In the valve industry, ensuring that every valve meets strict operational standards is critical for safety, efficiency, and reliability. This is where API 598 comes into play. As a key standard developed by the American Petroleum Institute (API), API 598 governs the testing and inspection procedures for valves used in various industries, from oil and gas to water treatment and manufacturing. This guide explores the significance of API 598, its role in the valve industry, and its impact on valve performance, quality assurance, and regulatory compliance.

What is API 598 Standards?

API 598 is a standard developed by the American Petroleum Institute (API) that outlines the procedures for the inspection and testing of valves, particularly for pressure-containing components. The standard covers a wide range of valve types, including gate, globe, ball, and check valves, among others, and provides detailed instructions for ensuring that these valves meet the operational and safety requirements of various industrial applications.

The primary focus of API 598 is to specify the testing protocols that determine a valve’s ability to function under pressure and seal effectively. These tests, which include pressure tests, leakage tests, and visual inspections, are crucial for verifying that valves meet their design specifications and function correctly in their respective systems.

Valve Types Covered by API 598

Gate Valves

Gate valves are widely used to start or stop the flow of liquids and gases. API 598 specifies rigorous testing to verify the valve’s ability to prevent leakage under various pressure conditions. These valves typically undergo shell tests, backseat tests, and closure tests to ensure their integrity and functionality.

Globe Valves

Globe valves, known for their precise flow control, are subject to similar testing procedures as gate valves. The standard ensures that these valves perform effectively under high and low-pressure conditions and provides guidelines for closure and backseat tests to guarantee a leak-proof seal.

Ball Valves

Ball valves are often used for quick shut-off in pipelines. API 598 outlines tests to check their sealing capabilities, especially in floating ball and trunnion-mounted designs. The valve must pass both shell and closure tests, with particular attention paid to leakage rates during high and low-pressure conditions.

Check Valves

Designed to allow flow in only one direction, check valves are crucial for preventing backflow. The API 598 standard details both shell and closure tests, emphasizing the unique leakage criteria for check valves due to their passive sealing mechanisms. Special focus is placed on verifying the valve’s ability to withstand high-pressure and low-pressure conditions.

Plug Valves

Plug valves, often used in throttling applications, are tested according to API 598 for leakage at both high and low pressures. These valves are subject to shell tests and specific closure tests to verify their performance in regulating the flow of fluids.

Butterfly Valves

Butterfly valves, used for flow regulation in large diameter pipes, must undergo rigorous shell tests and closure tests. API 598 ensures that these valves meet the necessary standards for pressure containment and leakage control across their entire range of operation.

Resilient Seated Valves

These valves, often incorporating soft or composite seating materials, are also covered by API 598. The standard specifies tests such as shell and closure tests to ensure that these valves meet the performance criteria, especially in applications where soft sealing is necessary.

Mandatory Tests Under API 598

When a valve is manufactured under API 598, several tests are mandatory to ensure structural integrity and sealing performance. These tests help guarantee safety, reliability, and consistency across batches.

Visual Examination Requirements and Typical Checkpoints

Before any pressure testing begins, a thorough visual examination of all pressure-containing parts is required. According to the standard:

You should check:

• The valve body, bonnet, covers, and closure elements for casting defects, cracks or porosity.

• That all markings and nameplates are correct (e.g. correct material, pressure rating, size, manufacturer) and clearly legible.

• Dimensional accuracy and conformity to design drawings or manufacturing specifications.

• Surface condition, especially coating or lining integrity where applicable.

Only after a valve passes visual inspection may pressure tests commence.

Shell Strength Test: Objective, Setup and Failure Modes

The shell test (sometimes called the body or shell-strength test) is the foundational pressure test under API 598. Its primary objective is to verify that the valve’s body, bonnet, and fixed joints can withstand pressure without structural failure or leakage.

Setup and parameters

• The valve is typically tested with the closure in the open position.

• For cast-steel valves (commonly used in industry), the test pressure is at least 1.5 times the pressure rating at 38 °C (100 °F), rounded up as per the relevant pressure standard.

• The test fluid may be water, inert gas, air, or other non-corrosive liquids with appropriate viscosity.

• Test duration depends on valve size: smaller valves might be tested for shorter periods (e.g. 15 seconds), while larger valves require longer hold times.

What constitutes failure

• Any visible leakage (drops, wetting, external seepage) from body, bonnet, joints or castings is unacceptable.

• Permanent deformation, cracking or structural damage during pressurisation also fails the test.

