Temperature, pressure, level, and flow instruments all sense a process parameter and produce a signal for indication or controller input.
If we want to control a process parameter, the controller output must convert to a signal that can translate to and subsequently drive a control valve. The control valve is a final control element. A final control element is any device or element that changes the value of a manipulated variable. Valves and heaters are common examples. Let's look at control valves and the devices that process the signal supplied to the control valve.
Achieve the programmed
In this illustration you can see the controller output sends an electronic signal to the current-to-pressure transducer (I/P), which sends a pneumatic signal to the control valve.
The control valve position changes in response to the signal to adjust flow to the setpoint. As the flow changes, it is sensed by the flow transmitter. When the flow sensed is equal to setpoint, the valve position remains the same. Any time there is a disturbance to the system or a change in setpoint, the flow control loop automatically responds to achieve the programmed setpoint. A block diagram of this concept is here.
The final control element can be proportional control or ON-OFF control. For ON-OFF control, a controller output relay changes the state of the relay contact, which completes the circuit for a solenoid valve to energize. The solenoid valve opens to allow air to open (or close) a control valve.
The first component in the final control subsystem is the signal conditioner. The signal conditioner amplifies and, if necessary, converts the signal for compatibility with the actuator.
Typical devices used as signal conditioners include current-to-pneumatic transducers, current-to-voltage (I/E) transducers, amplifiers (electronic or pneumatic), relays, digital-to-analog converters, or analog-to-digital converters. The most common signal conditioner in a proportional control loop is an I/P transducer.
A typical I/P transducer is a force balance device in which a coil suspends and hangs in the field of a magnet. Current flowing through the coil generates axial movement of the coil, which causes movement of the beam. The beam controls the backpressure against the nozzle by controlling the restriction of airflow through the nozzle. This backpressure acts as a pilot pressure to control the outlet pressure.
The zero adjustment causes the beam to move relative to the nozzle. The span adjustment is a potentiometer that limits the current through the coil. The I/P transducer must be supplied with instrument air within the range specified by the manufacturer, usually at least 20 psig.
The typical I/P transducer is calibrated for a 4-20 mA input = 3-15 psig output. Most I/P transducers can be configured for direct action (output pressure increases as input signal increases) or reverse action (output pressure decreases as input signal increases).
Mechanically to the valve
The next component in the final control subsystem, if applicable, is the actuator. The actuator receives the conditioned signal and changes it to some form of mechanical energy or motion.
Typical devices used as actuators include solenoids, pneumatic valve positioners, AC and DC motors, stepper motors, hydraulic motors, and hydraulic pistons. Many control valves include a pneumatic valve positioner.
A valve positioner is a device used to increase or decrease the air pressure (from the I/P) operating the control valve actuator. Positioners usually mount to the control valve actuator and connect mechanically to the valve stem for position indication.
A positioner is a type of air relay, which acts to overcome hysteresis, packing box friction, and effects of pressure drop across the valve. It assures exact positioning of the valve stem and provides finer control. There are many types of positioners. The basic principles of operation are similar for all types.
The instrument pressure (from an I/P, for example) acts on the input module, which controls the flapper-nozzle system of the relay. Supply pressure applies to the relay and the output pressure of the relay goes to the control valve actuator.
Most positioners can set up and function for direct or reverse action. For a direct-acting positioner, increasing the instrument pressure causes the input module to pivot the beam. The beam pivots the flapper and restricts the nozzle. The nozzle pressure increases and causes the relay assembly to increase output pressure to the actuator.
With a direct-acting actuator, the increased pressure moves the actuator stem downward. The positioner connects mechanically to the stem of the valve. Stem movement feeds back to the beam by means of a feedback lever and range spring, which causes the flapper to pivot slightly away from the nozzle to prevent further increase in relay output pressure.
Note that some positioners accept a milliamp input and include an integral I/P transducer.
The last component in the final control subsystem is the final control element. Let's look at control valves (Other final control elements include servo valves, heaters, conveyors, auger feeds, and hopper gates.).
There are many different types, sizes, and applications for control valves. Selecting the correct control valve for a specific application is crucial to proper system performance. Under sizing and over sizing are common problems.
There are many valuable resources available to assist with proper selection, not the least of which is a control valve sales engineer. Here's a typical control valve.
