Everything You Need to Know About Pneumatic Control Valves

A pneumatic control valve uses compressed air as its power source and a pneumatic cylinder as its actuator. With the aid of accessories such as electric valve positioners, converters, solenoid valves and hold-open valves, it drives the valve to achieve on/off or proportional control. It receives control signals from industrial automation control systems to regulate various process parameters of the medium in the pipeline, such as flow rate, pressure and temperature. The key features of pneumatic control valves are their simple control mechanism, rapid response, and intrinsically safe nature, which eliminates the need for additional explosion-proof measures.

Pneumatic control valves typically consist of a pneumatic actuator and a control valve, which are installed and commissioned together. Pneumatic actuators can be classified into single-acting and double-acting types; single-acting actuators contain a return spring, whereas double-acting actuators do not. In the event of a loss of air supply or sudden failure, a single-acting actuator will automatically return to the valve’s initially set open or closed position.

Pneumatic control valves are classified by their operating mode into air-to-open and air-to-close types, also known as normally open and normally closed types. The air-to-open or air-to-close operation of a pneumatic control valve is typically achieved through the direct or reverse action of the actuator and different assembly configurations of the valve body.

Operating modes of pneumatic control valves:

In the air-to-open (normally closed) type, when the air pressure on the diaphragm head increases, the valve moves in the direction of increased opening; when the upper limit of the input air pressure is reached, the valve is in the fully open position. Conversely, when the air pressure decreases, the valve moves in the closing direction; in the absence of input air, the valve is fully closed. Therefore, we commonly refer to air-to-open control valves as fail-to-close valves.

The air-to-close (normally open) type operates in the opposite direction to the air-to-open type. When air pressure increases, the valve moves towards the closed position; when air pressure decreases or is absent, the valve moves towards the open position or until fully open. Therefore, air-to-close control valves are commonly referred to as fail-open valves.

The choice between air-to-open and air-to-close valves is determined from the perspective of process safety. It depends on whether it is safer for the control valve to be in the closed position or the open position when the air supply is cut off.

For example, in the combustion control of a heating furnace, where a control valve is installed on the fuel gas pipeline to regulate fuel supply based on the furnace chamber temperature or the temperature of the heated material at the furnace outlet, it is safer to select an air-to-close valve. This is because, should the air supply cease, it is preferable for the valve to be in the closed position rather than fully open. If the air supply is interrupted whilst the fuel valve is fully open, this could lead to overheating and pose a safety hazard. Another example is a heat exchanger cooled by cooling water, where hot material is cooled through heat exchange with the cooling water within the exchanger. The control valve is installed on the cooling water pipe, and the cooling water flow is controlled based on the temperature of the material after heat exchange. In the event of a gas supply interruption, it is safer for the control valve to be in the open position; therefore, an air-to-close (FO) control valve should be selected.

Valve Positioners

Valve positioners are essential accessories for control valves and are widely used in conjunction with pneumatic control valves. They receive the output signal from the controller and use this to control the pneumatic control valve. Once the control valve actuates, the displacement of the valve stem is fed back to the valve positioner via a mechanical mechanism, and the valve position status is transmitted to the upper-level system via an electrical signal. Valve positioners can be classified according to their structural form and operating principle into pneumatic valve positioners, electro-pneumatic valve positioners and intelligent valve positioners.

Valve positioners increase the output power of control valves, reduce signal transmission lag, accelerate the movement of the valve stem, improve valve linearity, overcome friction forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the correct positioning of the control valve.

Actuators are classified into pneumatic and electric types, and are further divided into linear and rotary types. They are used to automatically or manually open and close various valves, dampers and similar devices.

