. For a structured overview of all key valve standards — from API and ASME technical specifications through to European PED regulatory requirements and EN material certification standards — visit our Valve Standards pillar page. It provides a single reference hub with direct links to each standard’s dedicated cluster page, supporting efficient multi-standard specification development for valve engineers and procurement professionals.

Related Standard Pages

The following pages provide in-depth technical coverage of the standards most directly related to API 602 in industrial valve specification and procurement:

  • API 600 Cast Steel Gate Valves — The companion standard to API 602, covering bolted bonnet cast steel gate valves for NPS ½ through NPS 24. Essential for engineers establishing the API 600 vs. API 602 selection boundary and material philosophy in a project valve specification.
  • API 6D Pipeline Valve Standard — Understand the distinction between API 602 process plant forged steel valves and API 6D pipeline valves, and the service conditions that determine which standard applies.
  • API 598 Valve Testing — The factory acceptance testing standard referenced by API 602 for seat leakage acceptance criteria, covering test procedures, leakage classifications, and documentation requirements for all valve types.
  • ASME B16.34 Pressure-Temperature Ratings — The essential companion standard for P-T rating verification of API 602 forged steel valves, providing material group classification and allowable working pressure tables across the full temperature range.
  • ASME B16.10 Face-to-Face Dimensions — Dimensional interchangeability standard governing the face-to-face lengths of API 602 flanged-end valves, ensuring replacement compatibility within standard process plant piping spools.
  • PED 2014/68/EU European Pressure Equipment Directive — Regulatory compliance framework for API 602 forged steel valves supplied to EU-based projects, including risk category determination, CE marking obligations, and conformity assessment route selection.
  • EN 10204 3.1 Material Certificates — Material traceability documentation standard required for API 602 forged steel valves on European and PED-regulated projects, providing heat-specific chemical composition and mechanical property test data for all pressure-containing components.