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,API 598 standard covers inspection, examination, supplementary examinations, and pressure test requirements for resilient-seated, nonmetallic-seated (e.g. ceramic), and metal-to-metal-seated valves of the gate valve, globe valve, plug valve,ball valve, check valve, and butterfly types. 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.
1.1- Resilient seats are considered to be:
a) soft seats, both solid and semisolid grease type (e.g. lubricated plug);
b) combination soft and metal seats (e.g. laminated seat rings);
c) any other type of seat material designed to meet resilient seat leakage rates as specified in Table 5.
API Standard 598 supplements the API standards that reference it, but it may also be applied to other types of valves by agreement between the purchaser and the valve manufacturer. Reference Annex A for information to be specified by the purchaser.
The inspection requirements pertain to examinations and testing by the valve manufacturer and any supplementary examinations that the purchaser may require at the valve manufacturer’s plant. The test requirements cover both required and optional pressure tests at the valve manufacturer’s plant or at a facility mutually agreeable to both the manufacturer and the purchaser.
The following tests and examinations are specified in this standard:
a) shell test,
b) backseat test,
c) low-pressure closure test,
d) high-pressure closure test,
e) double block and bleed high-pressure closure test,
f) visual examination of castings,
g) high-pressure pneumatic shell test.
Key Tests in API 598 Standards
Shell Test:
This is the primary test for checking the structural integrity of a valve. It involves pressurizing the valve body to ensure it can withstand the intended operating pressure without leaking. The test is typically performed using air, inert gas, water, or other non-corrosive fluids. The goal is to verify that the shell is free from leaks and defects.
Backseat Test:
This test is designed to ensure the valve’s backseat provides a proper seal. It’s particularly important for valves with a stem seal that prevents leakage when the valve is in the closed position. The test checks the ability of the valve to maintain a leak-free seal at the backseat under specified pressure conditions.
Low-Pressure Closure Test:
This test checks the valve’s ability to close and hold a seal under low pressure. It’s essential for valves that operate in low-pressure environments or systems. The low-pressure closure test uses air or inert gas and ensures that the valve maintains its sealing capability during operation under these conditions.
High-Pressure Closure Test:
In contrast to the low-pressure closure test, the high-pressure closure test is performed to verify that the valve can hold a seal under high pressure. This test is crucial for valves operating in high-pressure systems, such as in oil and gas pipelines or chemical processing plants. Water or air is typically used for this test.
Double Block and Bleed Test:
This test is crucial for ensuring that the valve can effectively isolate sections of a pipeline while allowing for pressure release. It involves testing the valve’s ability to hold two seals simultaneously while ensuring that any trapped fluids between the seals can be safely bled off. This test is vital in preventing accidents and ensuring safe operation in critical systems.
Visual Examination:
A detailed visual examination is conducted to inspect the external and internal parts of the valve, checking for any visible defects or damage. This includes verifying that the materials used in the valve conform to specifications and that there are no signs of wear, cracks, or corrosion.
High-Pressure Pneumatic Shell Test:
In some cases, a high-pressure pneumatic test is performed instead of the standard shell test. This test checks the valve’s ability to withstand high pressure using compressed air or inert gas, often for valves used in high-risk applications where gas leakage could be a concern.
Testing Fluids in API 598 Standards
API 598 specifies a set of stringent requirements for the testing of valves to ensure their performance, reliability, and safety in high-pressure environments. One key aspect of these tests is the selection of appropriate testing fluids, which must align with the valve’s intended application while adhering to strict standards.
Types of Fluids Used for Testing
Shell Test Fluids:
For the shell test, which evaluates the strength and integrity of the valve’s pressure-containing structures, the recommended fluids include air, inert gas, kerosene, water, or other non-corrosive liquids. These fluids must have a viscosity no higher than that of water and be within a specified temperature range (5°C to 50°C or 41°F to 122°F). The shell test ensures that no leakage occurs from the valve body or fixed joints.
Backseat Test Fluids:
The backseat test checks for leakage through the stem or shaft seals. For both low-pressure and high-pressure backseat tests, air or inert gas is typically used. This test is essential for validating the sealing capabilities of the valve’s stem or shaft area.
