Control valve automation has made significant progress in the past few years. Although the control valve itself has basically not changed, more emphasis is placed on the automation and accurate positioning of these instruments.
This paper introduces the importance of control valve automation in modern process factory and the precautions to ensure the best valve performance. There are many factors at work when selecting equipment or technology for automatic valves. They include specific application requirements, electrical certification, safety integrity level (SIL), digital protocol and traditional analog communication.
introduce
The valve positioner is the key component of the final control element of the process loop - control valve. The positioner converts the electrical signal into a pneumatic signal to control the actuator displacement. This ultimately keeps the valve in an intermediate position in response to variable process control signals.
The manufacturer of control valve positioner must meet the strict requirements for efficiency, reliability and reducing energy consumption, and produce cost-effective instruments that meet the requirements of the industry. The demand for predictive diagnosis is the demand for valve positioners. It is important to predict failures before catastrophic events of valves, control signals and positioners occur and the plant is shut down.
Recent progress
The control valve industry does not innovate as fast as other technology-centered fields. Designers did not make a major breakthrough in originality, but slowly adjusted and improved products over time, providing a slow but sustained development speed.
In the past decade, the development of intelligent and digital valve positioners has exceeded the performance capabilities of mechanical and electro pneumatic systems. The progress of digital communication protocol enables intelligent controller technology to make progress and integrate more complex functions. For example, positioner manufacturers are moving from designs that require direct contact with the valve stem to non-contact Hall effect sensor technology. The improvement of equipment diagnosis plays an important role in promoting the transition from traditional corrective and plan based maintenance to predictive maintenance. Diagnosis using a valve positioner becomes more complex and provides the ability to test the valve to determine whether maintenance or replacement is required.
The development of digital fieldbus protocol and low-power central processing unit has accelerated the development of valve positioner as a computer. Just as process sensors and controllers gain intelligent functions, valve positioners and controllers follow the same route.
Intelligent control valve positioner with internal trend, diagnosis, alarm and other functions is adopted, and external software is not required in many cases. This method is more failsafe and safer for the end user, because changing the setting parameters must be physically changed on the valve positioner. In addition, saving the valve history data in the positioner instead of the distributed control system ensures that valuable information will not be lost when the positioner is removed from the service. The ability to configure locators locally also eliminates the need for expensive handheld devices.
Some of the most important innovations in automatic valve monitoring are related to the air source (i.e. measuring and recording the air source pressure of the positioner and providing an alarm when it is lower than the set pressure), output pressure (i.e. measuring and recording the output air pressure of the actuator and providing an alarm if it does not exist after a period of time), and discharge (that is, detect the leakage of the corrugated section of the control valve, if there is leakage, close the valve without external control), and the emergency shutdown (ESD) solenoid valve (that is, test and determine whether the solenoid valve will act as required in an emergency).
A particularly important enhancement of automatic valve technology is partial stroke test (PST), a function in the positioner to open the ESD valve from 100% to the predetermined position (80%) , and then return to 100%. This means that the valve will move when needed. The new technology allows the factory to automatically execute the PST from an external source or according to a set time interval, and display a report indicating that the test passed. In addition, the current generation of intelligent positioners can be controlled by using discrete input and discrete output trigger output, while valve friction monitoring indicates the internal friction of the valve Increase or decrease, and the type of hysteresis it causes.
For the improvement of instrument and system operation, the valve signature test can be carried out to accurately evaluate the condition of internal components of the valve. This helps to eliminate unnecessary equipment service, spare parts procurement and valve replacement. The trend and histogram can be run in the background to track the performance of the positioner, and the alarm status list can provide alarm records. These advanced functions enable the process Equipment planners can determine the overall operation of their instrument assets and then formulate effective maintenance strategies.
Some positioner manufacturers have taken the lead in introducing advanced fugitive emission monitoring solutions, which help to avoid the failure of the bellows seal and packing of the valve and the possibility of harmful gas emission. These solutions enable the positioner to feel the bellows seal housing internally, open or close the valve, or move it to a predetermined position under too high pressure If the positioner senses a leak, it can start an alarm to remind the operator of the status of the valve. Through leak monitoring and early occurrence of fugitive emissions, the factory can immediately arrange maintenance to minimize air pollution and avoid fines.
Finally, energy consumption is an important issue for industrial equipment. When designing the control system, the power of the power supply is 1.5 times that of the current requirements for supplying power to a single loop. The factory gradually increases the power consumption demand for each equipment, which affects the field instruments. The instrument manufacturers deal with this situation by developing new and energy-saving positioner designs. Early valve positioners need to It takes about 2 to 3 w to operate, while the current generation of positioners, even with advanced functions, only need 0.5 to 0.75 w to operate.
Choose the right solution
Modern control valve positioners are designed to allow them to be used as switchgear or for any control combination, including adjustment, regulation, mixing and even isolation. Valve positioners are highly engineered instruments and should not be regarded as commodities. Solving the performance problems of control valves will have a significant impact on plant efficiency, overall profitability and asset life cycle cost.
Process equipment has two main requirements for control valves: ease of use and cost-effectiveness. However, some key factors play a role in the selection of equipment for automated valves:
? specific application requirements
? explosion proof
? intrinsic safety
Split installation
? high temperature
? low temperature
? Electrical Certification
?FM
?CSA
?ATEX
?usCSA/FM
? SIL level
? safety system
Communication drives automation in complex industrial facilities. The control valve positioner must be able to communicate with all recognized field device protocols, from traditional simulation to Hart, MODBUS, PROFIBUS, foundation fieldbus and industrial Ethernet.
Valve positioner technology with integrated digital communication provides plant operators with higher visibility and control over key assets. Intelligent valve positioner provides enhanced functions and practical advantages in improving plant performance and operation efficiency.
The main reason why digital positioners are popular is that they can do much more than controlling the position and opening of the valve. The latest positioners can also collect data about the valve to automatically remind users of the performance and assembly of the valve.
Looking ahead
The natural response to the increasing economic, environmental and competitive pressure is to realize the modernization of automation technology. Therefore, many companies are investing in powerful and most advanced control instruments.
Technology can help factories simplify operations and improve their efficiency. Valve automation is an example. Suppliers of automated valve solutions develop functions to meet changing industry requirements. New forms of communication are coming and tools are available to meet additional diagnostic requirements. The transformation from pneumatic linear control valves to electrical linear control valves has come. Control valves naturally fit the number of valves Word intelligence, which may be one of the most important elements of a successful plant asset management strategy. With more intelligent valve automation, end users can better diagnose and repair problems before faults occur.
conclusion
Strengthening the automation of control valve will help all kinds of manufacturers to continuously improve process efficiency and product quality while protecting personnel, equipment and environment.
The latest generation of intelligent control valve positioners enables industrial organizations to conduct more diagnosis locally and better understand the overall performance of the valve at a lower cost.
Any decision to use the positioner on the valve should be made by knowledgeable and experienced control valve experts who can make suggestions on the correct positioner type and installation technology, which is very important to optimize the control valve.