What is Control Valve?
A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
In automatic control terminology, a control valve is termed a “final control element”.
By its name, a control valve is a valve with the purpose of control. A typical control valve application could be for flow, temperature, pressure, or liquid level control. The control valve is opened and closed by a control signal which can be pneumatic, electric, or hydraulic.
Control valve use In the Process industry whether it is oil and gas, refinery, petrochemical, chemicals, specialty chemicals, pharmaceutical, power, etc., to control the process or process related utilities some control elements are required. Control valves are used as the final control element in most fluid handling methods.
How Control Valves Work?
Control valves regulate the flow of a liquid or gas by opening or closing internal passages. They form part of a control loop used to control a process. The control valves respond to instructions from the controller and adjust the internal openings accordingly.
To “control” the liquid or gas, a control valve can open or close fully (on/off), or slightly (modulating).
Modulating valves can open or close slightly. They do this in order to change the size of the opening through which the media is flowing. They may also be moved to fully closed or fully open. However, a valve by itself will not do anything. In order to open or close, the valve actuator must receive a signal from an outside source.
This is provided by one of the following:
- Pneumatically actuated control valve: a pneumatic pilot or liquid level controller
- Electrically actuated control valve: an electric pilot, electric liquid level controller, Programmable Logic Controller (PLC), or Remote Terminal Unit (RTU)
Purpose of control valves
A control valve varies the rate of flow passing through itself. The valve stem moves, altering the size of the passage, and this increase decreases or holds steady the flow.
The control valve opening is altered whenever the process parameter being controlled does not equal the value it is meant to be (the setpoint).
How Does a Control Valve Hold Back Pressure?
Here’s how a pneumatically actuated High-Pressure Control Valve installed on a separator holds back pressure:
- Pneumatic Pilot. The signal is provided by a pneumatic pressure pilot. The pilot on this package senses upstream pressure and sends a corresponding signal (typically processed supply gas) through tubing into the actuator of the valve.
- Pneumatic Actuator. The actuator reacts to this pilot signal by either closing or opening the valve.
- Electric Pilot. In the case of an electrically actuated High Pressure Control Valve installed on a separator to hold back pressure, the signal is provided by an electric pilot (or PLC). The electric pilot receives an analog 4-20mA signal from a sensor, which represents a process condition. The pilot then conditions this signal and sends a corresponding signal (4-20mA loop) to the electric actuator of the valve.
- Electric Actuator. The actuator reacts by either opening or closing the valve.
Operation of Control Valve
The opening or closing of automatic control valves is usually done by electrical, hydraulic, or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening.
Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switchgear, and hydraulically-actuated valves required high-pressure supply and return lines for the hydraulic fluid.
The pneumatic control signals are traditionally based on a pressure range of 3–15 psi (0.2–1.0 bar), or more commonly now, an electrical signal of 4-20mA for industry, or 0–10 V for HVAC systems.
Electrical control now often includes a “Smart” communication signal superimposed on the 4–20 mA control current, such that the health and verification of the valve position can be signal-led back to the controller. The HART, Fieldbus Foundation, and Profibus are the most common protocols.
An automatic control valve consists of three main parts in which each part exists in several types and designs:
- Valve actuator – which moves the valve’s modulating element, such as ball or butterfly.
- Valve positioner – which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear.
- Valve body – in which the modulating element, a plug, globe, ball or butterfly, is contained.
What Are the Important Control Valve Parts?
Here is a look at some of the key internal parts of our pneumatic Stem Guided High-Pressure Control Valve. These are linear valves that have a stem that guides the ball into the seat.
They can operate in modulating control or on/off applications.
- Bonnet: The bonnet is the top of the valve, which houses the diaphragm and actuator spring.
- Spring: The spring holds tension on the stem, keeping the valve in its failure position.
- Control Valve Actuator: The control valve actuator receives a pneumatic signal from a pressure pilot or another pneumatic controller and adjusts the valve position accordingly. This actuator can be disassembled and inverted to convert the valve from fail-closed to fail open or vice versa.
- Stuffing Box Assembly: The stuffing box assembly houses the valve packing. The valve packing provides a seal that prevents leakage from the stuffing box while allowing the stem to move up and down as needed to open and close the valve.
- Yoke and Travel Indicator: The yoke and travel indicator give a visual identification of the valve’s position at any given moment.
- Stem and Seat: The ball and seat are what block off the flow through the valve. These are vital pieces and also the items most susceptible to wear and tear from contact with abrasive or corrosive production.
Why Control Valves used?
Process plants consist of hundreds, or even thousands, of control, loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variables such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product.
Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable.
To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point.
A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs. When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller.
Types of Control Valve
The most common valve types in flow control industries include:
- Gate valves
- Globe valves
- Pinch valves
- Diaphragm valves
- Needle valves
- Butterfly valve
- Plug valve.
- Ball valve.
Continue reading to learn more about each of these types of flow control valves and their functions.
1. Gate Valves
Gate valves are general service valves primarily used for on/off, non-throttling service. Specifically, gate valves are used in applications requiring a straight-line flow of fluid with minimum restriction is desired. Gate valves operate when the user rotates the stem in a clockwise to close (CTC) motion or a clockwise to open (CTO) motion.
The gate moves up or down on the threaded step when an operator moves the stem, which is why it is a multi-turn valve; the valve must turn several times for it to go from open to closed, and it is a slow operation that prevents water hammer effects.
Engineers also utilize gate valves when minimum pressure loss and a free bore are required. Typical gate valves have no obstruction in the flow path, which results in a minimal loss of pressure.
