Analysis of 6 common reasons for valve positioner

2018-04-18 15:51:18 171

In the auxiliary device of the regulating valve, the most important and practical valve positioner is. Valve positioner is one of the key accessories of control valve. It has the function of valve positioning, not only to overcome the friction of the stem, but also to overcome the unbalanced force caused by the change of medium pressure, so that the valve can quickly follow the valve and correspond to the control signal output of the regulator so as to realize the fast positioning of the regulating valve and improve the adjustment quality. With the development of intelligent instrument technology, the wide application of intelligent technology and electronic technology in traditional instruments has greatly improved the function and performance of instruments.

The principle of valve positioner: the feedback lever feedback valve opening position changes, when the electromagnetic torque produced by the input signal is equal to the torque produced by the feedback system of the locator, the force balance system of the locator is in the balance state, the locator is in the stable state, and the input signal is proportional to the valve position at this time. When the input signal changes or the fluid force of the medium changes, the balance state of the force balance system is broken. The force of the magnetoelectric component and the feedback loop caused by the change of the position of the valve stem are in the unbalanced state. The output pressure of the locator gas source is changed because of the effect of the nozzle and the baffle. The change of the chamber pressure of the actuator promotes the movement of the actuator, so that the valve stem is positioned in a new position, and corresponds to the input signal again, so as to achieve a new equilibrium state. The structure of the feedback lever of the locator in use (such as the cam curve) can change the positive and negative effects of the regulating valve, the flow characteristic and so on, so as to improve the performance of the regulating valve.

There are many kinds of locale valve locators, such as pneumatic valve locator, electric valve locator, valve locator with film actuator, valve locator with piston actuator, powerful balance valve locator, displacement balance valve locator, wide use of valve locator. In the production process, it is inevitable that there will be various faults. For the sake of quality, quantity and safety, we must eliminate all possible failures of the locator in time. To eliminate the malfunction of the valve positioner, we must correctly judge the part of the valve positioner and the malfunction of that component. There are usually two fault analysis methods as follows: first, according to the transfer function of the valve locator, the valve positioner is analyzed one by one and one by one, which is not suitable for on-site maintenance, but it is very effective for the analysis of difficult problems; two is based on the comprehensive analysis and judgement of the troubleshooting of the overhauler. This method is most suitable for on-site maintenance. The following common causes of failure of the valve positioner are analyzed as follows:

1. valve positioner has signal input, but no output pressure signal.

(1) electric / gas locator, foreign body between armature and coil rack.

(2) blockage of constant throttling holes.

(3) bad fit of nozzle baffle or damage of nozzle baffle.

(4) the diaphragm of the amplifier (metal diaphragm or rubber diaphragm) is damaged.

(5) the gas connection is incorrect (including the amplifier).

(6) the input / output signal of the electric / pneumatic positioner is reversed.

(7) the diode in the junction box of the locator is open or wiring is bad.

(8) the size of the gas source pressure is not required.

(9) the excess air consumption of the amplifier is too large.

(10) the polarity of the magnetic pole is different in the installation of the electric / gas positioner.

(11) the pretightening force of the amplifier is overweight.

(12) slide valve in slide valve amplifier is stuck by foreign object.

(13) manual / automatic switchover is not in position (non manual and non automatic).

(14) the electrical / gas locator is short circuited by the electrical signal.

(15) the balance spring installation, the debugging is not good.

2. lower stroke locator output pressure change slowly

(1) the taper valve of the amplifier has a smaller taper.

(2) long term use of amplifier diaphragm leads to elastic hysteresis.

(3) the sensing element (bellows or membrane box) of pneumatic positioner is used for a long time, resulting in elastic lag.

(4) the feedback spring produces elastic hysteresis.

3. up stroke locator gives a slow change of pressure

(1) the amplifier into the balloon valve is too deep.

(2) the air consumption of the amplifier is larger.

(3) the intake valve of the amplifier is fouled and the flow area is reduced.

(4) the ratio of the diameter of the constant throttle hole to the nozzle diameter is smaller than the rated value (technical requirement value).

(5) the coordination between the nozzle and the baffle is not good.

(6) there is slight friction between the armature and the coil rack.

4. positioner is not linear

(1) improper selection of feedback cam or spring.

(2) the installation of the feedback mechanism is not good.

(3) the feedback cam or spring is not properly installed.

(4) there is contamination in the nozzle or baffle.

(5) the slide valve in the slide valve amplifier has friction with its contact surface.

(6) there is a slight leakage in the back pressure.

(7) improper installation of the whole machine.

(8) the feedback connecting rod surface regulating valve has a card phenomenon.

5. no input signal, the locator has output pressure

(1) the nozzle is blocked.

(2) the contamination of the intake ball valve causes the card not to die or the sealing surface is damaged.

(3) the diameter of the constant throttle hole and the diameter ratio of the nozzle are larger than the rated value.

(4) there are problems in the connection of the air road plates of the amplifier.

(5) the deformation or improper installation of the metal diaphragm of the amplifier causes the stem to be opened at the top of the balloon valve.