By strictly controlling the pressure and temperature of both heating. And cooling environments in heat exchangers, the use of automatic control valves can help ensure efficient heat transfer.
Start-up, linear modulation control valves have been used in processes to accurately control. The flow and pressure of liquids since the eighteenth century. When reliable actuator designs and automatic controllers have been introduced. The activation of pneumatic or electronic valves via remote signals has become commonplace.
Automatic positioning of the valve increases the accuracy of the system, greatly affecting product quality. And reducing waste related to the manufacturing process. When specific feed sensors are added, the accuracy of the system again increases.
Although performance has improved in this way, the control valves. And drive designs still determine the accuracy of the entire system. And changes in the drive mechanism design have not changed significantly. Their precision control. The positioning valve addition has radically altered how the actuated control valve responds in a controlled process loop.
By adding a pneumatic or pneumatic positioning valve to the valve, both single. And double acting actuators can be positioner and controlled precisely. Valves can become an integral part of a digital control system, relaying both valve information. And diagnostic information to ensure that the loop maintains the set point within the desired precision. This article details many of the advantages of installing and applying a positioning device for a basic pneumatic control valve.
The operation of the control valve includes the positioning of its plug relative to the static seat in the valve. The actuator is connected directly to the valve plug through the body. And moved the valve plug to the desired control position. A pneumatic or electric actuator is often used to control the position of the plug. Pneumatic actuator controls operate in either open or air mode. Open air valves are usually closed by springs and need air pressure to open, air valves normally open by springs and needing air pressure to close them. The mechanical design of the valve combination and the actuator determines how the actuator operates. But the unwanted safety condition determines which type is applied (in the case of air of machinery).
A computer, controller, PLC, thermostat or other electrical control device sends an analogue electrical signal directly to an electric drive or via a voltage / compressor or electric-pneumatic locator for pneumatic control. The signal is based on the desired system setting point and regulates the valve in a more linear or closed manner. By automating the valves in this way, they can be used to directly. Or indirectly control the temperature, pressure and flow in a closed or open loop system.
The heat exchanger is a common closed-loop control application where both pneumatic. And electronic control valves can be adjusted to regulate water, steam and condensate.
In order for the pneumatic control valve to be connected to the control signal generated by the control device. There are three methods of positioner the valve: a voltage / voltage converter; converters and compressed air combination. Or electronic valve locating device. Selecting a valve positioner to control offers some advantages over just using one adapter.
Because modern heat exchanger design requires higher efficiency, it is necessary to improve the performance of the control valve. Modular and accurate control of valves with rapid response and accuracy of about 2% of set value. Increased effluent discharge and removal. Some advantages of using valve positioner valves include:
A heat exchanger does not respond fast enough to change the temperature due to the process of changing the load leading to the steam system that can be caused by something as simple as an incorrect hot water faucet or steam valve or Valve actuators are used. Sometimes, changing from a power-operated valve to a pneumatic-driven valve using a locator can make a difference, as long as the heat exchanger still meets the power requirements.
If the differential pressure is higher or variations in valve pressure cause the valve positioner to change, the positioner will automatically adjust the air pressure for the actuator to “reposition” the adapter control signal. When a positioning device is used, the valve has a closed loop controller based on the input signal and the position feedback of the leg. This is done via a positional information connector, a pneumatic amplifier, and an output pressure regulator for the actuator according to the input control signal. This closed loop control circuit is inseparable from the positioner and thus maintains the locking force and the open force (thrust) in accordance with the position of the valve and the valve.
A larger valve valve or piston valve may require increased air volume, increased flow and vice versa. And increased air pressure not due to conventional create. For example, a larger pneumatic diaphragm actuator may have an internal mass of 0.30 ft3 to be filled to move the valve to the open or closed positioner. If a navigation device is used with the next flow and reverse 0.07 scfs, the valve opens or closes within 4.3 seconds. If you do not use the locator, the opening or closing time will be greatly reduced. Most positioner converters are only rated for 0.02 scfs forward flow. And have even less reverse flow volumes.
Positioner device also supplies the air pressure to the actuator at or near the supply pressure to bring the valve to the desired positioner. They become “forces” regardless of the air supply (psi) to the actuator to move the valve where it needs to reach the balance of force. Positioner converters can not and do not work this way. The converter only provides output pressure based on the linear electric control input signal. Typically, their pressure range is around 30 psi with a normal range of 3-15, 3-27 or 6-30. The air pressure can only change 2-3 psi. While the electric-pneumatic locator can receive the same type of linear input control signal. It is not bound by the calibrated operating range limitations.
Being able to bring these higher pressures is especially useful in overcoming the negative effects of delays. And dead bands that are always present in the combination of actuators and valves. Friction is the most common contributor to negative influences and is always against atmospheric pressure emanating from control signals in dynamic conditions. The positioner always compares the desired point to the positioner of the foot and adjusts the air pressure for the drive to minimize friction effects.
The concept of control action is easier to understand by considering an air operated pneumatic control valve to open with the pneumatic-electric setting. With direct control action, when the input signal to the positioner increases too, the compressed air output from the positioner to the actuator. The valve opens and adjusts when the flow through the valve increases or decreases accordingly.
With reverse control action, when the input signal to the positioner increases, the compressed air output from the positioner to the drive decreases. Sometimes when the design system changes due to pipe rerouting or changing control logic in the controller, the computer or DDC requires a control valve to operate in this way. While using the locator can not change the function of the valve, it can switch the control action from direct to reverse and reverse to reverse.
The fixed flow characteristics are set (design characteristics) of the control valve determined by the type of plug, seat, cage, etc. used. Sometimes the linearization of a nonlinear current characteristic is necessary so that the incremental valve process is constant, regardless of the output of the controller. Modifying or adjusting the flow profile is necessary to tighten control precisely and improve performance. Depending on the design and type of locator, linearization can be accomplished by transferring a mechanical cam or by digital / electronic reprogramming to a new performance curve. Using locators to change existing flow characteristics may be a more cost effective solution than installing new valves or changing valves.
Reduced lift or travel limitation of valves is sometimes required in control systems for safety or performance reasons. Sometimes the valves are overly confused, resulting in inefficient operation at less than 5-10% of their trip. Similar to dividing the input signal to accommodate the full stroke of the valve as described in the previous section. The output signal of the positioner’s pneumatic positioner can be compressed to match the electrical signal input full. This allows the input resolution of the control signal to be applied to the first 5-10% of the stroke valve. But there is a balance between the ability of the valve to react to the signal changes. Smaller controls and more input resolution. Hysteresis and deadband effects are greatly amplified and can affect valve performance with a high “turndown”.
Sometimes the travel or lift of the valve is restricted for safety or process protection reasons. If the positioner and calibration of the locator are changed for these reasons. The mechanical stopping installation must be followed immediately. Restrictive travel mechanisms ensure that the valve will not open outside this point in case of positioner errors and does not cause unsafe conditions.