Installation Principles for Pneumatic Control Valves
(1) Pneumatic control valves must be installed at a certain height above the floor, with sufficient space above and below the valve to allow for disassembly, assembly and maintenance. For control valves fitted with pneumatic valve positioners and handwheels, it is essential to ensure ease of operation, observation and adjustment.
(2) Control valves should be installed on horizontal pipelines and positioned vertically in relation to the pipeline. Generally, the valve should be supported from below to ensure stability and reliability. In special circumstances where a control valve needs to be installed horizontally on a vertical pipe, the valve must also be supported (except for small-bore control valves). During installation, care must be taken to avoid subjecting the control valve to additional stress.
(3) The operating environment temperature for the control valve must be between –30°C and +60°C, with a relative humidity not exceeding 95%.
(4) There should be a straight pipe section upstream and downstream of the control valve, with a length of not less than 10 times the pipe diameter (10D), to prevent the straight pipe section being too short and affecting the flow characteristics.
(5) If the diameter of the control valve differs from that of the process piping, a reducer should be used for connection. For small-diameter control valves, threaded connections may be used. The fluid direction arrow on the valve body must align with the actual flow direction.
(6) A bypass pipe must be installed. The purpose is to facilitate switching or manual operation, and to allow maintenance of the control valve without shutting down the process.
(7) Before installing the control valve, thoroughly remove any foreign matter from the pipeline, such as dirt or welding slag.

Common Faults and Troubleshooting
1. Control valve fails to operate
First, check whether the air supply pressure is normal and investigate any faults in the air supply. If the air supply pressure is normal, check whether the amplifier in the positioner or the electro-pneumatic converter is producing an output; if there is no output, the constant orifice in the amplifier may be blocked, or moisture from the compressed air may have accumulated at the ball valve in the amplifier. Clear the constant orifice with a fine steel wire, remove any debris, or clean the air supply.
If all the above are normal, and there is a signal but no movement, the actuator is faulty, the valve stem is bent, or the valve plug is stuck. In such cases, the valve must be dismantled for further inspection.

2. Control Valve Jamming
If the valve stem’s reciprocating movement is sluggish, this may be due to highly viscous substances within the valve body, coking and blockage, packing that is too tightly compressed, ageing of the PTFE packing, or a bent or scratched valve stem. Blockage faults in control valves most frequently occur in newly commissioned systems or during the initial phase following major overhauls. This is often caused by welding slag, rust or other debris within the pipeline causing blockages at the throttling or guiding points, thereby impeding the smooth flow of the medium. Alternatively, it may result from the packing being over-tightened during valve maintenance, increasing friction and leading to a failure to respond to small signals or an overshoot in response to large signals.
In such cases, the bypass line or control valve should be opened and closed rapidly to allow the debris to be flushed out by the medium. Alternatively, the valve stem can be clamped with a pipe wrench and rotated forcefully in both directions whilst applying an external signal pressure, to allow the valve plug to pass through the jammed area. If this does not resolve the issue, increasing the air supply pressure and drive power whilst repeatedly moving the valve up and down several times should resolve the problem. If the valve still fails to operate, it will need to be disassembled. Naturally, this task requires a high level of technical expertise and must be carried out with the assistance of qualified technical personnel; otherwise, the consequences could be even more severe.

3. Valve Leakage
Leakage in control valves generally falls into several categories: internal leakage, packing leakage, and leakage caused by deformation of the plug and seat. These are analysed below.
(1) Internal Leakage
If the valve stem is of an inappropriate length—for example, if the stem of an air-operated valve is too long—the upward (or downward) travel of the stem may be insufficient. This creates a gap between the plug and the seat, preventing full contact and resulting in a poor seal and internal leakage. Similarly, in air-to-close valves, if the stem is too short, a gap may form between the plug and the seat, preventing full contact and resulting in an incomplete seal and internal leakage. Solution: The control valve stem should be shortened (or lengthened) to ensure the valve is the correct length, thereby eliminating internal leakage.
(2) Packing Leakage
After the packing is installed in the packing chamber, axial pressure is applied via the gland. Due to the plastic deformation of the packing, radial forces are generated, causing it to make close contact with the valve stem. However, this contact is not entirely uniform; some areas are loose, others are tight, and in some cases, there is no contact at all. During operation, relative movement occurs between the valve stem and the packing; this is known as axial movement. Under the influence of high temperatures, high pressures and highly permeable fluid media, the packing chamber is one of the areas most prone to leakage in control valves. The primary cause of packing leakage is interface leakage; in the case of textile packing, seepage may also occur (where the pressurised medium escapes through minute gaps between the packing fibres). Interface leakage between the valve stem and the packing is caused by factors such as the gradual reduction in contact pressure and the ageing of the packing itself; in such cases, the pressurised medium escapes through the contact gap between the packing and the valve stem.
To facilitate packing installation, a chamfer is machined at the top of the stuffing box, and a metal protective ring with a small clearance and high erosion resistance is placed at the bottom. It is important to ensure that the contact surface of this protective ring with the packing is not sloped, to prevent the packing from being forced out by the pressure of the medium. The surfaces of the stuffing box in contact with the packing must be precision-machined to improve surface finish and reduce packing wear. Flexible graphite is selected as the packing material due to its excellent airtightness, low friction, minimal dimensional change over prolonged use, low wear and charring, ease of maintenance, and the fact that friction remains unchanged after retightening the gland bolts. It also offers good pressure and heat resistance, is not susceptible to erosion by the internal medium, and does not cause pitting or corrosion of the metal in contact with the valve stem and the interior of the packing chamber. This effectively protects the seal of the valve stem stuffing box, ensuring the reliability of the packing seal and significantly extending its service life.
(3) Leakage due to deformation of the valve plug and seat
The primary cause of leakage in the valve plug and seat is casting or forging defects during the control valve manufacturing process, which can exacerbate corrosion. Furthermore, the passage of corrosive media and the scouring action of the fluid medium can also cause leakage in the control valve. Corrosion primarily occurs in the form of erosion or cavitation. When corrosive media pass through the control valve, they erode and impact the valve plug and seat materials, causing them to become elliptical or otherwise deformed. Over time, this leads to a mismatch between the valve plug and seat, creating gaps that prevent a tight seal and result in leakage.

