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Why 3-15 psi air is supplied to valve positioner and not valve actuator directly?
Wiki User
∙ 13y ago
When supplied to the positioner, the travel of the positioner is much more accurate than when connected directly to the actuator, making for better measurements and more accurate interpretation of the process control system as a unit and as a whole.
Wiki User
∙ 13y ago
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What is positioner on control valve?
positioners are used to control the opening or closing of the actuator based on Electric , or pneumatic signals
What are the Application of control valve with positioner?
One of the application of the control valve with positioner is that it is used to control the opening and closing of the actuator based on electric or pneumatic signals. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators.
What are the main parts of control valves?
The main parts of control valve are actuator, diaphragm casing, bonnet,yoke,stem,seat,travel indicator,positioner
How does control valve positioner work?
Positioners are devices that contribute to the control valve in positioning the actuator with the help of a control signal. They respond according to the input signal they receive pneumatically or electrically. These positioners would provide output power to the actuator. Positioners move control valves to a specified position. A control valve positioner is an interpreter between the control valve assembly and the control system that translates the output signal from the control system and adjusts the air to the actuator which then moves the valve to the desired position requested by the control system. Therefore, the positioner will fight against any other forces acting on the valve stem to reach crisp and accurate stem positioning based on the command signal. A properly functioning positioner ensures the control valve will be well-behaved and obedient to the command signal. Control valve positioner types These positioners come in different types: pneumatic, electronic, electro-pneumatic, and digital. Pneumatic devices use a pneumatic input signal to send pressure for opening or closing the valve. They use the flapper/nozzle system and require compressed air to make everything work correctly. A signal of 3-15 pounds per square inch is needed to receive the data from the controller. The input signal tells the bellows to expand or compress, depending on the signal. The bellows move the flapper assembly and the beam measuring the valve stem feedback through the cam. In this process, the flapper will move closer or further from the nozzle. Therefore, the relay will increase or decrease the air output to the valve actuator. When the valve stem moves, it sends feedback through the beam. Then the air output will decrease or increase again, to position the valve appropriately according to the input signal. Electric valve positioners send and receive electrical signals. There are three electric actuation types: single-phase and three-phase alternating current (AC), and direct current (DC) voltage. Electro-pneumatic positioners receive an analog signal, usually 4-20 milliamps from a controller. The signal creates an electromotive force according to the electrical signal applied to a coil. This force moves the flapper and increases or decreases the air to the valve actuator. Here is also feedback from the valve stem and positioner to decrease or increase the output to the valve actuator when required. Digital or “smart” positioners use a microprocessor to position the valve actuator and monitor and record data. Thus, they are very accurate. They bring a lot of benefits to the user. They use less air than analogue positioners and provide more flexibility for setup, plus they allow for online digital diagnostics which as a result increases the life of the valve, reducing maintenance and unscheduled downtime. Best of all, these smart positioners use loop power and offer a handful of field protocols for you to choose from. Advantages of control valve positioner As valve positioners know the valve’s exact position, they provide more precise control than can be achieved by an actuator on its own. Positioners make control valves respond more rapidly to changes in the process variable, which as a result, minimizes the amount of time operating by the system above or below the set point. Positioners can cope with large variations in forces acting on plug. Varying differential pressure across the valve shows instability in the control loop. A positioner helps stabilizing valve position. A positioner allows you to put distance between the controller and the control valve; therefore, more flexible configurations and functions will be provided. They remove the effects of large distances between valve and positioner. They minimize the effects of friction. They eliminate hysteresis which reduces productivity. It allows for split ranging. For instance, one controller for two valves. It allows use of piston actuators with high instrument air supply pressures. It allows a wide range of flow variation. For instance, operate at less than 10% travel under normal conditions. It allows the use of characteristic cams in rotary valves. When should a positioner be installed? A positioner should be considered in the following circumstances: To increase the pressure that a particular actuator and valve can close against. When exact valve positioning is required. To linearize a non-linear actuator. To speed up the valve response. The positioner uses higher pressure and greater air flow to adjust the valve position. When controlling with wide throttling range. When valves are handling sludge or solids in suspension.
What is the principle of positioner in control valve?
to amplify the input pneumatic signal
What is the difference between a Single acting and double acting valve?
Normally a control valve is referred to by its 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 a valve as fail open or fail close, the valve actuator has to be spring loaded. So by having the spring on top or bottom of the actuator piston will we determine if it will be an FO or an FC valve. This kind of valve is also called single acting valve 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 too weak 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 an explosion. To ensure 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 in having very accurate and stable control on a high flow line since the pressure from the positioner does the actual control unlike spring controlling one way and positioner controlling the other way as is in the 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 an emergency. In shutdown valves (open/close ESDVs) the same is true and sometimes at critical and high pressure points we use hydraulics instead of pneumatic as the double acting agent to make sure the valve will close during the emergency. So to summarize the double action in ESD and control valve, it suffices to say that this double action is there just to make sure that the valve will do what it was designed for. Call it an extra fail safe if you want. It might not be needed in theory since a single acting valve should do the trick just as well, but in practice you are at times very glad that 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.
What is the importance of a valve actuator?
Valve actuator are important for installing certain pipes correctly. With out them you can be in for major pipe damage and thousands in repairs.
What is the difference between a valve and actuator?
actuator is a mechanism that will responds to hydraulic,pneumatic and electrical signal ,it is used to control the valve that to be in open or close condition.
What is the differences of direct acting actuator and reverse acting actuator?
When a pressure signal is applied, the valve shaft moves in the direction opposite that of the direct-acting actuator, but it follows the same operating principle. In the reverse-acting actuator, the valve shaft is pulled into the actuator by the application of a pressure control signal.
What the function of motor operated valve actuator?
The main function of a valve actuator is to control the valves ON to Close or modulating the valve,and the motor operated valve actuator is one important kind of valve actuator,the main different between kind valve actuators is it drive by electric power.
Which type of solenoid valve used for double acting actuator?
a Three Way Solenoid valve is required to operate a double acting Pneumatic Actuator , depending of the orientation of the Actuator could operate as a Fail to open or Fail to Close .
What is actuator in turbine?
Which actually Actuates the Steam Control Valve
