What Is A Control Valve?
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 variable 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 setpoint. 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.
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam,water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.
Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a varietyof additional valve accessories, which can include positioners, transducers,supply pressure regulators, manual operators, snubbers, or limit switches.
Whether it is called a valve, control valve or a control valve assembly is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most important part of the loop. It is useful to think of a control loop as an instrumentation chain. Like any other chain,the whole chain is only as good as its weakest link. It is important to ensurethat the control valve is not the weak est link.
Following are definitions for process control, sliding-stem control valve,rotary-shaft control valve, and other control valve functions and character istics terminology.
Process Control Terminology
Accessory: A device that is mounted on the actuator to complement the actuator’s function and makeit a complete operating unit. Examples include positioners, supply pressure regulators, solenoids, and limit switches.
Actuator*: A pneumatic, hydraulic,or electrically powered device that supplies force and motion to open or close a valve.
Actuator Assembly: An actuator,including all the pertinent accessories that make it a complete operating unit.
Backlash: The general name given to a form of dead band that results from a temporary discontinuity between the input and output of a device when the input of the device changes direction. Slack, or looseness of a mechanical connection is a typical example.
Capacity* (Valve): The rate of flow through a valve under stated conditions.
Closed Loop: The interconnection of process control components such that information regarding the process variable is continuously fed back to the controller set point to provide continuous, automatic corrections to the process variable.
Controller: A device that operates automatically by use of some established algorithm to regulate a controlled variable. The controller input receives information about the status of the process variable and then provides an appropriate output signal to the final control element.
Control Range: The range of valve travel over which a control valve can maintain the installed valve gain between the normalized values of 0.5 and 2.0.
Control Valve Assembly: Includes all components normally mounted on the valve: the valve body assembly, actuator, positioner, air sets, transducers, limit switches, etc.
Dead Band: The range through which an input signal can be varied,upon reversal of direction, without initiating an observable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device. For the valve Figure 1-1. Process Dead Band A7152 / IL assembly, the controller output (CO) is the input to the valve assembly and the process variable (PV) is the output as shown in figure 1-1. When the term Dead Band is used, it is essential that both the input and output variables are identified, and that any tests to measure dead band be under fully loaded conditions. Dead band is typically expressed as a percent of the input span.
Dead Time: The time interval (Td) in which no response of the system is detected following a small (usually 0.25% - 5%) step input. It is measured from the time the step input is initiated to the first detectable response of the system being tested. Dead Time can apply to a valve assembly or to the entire process.
Disk: A valve trim element used to modulate the flow rate with either linear or rotary motion. Can also be referred to as a valve plug or closure member.
Gain: An all-purpose term that can be used in many situations. In its most general sense, gain is the ratio of the magnitude of the output change of a given system or device to the magnitude of the input change that caused the output change. Gain has two components: static gain and dynamic gain.
Static gain is the gain relation ship between the input and output and is an indicator of the ease with which the input can initiate a change in the Figure 1-2.
Linearity*: The closeness to which a curve relating to two variables approximates a straight line. (Linearity also means that the same straight line will apply for both upscale and downscale directions. Thus, dead band as defined above, would typically be considered a non-linearity.)
Linear Characteristic*: An inherent flow characteristic that can be repre-sented by a straight line on a rectangular plot of flow coefficient (Cv) versus rated travel.
Packing: A part of the valve assembly used to seal against leakage around the valve disk or stem.
Positioner*: A position controller (servomechanism) that is mechanically connected to a moving part of a final control element or its actuator and that automatically adjusts its output to the actuator to maintain a desired position in proportion to the input signal.
Relay: A device that acts as a power amplifier. It takes an electrical, pneumatic, or mechanical input signal and produces an output of a large volume flow of air or hydraulic fluid to the actuator. The relay can be an internal component of the positioner or a separate valve accessory.
Trim*: The internal components of a valve that modulate the flow of the controlled fluid.