It is essential to ensure the correct selection of materials for the valve plug and seat. Corrosion-resistant materials must be chosen, and products exhibiting defects such as pitting or pinholes must be strictly rejected. If the deformation of the valve plug or seat is not severe, fine sandpaper can be used to grind away the marks, improving the surface finish of the seal to enhance sealing performance. If the damage is severe, the valve should be replaced with a new one.

4. Oscillation
Insufficient spring stiffness in the control valve, combined with an unstable and rapidly fluctuating output signal, can easily cause the valve to oscillate. Furthermore, if the natural frequency of the selected valve matches that of the system, or if the piping or mounting base vibrates violently, the control valve will vibrate in response. Inappropriate valve selection, where the control valve operates at a small opening, results in significant fluctuations in flow resistance, velocity and pressure. When these exceed the valve’s stiffness, stability deteriorates and, in severe cases, oscillation occurs.

As the causes of oscillation are multifaceted, each situation must be analysed on a case-by-case basis. For minor vibrations, stiffness can be increased to eliminate the issue, for example by selecting a control valve with a high-stiffness spring or switching to a piston actuator; If the pipeline or base is vibrating violently, the vibration interference can be eliminated by adding supports; if the valve’s natural frequency matches that of the system, replace the control valve with one of a different design; oscillations caused by operation at a small opening are due to incorrect selection, specifically because the valve’s flow capacity (C-value) is too high. It is necessary to reselect the valve, choosing one with a lower C-value, or to adopt split-range control or a master-slave valve configuration to overcome the oscillations generated when the control valve operates at a small opening.

5. Excessive Noise from Control Valves
When fluid flows through a control valve, excessive pressure differential between the inlet and outlet can cause cavitation in components such as the valve plug and seat, resulting in fluid noise. If the flow capacity (C-value) has been selected too high, a control valve with a suitable flow capacity must be selected to overcome the noise caused by the valve operating at a small opening. Several methods for eliminating noise are outlined below.

