Why have to use Valve Positioners?
An integral part of the fluid flow control system are the positioner, control valve, and actuator are completely controllable the flow of fluid.
The positioner is one of the most widely used equipment in basic industrial plants. The positioner is responsible for closing or opening, adjusting the amount of compressed air corresponding to the control signal into the actuator.
So, if you want to buy one of an integral part please read this tip.
A valve positioner is a control system used to set up the valve’s stem reaches a position balanced to the output signal from the process variable instrument controller. The main task of the positioner to compare signal of a signal control to the position of the valve actuator and moves the actuator accordingly.
How to fix it? The positioner is fitted to the yoke of the actuator and is linked to the spindle of the actuator by a feedback arm in order to monitor valve position. When a control signal differs from the valve actuator’s position, the valve positioner sends the necessary power to move the actuator until the correct position is reached. This uses a high air supply.
(1) Increase control system resolution i.e. fine control
(2) Facilitate operation when the higher number in the actuator bench-set range is greater than 15psig. i.e. 10 – 30psig, 6 – 30psig
(3) Allow the use of characteristic cams in rotary valves
(4) Minimize valve stem packing friction effects and the resulting hysteresis, particularly for high-temperature packing materials such as graphite
(5) Negate flow-induced reactions to higher pressure drops. i.e compensate for internal force imbalances
(6) Increased speed of response to a change in the process; allows faster loading and venting.
(7) Allow for split ranging. i.e. one controller for two valves
(8) To overcome seating friction in rotary valves
(9) Allow distances between controller and control valve
(10) Allow operate at less than 10% travel under normal conditions. i.e. Allow the wide range of flow variation.
(11) Allow increased usage of 4 – 20mA electronic signal
(12) Permit use of piston actuators with high instrument air supply pressures.
Pneumatic Positioners, electronic, electric, pneumatic and digital. Positioning valves. Depending on a vary of valve positioners that pneumatic input signal range, maximum supply pressure, milliampere input signal range, split range, operating temperature, and output action.
Several products are designed for automotive, aerospace, marine, medical, or military applications. But, other products are suitable for food processing or pharmaceutical applications. So that, valve positioners differ in terms of applications, features, and approvals.
General reviews of the valve locator include trademarks of the Canadian Standards Association (CSA) and Underwriters Laboratories (UL), an independent inspection body. Additional standards can be found at the IHS Standards Store.
More details here: http://www.naman-tech.vn/positioner/
The pneumatic valve positioner accepts an input pneumatic signal from a control device and sends an increased or decreased output signal to a control valve to ensure the control valve plug travels correctly and is positioned properly on the valve seat. The schematic below illustrates how the valve positioner is connected to the control valve.
Look at the diagram, the valve positioner is attached to a diaphragm-actuated sliding stem control valve. The supply pressure is connected to a pneumatic relay. A fixed orifice restriction in the relay limits flows to the nozzle such that when the flapper is not restricting the nozzle, air can bleed out faster than is being supplied.
The input signal from the control device is connected to the bellows. When the input signal increases, the bellows expand and moves the beam. The beam pivots about the input axis moving the flapper closer to the nozzle. The nozzle pressure increases and through the pneumatic relay action increases the output pressure to the valve actuator. The increased output pressure on the actuator causes the actuator stem to move downward. Stem movement is then fed back to the beam by means of a cam. As the cam rotates, the beam pivots about the feedback axis to move the flapper slightly away from the nozzle. The nozzle pressure decreases and reduces the output pressure on the actuator. Stem movement continues backing the flapper away from the nozzle until equilibrium is achieved.
When the input pressure decreases, the bellows contracts (aided by an internal range spring) and the beam pivots about the input axis to move the flapper away from the nozzle. Nozzle pressure decreases and the pneumatic relay permits the release of diaphragm casing pressure to the atmosphere causing the actuator stem to move upward. Through the cam, stem movement is fed back to the beam to re-position the flapper closer to the nozzle. When equilibrium conditions are attained, stem movement stops and the flapper is positioned to prevent any further decrease in diaphragm case pressure.
In a reverse-acting valve positioner, the same basic operating principle above applies except that as the input signal increases, the diaphragm casing pressure is decreased. A decreasing input signal causes an increase in pressure to the diaphragm casing of the control valve.