Control Valve Leakage Testing, Types, and Calculation Standards

• A control valve is a power-operated device that is used to control and manipulate the flow of media, including water, gas, oil, and the stem. 
• The device is by far the most often utilized final instrument for controlling pipeline transit worldwide and is an essential part of a control loop. 
• It is controlled by electrical, hydraulic, and pneumatic means by means of a controller that sends out a signal to change the flow.
• A control valve needs a device to convert the electrical signal that the controller, which is typically a PLC, sends in order for it to function. 
• Because this valve is meant to throttle rather than shut entirely, each one has a different capacity to shut off, which causes the control valve seat leakage must be calculated.
• Leaks from control valves can be broadly classified into two categories: leaks within the system and fugitive emissions to the environment. 
• Leakage within it is hazardous to the entire process, even though fugitive emissions might be dangerous to the environment and industry.
• The manufacturer inspects and tests every valve to make sure it complies with the necessary valve leakage criteria. 
• The buyer may also request optional testing, and frequent testing is essential for continued valve maintenance and safety. It is frequently advised to test valves no more frequently than once every 12 months. 
• The precise interval, however, can change depending on the state of the valve, the state of the service, and the required degree of performance.
• A hydrostatic test, in which a liquid test medium, like water or kerosene, is used to measure valve leakage, or a pneumatic test, in which a gas test medium, like air or nitrogen, is used. 
• The standards define the Maximum permissible leakage (MAL) for valves under the specified testing conditions because zero leakage is rarely, if ever, feasible.
• The MAL is often determined by valve size for both hydrostatic and pneumatic tests; a small amount of leakage through a valve with a small effective orifice presents a significantly higher risk than the same amount of leakage through a valve with a wide effective orifice. 
• MAL might also depend on the pressure category and valve class. It is imperative that your valves be examined by highly qualified and trained specialists, as the results of valve leakage tests can differ depending on the tester’s technique.

To familiarize you with a control valve seat leakage calculation, this article will guide you through all kinds of leakages and testing.

Classification of Control Valve Leaks

• The type of control valve determines its ability to close. Some double-seated control valves are not as good at shutting as others. 
• What happens to a certain valve that you buy from an established control valve manufacturer depends on a number of factors, including actuator thrust, fluid type, seat material, guiding, and pressure drop. 
• The European equivalent standard, IEC 60534-4, and the ANSI standard FCI 70-2 2006 describe a number of classes for six-seat valve leakage test procedures.

Class I:

• It is also known as dust-tight and is made of resilient seated valves and metal. Similar to Class II, III, and IV, they were not subjected to a shop test during construction.

Class II:

• This type of valve establishes the maximum allowable control valve seat leakage calculation, which is typically related to commercial double-seat or balanced single-seat control valves that use piston ring seals or metal-to-metal seats.

Class III:

• This type of valve has an increased seat level and seal tightness in addition to the maximum allowable leakage, which is typically connected to Class II. As with Class II, it is designed for valves of a similar kind.

Class IV:

• The maximum allowable leakage for both balanced and unbalanced commercial single-seat valves is determined by Class IV leakage class. 
• It features metal-to-metal seating, additional sealing techniques, and particularly tight piston rings. 
• This control valve seat leakage calculation rate, also referred to as metal-to-metal, indicates what to expect from a valve having a metal seat and metal plug.

Class V:

• This category of valve leaks is designated for important applications where the valve must be closed for lengthy periods of time without obstruction and with a high differential pressure across the seating. 
• It necessitates special valve leakage test protocols, assembly techniques, and design for metal seats, unbalanced single-seat control valves, and balanced single-seat designs with unusual seal and seat rigidities.

Class VI:

• The maximum allowable seat for industrial valves with resilient seating—balanced or unbalanced single-seat designs with identical gaps or O-ring seals—is determined in the final valve leakage test procedure class. 
• Here, resilient material designed for resilient seating valves is used to produce the seats, the shut-off disc, or both. 
• This sort of leakage rate, also known as the soft seat classification or soft seat valves, occurs when either the plug, the seat, or both are constructed from composition materials like polytetrafluoroethylene (PTFE).

Leakage Tests & Calculations in a Control Valve

A control valve manufacturer inspects and tests each valve thoroughly to ensure compliance with the necessary standards. Optional or regular testing for valve safety and maintenance can also be requested. Generally, it is recommended to test a valve annually. However, this interval may vary based on the valve’s condition, service, and performance level.

Bubble Test:

• This test is used for Class VI seat leakage, measuring the rate of air or nitrogen leaking past the valve seat by counting bubbles escaping from a tube submerged in water. 
• The maximum allowable bubble rate for a 6-inch valve is 27 bubbles per minute. 
• Achieving Class VI shut-off is only possible with a soft seat rather than a metal-to-metal seal between the plug and the seat. 
• Limitations of this method include a restricted operating temperature range and inability to withstand nuclear radiation.

Hydrostatic or Pneumatic Test:

• In a hydrostatic valve leakage test, the medium is a liquid, such as kerosene or water. 
• In a pneumatic test, the medium is usually a gas, such as nitrogen or air. Achieving zero leakage is unlikely; instead, the maximum allowable leakage (MAL) is defined. 
• For both tests, MAL is based on the valve size – less leakage through a valve with a small effective orifice is riskier compared to the same leakage through a larger orifice. 
• MAL can also depend on the valve class and pressure.

Typical control Valve leakage Testing Procedure

Step 1: Filling

• Fill the valve with the appropriate testing fluid, ensuring it is at a specific temperature that matches the test requirements. 
• The testing fluid can be a liquid, such as water or kerosene, or a gas, such as nitrogen or air, depending on the type of test being conducted (hydrostatic or pneumatic). 
• Ensuring the correct temperature is crucial because it can affect the fluid’s properties and the test’s accuracy.

Step 2: Pressurizing

• Apply a defined pressure level to the valve for a specified length of time. This pressure is determined based on the valve’s design specifications and the standards it must meet. 
• The pressure application should be steady and controlled to avoid any sudden surges that could potentially damage the valve or skew the test results. 
• The duration of the pressurization is also critical, as it needs to be long enough to identify any potential leaks.

Step 3: Measuring

• Measure the leakage across the element of interest, whether it be the stem, seat, or closure mechanism. 
• This is typically done using visual examination and precise measuring tools. During this phase, it is essential to monitor for any fluid escaping from the valve. 
• According to most standards, any visually detectable leakage is not permissible. 
• Measuring instruments may include bubble testers for gas leaks or calibrated containers for liquid leaks to quantify the leakage accurately.

Step 4: Inspection

• Perform a thorough visual inspection of the valve to ensure it has not been damaged during the testing procedure. 
• This inspection should check for any signs of wear, deformation, or other damage that could have occurred due to the pressure applied during the test. 
• The valve’s integrity must be maintained, and any damage identified should be addressed before the valve is put into service. 
• This final inspection ensures the valve’s reliability and safety for future operation.

By following these detailed steps, valve manufacturers and maintenance teams can ensure that each valve meets the necessary performance and safety standards before it is installed or returned to service.

Various Types of Control valve Leakage Tests

• The methods for testing control valve leakage vary depending on the type of valve and its specific elements. Here are some common leakage tests, elaborated:

Seat Leakage Test

• This test is primarily used for pressure relief valves to ensure that the valve seat can properly seal against the valve disc under pressure conditions. 
• The test involves applying pressure to the valve and measuring any leakage past the seat. 
• The amount of leakage is then compared to acceptable limits defined by relevant standards. 
• This test is crucial for maintaining safety and operational efficiency, as seat leakage in pressure relief valves can lead to malfunction or insufficient pressure relief during overpressure situations.

Backseat Test

• The backseat test is used for valves that have a backseat, such as gate and globe valves. 
• A backseat is a secondary sealing surface located on the stem of the valve, which provides an additional seal when the valve is fully open. 
• During the test, the valve is fully opened, and pressure is applied to check for leakage past the backseat. 
• This test ensures that the backseat can provide a tight seal, preventing leakage around the stem packing area, which is particularly important in high-pressure applications.

Shell Leakage Test

• The shell leakage test is used for valves in fully open or fully closed positions, such as stop, isolation, and check valves. 
• This test involves applying pressure to the entire valve body (shell) to check for leaks through the valve body, bonnet, and end connections. 
• The purpose is to ensure the structural integrity and leak-tightness of the valve housing. It is essential for verifying that the valve can withstand the maximum pressure it will encounter during operation without any leaks, ensuring reliability and safety in service.

Closure Test

• The closure test is used to evaluate the effectiveness of the valve’s closing mechanism. This test is applicable to various types of valves, including gate, plug, globe, and ball valves. 
• During the test, the valve is closed, and pressure is applied to one side of the valve. The other side is observed for any leakage. 
• The amount of leakage is measured and compared against the acceptable limits defined by standards. 
• This test is vital for ensuring that the valve can achieve a complete shutoff, preventing process media from leaking past the valve when it is supposed to be closed.

Each of these tests is designed to verify specific aspects of valve performance and integrity, ensuring that the valves can operate safely and effectively under their intended service conditions.]

Valve Inspection and Testing Standard – API 598 

API 598 is a standard established by the American Petroleum Institute that outlines the inspection and testing procedures for various types of valves, ensuring they meet safety and performance criteria. 

Key elements include:

• Scope: Applies to gate, globe, plug, ball, check, and butterfly valves.
• Visual Inspection: Ensures proper assembly, absence of defects, and correct identification markings.
• Pressure Testing:
  1. Shell Test: Verifies structural integrity using hydrostatic pressure (usually 1.5 times the rated pressure); no leakage allowed.
  2. Seat Leakage Test: Checks sealing capability at rated pressure; allowable leakage rates vary by valve type and class.
• Test Medium: Uses water, air, or an inert gas; test medium temperature must be controlled.
• Test Duration: Specifies minimum duration for each pressure test.
• Test Equipment: Requires calibrated instruments for accuracy.
• Documentation: Maintains comprehensive records of inspections and tests.
• Optional Testing: Allows for additional tests upon purchaser’s request.

This standard ensures valves are reliable and safe for use in industries such as oil and gas, petrochemical, and power generation.

For detailed information, you can access the full API 598 document by below Link

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Comparison of Control Valve Leakage Classes with Maximum Test Leakage and Test Pressure

Below is a comparison table outlining leakage classes for control valves, specifying maximum test leakage and corresponding test pressure standards

Please note that the test pressures for Classes II, III, IV, and V are based on ANSI/FCI 70-2 standards and may vary depending on specific valve sizes and configurations. The test pressure for Class VI is determined through a bubble tightness test rather than a specific pressure value.

Class IV Control Valve Leakage Rates: Maximum Allowable Leakage in ml/min and Equivalent Bubbles per Minute

Table outlining the leakage rates for Class IV control valve tests based on the nominal seat diameter, provided in milliliters per minute (ml/min) and bubbles per minute (bubbles/min):

Note:

• The leakage rate is calculated for Class IV valves, which typically allow a leakage rate of 0.01% of the valve’s full open capacity (Cv).
• The “Equivalent Bubbles per Minute” is an approximate conversion based on the general assumption that 1 ml/min of leakage is roughly equivalent to 40 bubbles per minute in typical test conditions. Actual values can vary based on specific test setups and conditions.

Always refer to the specific standards and manufacturer guidelines for precise testing parameters and leakage rate values.

Frequently Asked Questions (FAQ) 

What is control valve leakage testing, and why is it important?

Control valve leakage testing is a procedure to assess the sealing integrity of control valves to prevent fluid leakage during operation. It’s crucial for ensuring process safety, environmental protection, and equipment reliability.

What are the different types of control valve leaks?

Control valve leaks can be classified into two main categories: leaks within the system and fugitive emissions to the environment. Leaks within the system can compromise process efficiency and safety, while fugitive emissions can pose environmental and regulatory concerns.

How are control valve leaks classified according to standards?

Control valve leaks are classified into different classes based on their ability to close and seal effectively. Standards such as ANSI FCI 70-2 and IEC 60534-4 define leakage classes ranging from Class I (dust-tight) to Class VI (soft seat) based on specific test procedures and allowable leakage rates.

What is the maximum allowable leakage (MAL) in control valve testing?

The maximum allowable leakage (MAL) is the maximum acceptable level of leakage determined by valve size, pressure category, and class. It is defined in standards and test procedures to ensure consistent performance and safety.

What are the common methods for control valve leakage testing?

Common methods for control valve leakage testing include hydrostatic testing using liquid test media (e.g., water) and pneumatic testing using gas test media (e.g., air or nitrogen). These tests assess the valve’s sealing performance under specified conditions.

How often should control valves be tested for leakage?

It is generally recommended to test control valves annually to ensure continued performance and safety. However, the testing interval may vary based on factors such as valve condition, service conditions, and regulatory requirements.

What is API 598, and how does it relate to control valve testing?

API 598 is a standard established by the American Petroleum Institute (API) that outlines inspection and testing procedures for various types of valves, including control valves. It provides guidelines for visual inspection, pressure testing, test medium, duration, equipment requirements, and documentation.

What are some common challenges in control valve leakage testing?

Common challenges in control valve leakage testing include ensuring accurate test conditions, interpreting test results reliably, addressing variations in testing procedures, and maintaining test equipment calibration and reliability.

How can control valve leakage testing contribute to overall process efficiency and safety?

Control valve leakage testing helps identify potential leaks that could affect process efficiency, product quality, and safety. By detecting and addressing leaks promptly, operators can minimize downtime, reduce environmental impact, and enhance overall operational reliability.

Where can I find more information on control valve leakage testing and standards?

Additional information on control valve leakage testing, types, and calculation standards can be found in industry publications, standards documents such as API 598, and resources provided by valve manufacturers and professional organizations specializing in fluid control and instrumentation.