The pneumatic signal from the positioner (or I/P if a positioner is not used) applies directly to the actuator. For this control valve, the air enters above the diaphragm and pushes against spring pressure to close the valve. The valve fully closes when the plug seats tightly against the seat ring.
As air pressure decreases, the spring pressure causes the diaphragm, stem, and plug to move upward, opening the valve. This means a loss of pressure would cause the valve to open. This is a fail-open valve.
Different configurations of air inlet, spring location, and valve seat arrangement result in different fail positions and determine whether the valve is direct- or reverse-acting. For example, this same valve, with the plug below the seat ring (reverse-seated), would open with increased air pressure and would fail closed on loss of air pressure.
So, all components in the final control subsystem must be configured correctly for the system to work properly. The fail-safe positions must be correct for the application, and the action must produce the desired results. These configurations must be properly documented and utilized during calibration, loop checks, or troubleshooting.
Attune I/P transducer
The figure below shows the setup for a bench calibration of an I/P transducer. The air supply connected to the input must be in accordance with manufacturer's specification (typically between 20-100 psig).
The pressure standard connects to the air outlet, and a mA simulator connects to the current input. It is important for the I/P transducer to be oriented the same way as the installed position in the field. A change in orientation will introduce error in most I/P transducers.
If the calibration takes place in the field, one uses the existing supply air. It is convenient to tee into the air outlet so one can check the control valve position at the same time. Of course, you need to ensure the system is in a safe condition before you open and close the valve.
Once the setup is established, apply the mA inputs for each desired test point, such as 4.0, 8.0, 12.0, 16.0, and 20.0 mA. Record the corresponding outlet pressure at each test point. For a 4-20 mA input = 3-15 psig output I/P, the corresponding outputs would be 3.0, 6.0, 9.0, 12.0, and 15.0 psig.
Some facilities adjust the 0% test point so a slightly higher mA input results in the 0% output. For example, 4.10 mA may result in a 3.0 psig output. This ensures the valve is in the closed state with a controller output of 4.0 mA.
Upon ascertaining the as-found readings, evaluate the results against the required specification. If required, perform zero and span adjustments until no further adjustment is required. Then, repeat all test points to record as-left readings.
Many organizations do not require periodic calibration of I/P transducers, positioners, or control valves. The justification is the control signal will adjust the output until the required setpoint is achieved based on the process measurement. This is true, but you want to make sure the output loop is performing correctly. The best way to do so is to check the calibration periodically.
Calibrate valve positioner
Calibration of the valve positioner can be performed at the same time as the I/P in a loop calibration. Simply tee in the pressure module at the I/P outlet in the I/P calibration. Record the valve position at each test point.
If calibrating the valve positioner separately, connect an input test pressure regulator or hand pump, and monitor the input pressure applied with a pressure standard. If there is no supply air, connect the required supply air to the positioner. Apply the pressure for the desired test points and record valve position.
For example, assume our valve positioner is 3-15 psig input = 0-100% valve position. In this case, apply 3.0, 6.0, 9.0, 12.0, and 15.0 psig. The expected valve positions should be 0, 25, 50, 75, and 100%, respectively.
The valve position indicator on the stem usually marks off in 5% or 10% increments. Therefore, a best estimate of the valve position may be all you can obtain. In other cases, a valve position detector provides a remote indication to a DCS. In such cases, ensure both indicators are working properly.
Many organizations do not require calibration of valve positioners for these reasons. There's much documentation that control valve positioner performance is responsible for significant loss in system efficiency and, therefore, increased costs.
To provide guidance on methods for testing positioners and control valve performance, ISA has developed a standard, ANSI/ISA-75.25.01-2000, Test Procedure for Control Valve Response Measurement for Step Inputs.
As to control valve calibration, the process is similar to positioner calibration in that one applies a pressure signal to the actuator and then tallies the resulting valve position. This step can take place with the positioner calibration, if applicable, and it can happen in conjunction with I/P calibration.
Remember to ensure the system is in a safe condition if performing the calibration in the field. In addition, know the correct action, direct or reverse, and fail position before starting. CE
Nicholas Sheble (firstname.lastname@example.org) edits the Certification department for InTech magazine. This article is from Michael Cable's book Calibration: A Technician's Guide, ISA Press 2005. Cable is a Level 3 Certified Control System Technician and is the validation manager at Argos Therapeutics.