Reposting: Some questions about the instrument

This post was last edited by backhamm1982 on 2010-8-4 09:33. Proper installation of the valve core + positive-action actuator for the positioner + positive-action actuator (with the spring on top) = positive action (air-operated). If any one of these elements is reversed, the effect will be reversed as well.

Reply to 2# backhamm1982: Upon careful thought, it seems I was wrong in what I said regarding my rating. What you said is probably incorrect, right? Three positive actions should ultimately result in a negative action, which is a pneumatic shut-off valve.

It doesn’t matter whether it’s called positive or negative action; as long as the signal causes the valve to open, that is what the original poster refers to as positive action.

It’s not clear whether the poster is a manufacturer or a designer; the issue of being a manufacturer should be clear, right? It’s a matter of manufacturing processes. If one is a designer, they should understand under what operating conditions the gas should be turned on and under what conditions it should be turned off. First of all, it should be understood that the so-called \"open on loss of air\" and \"close on loss of air\" refer to FO and FC, that is, FAIL OPEN and FAIL CLOSE – meaning the valve opens when there is a loss of air pressure and the valve closes under the same condition. This is done primarily to ensure production safety in the event of a failure in the air supply. If the process requires that gas supply failures, or an emergency shutdown or accidental stop of the entire unit, result in the cessation of flow of the medium in the pipes or prevent any leakage, then FC is used ； If the process requires the pipeline to be emptied in case of a gas supply failure, or during an emergency shutdown or accidental stop of the entire plant, then FO is used. The actuators in a control loop are primarily valves. On-off valves are mainly composed of a cylinder and a valve body, while control valves consist of a diaphragm head, a valve body, and a valve positioner; both types require an air supply. An electric valve is also a type of actuator, consisting of an electric motor and a valve body; it is generally used in air conditioning systems or in situations where there is no gas supply available.

Reply to 5# chq198710: Hello, one more question – does the installation method of the valve core and actuator determine whether it is air-operated for opening or closing? Does a valve positioner affect air-open and air-close functions?

Reply to 6# xyhunter: The locator has nothing to do with whether it is air-operated or electrically operated

For control valves, the diaphragm is located within the diaphragm head, dividing it into two chambers (a and b). The way in which air is introduced into these chambers a/b and the manner in which the valve spool is mounted determine whether it is an FO or FC type； For single-acting on-off valves, there is a spring inside the cylinder; when the air is released, the spring returns to its original position. The way in which the spring and the valve element are mounted determines whether it is of the FO or FC type ； For double-acting on-off valves, there is no spring inside the cylinder; the cylinder maintains its position when there is no air pressure, and the way in which the valve element is installed determines whether it is of the FO or FC type ； A valve positioner is a component mounted on control valves or two-way on-off valves; its function is to ensure that the valve opening reaches the level corresponding to the output signal, regardless of whether it is FO or FC.