Closure Test Fluids:
In closure tests, where the valve’s closure mechanism is tested for leakage, both low-pressure and high-pressure closure tests are required. Fluids such as air, inert gas, or non-corrosive liquids are used, depending on the test’s pressure requirements. For valves that are designed with non-metallic or ceramic seats, the allowable leakage rate during closure tests must adhere to the same standards as metal-seated valves of similar size.
Pressure and Duration Requirements In API 598
API 598 outlines the testing procedures for valves to ensure their performance and integrity under different conditions. The test pressures and durations required by the standard are critical for verifying valve reliability, particularly for the pressure containment and sealing capabilities of valves used in various industrial applications.
Test Pressures:
Shell Test:
Valves are subjected to a pressure higher than their rated working pressure to verify the structural integrity of the valve’s body. For example, steel and alloy valves undergo testing at 1.5 times the rated pressure at 38°C (100°F), while iron valves are tested at the listed pressures for shell tests.
| Valve Type | Shell Test Pressure (Minimum) | Test Fluid | Remarks |
| Flanged Valves | 1.5 times the rated pressure at 38°C (100°F), rounded to the next higher bar (25 psi) | Water, air, inert gas | Common for a wide range of valve sizes and pressure classes. |
| Butt Weld Valves | 1.5 times the rated pressure at 38°C (100°F), rounded to the next higher bar (25 psi) | Water, air, inert gas | Same standard as for flanged valves. |
| Threaded and Socket Weld Valves | 1.5 times the rated pressure at 38°C (100°F), rounded to the next higher bar (25 psi) | Water, air, inert gas | Limited to Class 2500 or lower per ASME B16.34. |
| Class 800 Valves | 1.5 times the rated pressure at 38°C (100°F), rounded to the next higher bar (25 psi) | Water, air, inert gas | Specific to Class 800 valves as per API Standard 602. |
| API Standard 609 Category A Valves | 1.5 times the maximum Cold Working Pressure (CWP) | Water, air, inert gas | Shell test pressure for valve categories specified by API 609. |
| High-Pressure Valves | 1.5 times the rated pressure at 38°C (100°F), rounded to the next higher bar (25 psi) | Water, air, inert gas | Used for valves in high-pressure applications, often exceeding 2500 psi ratings. |
Key Notes:
Backseat Test: This test ensures that no leakage occurs past the stem or shaft seals, which is critical to preventing pressure loss or contamination.
Low-Pressure Closure Test: Performed at the valve’s rated pressure to check for leakage at the seating surfaces under normal operating conditions.
High-Pressure Closure Test: A more rigorous test done at 1.5 times the rated pressure to verify that the valve can handle higher pressures without leaking.
Double Block and Bleed Closure Test: Used for double block valves to ensure that both seating surfaces are sealed and that any fluid in the cavity can be safely vented.
Visual Leakage Test: During all closure tests, no visible signs of leakage are permitted. Special methods like volumetric devices may be used for detection.
Test Duration:
| Valve Size (NPS) | Shell Test (seconds) | Backseat Test (seconds) | Closure Test (seconds) |
| ≤ 2 | 15 | 15 | 15 |
| 2.5 to 6 | 60 | 60 | 60 |
| 8 to 12 | 120 | 60 | 120 |
| ≥ 14 | 600 | 60 | 120 |
Common Issues Addressed by API 598 Testing
1. Valve Leakage
One of the primary issues API 598 tests address is leakage. Leakage can occur at various points, such as the valve seat or body. The test identifies and quantifies the leakage rate of valves, ensuring they meet acceptable standards. For valves with resilient seats, the standard typically requires “zero” leakage, meaning no visible drops or bubbles of fluid should escape during the test. However, in practice, some level of leakage is often acceptable, particularly for metal-seated valves. The test pressure and duration play a significant role in detecting these issues, helping identify even minute leaks that could compromise performance.
2. Seal Integrity
API 598 tests also focus on ensuring that seals, such as the valve body and seat seals, remain intact under the specified pressure conditions. A compromised seal could lead to valve failure, affecting the flow control and safety of the system. By subjecting valves to hydrostatic or gas tests, API 598 verifies that the seals are functioning as expected and can withstand the pressures in operational conditions without failure.
