This article is about What is Control Valve?, Control Valve Types, Control Valve Sizing Basics and Design Guidelines for Control Valves for plants and refineries.
What is Control Valve?
A control valve is a mechanical device used in various industries to regulate the flow, pressure, temperature, or level of a fluid (liquid or gas) in a system. It acts as a modulating device and can adjust the flow rate automatically based on the input signals it receives from a controller. Control valves play a crucial role in maintaining process stability and efficiency, allowing precise control of the fluid’s characteristics during different operational conditions.
Control Valve Types
There are several types of control valves, each designed for specific applications and functions. The main types of control valves include:
- Globe Valves: These are commonly used for controlling flow in a linear manner. They have a globe-shaped body with a movable disk (plug) that regulates the flow by moving up and down.
- Butterfly Valves: These valves use a circular disc mounted on a shaft to control flow. When the disc is parallel to the flow, it allows maximum flow, and when it’s perpendicular, it restricts flow.
- Ball Valves: Ball valves use a spherical disc with a hole (port) to control flow. They are quick-acting and can provide tight shut-off.
- Diaphragm Valves: These valves use a flexible diaphragm to control flow. The diaphragm moves up and down to regulate the flow area.
- Plug Valves: Plug valves have a cylindrical or tapered plug that rotates to control the flow.
- Needle Valves: These valves have a slender, tapered point at the end of the valve stem that fits into a small orifice to precisely control flow.
- Control Ball Valves: These are modified ball valves with additional features to provide more precise control over flow.
- Control Butterfly Valves: Similar to butterfly valves, but with added features for better flow control.
- Control Globe Valves: Similar to globe valves, but designed for more accurate flow regulation.
Each type of control valve offers distinct advantages and is chosen based on specific application requirements, flow characteristics, and process conditions.
Read Also: Typical Requirements for Control Valves used in Integrated Automation System
Control Valve Sizing Basis
Control valves will be sized based on the fluid characteristics and process requirements. The following table provides general guidelines for control valve sizing.
Table 6 – Guidelines for Control Valve Sizing
| Valve Characteristic | Sizing Criteria |
| Pressure Drop – based on design normal flow rate. | At least 33% of total system dynamic loss or 0.7 bar, whichever is greater. In fixed pressure systems and vacuum systems such as refrigeration and steam systems, control valves shall be sized for a pressure drop equal to the pressure drop available. For long lines, such as rundown systems going offsite, pressure drop may be 15% of the system pressure drop, provided variation in flow rate is small or in system where total pressure is less than 0.7 bar. Here the pressure drop in the control valves may be 0.14 – 0.21 bar. For control valves in the steam line to reboilers, the allowable pressure drop will be 5% to 10% of the initial absolute steam pressure. |
| Minimum Size Body | 1-Inch (except for Packaged Equipment) with approval |
| Valve Opening | Valve opening not to exceed 70- 90% for maximum specified flow. Valve opening not to be less than 10-30% for minimum specified flow. |
Control Valve Block and By-Pass Valve Sizing
The following table may be used as a guide in sizing control valve block valves and the control valve
by-pass valves. In systems with a high pressure drop, if the control valve is two or more sizes smaller than the line size, the block and by pass valves may be of an intermediate size (size between pipe size and control valve body size).
Table 7 – Control Valve Block and By-Pass Valves
Drains
The drain for control valve stations will be located between the control valve and the downstream
block valve. However, in some cases where dictated by process arrangement, the drain may be
provided between the control valve and upstream block valve.
There may also be cases where two drain valves are required, one upstream and one downstream of the control valve, for plugging services where back flushing of the control valve is required on a regular basis.
Special considerations for control valves
1. Control valves will be furnished with either a hand wheel, or with block valves for isolating the
control valves and a bypass valve for hand control. Company approval is required when hand
wheel only is to be provided.
2. All three-way control valves shall be furnished with a hand wheel for manually operating the
valves. Company approval is required to use three-way valves.
3. In case of large size control valves (where cost of bypass/block valves may be very high) and shutdown is allowed, block valves and bypass arrangement may not be necessary, subject to Company approval.
4. Control valves shall be provided with block and bypass valves for the following:
- Services handling dirty, erosive, or corrosive fluids, services with high pressure drop where frequent planned maintenance is expected, or services having materials having a high viscosity.
- Boiler feed water services.
- Steam service with a pressure drop across the control valve greater than 8.5 bar.
- For control valves 2” and smaller.
- Other cases to be identified by Company.
- Ethylene furnace feed and dilution steam control valves do not require block valve and bypass valve.
- Control valves that may be removed from service during operation of the unit shall be furnished with single block valves and valve drain or valve vent and drain.
- Control valves in toxic or lethal services shall be furnished with block valves located adjacent to the control valves.
- Control valves, which are spared or operated in parallel services, shall be furnished with upstream and downstream block valves. Bypass valves and hand wheels may not be furnished, subject to Company approval.
- Ball valves may be used in place of globe valves for plugging services.
Read Also: Distributed Control System (DCS) Historization and Trending
FAQs about Control Valve
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What is Flow Control Valve?
A flow control valve is a type of control valve used to regulate the flow rate of fluids. It adjusts the flow passage to control the amount of fluid passing through, ensuring precise flow rates in various applications.
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What is thermostatic valve?
A thermostatic valve is a type of valve that automatically controls the flow of fluids based on temperature changes. It uses a temperature-sensitive element to adjust the valve opening, maintaining a constant temperature in the system it is installed in.
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What is proportional valve?
A proportional valve is a type of control valve that regulates fluid flow in proportion to the input signal it receives. It can provide precise control over the flow rate, allowing for continuous adjustment to achieve specific desired setpoints. These valves are commonly used in various industrial applications, including automation, process control, and fluid power systems.
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What are pneumatic valves?
Pneumatic valves are a type of control valve that use compressed air to operate and control the flow of fluids or gases in a system. They are commonly used in industrial automation, manufacturing, and various other applications.u003cbru003eu003cbru003ePneumatic valves are known for their reliability, quick response time, and ability to handle high-pressure applications. They come in different types, including solenoid valves, control valves, and directional control valves, each serving specific functions in pneumatic systems.
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What is thermostatic mixer valve?
A thermostatic mixer valve is a type of valve used in plumbing and heating systems to control the temperature of water. It blends hot and cold water to maintain a stable and precise output temperature, which can be set by the user. u003cbru003eu003cbru003eThe valve contains a temperature-sensitive element that expands or contracts based on the water temperature, regulating the flow of hot and cold water accordingly to achieve the desired temperature. Thermostatic mixer valves are commonly used in showers, baths, and other water fixtures to prevent scalding and ensure a consistent water temperature.
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What is throttle valve?
Throttle valves are a type of control valve designed to regulate the flow rate of a fluid in a pipeline. They work by restricting the passage of the fluid, thereby controlling its flow rate and pressure. Throttle valves can be manually operated or automated, allowing for precise adjustments to achieve the desired flow conditions. These valves are commonly used in various industries, including oil and gas, chemical, and water treatment, to control the flow of liquids or gases in a system.