(1) Method for Eliminating Resonance Noise
Only when a control valve resonates does the superposition of energy produce intense noise exceeding 100 decibels. In some cases, this manifests as strong vibration with relatively low noise; in others, the vibration is weak whilst the noise is extremely loud; and in some instances, both vibration and noise are significant. This noise produces a single-tone sound, typically with a frequency of 3,000 to 7,000 hertz. Clearly, eliminating resonance will naturally cause the noise to disappear.
(2) Method for Eliminating Cavitation Noise
Cavitation is a primary source of fluid dynamic noise. During cavitation, the collapse of bubbles generates high-speed shock waves, causing intense local turbulence and producing cavitation noise. This noise has a relatively wide frequency range and produces a rattling sound, similar to that produced when sand or gravel is present in a fluid. Eliminating or reducing cavitation is an effective method for eliminating or reducing noise.
(3) Method of Using Thick-Walled Piping
The use of thick-walled pipes is one of the acoustic treatment methods. Using thin-walled pipes can increase noise by 5 dB, whilst using thick-walled pipes can reduce noise by 0–20 dB. For the same pipe diameter, the thicker the wall; for the same wall thickness, the larger the pipe diameter; the better the noise reduction effect. For example, in a DN200 pipeline, with wall thicknesses of 6.25, 6.75, 8, 10, 12.5, 15, 18, 20 and 21.5 mm respectively, noise reduction is -3.5, -2 (i.e. an increase), 0, 3, 6, 8, 11, 13 and 14.5 decibels respectively. Naturally, the thicker the wall, the higher the cost.
(4) Use of sound-absorbing materials
This is also a relatively common and highly effective method of acoustic treatment. Sound-absorbing materials can be used to wrap the noise source and the pipeline downstream of the valve. It must be noted that, as noise propagates over long distances via fluid flow, the effectiveness of noise reduction ceases wherever the sound-absorbing material ends or thick-walled piping begins. This method is suitable for situations where noise levels are not very high and the pipeline is not very long, as it is a relatively costly approach.
(5) Series Silencer Method
This method is suitable for attenuating aerodynamic noise; it can effectively eliminate noise within the fluid and suppress the noise level transmitted to the solid boundary layer. It is most effective and economical in applications with high mass flow rates or high pressure drop ratios across the valve. The use of absorptive series silencers can significantly reduce noise levels. However, for economic reasons, attenuation is generally limited to approximately 25 decibels.
(6) Soundproof Enclosure Method
This method utilises soundproof enclosures, rooms or buildings to isolate the noise source within, thereby reducing the noise level in the external environment to an acceptable range.
(7) Series Throttling Method
In applications where the pressure ratio of the control valve is high (△P/P1 ≥ 0.8), the series throttling method is employed. This involves distributing the total pressure drop between the control valve and a fixed throttling element downstream of the valve. Examples include the use of diffusers or perforated restrictor plates, which are the most effective methods for noise reduction. To achieve optimal diffuser efficiency, the diffuser must be designed (in terms of shape and dimensions) according to the specific installation conditions, ensuring that the noise level generated by the valve matches that of the diffuser.

(8) Select low-noise valves
Low-noise valves operate by gradually decelerating the fluid as it passes through the tortuous flow path (multi-channel, multi-groove) of the valve plug and seat, thereby preventing supersonic flow at any point within the flow path. A variety of low-noise valve types and designs (including those specifically designed for particular systems) are available for selection. Where noise levels are not particularly high, low-noise globe valves should be selected, as these can reduce noise by 10–20 decibels and represent the most economical option.

Faults in Valve Positioners
Conventional positioners operate on the principle of mechanical force balance, specifically using orifice-flap technology, and are prone to the following types of faults:
(1) As they operate on the principle of mechanical force balance, they contain numerous moving parts that are susceptible to temperature and vibration, leading to fluctuations in the control valve;
(2) With orifice-flap technology, the orifice holes are very small and can easily become blocked by dust or impurities in the air supply, preventing the positioner from functioning correctly;
(3) When operating on the principle of force balance, the spring’s elastic coefficient may change in harsh field conditions, causing non-linearity in the control valve and resulting in reduced control quality.
(4) Intelligent positioners consist of components such as a microprocessor (CPU), A/D and D/A converters; their operating principle differs entirely from that of conventional positioners, as the comparison between the setpoint and actual value is purely an electrical signal rather than a force balance. Consequently, they are able to overcome the drawbacks of the force balance mechanism found in conventional positioners. However, when used in emergency shutdown applications, such as emergency shut-off valves or emergency vent valves, these valves are required to remain stationary in a specific position and only need to operate reliably when an emergency arises. Prolonged retention in a single position can easily cause the electrical converter to lose control, creating a hazardous situation where the valve fails to respond to small signals. Furthermore, the position-sensing potentiometers used in valves operate in the field, where resistance values are prone to fluctuation, creating a risk of failure to act in response to small signals or full opening in response to large signals. Consequently, to ensure the reliability and availability of smart positioners, they must be tested frequently.