3. Pressure Endurance
Valves are subjected to pressure endurance testing to confirm that they can withstand the pressures for which they are rated. This includes shell and closure tests, where valves must hold the test pressure without deforming or leaking over a specified period. The test duration is critical because it simulates real-world conditions, ensuring that valves will not fail when exposed to long-term operational stresses.
4. Valve Body and Structural Integrity
During testing, valve bodies are inspected for potential defects like cracks, weld failures, or material inconsistencies that could lead to a structural failure during service. These structural issues are often not visually obvious and can only be detected through rigorous testing under pressure. API 598 provides detailed requirements for testing under both liquid and gas conditions, ensuring that the body remains intact and functional.
5. Functional Performance
Beyond basic pressure and leakage tests, API 598 evaluates the functional performance of valves, particularly their ability to seal properly at different operational pressures. This includes tests like the back-seat test and closure test, which confirm that the valve functions as intended under both high- and low-pressure conditions. This is particularly important for valves used in high-stakes applications like oil and gas processing, where failure could lead to significant operational hazards.
API 598 Standards Allowable Leakage Rates
API 598 defines leakage rates for valves in terms of two primary tests: the liquid test (usually water or other fluids) and the gas test (typically air or gas). The allowable leakage is specified for different valve sizes (NPS) and is given in two units: milliliters per minute (ml/min) for liquid tests, and bubbles per minute (bub/min) for gas tests.
Table for Valve Leakage (Liquid and Gas)
Maximum Allowable Leakage Rates for Closure Tests c
| Valve Size (NPS) | Liquid Leakage (drops/min) | Liquid Leakage (ml/min) | Gas Leakage (bubbles/min) | Gas Leakage (ml/min) |
| 2 | 0 | 0 | 0 | 0 |
| 2.5 | 5 | 0.31 | 10 | 0.1 |
| 3 | 6 | 0.38 | 12 | 0.12 |
| 4 | 8 | 0.5 | 16 | 0.16 |
| 5 | 10 | 0.63 | 20 | 0.2 |
| 6 | 12 | 0.75 | 24 | 0.24 |
| 8 | 16 | 1 | 32 | 0.32 |
| 10 | 20 | 1.25 | 40 | 0.4 |
| 12 | 24 | 1.5 | 48 | 0.48 |
| 14 | 28 | 1.75 | 56 | 0.56 |
| 16 | 32 | 2 | 64 | 0.64 |
| 18 | 36 | 2.25 | 72 | 0.72 |
| 20 | 40 | 2.5 | 80 | 0.8 |
| 24 | 48 | 3 | 96 | 0.96 |
| 26 | 52 | 3.25 | 104 | 1.04 |
| 28 | 56 | 3.5 | 112 | 1.12 |
| 30 | 60 | 3.75 | 120 | 1.2 |
| 32 | 64 | 4 | 128 | 1.28 |
| 36 | 72 | 4.5 | 144 | 1.44 |
| 40 | 80 | 5 | 160 | 1.6 |
| 42 | 84 | 5.25 | 168 | 1.68 |
| 48 | 96 | 6 | 192 | 1.92 |
| 52 | 104 | 6.5 | 208 | 2.08 |
| 56 | 112 | 7 | 224 | 2.24 |
| 60 | 120 | 7.5 | 240 | 2.4 |
| 64 | 128 | 8 | 256 | 2.56 |
| 68 | 136 | 8.5 | 272 | 2.72 |
| 72 | 144 | 9 | 288 | 2.88 |
| 76 | 152 | 9.5 | 304 | 3.04 |
| 80 | 160 | 10 | 320 | 3.2 |
Key Notes:
Resilient-seated valves: These valves must not show any visible leakage through the seating surface. The leakage volume can be measured in milliliters per minute or by drop count.
Gas test: A result of “0 bubbles” means that less than one bubble is observed during the minimum specified test duration, signifying negligible leakage.
Liquid test: “0 drops” means no visible leakage is seen over the test duration.
Conversions: 1 milliliter is equivalent to approximately 16 drops, and leakage volume in milliliters can be converted to drops if needed.
