How can gas on-valves and gas off-valves be distinguished by appearance?
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How can one distinguish between air-operated open and air-operated closed functions of on-off valves and control valves from their appearance? Under what conditions are air-operated open valves used, and under what conditions are air-operated closed valves used? For the inlets and outlets of steam, heat transfer oil, circulating water, refrigerant, condensate water, hot water, etc., in a reaction kettle jacket, should valves be used with air operation for opening or closing?
1. How can one distinguish between air-open and air-close functions of on-off valves and control valves from their appearance? A valve with a pipe connected to the top of the cylinder is usually a gas-shut-off valve. The valve connected to the air pipe at the bottom of the cylinder is a normally open valve. This is just the actuator; what’s crucial is the structure of the valve body connected to it. For example, control valves usually have a valve stem that is perpendicular to the valve body, while butterfly valves are something different. But most follow this format. For safety reasons, it is still necessary to make a judgment in conjunction with the corresponding valve body. 2. Under what conditions are air-operated open valves used, and under what conditions are air-operated closed valves used? In most cases, during the design phase, accidents such as power outages that lead to shutdowns are taken into consideration; it is necessary to determine whether the process requires timely pressure relief or the closing of feed valves in order to automatically ensure the safety of the system. It is mainly established with this aspect in mind, namely the open or closed state of the gas. 3. Should the inlets and outlets for steam, heat transfer oil, circulating water, refrigerant, condensate water, hot water, etc., in the reactor jacket be controlled by air-on or air-off valves? It is mainly determined by the requirements of the manufacturing process, and is set with safety in mind.
In terms of appearance, for straight-stroke actuators (globe valves), those with air intake at the top are air-shut valves, while those with air intake at the bottom are open valves; it is not possible to tell from the appearance whether it is an angle-stroke actuator (usually butterfly valves or ball valves). The decision of whether a particular valve should be set to open or close in response to air pressure is primarily based on process safety considerations – that is, it determines whether the valve should remain open or closed in the absence of any power supply (air pressure, voltage, current).
For air supply valves that need to be opened in an emergency situation, and those that need to be closed under the same conditions, in simple terms, if the purge air supply to a control valve is interrupted, it is safer for that valve to be closed rather than open.
Check whether the diaphragm takes in air from the top or from the bottom; air intake from the bottom means it is a gas-on type, while air intake from the top means it is a gas-off type!
When the gas is turned on, the valve is in a normally closed state; gas is supplied when it needs to be opened. Air shutdown: the valve is in an open state; ventilation is provided when it needs to be closed. The choice depends on the process; if the valve needs to remain open most of the time with occasional closing, select a gas-shut type. There is also another type called double-acting, where both the opening and closing are operated by air.
Judging from the air inlet, it is such that the air supply valve opens when there is wind and closes when there is no wind, while the air shut-off valve does the opposite
It is not possible to tell from the appearance of the on-off valve whether it operates on air open or air close principle; the key is to determine whether the fault condition is in the open state or the closed state. If the fault condition is open, then it operates on air close principle; if it is closed, then it operates on air open principle. To determine whether a control valve operates on air open or air close basis, it depends on whether the air inlet is above or below the diaphragm; if it is above, the valve operates on air close, while if it is below, it operates on air open. For the jacket steam, heat transfer oil, circulating water, coolant, condensate water, hot water, etc. in the reaction kettle, it depends on the process requirements
For single-acting actuators, whether of linear or rotary type, it is recommended to check the valve position in the absence of instrument air supply. If the valve is normally closed, it is a gas-open valve; if it is normally open, it is a gas-close valve. (Observing the air inlet is only applicable to diaphragm actuators; this method is not effective for piston-type angular stroke cylinders.) As for the choice between air-operated open, air-operated closed, and double-acting types, it is determined based on the operating conditions: if positioning is required even when there is no air supply, a double-acting type is chosen; if the valve should close when there is no air supply, it is a single-acting air-operated open type; and if the valve should stay open when there is no air supply, it is a single-acting air-operated closed type.
Through the on-site switch, when pressure relief is required, the valve operates as a gas-shut valve; otherwise, it is set as a gas-open valve. Whether it remains a gas-shut valve or not is determined with the aim of keeping the system in a safe state in case of system abnormalities