API 598 Testing — Valve Inspection & Test Requirements Explained

API 598 Testing — Valve Inspection & Test Requirements Explained

Factory testing is the last line of defense before a valve leaves the manufacturing facility and enters service in a critical industrial pipeline or process system. API 598 is the industry’s most widely referenced standard for valve inspection and testing, providing a consistent, reproducible basis for verifying that every valve — regardless of manufacturer — meets defined criteria for structural integrity and sealing performance. This page provides a complete breakdown of API 598: what it covers, how each test is conducted, how leakage limits are defined, and how the standard integrates with other key specifications such as API 6D and ASME B16.34.

For a broader overview of all major valve standards, visit our Valve Standards pillar page.

What Is the API 598 Testing Standard?

API 598 Testing Standard Overview

API 598 — formally titled Valve Inspection and Testing — is published by the American Petroleum Institute (API) and establishes the minimum requirements for the inspection, examination, and pressure testing of industrial valves at the point of manufacture. Its primary purpose is to define a standardized factory acceptance testing (FAT) framework that gives purchasers confidence that a valve has been verified for pressure integrity and leak tightness before shipment.

The standard specifies the types of tests required, the test pressures to be applied, the acceptable leakage rates for different valve and seat configurations, the test durations, and the documentation that must accompany each tested valve. API 598 is applicable to gate, globe, check, ball, butterfly, and plug valves — making it one of the most broadly applicable valve testing references in the industry.

API 598 does not define valve design requirements; instead, it complements design standards such as API 6D (pipeline valves) and ASME B16.34 (process valves) by providing the detailed testing methodology that those standards reference or invoke. In many pipeline and process projects, API 598 is explicitly cited in the purchase order as the governing test standard, regardless of which design standard applies.

Which Valves Does API 598 Apply To?

Primary Applicable Valve Types

API 598 covers a wide range of valve types commonly used in oil and gas, petrochemical, power generation, and general process industries. The valve types addressed by the standard include:

  • Gate valves — including full-bore and reduced-bore designs in both cast and forged construction
  • Globe valves — for throttling and isolation service
  • Check valves — swing, piston, and tilting disc types
  • Ball valves — floating and trunnion-mounted designs
  • Butterfly valves — concentric and high-performance eccentric designs
  • Plug valves — lubricated and non-lubricated types

The standard applies irrespective of valve size or pressure class, though specific leakage rate allowances and test durations vary depending on the valve type, seat material, and nominal pipe size. For pipeline ball valves and gate valves manufactured under API 6D, API 598 testing requirements are typically incorporated by reference within the API 6D specification itself, confirming its role as a universal testing baseline.

When Is API 598 Testing Required?

API 598 testing is performed as part of the factory acceptance process before a valve is shipped to site. In practice, the trigger for API 598 testing is typically one of the following:

  • Project purchase specification: The buyer’s engineering specification or valve datasheet explicitly requires all valves to be tested in accordance with API 598, often listed as a mandatory quality hold point or witness point in the inspection and test plan (ITP).
  • Referenced by a design standard: Standards such as API 6D and API 600 directly invoke API 598 testing requirements, meaning compliance with the design standard automatically requires API 598 testing.
  • Manufacturer’s standard practice: Many valve manufacturers test all production valves to API 598 as part of their quality management system, even when not explicitly required by the customer.

When multiple standards apply simultaneously — for example, when a valve must comply with both API 6D and the European PED 2014/68/EU — the more stringent test requirement prevails for each test parameter. In such cases, the test certificate should reference all applicable standards to demonstrate dual compliance. Purchasers should explicitly state the priority order of applicable standards in the purchase specification to avoid ambiguity during vendor document review.

Core Testing Requirements Under API 598

Shell Test (Hydrostatic Body Test)

The shell test — also referred to as the hydrostatic body test or pressure test — is a mandatory test that verifies the structural integrity of the valve’s pressure-containing components: the body, bonnet, cover, and all associated pressure-boundary welds and castings. During the shell test, the valve body cavity is pressurized with water (or another approved test medium) at a defined test pressure while both end connections are blanked off and the closure element (gate, ball, or disc) is held in the partially open position.