Maximum Allowable Leakage Rates for Closure Tests c
| Valve Size | All Resilient | Metal Seated Valves Except Check | Metal Seated Check Valves | ||||
| DN (mm) | NPS (inch) | seated Valves | Liquid Test a (drops/min) | Gas Test a (bubbles/min) | Liquid Test (cc/min) | Gas Test (m3/h) | Gas Test (ft3/h) |
| ≤ 50 | ≤ 2 | 0 | 0b | 0b | 6 | 0.08 | 3 |
| 65 | 2002-1-2 | 0 | 5 | 10 | 7.5 | 0.11 | 3.75 |
| 80 | 3 | 0 | 6 | 12 | 9 | 0.13 | 4.5 |
| 100 | 4 | 0 | 8 | 16 | 12 | 0.17 | 6 |
| 125 | 5 | 0 | 10 | 20 | 15 | 0.21 | 7.5 |
| 150 | 6 | 0 | 12 | 24 | 18 | 0.25 | 9 |
| 200 | 8 | 0 | 16 | 32 | 24 | 0.34 | 12 |
| 250 | 10 | 0 | 20 | 40 | 30 | 0.42 | 15 |
| 300 | 12 | 0 | 24 | 48 | 36 | 0.5 | 18 |
| 350 | 14 | 0 | 28 | 56 | 42 | 0.59 | 21 |
| 400 | 16 | 0 | 32 | 64 | 48 | 0.67 | 24 |
| 450 | 18 | 0 | 36 | 72 | 54 | 0.76 | 27 |
| 500 | 20 | 0 | 40 | 80 | 60 | 0.84 | 30 |
| 600 | 24 | 0 | 48 | 96 | 72 | 1.01 | 36 |
| 650 | 26 | 0 | 52 | 104 | 78 | 1.09 | 39 |
| 700 | 28 | 0 | 56 | 112 | 84 | 1.18 | 42 |
| 750 | 30 | 0 | 60 | 120 | 90 | 1.26 | 45 |
| 800 | 32 | 0 | 64 | 128 | 96 | 1.34 | 48 |
| 900 | 36 | 0 | 72 | 144 | 108 | 1.51 | 54 |
| 1000 | 40 | 0 | 80 | 160 | 120 | 1.68 | 60 |
| 1050 | 42 | 0 | 84 | 168 | 126 | 1.76 | 63 |
| 1200 | 48 | 0 | 96 | 192 | 144 | 2.02 | 72 |
Note:
a - For the liquid test, 1 mL is considered equivalent to 16 drops. For the gas test 1 mL is considered equivalent to 100 bubbles.
b - There shall be no leakage for the minimum specified test duration (see Table 4). For liquid test, 0 drops means no visible leakage per minimum specified test duration. For standard gas test, 0 bubbles means less than 1 bubble per minimum specified test duration. For high-pressure pneumatic closure test refer to paragraph 5.4.
c - Leakage rates for sizes above DN 1200 (NPS 48) shall be calculated by the following formulas: Liquid Test for Metal Seated Valves except Check: 2 x NPS (drops/min) Gas Test for Metal Seated Valves except Check: 4 x NPS (bubbles/min) Liquid Test for Metal Seated Check Valves: 3 x NPS (cc/min) Gas Test for Metal Seated Check Valves: 0.042 x NPS (m3/h) Gas Test for Metal Seated Check Valves: 1.5 x NPS (ft3/h)
Refer to ISO 5208 leakage rate.
Shell Test Pressures
| Valve Type | Class | Shell Test Pressure (Minimum) | |
| (Steel and nonferrous alloys) | Bar Gauge | Pounds per Square Inch | |
| Flanged | 150 to 2500 | b | b |
| Butt weld | 150 to 4500 | b | b |
| Threaded and socket weld | 800 | c | c |
| 150 to 4500 | b | b | |
Note:
a - ASME B16.34 limits threaded-end valves to Class 2500 and lower.
b - Per ASME B16.34, the shell test pressure shall be 11/2 times the pressure rating at 38 °C (100 °F),rounded off to the next higher bar (25 psig). The attachment of hubs, flanges, or other end connections with ambient working pressures lower than the primary valve assembly will require lower test pressures.
c - For Class 800 valves, the shell test pressure shall be 11/2 times the pressure rating at 38 °C (100 °F), rounded off to the next higher bar (25 psig) (see API Standard 602[4]).
d - Shell test pressure for API Standard 609 Category A valves shall be 11/2 times the maximum CWP of the valve.
