API 6D vs API 598
The valve industry is an integral part of many industrial sectors, from oil and gas to chemical processing and water treatment. Valves are critical components used to control the flow of liquids and gases in various systems, ensuring efficiency, safety, and performance. One of the most important factors in producing high-quality, reliable valves is adhering to industry standards. Two key standards in the valve manufacturing process are API 6D and API 598, set by the American Petroleum Institute (API). These standards ensure that valves meet specific requirements for safety, functionality, and quality, and they play a significant role in enhancing operational performance. API 6D is the American Petroleum Institute’s specification for pipeline and pipeline valves in the petroleum and natural gas industries. It contains requirements for the design, manufacture, assembly, documentation, and testing of ball, gate, plug, and check valves for pressure ratings up to ASME class 2500. In this article, we will review what constitutes an API 6D ball, gate, plug, and check valve, design considerations, as well as API 6D vs API 598.
In this blog, we’ll provide an overview of these two important API standards, explain their significance in the valve industry, and explore how they impact valve manufacturing and testing. By the end of this article, you’ll gain a deeper understanding of API 6D vs API 598, and why following these standards is crucial for the production of high-quality valves.
What is API 6D?
API 6D is a standard developed by the American Petroleum Institute that applies specifically to pipeline valves used in the oil and gas industry. This standard establishes requirements for the design, materials, manufacturing, testing, and operation of valves used in pipeline systems. The goal of API 6D is to ensure that these valves perform reliably under the extreme conditions often found in the pipeline sector.
Key Features of API 6D:
Design Requirements: API 6D specifies the design parameters for valves, ensuring they meet the necessary structural and mechanical properties for pipeline use. This includes considerations for valve body strength, pressure rating, and operational capacity.
Materials Specifications: The standard outlines acceptable materials for valve construction, ensuring valves are durable and resistant to corrosion, abrasion, and other forms of wear that can occur in pipeline systems.
Testing Protocols: API 6D also defines the testing methods required to verify the functionality and safety of pipeline valves, including pressure testing and sealing tests.
Performance Standards: Valves must meet stringent performance criteria, including tight shutoff, leak resistance, and the ability to withstand high-pressure conditions.
By adhering to API 6D, manufacturers ensure their valves are capable of withstanding the harsh environments and high pressures typically encountered in pipeline systems, providing peace of mind for operators and reducing the risk of valve failures.
API 6D Ball Valve
API 6D ball valves are ideal for shutoff applications in the petrochemical, power, and oil & gas industries. Their design delivers high performance and reliability over extended periods. For API 6D, ball valves are available for only the following pressure ratings: class 150, class 300, class 600, class 900, class 1500, and class 2500.
Sizes range from NPS 2 to 36 inches or DN 50 to 900 mm in both full bore and reduced bore. The standard specifies that the obturator of the valve shall rotate on an axis perpendicular to the direction of flow. Also, the wrench and/or the position indicator shall be either in line or transverse with the pipeline when the valve is open and closed respectively. Importantly, the design of this indicator shall not permit assembly in a way that it falsely interprets the valve position. Typical configurations of ball valves from the API 6D are top-entry, three-piece, and welded body ball valves.
API 6D Check Valve
API 6D check valves are full port swing check valves that primarily serve in industrial facilities to prevent backflow. Also, their closure makes them easy to troubleshoot and maintain, with top-entry access to all valve internals. When fully open, the closure mechanism must fully clear the path of flow. There are different types including wafer, axial flow, and lift type either with full or reduced-opening configuration. API 6D check valves are available for the same pressure ratings as the ball valves, but their size range is smaller, spanning from NPS 2 to 24 inches or DN 50 to 600 mm.
The standard recommends check valves as a secondary means of isolation at injection points for sealant, lubrication or flushing. When used in this capacity, they shall be rated higher than the pipeline and injection pressure. Any locking position of check valves shall be in the open position only and the position of its obturator should not be influenced by the flow’s dynamic forces.
Also, API 6D check valves are to have a clear marking of the flow direction on its body. The supplier should ensure the disc is secure during shipping. Prior to pigging, operators must ensure the valve can accept such an operation.
API 6D Gate Valve
Generally, API 6D gate valves come as a single-piece or multi-piece construction for slab-gate valves and expanding-gate valves respectively. In addition to the primary stem sealing, they are to come with a secondary sealing feature in the form of a back seat.
API 6D specifies that the design of gate valves shall be such that forces generated from internal pressure shall not alter the position of the obturator. The standard also recommends shipping of reverse-acting through-conduit gate valves in the open position, unless fitted with a fail-to-close actuator. As for other types of gate valves, shipping shall be in the full-closed position.
API 6D Plug Valve
According to API 6D, plug valves shall have a conical or cylindrical obturator that rotates on an axis perpendicular to the flow direction. It is available in the usual size range of NPS 2 to NPS 36, but most larger sizes are limited to the venturi configuration.
For plug valves, the standard specifies that the wrench or position indicator shall be in line with the pipeline when the valve is open. But when the valve is closed, it shall be transverse. In addition, its design shall be such that the components of the indicator/wrench cannot be assembled to falsely indicate the valve position. Valves without position stops shall make provision for the verification of open and closed alignment with the actuator removed. During hydrostatic and low-pressure gas seat tests, leakage for lubricated plug valves shall not exceed ISO 5208 Rate A (no visible leakage).
API 6D Valve Design
Beyond specifications for individual valves, API 6D has general recommendations covering various aspects of valve design. Some of them include lifting, allowable stress and deflections, as well as material specifications.
Lifting of Valves
The standard specifies that valves of NPS 8 or larger shall have lifting points. It is the responsibility of the manufacturer to verify the suitability of these lifting points for the complete valve and operator assembly. But, if the purchaser is responsible for the supply of the operator assembly, then the purchaser shall provide adequate information to enable the manufacturer to verify the suitability.
Allowable Stress and Deflection
To minimize the risk of failure, API 6D recommends allowable stresses and deflections for the drive train components of valves. Some of them are as follows:
The design of the drivetrain shall place its weakest component outside the pressure boundary.
When delivering the design thrust or torque, the tensile stresses in drivetrain components and stem extensions shall not exceed 67% of SMYS. Also, shear, torsion, and bearing stresses shall not exceed limits specified in ASME Code Section VIII, Division 2, Part AD-132. These limits do not include proprietary bearings or other components with high bearing strength capacity.
A strength efficiency factor of 0.75 shall be used for fillet welds.
Deflections of the drivetrain shall not prevent the obturator from reaching the fully open or fully closed position.
The manufacturer shall demonstrate by testing or calculation that external loads or loads from design pressure do not impair sealing or functionality. Because allowable stress and deflection limits of design codes alone might not result in valve functionality.
Materials
Some of the specifications of API 6D valve materials are as follows:
Pressure-retaining parts shall be made of materials consistent with the pressure-temperature rating.
All process-wetted parts and lubricants shall be compatible with the commissioning and service fluids specified by the purchaser.
Selection of elastomeric materials for hydrocarbon gas service at pressures of PN 100 (class 600) and above shall consider the effect of explosive decompression.
As for metallic parts, material selection shall be geared to avoid corrosion and galling. Forged materials shall be hot worked and heat treated to produce uniform grain size and mechanical properties in the finished product.
For sour service specifications, materials for pressure-containing, pressure-controlling parts, and bolting shall meet the requirements of ISO 15156 (all parts).
What is API 598?
While API 6D focuses on pipeline valves, API 598 is specifically concerned with valve testing, including the tests required to ensure valves meet the necessary sealing and pressure standards. It outlines the procedures for testing valves across various industries, such as oil and gas, petrochemical, and power generation.
API 598 defines the requirements for testing different valve types, including gate, globe, check, ball, and plug valves. The standard is widely used for verifying valve performance before they are installed in systems, ensuring that they meet the expected operational standards.
Key Features of API 598:
Valve Leak Testing: One of the primary requirements of API 598 is the leak testing of valves. The standard defines methods for testing a valve’s ability to seal properly, including seat leakage and body leakage tests. This ensures valves can effectively control flow and pressure without leaks.
Hydrostatic and Pneumatic Testing: API 598 specifies both hydrostatic and pneumatic testing methods to check for leaks and verify valve performance under pressure. Hydrostatic tests use water, while pneumatic tests use air or gas to test the valve’s sealing capabilities.
Functional Testing: The standard also includes functional tests to ensure the valve’s operation is smooth and reliable under normal and extreme conditions. This includes testing the valve’s opening, closing, and sealing performance.
By adhering to API 598, manufacturers can ensure that their valves meet the necessary quality and safety standards, helping to avoid failures during operation and enhancing system reliability.
(For detailed information on API 598 and its valve testing standards, click here)
Testing Methods: API 6D vs API 598
API 6D Testing Methods
API 6D is specifically tailored for valves used in pipeline applications, typically in the oil and gas industry. It focuses on ensuring that valves can withstand the extreme pressures and operational conditions found in pipeline systems. The testing methods specified under API 6D are rigorous, reflecting the high-stakes environment in which these valves operate.
The primary tests included under API 6D are:
Hydrostatic Shell Test: This test ensures the valve body can handle the internal pressures of the pipeline without leaking. The test is conducted by applying pressure that is 1.5 times the valve’s rated pressure.
Seat Hydrostatic Test: This checks for leakage at the valve seats to ensure a tight seal when the valve is closed. The test pressure is typically 1.5 times the rated pressure.
Stem Backseat Test: Ensures that the valve stem can withstand pressure applied to the backseat, typically done at 1.1 times the rated pressure.
The duration of these tests depends on the size of the valve. Larger valves, for example, undergo more prolonged testing. For instance, a valve with a 20 NPS or larger size could undergo casing tests for up to 1,800 seconds to ensure full integrity under high-pressure conditions.
API 598 Testing Methods
API 598, while also concerned with valve testing, serves a broader industry scope. It provides a general framework for inspecting valves across various industries, including oil and gas, but also for waterworks, chemical processes, and power generation. The testing methods in API 598 focus on ensuring that valves are leak-free and function as designed, making it more versatile than API 6D, which is specific to pipeline valves.
The key tests specified in API 598 are:
Shell Test: Similar to API 6D’s shell test, this test applies pressure to the valve body to detect any leakage.
Backseat Test: A test that checks the integrity of the valve stem’s backseat, ensuring no leaks occur when the valve is closed.
Low and High-Pressure Closure Tests: These tests are meant to verify the sealing capability of the valve at various pressures, including both low-pressure and high-pressure conditions.
Visual Inspection: This initial examination ensures that there are no obvious defects in the valve that could impact its performance.
The test durations under API 598 tend to be shorter than those required by API 6D. For example, smaller valves (under 2 NPS) are tested for only a few minutes, whereas larger valves may require up to 10 minutes for testing. The test mediums are also more flexible under API 598, allowing air, water, inert gases, or even kerosene to be used for testing
API 6D vs API 598: Understanding the Differences
While both API 6D and API 598 are crucial standards for valve manufacturing and testing, they serve different functions. API 6D is primarily concerned with the design, materials, and manufacturing requirements for valves used in pipeline systems. In contrast, API 598 focuses on testing the performance of these valves to ensure they meet the required quality and operational standards.
Here’s a comparison of API 6D vs API 598:
| Differences | API 6D | API 598 |
| Purpose | Provides specifications for pipeline valves, especially those in the petroleum and natural gas industry. | Covers inspection, examination, and pressure testing of valves in general. |
| Industry Application | Stringent standards for examination and testing of pipeline valves. Also includes longer pressure times, more test items, and sophisticated operating methods. | Common testing regulations for valves designed with API 602, API 608, and API 609. |
| Valve Coverage | Gate, ball, plug, and check valves. | Globe, gate, plug, ball, check, and butterfly valves. |
| Test Coverage | Includes tests such as stem backseat test, hydrostatic shell test, hydrostatic seat test. | Covers closure examinations, shell examinations, backseat examinations |
| Test Duration | Generally provides longer test times as follows: backseat test (120-300s), shell test (120-1,800s), and seat test (120-300s). | Test duration is directly proportional to valve size, but shorter test times than API6D. Range for each test is as follows: shell (15-300s), backseat (15-60s), closure for check valves (60-120s), and other closure tests (15-120s) |
| Test Medium | For hydrostatic shell and backseat tests, the medium is freshwater with a corrosion inhibitor. If it is a low-pressure closure test, the medium is air or nitrogen. While high-pressure closure tests use freshwater or inert gas. | When doing hydrostatic shell and backseat examinations, the medium is air, inert gas, water, kerosene, or non-corrosive fluid. Then, air or inert gas serves for low-pressure closure tests. Whereas air, inert gas, water, kerosene, or non-corrosive fluid serve for high-pressure closure tests. |
Why API 6D and API 598 Matter for Valve Manufacturers
Complying with API 6D and API 598 standards is essential for valve manufacturers for several reasons:
Safety and Reliability:
Both standards ensure that valves meet rigorous testing criteria, which is critical for preventing leaks, ensuring proper function, and maintaining operational safety.
Quality Assurance:
Following these standards provides a framework for manufacturers to deliver high-quality products that are tested to industry-leading specifications. This helps build customer trust and supports brand reputation.
Market Competitiveness:
Valves that comply with API standards are more likely to meet regulatory requirements and be accepted in both local and international markets. Manufacturers who adhere to these standards can expand their market reach and attract more customers in highly regulated industries.
Operational Efficiency:
By adhering to API 6D and API 598, manufacturers ensure that the valves they produce can withstand the operational stresses they will face. This reduces the likelihood of failures and costly maintenance, ensuring long-term reliability in systems where valve failure could result in severe consequences.
Conclusion
In summary, adhering to the stringent requirements of API 6D and API 598 is essential for valve manufacturers to ensure the safety, reliability, and performance of their products. API 6D sets high standards for valves used in demanding pipeline applications, focusing on rigorous testing procedures and material specifications. On the other hand, API 598 provides a more versatile testing framework for a wider range of valve types across various industries. By complying with these standards, manufacturers not only ensure product quality but also gain a competitive edge in the market, meeting both regulatory requirements and customer expectations.