Passing the shell test confirms the pressure-boundary integrity of the valve body, before assessing sealing performance.

Backseat Test (Where Applicable) and Stem Leakage Control

For valves that include a backseat feature (mainly rising-stem valves), a backseat test is mandatory under API 598. This verifies that the stem packing or gland does not leak under pressure.

Procedure highlights

• Valve is mounted with ends closed and the closure fully open; stem packing gland is loosened (or packing may be removed) to expose potential leakage paths.

• A hydrostatic pressure (often at or above 110 % of the maximum allowable working pressure at 38 °C) is applied.

• Test duration often matches that of the shell test for the same size range.

Pass / fail criteria

• No visible leakage from the packing gland, stem, or bonnet — liquid drops are forbidden; if gas is used, no bubbles should appear using the chosen detection method.

• The test confirms the integrity of the stem seal, which is vital for long-term leak prevention during service.

Closure/Seat Tests: Low-Pressure and High-Pressure Versions

Once body integrity and stem sealing are confirmed, the next step is testing the valve’s ability to seal the flow path. API 598 requires closure or seat tests — in low-pressure and (where relevant) high-pressure versions.

Low-Pressure Closure Test

• Typically uses air or inert gas as test medium.

• Pressure is around a defined low-pressure value (for example around 80 psig / 5.5 bar gauge ± tolerances, depending on valve size and rating) for gas tests.

• Focus is detecting any leakage past the seat under normal or moderate conditions.

High-Pressure Closure Test

• Usually performed with liquid (water or non-corrosive fluid) at 1.1 times the rated pressure (at 38 °C) — this ensures the valve can handle or contain pressure above normal operating levels. 

• Duration depends on valve size; for many valves, a typical hold time is 60 seconds for smaller/medium sizes.

Acceptance criteria for seat leakage

• For liquid tests: no visible droplets, wetting or seepage from seat interface or downstream passage.

• For gas tests: zero bubbles per minute (or below a minimal threshold defined by API 598) — particularly strict for resilient (soft) seats, which often require bubble-tight performance.

• Plastic deformation of resilient seats or seals is generally acceptable, but structural damage to sealing surfaces, body or disc is not.

These tests together ensure that the valve both holds pressure safely and closes reliably under service conditions.

Test Media, Pressures and Durations

When executing valve tests under API 598, choice of test medium (liquid or gas), the applied pressure, and the duration of the test are all critical. These factors determine whether the valve will pass or fail inspection.

Liquid Test vs Gas Test: When Each is Applied

• Liquid (hydrostatic) tests are the default for strength (shell) tests and high-pressure seat/closure tests. Water or other non-corrosive fluids of similar viscosity are commonly used.

• Gas (pneumatic) tests — often using air or inert gas — are typically applied for low-pressure seat/closure checks or, when needed, high-pressure pneumatic shell tests (if specified in the purchase order).

• The choice depends on the valve design, seat material (metal vs soft/seated), client requirements, and safety considerations (gas tests require careful handling).

In many cases, a comprehensive test sequence may include both hydrostatic and pneumatic tests to ensure structural integrity and seat sealing.

Typical Test Pressures for Different Pressure Classes and Materials

• For valves made of ductile iron, cast iron, steel or non-ferrous alloys, the shell (body) test pressure generally follows a rule of 1.5 × rated pressure at 38 °C (100 °F). For steel valves, the rated pressure is often derived per the relevant product standard (e.g. the corresponding ASME class).

• Pneumatic (high-pressure) shell tests, if required, are often set at 110% of the maximum allowable working pressure (MAWP) at 38 °C, provided appropriate safety precautions are observed.

• For seat (closure) tests:

  • High-pressure closure tests (liquid) are typically conducted at 1.1 × rated pressure.

  • Low-pressure closure tests (gas) are often done at a relatively modest pressure, commonly in the range of ~ 6–8 bar (approx. 80–100 psi) gauge, though actual pressure depends on valve size and specified requirements. 

This approach ensures the valve is tested both under realistic operating (or slightly above) pressures and under more conservative, low-stress conditions — helping detect different kinds of leak paths or seat failures.

Minimum Holding Times and Stabilisation Requirements

Once the valve is pressurised, API 598 specifies minimum durations during which the pressure must be held and the valve inspected.

Before starting a liquid (hydrostatic) test, the valve must be fully filled with test fluid and any trapped air vented — this stabilisation ensures accurate detection of leaks.

For closure tests using gas and volumetric leak detection methods (e.g. bubbler tubes), the measuring system must be properly stabilised before the actual timed test begins, so that bubble counts represent steady-state leakage and not transient effects.

API 598 Allowable Leakage Rates and Acceptance Criteria

When a valve is tested under API 598, the acceptance criteria depend on whether the valve has a soft (resilient) seat or a metal seat. The standard defines limits — typically in drops per minute (for liquid tests) or bubbles per minute (for gas tests) — and requires “no visible leakage” in many cases.

Soft-seated vs Metal-seated valves: Leakage Criteria

Soft-seated valves (e.g. those using elastomeric/PTFE seats)

• For resilient-seated valves, API 598 requires zero visible leakage during the test. That means no drops (in liquid) or bubbles (in gas) for the minimum test duration.

• This “bubble-tight” performance is often required where tight shut-off is essential, such as gas, chemical, or critical fluid services.

Metal-seated valves (metal-to-metal seats, sometimes with soft-seat option where seat ring is non-metallic), e.g. gate, globe, plug, check and butterfly valves with metal seats

• The standard allows a small, defined leakage rate rather than absolute zero. Acceptable rates vary based on valve size (nominal pipe size, NPS).

• For shell and backseat tests (which check body integrity and sealing around stem/seals), no visible leakage is permitted. But for seat (closure) tests, limited leakage may be tolerated under defined limits.

Example Leakage Allowances by Valve Size

Here is a simplified illustration of typical maximum allowances under API 598 for seat (closure) tests. Actual criteria may depend on manufacturer or purchaser agreement.

Because metal-seated valves have differing acceptable rates depending on size and application, purchasers often need to specify clearly what leakage limit is acceptable for a given project.

Why Choose Tanggong Valve for API 598 Compliant Projects

Selecting the right valve supplier is as important as selecting the right valve. Buyers want confidence that every product has been tested thoroughly, inspected properly, and backed by proven performance. Our manufacturing and testing setup is designed to deliver that reassurance.

Overview of Our In-house Test Facilities and Ranges

We manage every stage of pressure testing in-house, which gives us tighter control over quality and faster delivery times. Our facilities cover the full spectrum of API 598 testing, from small-bore valves to large industrial units.

Our test capability includes:

• Hydrostatic and pneumatic benches covering a wide pressure range for all API 598 shell, backseat and closure tests.

• Dedicated fixtures for gate, globe, check, ball and butterfly valves to ensure accurate and repeatable test conditions.

• Calibrated instrumentation for pressure, flow, and leak detection, updated in line with international metrology requirements.

• Test media compatibility for water, inert gases and customer-specified fluids when project documentation calls for it.

Because all testing takes place within our own facility, we maintain complete traceability, from raw material certification through final inspection. This forms part of our long-term quality records, which are available to clients upon request.

Typical Industries, Projects and Approvals Where Our Valves Have Passed API 598 Tests

Our valves are used in a wide range of industries that depend on consistent sealing performance and reliable pressure containment. API 598 compliance is often a baseline requirement for these sectors, particularly where downtime or leakage carries high operational or safety risks.

You will typically find our API 598-tested valves operating in:

• Oil and gas production and refining

• Petrochemical and chemical processing

• Power generation, including steam and combined-cycle plants

• Water treatment and desalination

• Pulp and paper

• District heating and general industrial utilities

Many projects within these industries require strict inspection regimes, including client witnessing, third-party verification or additional test documentation. Our valves have been accepted on projects with these enhanced requirements, supported by:

• Complete inspection and test plans (ITPs) aligned with API 598

• Third-party witnessing from globally recognised agencies when requested

• Detailed material and pressure test certification for project handover

Maintaining reliable performance across such a wide range of applications demonstrates not just compliance, but consistency. Our goal is to give engineers and procurement teams confidence that every valve they receive has met the same rigorous standard.

Conclusion

API 598 is an indispensable standard in the valve industry, ensuring that valves meet the necessary requirements for pressure containment, leakage prevention, and overall performance. Its role in maintaining safety, operational efficiency, and regulatory compliance cannot be overstated. For professionals in the valve manufacturing and operations sectors, adhering to API 598 is essential for producing high-quality, reliable valves that meet global standards and mitigate risks.

TangGong Valve Group guarantees that your valves meet API 598 standards for optimal performance, safety, and leak-tightness—contact us today to discover how our expert testing services can enhance your valve reliability and compliance.