Backseat and Closure Test Pressures
| Test | Test Pressure | |
| Bar Gauge | Pounds per Square Inch Gauge (psig) | |
| Valves Except Butterfly and Check | ||
| High-pressure closure and backseat a | b | b |
| Low-pressure closure and backseat a | 5.5 ± 1.5 | 80 ± 20 |
| Butterfly Valve | ||
| High-pressure closure | c | c |
| Low-pressure closure | 5.5 ± 1.5 | 80 ± 20 |
| Check Valve | ||
| High-pressure closure | ||
| Carbon, alloy, stainless steel, and special alloys | b | b |
| Low-pressure closure (see Table 1) | 5.5 ± 1.5 | 80 ± 20 |
Note:
a - The backseat test is required for all valves that have the backseat feature, except for bellows seal valves.
b - 110 % of maximum allowable pressure at 38 °C (100 °F) in accordance with the applicable purchase specification.
C - 110 % of design differential pressure at 38 °C (100 °F) in accordance with the applicable purchase specification.
d - Single values shown are minimum test pressures. Values with a tolerance indicate both minimum and maximum test pressures.
Duration of Required Test Pressure
| Valve Size | Minimum Test Duration (Seconds) a | ||||
| DN | NPS | Shell | Backseat (for Valves with Backseat Feature) | Closure Check Valves (API 594[1]) | Closure Other Valves |
| ≤ 50 | ≤ 2 | 15 | 15 | 60 | 15 |
| 65 to 150 | 21/2 to 6 | 60 | 60 | 60 | 60 |
| 200 to 300 | 8 to 12 | 120 | 60 | 120 | 120 |
| ≥350 | ≥ 14 | 300 | 60 | 120 | 120 |
Note: a - The test duration is the period of inspection after the valve is fully prepared and is under full pressure.
Key Benefits of Adhering to API 598 Standards
Adhering to API 598 standards for valve inspection and testing provides numerous benefits across various industries, particularly in critical sectors like oil and gas. This standard sets clear, rigorous guidelines for ensuring the integrity, safety, and reliability of valves used in high-pressure and hazardous environments. Below are the core advantages:
Enhanced Safety and Reliability
The API 598 standard ensures that valves meet strict testing procedures, including shell tests, backseat tests, and closure tests, which help prevent valve failures in operation. By confirming that valves can withstand high pressures and seal effectively under different conditions, the standard minimizes the risk of leaks or catastrophic failures, thus enhancing the safety of both personnel and the environment.
Improved Quality Control
API 598 emphasizes visual inspections, pressure testing, and certification processes, ensuring that valves meet the necessary standards before being put into service. This rigorous testing and inspection process helps guarantee that valves are built to last and function correctly throughout their operational life, reducing the risk of performance failures and costly repairs.
Consistency Across the Industry
API 598 provides a uniform set of testing protocols and standards that valve manufacturers and users can rely on globally. This standardization helps ensure that valves are compatible across different systems and suppliers, facilitating smoother operations and integration across multiple industries, including oil, gas, water, and petrochemical sectors.
Long-Term Cost Savings
By reducing the likelihood of valve failure through comprehensive testing, adhering to API 598 standards can lead to significant cost savings over time. Companies can avoid the financial burden associated with unscheduled downtimes, emergency repairs, and replacements that result from substandard valve performance.
Compliance with Regulatory Requirements
Many industries, particularly those related to oil and gas, have stringent safety and environmental regulations. API 598 compliance ensures that valves meet these regulations, helping businesses avoid legal and financial penalties. Additionally, the certification associated with API 598 offers documentation that can be used to prove compliance during audits or inspections.
Optimized Performance
The testing procedures outlined in API 598—such as the verification of sealing capabilities at both high and low pressures—ensure that valves function optimally under the conditions they will face in real-world operations. This leads to better flow control, pressure regulation, and overall system performance, ensuring operational efficiency.