Under API 598, the shell test pressure is typically set at 1.5 times the valve’s pressure-temperature rated working pressure at 38°C (100°F), consistent with the requirements of ASME B16.34. The test is held for a minimum duration that varies with valve size — ranging from 15 seconds for small-bore valves to 60 seconds or more for larger sizes — during which zero visible leakage through the body wall, bonnet joint, or gland is permitted. Any weeping, seeping, or external leakage constitutes a test failure, and the valve must be repaired, reworked, or rejected before the test can be repeated. All shell test results are recorded on the factory test certificate, which forms part of the valve’s permanent quality documentation package.

Seat Leakage Test

The seat leakage test — also called the seat test or closure test — verifies the sealing performance of the valve’s closure element against its seats when the valve is in the fully closed position. This is arguably the most commercially significant test in API 598, as it directly confirms whether the valve will provide the required shutoff performance in service.

API 598 defines two fundamentally different leakage acceptance criteria based on seat material type:

  • Soft-seated valves (PTFE, elastomer, or other resilient seat materials): Required to achieve zero measured leakage — commonly described as “bubble-tight” shutoff. This is the standard expectation for soft-seated ball valves and butterfly valves used in most process applications.
  • Metal-seated valves (gate, globe, and metal-seated ball valves): Permitted a defined maximum leakage rate expressed in cubic centimeters per minute (cc/min) or as a number of bubbles per minute, scaled by valve size. The allowable leakage increases with valve nominal bore, reflecting the practical manufacturing tolerances of metal-to-metal seating surfaces.

The seat leakage test is typically performed using water at a test pressure equal to 1.1 times the maximum rated working pressure at 38°C, or using air or nitrogen at low pressure (typically 5.5 to 8 bar) for pneumatic leakage tests where bubble counting is the acceptance method. The test is applied to each end of the valve sequentially, with the pressure applied upstream and leakage measured at the downstream end. Both directions may be tested for bidirectional valves. All measured leakage rates must fall within the API 598 tabulated limits for the valve type, size, and seat material to be considered acceptable.

Additional Inspection Requirements

Beyond the primary shell and seat tests, API 598 also specifies several supplementary inspection activities that must be completed as part of the overall factory acceptance process:

  • Backseat test: Applicable to gate and globe valves equipped with a backseat feature. The test verifies that the backseat provides an effective secondary stem seal when the valve is driven to the fully open position, allowing packing replacement under pressure. The backseat test is performed at the same pressure as the shell test.
  • Visual and dimensional inspection: Every valve must be visually inspected for surface defects, casting discontinuities, weld quality, and coating integrity. Dimensional checks confirm that the valve’s face-to-face length, end connection dimensions, and bore comply with the applicable standard (e.g., ASME B16.10 face-to-face dimensions).
  • Marking inspection: Verification that all required nameplate and body markings are present, legible, and correct — including pressure class, material designation, size, and applicable standard references.

API 598 Test Equipment & Execution Process

Factory Test Equipment Requirements

Performing API 598 tests correctly requires purpose-built test equipment that can safely and accurately apply the required test pressures and measure leakage rates within defined tolerances. Key equipment requirements include:

  • Hydrostatic test bench: A rigid fixture capable of supporting the valve, sealing both end connections with test blinds or flanged closures, and applying hydraulic pressure through a calibrated pumping system. The test bench must be rated for pressures exceeding the maximum test pressure for the valve class being tested.
  • Pressure measurement instruments: Calibrated pressure gauges or transducers with ranges appropriate to the test pressure, maintained within current calibration certification. Gauges used for shell and seat tests must have a full-scale range no greater than four times the test pressure to ensure measurement accuracy.
  • Test media: The standard test medium for hydrostatic testing is water — typically with a corrosion inhibitor added — at a temperature between 5°C and 52°C (41°F to 125°F). Pneumatic testing using air or nitrogen is permitted as an alternative for seat leakage tests, particularly for valves that cannot be fully dried after hydrostatic testing (e.g., cryogenic service valves). Pneumatic tests carry a higher safety risk due to the stored energy in compressed gas, and appropriate safety precautions — including pressure relief devices and exclusion zones — must be in place.
  • Leakage measurement equipment: For pneumatic seat tests, calibrated bubble counters or underwater submersion vessels are used. For hydrostatic seat tests, graduated collection vessels or flow meters measure the volume of leakage over the test duration.

API 598 Test Procedure Step-by-Step

A complete API 598 factory test sequence follows a structured process to ensure all required verifications are completed in the correct order and results are properly documented:

  1. Pre-test inspection: Visual examination of the valve for surface defects, correct assembly, and completeness. Verification of nameplate markings and material traceability documentation against the purchase order.
  2. Shell test: The valve is mounted in the test fixture with the closure element partially open. The body cavity is pressurized to the required shell test pressure and held for the specified minimum duration. The external surfaces are inspected for leakage. Results are recorded.
  3. Backseat test (if applicable): For gate and globe valves with a backseat feature, the valve is driven fully open and the shell test pressure is applied to verify backseat integrity.
  4. Seat leakage test — Direction 1: The valve is closed and pressure is applied from one end. Leakage at the downstream end is measured and compared against API 598 allowable limits for the valve type and size.
  5. Seat leakage test — Direction 2 (if bidirectional): The test is repeated from the opposite end for valves rated for bidirectional shutoff.
  6. Depressurization and disassembly (if required): Valves that fail any test are depressurized, inspected, reworked or repaired, and retested. The number of permissible retest attempts may be limited by the purchase specification.
  7. Post-test inspection and documentation: Following successful completion of all tests, the valve is drained, dried, and visually inspected. The test record — including test pressures, durations, measured leakage values, and pass/fail results — is compiled and signed by the manufacturer’s quality representative. A third-party inspector or purchaser’s representative may co-sign the test certificate where required by the ITP.

API 598 Compared with Other Standards

API 598 vs. API 6D (Pipeline Valve Standard)

API 598 and API 6D serve complementary but distinct roles in the pipeline valve qualification process. API 6D is a comprehensive design and manufacturing standard that defines what a pipeline valve must be — covering design requirements, materials, pressure ratings, dimensional requirements, and the types of tests that must be performed. API 598, by contrast, defines how those tests must be executed — specifying test pressures, leakage limits, test durations, and acceptance criteria in precise, measurable terms.

In practice, API 6D explicitly references API 598 as the basis for factory pressure testing, which means a valve claiming API 6D compliance must be tested in accordance with API 598 methodology. However, API 6D may impose additional or more stringent test requirements beyond the API 598 baseline — for example, requiring cavity pressure relief testing for DBB configurations or additional low-temperature testing for arctic service valves. Engineers reviewing valve test documentation should therefore verify compliance with both standards rather than treating them as interchangeable.

API 598 vs. ASME B16.34 & EN Standards

ASME B16.34 includes its own production testing requirements for valves manufactured under that standard, covering shell pressure tests and seat leakage tests. While the ASME B16.34 testing requirements are broadly consistent with API 598, there are differences in test pressure factors, leakage classifications, and acceptable test media that must be understood when specifying dual-standard compliance.

European standards — particularly those harmonized under the PED 2014/68/EU — may require additional conformity assessment activities beyond factory pressure testing, including design examination by a Notified Body and review of material certifications to EN 10204 3.1. When a project requires both API 598 compliance and PED CE marking, the test certificate must reference both frameworks, and the documentation package must satisfy the requirements of each independently. In all cases of conflicting requirements, the purchaser’s specification should clearly state which standard takes precedence for each test parameter.