2. Globe Valves
A linear motion valve, globe valves stop, start and regulate flow. Globe valves initiate closure via a plug featuring a flat or convex bottom that is lowered onto a horizontal seat situated in the center of the valve. When a user opens the valve, the plug raises to allow fluid to flow.
Globe valves are used for on/off and throttling applications because the disk of the valve can be removed from the flow path completely or it can completely close the flow path. While this type of flow control valve does produce slightly higher pressure drops than straight-through valves like gate, plug, and ball valves, they are applicable in situations where the pressure drop through the valve is not a controlling factor.
3. Pinch Valves
A cost-effective flow control valve, pinch valves are ideal for applications of slurries or liquids containing significant amounts of suspended solids. Pinch valves seal using one or more flexible elements like rubber tubes that become pinched to turn off the flow.
These rubber sleeves are the valve’s only wetted part, and their flexibility allows pinch valves to close tightly around entrapped solids. Air or hydraulic pressure is placed directly on the elastomer sleeve to actuate pinch valves. A pinch valve’s body acts as a built-in actuator, which eliminates expensive hydraulic, pneumatic, or electric operators and results in the cost-effectiveness of this type of flow control valve.
4. Diaphragm Valves
Diaphragm valves are characterized by a flexible disc that contacts a seat at the top of the valve body and forms a seal. The diaphragm is flexible and pressure-responsive; it transmits force to open, close, or control a valve.
While diaphragm valves are related to pinch valves, they use an elastomeric diaphragm rather than an elastomeric liner in the valve body. The elastomeric diaphragm is attached to a compressor and separates the flow stream from the closure element. Diaphragm valves are ideal for handling corrosive, erosive, and dirty services.
There are many advantages to using diaphragm valves: they are extremely clean, feature a leak-proof seal, have a tight shut-off, are easy to maintain, and reduce leakage to the environment. Diaphragm valves also may be repaired without interrupting a pipeline.
5. Needle Valves
Needle valves are volume control valves that restrict flow in small lines. Fluid moving through the valve turns 90 degrees and flows through an orifice that serves as the seat for a cone-shape-tipped rod. The orifice size changes when the user positions the cone in relation to the seat.
Needle valves are similar to globe valves in that they share a few design features and have similar benefits; for example, both needle valves and globe valves empower operators to change flow rate using a threaded rotating stem.
The difference between needle valves and globe valves is the precision that needle valves can achieve. In fact, needle valves are an ideal choice for calibration applications because they are capable of being fine-tuned.
Needle valves can provide positive shutoff in order to allow gauges and other measurement instruments to be installed or removed safely. That’s also why needle valves may be used in a range of industries, from petrochemicals to biofuels.
6. Butterfly valve.
A butterfly valve is operated by rotating a disk within the flow area and, due to this design, it does not have linear flow characteristics. This makes these valves less precise than the more common flow control valve types above.
For this reason, it can often be dismissed as a flow control valve choice even though it is useful in some applications that do not require a very high degree of accuracy. They are also a very affordable valve option, which makes it worthwhile to consider them in the right applications.
7. Plug valve.
Plug valves come in a variety of configurations and are operated by rotating a cylindrical or cone-shaped plug within the valve body to regulate the flow through a hollow area of the plug. For flow control applications the most common design is an eccentric plug valve, which uses a half plug to create a higher seating force with minimal friction as it is opened and closed. This has the advantage of greater shut-off capability which is ideal for flow control situations.
8. Ball valve.
Ball valves are commonly used in flow systems across numerous industries due to their low cost, durability, and excellent shutoff capability. Similar to butterfly valves, they are not as effective for flow control applications that require a high degree of accuracy and control.
One of the reasons for this is that a ball valve requires a high degree of torque to open and close that preventing an operator from making fine adjustments.
What are the factors that must be considered while installing a control valve, so that we can avoid future maintenance?
The control valve will function properly if the installation is done perfectly so we must consider certain factors while installing a control valve to operate the valve perfectly without any trouble they are
- We must avoid the valve installation where the vibrations or external stress may impair proper valve functions. So, we must install it in a place which is suitable for the valve if it is inevitable then we must consider proper provisions against these.
- We must make sure that the control valve is installed in a place where it has sufficient space to operate and can be easily accessible to do the maintenance too.
- We can provide support to the valve so that it won’t be a burden to the process pipe due to the weight of the valve or we can provide supports to the pipe runs.
- We should not install a valve in places where it will submerge in water, snow, or in places of excessive freezing
- We must take precautions against the corrosive nature of the atmosphere
How to do the quick check and maintenance of a control valve
What are the major parts that must be inspected in a control valve?
- Actuator diaphragm
- Actuator shafts
- Trim components such as plug seat cage and stem
- Packing box must be inspected which is in the bonnet
- Gasket seating surface
- NDT test for body valve
- We must check the condition of the valve positioner
- Bolts and nuts of the body
- Actuator spring
- Handwheel if present
- The surface of the flange gasket
Actuators are also affected by mechanical problems like other components and these problems could affect the process. So, we need to find the problems in the actuator and we can reduce or eliminate the problems, by checking the valve position indicator and we must compare it to the position that the valve controller is called for.
In some types of actuators, they have manual operators and it can be used to solve the actuator problem and in other types of actuators it would have manual bypass lines and it could be near to the failed components and we can solve the actuator problems by this. We must also check the spring of the actuator if the tension of the spring is not proper then it could lead to problems.
Diaphragm inspection, while doing this we must insect the springs. The actuators and positioners of the valve must be properly sized
While doing the routine maintenance we need to check the gland nuts and tighten them if there is any leakage through gland packing. We must also check the nuts on the top and bottom flanges if the gasket is leaking.