Normally a control valve is refer to by it's fail position. This means "what position will the valve move to should the supply air or control signal to the valve falls away". This is important to safe guard the process at various places so some valves will be fail open and some fail close. In order to have valve as a fail open or close the valve the actuator have to be spring loaded. So by having the spring on top or bottom of the actuator piston, will determine if it will be a FO or FC valve. This kind of valve is also called single action since it will only have one output from its positioner to either the top or bottom of the actuator. The positioner on the valve is also setup as a single acting positioner since it will only give a single action to the actuator, the reverse action will be done by the spring. The problem with this setup is that it is possible that the process might be so strong or the pressure so high (during a blow down or ESD shutdown in the plant) that the spring might in certain instances be to week to push the valve into the fail position quick enough, due to the back pressure from the process and can cause damage to the plant or even a explosion. To make sure that the valve will go to the fail position we install a double action positioner with two outputs. One goes to the top of the actuator and one to the bottom. This is also very helpful to do very accurate and stable control on a high flow line since the pressure from the position do the actual control and not spring control one way and positioner control the other way as in single acting control valves. It is also solving the problem that the valve will now be forced into the fail position by the spring as well as the positioner supply pressure during a emergency. In shutdown valves (open/close ESDV's) the same is true and sometime at critical and high pressure points we use hydraulics instead of pneumatics as the double acting agent to make sure the valve will close during a emergency. So to summarize the double acting action in ESD and control valve is just there to make sure the valve will do what it was designed for. Call it a extra fail safe if you want. In theory not needed since a single acting valve should do the trick just as well,but in practice you are at time very glad you did it especially if you look at the kind of pressures the valves are working on. With those kind of flows and pressures you don't want to leave anything to chance
Process Level Measurement Traditional sensors such as displacers and differential pressure transmitters are making way for new technology and in particular two-wire radar level transmitters. We look at some of the advances in different level technologies and highlight why pulse radar is rapidly becoming the preferred choice for process level measurement. Capacitive / Admittance level transmitters Capacitive level transmitters have been used on a variety of liquid and solid level applications for many years. If the process is understood and does not vary, and if a number of basic rules are followed, then capacitive level sensors can prove to be reliable and cost effective. In the past, applications with very viscous and conductive products have proved to be problematic. However, recent advances such as "phase selective admittance" processing within the transmitter oscillator have allowed automatic compensation for product build-up on the prob...
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