Seeking advice on the issue of selecting a high valve position for control valves
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In the equipment I work on, the adjustment of several parameters is done by selecting a higher valve position. I would like to know the principle behind this, as well as why this method is used for adjustment (it’s hardly ever used in actual production). What are the advantages and disadvantages of this method?
Most control valves used in refining and chemical processing plants are pneumatic control valves. These pneumatic control valves include air-open valves (FC) and air-close valves (FO). The choice between an air-open valve and an air-close valve depends on whether it is more beneficial for the safety of the plant that the valve be in the open position or the closed position in case of a failure of the control valve.
Is the poster referring to a situation where, for example, two PID controllers operate on the same valve, with the inputs of these two PID controllers being used to control the valve through an OR gate output? This situation is quite common in practical applications; for example, the control of compressor anti-surge valves and manual control are both preferred options. It ensures that surge is prevented while performing manual operation.
When several regulators control a single terminal regulator, such as a valve, some systems require a low-select or high-select mode, as in the dual-cross combustion control system.
I watched it but didn’t quite understand it. Could the original poster explain it in more detail?
There is a high selector; the signal from the control valve must pass through this high selector device in order to be sent to the valve, thereby preventing the valve from being operated when it is in a low position. This is one of the basic control schemes; there are also low-select, ratio, and other options!
High valve position selection means that one regulator controls two or more control valves; in other words, the valve with a larger opening degree is chosen
The high-or-low selection for the valve position or regulator output involves two or more control devices working together to operate one or more control valves. This approach is widely used in complex and advanced control systems, and it represents one of the basic control schemes. It yields excellent results, enabling intrinsic safety as well as high-quality regulation that meets the various requirements of different process conditions. The downside is that it requires a somewhat higher level of knowledge in operational management and basic instrument control skills from the operators; otherwise, they won’t be able to handle more complex configurations, which leads to manual operation of the valves and renders advanced control systems ineffective