API 598 in Procurement & Contract Specifications

How to Reference API 598 in a Purchase Specification

Correctly invoking API 598 in a valve purchase specification ensures that the manufacturer understands the testing obligations and that the resulting test documentation will satisfy project quality requirements. A well-written specification clause should identify the standard and edition, specify the test types required, define any witness or hold points, and state documentation requirements clearly.

The following example illustrates a typical API 598 reference clause in a valve procurement technical specification:

“All valves shall be inspected and pressure tested in accordance with API 598, latest edition, prior to shipment. Testing shall include shell (hydrostatic body) test, seat leakage test on both ends, and backseat test where applicable. Test pressures and acceptance criteria shall be as defined in API 598 Table 1 and Table 2 for the applicable valve type and pressure class. Test results shall be recorded on the manufacturer’s standard test record form and included in the final documentation package. The purchaser’s inspection representative shall be notified at least five (5) working days in advance of testing and shall have the option to witness all pressure tests as a Witness Point (W) in the Inspection and Test Plan.”

Procurement engineers should also specify the required edition of API 598 (citing the latest edition unless a specific edition is contractually frozen), the required format of the test certificate, the number of copies required, and whether third-party inspection by an approved inspection authority (such as Bureau Veritas, Lloyd’s Register, or TÜV) is required in addition to the manufacturer’s own quality control testing.

Frequently Asked Questions

Is API 598 Testing Mandatory?

API 598 testing is not universally mandatory by law — its applicability depends on the project specification, the governing design standard, and any applicable regulatory requirements. However, in practice, API 598 testing is effectively mandatory in the following situations: when the purchase specification explicitly requires it; when the valve is being supplied under a design standard such as API 6D or API 600 that references API 598; or when the valve is subject to third-party inspection under a quality plan that includes pressure testing as a mandatory hold point. For critical service applications — high-pressure, high-temperature, sour service, or safety-critical isolation — specifying API 598 testing is considered standard industry practice and is strongly recommended regardless of whether it is strictly required by contract. Omitting pressure testing documentation creates significant liability exposure for both the manufacturer and the end user.

Does API 598 Require Field Re-Testing After Installation?

API 598 is specifically a factory inspection and testing standard — it governs tests conducted at the manufacturing facility before the valve is shipped, and it does not address or require field testing after installation. Post-installation testing of valves is governed by separate codes such as ASME B31.3 (Process Piping), ASME B31.8 (Gas Transmission Pipelines), or owner-operator inspection procedures. That said, many project specifications require that valves be re-tested as part of the overall piping system pressure test (hydrostatic or pneumatic) after installation, which tests the valve in its as-installed condition as part of the complete pipeline system. These field tests are distinct from API 598 factory tests and are documented separately under the construction quality system.

Is There a Conflict Between API 598 and CE / PED Requirements?

There is no fundamental conflict between API 598 and the PED 2014/68/EU CE marking requirements, but they operate at different levels of the compliance framework. API 598 defines the technical testing methodology for pressure and leakage verification. The PED, by contrast, is a European regulatory directive that requires conformity assessment of the overall pressure equipment design and manufacturing process, resulting in CE marking. A valve can satisfy both requirements simultaneously: the manufacturer performs API 598 pressure tests to verify sealing and structural performance, while also completing the PED conformity assessment route (which may include design examination, material review per EN 10204 3.1, and quality system audit by a Notified Body) to obtain CE marking. Purchasers requiring both should specify this clearly in the valve datasheet and purchase order.

Related Resources & Further Reading

Valve Standards Pillar Page

API 598 is one component of a broader network of valve standards that govern design, materials, testing, and dimensions across the full spectrum of industrial valve applications. For a structured overview of all key standards — including API 6D, ASME B16.34, EN 10204 3.1, and PED 2014/68/EU — visit our Valve Standards pillar page. It provides direct links to each standard’s dedicated cluster page, making it a practical reference hub for engineers building multi-standard valve specifications.

The following related standard pages provide complementary technical detail that directly supports correct application of API 598: