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A power-operated device used for regulating and manipulating the flow of mediums, such as oil, gas, water and the stem is known as a control valve. The device is a critical component of a control loop and by far the most common final element used for controlling pipeline transport around the world. It is operated electrically, hydraulically, and pneumatically by receiving a signal from a controller to alter the flow.
The controller is usually a PLC that sends an electrical signal, because of which a control valve requires a device to convert it, thus making it operate. This valve is designed to throttle and not close completely, due to which all of them have varying shut-off capabilities, resulting in control valve seat leakage calculation.
There are basically two types of leakage from a control valve – fugitive emissions to the environs and leakage in the system. While fugitive emissions can be detrimental to the environment and industry, leakage within it is detrimental to the whole process.
This guide will take you through the different types of leakages and tests to familiarize you with a control valve seat leakage calculation.
Classification of Leakages in a Control Valve:
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The ability of a control valve to shut off has to do with its type. While a double-seated control valve has poor shutting power, others are different. Factors such as guiding, seat material, type of fluid, pressure drop, and actuator thrust influence how a particular valve purchased from a reputed control valve manufacturer shuts.
According to the ANSI standard FCI 70-2 2006 (European equivalent standard IEC 60534-4), there is a series of six-seat valve leakage test procedure classifications.
Class I
It is also referred to as dust-tight and materials such as metal and resilient seated valves. They are akin to Class II, III, and IV inbuilt and designed without any shop test.
Class II
In this class of valve, the maximum permissible control valve seat leakage calculation commonly associated with commercial double-seat or balanced single-seat control valves with metal-to-metal seats or piston ring seals is established.
Class III
In this class of valve, the maximum permissible leakage, generally associated with Class (4.2.2) II, is established, but with an elevated seat level and seal tightness. It is built for similar types of valves as in Class II.
Class IV
This leakage class establishes the maximum permissible leakage associated with unbalanced commercial single-seat and balanced single-seat valves. It has extra tight piston rings and other sealing methods along with metal-to-metal seats. Also known as metal-to-metal, it is a control valve seat leakage calculation rate where you can expect from a valve with a metal plug and a metal seat.
Class V
In this class of valve leakages, critical applications where the valve needs to be closed, without blockage and high differential pressure across the seating, for extended periods is specified. It demands unique designing, assembly, and valve leakage test procedures relating to metal seats, unbalanced single-seat control valves, and balanced single-seat designs with atypical seat and seal rigidity.
Class VI
It is the final valve leakage test procedure class where the maximum permissible seat, associated with resilient seating industrial valves, either balanced or unbalanced single-seat having the same gaps or O rings seals, is established. Here, the seats, shut-off disc, or both are created using resilient material intended for resilient seating valves.
Also referred to as the soft seat classification or soft seat valves, it is the type of leakage rate in which either the plug or the seat, or both of them are made using composition material, such as Polytetrafluoroethylene (PTFE).
Leakage Tests & Calculations in a Control Valve:
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A control valve manufacturer inspects and tests every valve thoroughly to conform to the needed standards. One may also request optional or regular testing for valve safety and maintenance. In a majority of cases, it is recommended to test a valve every year. However, the interval may vary based on a valve’s condition, service, and performance level.
Bubble Test
This test is used for Class VI seat leakage based on the exuding rate of air or nitrogen past the valve seat, calculated by the rate of bubbles escaping the tube submerged in water. The higher limit of bubble rate for a 6-inch valve is 27 bubbles per minute. Class VI shut-off is possible only with a soft seat rather than a strict metal-to-metal union between the plug and the seat.
The limitations of using this method include limited operating temperature range and the inability to bear nuclear radiation.
Hydrostatic or Pneumatic Test
Under the hydrostatic valve leakage test procedure, the medium is a liquid, such as kerosene or water. Under the pneumatic test, the medium is usually a gas, such as nitrogen or air. Zero leakage is seldom likely under this method; it states the highest allowable leakage (MAL).
For both hydrostatic and pneumatic tests, the MAL is interpreted by the valve size – less leakage through a valve with a small effective orifice carries many risks compared to an equal amount of leakage through a valve with a big effective orifice. MAL may be the function of a valve class and pressure.
Typical Valve Testing Procedure:
- The first step is filling a valve with testing fluid at a specific temperature.
- The next step is applying a defined pressure level for a stated length of time.
- Measuring the leakage across the element of interest (stem, seat, or closure procedure) using visual examination and measuring tools. According to most of the standards, no visually detectable leakage is permissible.
- The final step is to perform a visual inspection to ensure the valve has not defaced during the testing procedure.
Various Types of Leakage Tests:
The testing methods for calculating a control valve leakage depend on the type you’re using and the valve elements. Here are some of the most commonly performed tests of valve leakage:
- Seat Leakage Test – Used for pressure relief valves.
- Backseat Test – Used for valves having a backseat, including gate and globe valves.
- Shell Leakage Test – Used for valves in full open and full closed service, such as stop, isolation, and check valves.
- Closure Test – Used to test the closing mechanism of various valves, such as gate, plug, globe, and ball valves.
When is the Right Time to Repair a Control Valve?
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Internal Control Valves Leaks
The main reason for internal leaks in control valves is their design that keeps them from shutting off 100%. The ANSI/FCI Standard 70-2 has divided them according to six valve leakage test procedure rates and states the maximum permissible leakage (MAL) for each. So, if your valve is leaking within the allowable range, it probably doesn’t need repair yet.
But if your valve is leaking beyond the MAL, here are some possible reasons behind it:
- Improper actuator setup
- Worn out internal components
- The positioner is not calibrated correctly
- Distributed Control Systems (DCS) is wrongly configured.
External Control Valves Leaks
If a control valve is leaking from the outside, it might emancipate toxic fugitive emissions into the setting. In the refining world, issues with valve packaging are usually due to external leaks. Here are a few instances that cause packaging problems in chemical processing and other industries:
- Poor maintenance
- Thermal cycling
- Incompatible material
- Vibrating valve assembly
- High packing box temperature
Control Valve Sticks
In the event of a valve leak test, the operator must try to stop it by tightening the packaging, but it causes the valve to stick to a place. As the valve is sized for a specific purpose with a packing torque valve, it won’t work correctly if it is over torques. Positioner problems also manifest as sticky valves, especially in pneumatic valves, that comprise most of the control valves in operation.
Control Valve Fails to Work
Sometimes a control valve fails to work because its pneumatic diaphragm deteriorates. It is being used in an unsuitable condition, the actuator is of the wrong size, or the positioner is not calibrated correctly.
So, when you step into a market for purchasing from a control valve manufacturer, be sure to have all the necessary information needed to size and select the components, such as details about process conditions, including flow, media, and temperature. By not having this information, you will risk purchasing unsuitable equipment, resulting in you repairing the same valve assembly every year.
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Final Words
This article taught us everything about a control valve leakage classification, procedure, and tests, and most importantly, the right to get it changed. We hope it has given you an insight into the working and malfunctioning of this device. For further help or support, please don’t hesitate to contact us.
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