What are the selection principles for air-operated control valves in air-open and air-close modes?

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The choice between air-operated valves that open when air is supplied and those that close when air is supplied is not merely a design decision related to automation; it involves coordination between two fields, namely process engineering and automation. More precisely, it is the process engineering team that makes this decision. Experienced process engineers can determine whether a valve should operate in air-open or air-close mode with ease. In simple, straightforward terms, an air-open valve opens when air is available (and closes when there is no air supply), while an air-close valve closes when air is available (and opens when there is no air supply). ”The absence of a gas supply can be considered an emergency situation or a shutdown for maintenance. Generally, the choice between air-operated or gas-operated controls is made with the emphasis on \"safety\" in terms of process operation; this safety aspect is extremely important, and it is necessary to take into account various factors such as the process itself, the equipment, and the operating procedures, to ensure that production, equipment, and human safety are maintained during emergencies. Additionally, the requirements of the production process must also be considered, though there are also special cases. To give two examples: 1. Take the steam heaters mentioned earlier, such as the reboilers in distillation columns, as an example. The heating medium in these devices is steam. As a general rule, in case of abnormal operations – such as excessively high column pressure or bottom temperature – this control valve should be in a closed position during emergency situations or abnormal operating conditions. For instance, if the instrument air supply is suddenly lost, there is a problem with the compressor, or the pressure in the instrument air buffer tank becomes abnormal, having the valve in a closed position helps to prevent high-temperature steam from continuing to flow into the heater, thereby cutting off the heat source and ensuring the safety of the equipment. \"Close when there is no air supply.\" Obviously, this control valve should be of the type that opens when air is present (it only opens when air is supplied). Thinking more broadly, for the water supply control valve of the condenser at the top of the tower, should it be operated by air pressure to open or to close? To ensure that the condenser does not operate at high temperatures, or in cases where the control valve loses its air supply, in order to maintain normal pressure and temperature at the top of the tower, it is not permissible for the cooling water supply to be interrupted. Therefore, this control valve should remain open in emergency situations, which is the opposite of what happens with the control valve of the heater; the reason is the same: to ensure the normal and stable operation of the distillation tower and to prevent any adverse effects during emergencies. For safety reasons, this valve should operate in an \"open state when there is no air supply\" – obviously, this control valve should be a gas-actuated valve that closes only when air is supplied. In special cases, it may also be necessary to use a gas-shut valve for the heater’s steam control valve; for example, if the material in the tower bottom tends to crystallize at low temperatures, stopping the supply of steam will cause the material to crystallize as the temperature drops. In such emergency situations, in order to prevent blockages in the pipes and equipment and to buy time to deal with the issue, it is necessary to keep steam flowing so that the pipes and equipment do not get blocked, and this is why a gas-shut valve is used in such cases. This is a special case, determined by the specific process conditions. 2. Next, look at the bypass flow control valve at the outlet of the vortex pump. Generally speaking, bypass flow control is a common method; although it results in some loss of electrical energy, it remains a widely used approach for vortex pumps, which are pumps that handle low flow rates but high head pressures. How should the air-open and air-close settings for this valve be chosen? ? Those familiar with pump selection know that it is not allowed for the outlet valve of a vortex pump to be completely closed. If the outlet valve of such a pump becomes closed during operation, the pump will be damaged very quickly. Generally, there are check valves and globe valves at the pump’s outlet, in addition to a bypass control valve used to regulate flow. In the event that the valve element of the pump’s outlet globe valve gets stuck, this bypass control valve must remain open in order to prevent the pump’s outlet from being completely closed. Another scenario: if the air supply is lost and this control valve is in the closed position, then, with the outlet globe valve remaining at the same opening degree as under normal operating conditions, the pressure at the pump’s outlet will increase sharply, which is extremely harmful to the pump. Therefore, to ensure the safety of the vortex pump, this valve should remain open when there is no air supply; obviously, an air-operated valve should be chosen for this purpose. In short, the choice between air-open and air-close operation should be determined based on specific circumstances, by focusing on the main aspect of the conflict. For example, in the case of crystallization of the material in the tower bottom mentioned earlier, crystallization is the key factor at play here. The decision regarding this selection should not be left to the automation engineers alone; experienced automation designers will ask the process engineers about the operating conditions and procedures, such as how operations are carried out, what precautions must be taken in case of accidents, etc. Likewise, experienced process engineers, when providing information to the automation team, will not forget to specify whether the operation should be air-open or air-close in their documentation.