Pic708 can not control the control valve from DCS operation station. The operation interface shows that the point is in "initialization" state.
This phenomenon is generally caused by open circuit of DCS output circuit. If the positioner is damaged, the wiring is loose, the power is cut off, and the safety barrier is damaged, the fault can be caused. Because of the bad environment and large vibration, the first consideration is whether it is caused by the loose wiring of the positioner.
1. After handling the online maintenance work order, the on-site inspection found that the locator wiring was not loose. The working voltage of the positioner is 4.95v. The positioner is Fisher dvc6010 intelligent positioner, and the working voltage should be about 10V. Disconnect the wiring of the positioner and measure the open circuit voltage to be 17.5V, which is normal. The short-circuit current is only 0.78ma, and the normal value should be greater than 20mA. It seems that the power supply is abnormal. In order to determine whether it is the simple power supply problem or the abnormal power supply caused by the damage of the positioner, a signal generator is used to add the current signal to the positioner, and the regulating valve works normally. It can be concluded that the fault is before the positioner.
2. Check the cable from the positioner to the middle junction box to the machine room, and the line resistance is normal. Thus, the fault is reduced to between the DCS output card and the computer room terminal block.
If the terminal of DCS output card is short circuited, the fault of DCS interface disappears and the card fault is eliminated. In the output side of the short circuit barrier, the fault does not disappear. If the terminal is loosened and pressed again, the short circuit fault can disappear, but if it is not short circuited, the fault still exists, indicating that there is more than one fault. Short circuit the output side of the safety barrier, and the fault does not disappear. But this side is directly connected to the field positioner, and the circuit from this side to the positioner has been tested, and there is no problem. Can it be caused by the deterioration of barrier performance and load carrying capacity?
3. Disconnect the wiring at the output side of the safety barrier and connect a 250 ohm load resistor here. At this time, the fault disappears. The voltage is 1V and the current is 4mA. Remove the 250 ohm load resistor and restore the wiring. The instrument fault disappears and the locator voltage is 9.6V. It can be seen that the output side of the safety barrier also has poor contact. When it is disconnected and pressed again, the fault is eliminated.
Conclusion: in this case, the fault is caused by poor contact at the input end of the safety barrier. Because there are needle terminals in the wiring, after a period of time, due to the deformation and oxidation of metal materials, the joints become loose and the contact resistance becomes larger, resulting in the failure of the instrument. After finding out the causes, the device was used to stop and eliminate the defects, and all the wiring terminals in the machine room were fastened once. The problems of loose wire ends in many places were found out, and the hidden dangers were eliminated in time.
It is suggested that the wiring of control system wiring cabinet, intermediate wiring box and field instrument should be checked during the maintenance of the device, and the dust cleaning and terminal fastening should be carried out, especially in the area with large vibration.
The above is the reason why the DCS card xp313 of Zhejiang University central control can not operate the control valve. I hope it can help you. For more wonderful information, please click our website: http://www.eboudy.com