Control Valve Basics
A Control Valve is used to control process conditions such as flow, pressure, temperature and level. This is done by closing and opening in response to signals. These signals can come from sensors or controllers.
The opening and closing of the valve can be executed by electric, pneumatic (air) or hydraulic (liquids) actuators. Some actuators can also provide intermediate position (half open).
A simple control loop usually consists of the control valve, a controller and a sensor. The sensor detects the condition of the process and sends a signal to the controller. The controller will then decide if a change in the process condition is necessary, and if yes it will send a signal to the control valve to make the necessary adjustment.
The most basic control signal that is widely used in many industries is the 4-20mA signal.
Parts of a Control Valve
The three main parts are:
- Valve Body
Electric actuators use electricity to drive the valve. They are the most common type of actuators as electricity is widely available throughout the process plant. They have generally lower maintenance costs as compared to the other types of actuators.
In case of failure, electric actuators can be designed to open/close upon failure, or hold its last position.
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Pneumatic Actuators require a compressed air supply to work. Compressed air is an expensive commodity on site. The benefit is that it is suitable for use in explosive environment. It is also generally faster acting than electric actuators. Similarly, the fail-safe position of the actuators can be defined.
Pneumatic actuators come in 2 types: Piston type and Diaphragm type.
The actuator can be mounted onto a variety of valve bodies, depending on the requirement. Examples are:
- Ball Valves
- Gate Valves
- Globe Valves
- Needle Valve
- 3-way valves
- And many more
Pneumatic valves require a positioner. The positioner serves a few purposes. The first basic function is to receive the control signal and adjust the valve position to match the signal by applying air pressure to open or close the valve. Secondly, it can notify the controller of the valve’s actual current position via an electrical signal. This is also known as a feedback signal.
Sizing of Control Valve
The most basic consideration is capacity. To size a valve, first of all the desired flow rate and pressure drop across the valve must be known.
Kv = FlowRate / √ΔP
Where ΔP is the pressure drop
The flow coefficient, Kv is defined as the maximum amount of flow at a pressure differential of 1 bar at a given opening of the valve.
The pressure drop depends on many factors, such as the orifice area, shape, path, material of valve etc.
Using the Kv valve will allow us to calculate the flow at specified pressure drop, or vice versa.
As a rule of thumb, it is best to operate the valve between 20% to 80% opening most of the time.
There are many other factors to consider when it comes to selection and sizing. Contact us for further advice by clicking here