Introduction
At present, DCS control system is widely used in cement industry, the system runs safely and reliably, and its central processing unit rarely fails. However, due to unreasonable design or improper use and maintenance, component quality problems or software failures, the power module, storage module, input and output module and peripheral lines, often make the DCS control system failure. Now to a company DCS control system as an example, the emergence of a number of problems for case analysis, for peer reference.

01 Case one

On September 2, 2015, the 5000t/d kiln system of the company was overhauled. After 1.5 hours of feeding production, the hopper elevator, air chute and raw material bin flow control valve jumped and stopped at the same time. It was shown in the central control room that the kiln main-transmission was not ready. The on-site inspection showed that the circuit breaker of the kiln main-transmission tripped, which was suspected to be overload. The setting value of the circuit breaker was adjusted to the maximum value of 1600A(the rated current of the kiln main-transmission of the company was 1060A). However, after running for about 1.5 hours, the kiln main transmission jumped again. This time, the breaker did not trip and the rectifier cabinet showed no fault. At the same time, the kiln bucket elevator, the kiln air chute and the raw material bin flow control valve jumped at the same time. The operating station of the central control room shows that the fault is LCS02 system hardware failure, and check the cause of the fault in the system hardware of the engineer station:

The fault record is:

(1) At 13:43 on September 2, there was a “distrulse I/O Stationfailure” fault;

(2) There was an “I/O access error when transferring the process image” at 13:45 on September 2.

Accordingly, it is suspected that there is something wrong with the I/O system. It is checked that there is instantaneous loss of response signal and drive signal in the No.2 slot and No.3 slot of the No.2 cabinet of the I/O system at the end of the kiln. The equipment corresponding to No. 2 and No. 3 slots is the related switching signal of kiln master transmission, kiln bucket elevator, kiln air chute and raw material bin flow control valve.

Solution:

(1) Replug the module in the corresponding slot 2 and slot 3 once.

(2) Check the grounding of the signal shielding cable in No.2 and No.3 slots, and it is found that the grounding of DCS system is confused with the electrical safety grounding of the cabinet. After the two kinds of grounding are processed separately, there is no kiln owner transmission stop phenomenon.

a“Module error entering state”

b“Asynchronous PLC error entering state”

“No Access to the SQL Server，Data Loss”

“LCS@(3)/LCS01-ET04、ET05、ET07、ET08、ET09 Station Failure”

02 Case two

On July 11, 2016, when the roller mill system was started up and opened to the frequency converter of the roller mill separator according to the technological process, all equipment of the roller mill system (including high temperature fan, exhaust fan at the end of the kiln, electric dust collection at the end of the kiln, conveying and dust collection equipment at the top of the raw material warehouse) instantly stopped. After checking the CPU and I/O system modules of the control station of raw material mill site, no fault was found. After restarting the high-temperature fan and roller mill system, the fault still existed when the frequency converter of roller mill separator was started in sequence. It was suspected that the high-order harmonic interference of the frequency converter had an impact on the DCS system. Look for the system fault in the ES station of the upper computer, and the fault phenomena are as follows:

(1)LCS01 field control station reports system failure

A “Module error if state”

B “Asynchronous PLC Error Entering State”

(2) ES source of trouble

“No Access to the SQL Server, Data Loss”

(3) Control station rack failure

“LCS@(3)/LCS01-ET04, ET05, ET07, ET08, ET09 Station Failure”

A few days ago, the high temperature fan and roller mill slag spilling group had a jump stop phenomenon. After the jump stop, we checked the field control station, and found no problem. Only after the jump stop, the motor block in the picture of the operation station appeared a red alarm, and the alarm was cancelled and returned to normal. At that time, it was only suspected that the corresponding input or output terminals of the jump stop equipment might be loose, and the vibration of the roller mill was lost, causing the equipment to jump stop. After the jump stop, the vibration was eliminated, so as to restore the ready.

From the failure on July 11, we can see that starting the frequency converter will cause the system to stop. Therefore, we have checked the shielded grounding of the primary circuit of the frequency converter, and found that the shielded grounding of the frequency converter is not reliable. Carefully checking, we found that the system grounding of DCS also has the phenomenon of virtual connection. After the primary shielding of the frequency converter and the reliable grounding of the DCS system, there is no more above fault phenomenon.

03 Case three

On October 8, 2016, the CO concentration of the coal mill bag collector either showed a bad value or a negative value. After detection, the actual current signal is 178mA, beyond the range of 4 ~ 20mA. It is suspected to be caused by signal interference. After checking the shield grounding, it is found that the shield of the input signal loop has bad grounding. After reprocessing, the CO concentration display returns to normal.

04 Case four

On October 30, 2016, the temperature and pressure at the inlet and outlet of the electric dust collector at the end of the kiln of the company showed bad values at the same time. After inspection, the input channel of the analog quantity burned out, and it returned to normal after replacing the input channel. However, two months later, when the kiln was stopped to repair the electric dust collector, the temperature and pressure at the inlet and outlet of the electric dust collector at the end of the kiln appeared bad values at the same time. It was suspected that the over-voltage generated during welding was transferred to the analog input channel due to the inspection body of the electric dust collector, which caused the input channel to be damaged. Inspection found that the installation company was not grounded when connecting the above signal’s shielding line. After handling the grounding, there has never been a phenomenon of burnt out channel.

The above four examples show that the independent grounding and reliable grounding of the DCS system are very important for the safe and reliable operation of the DCS system. Therefore, the construction of the DCS system grounding, the laying of the grounding cable and the processing of the system grounding terminal should strictly follow the requirements of the design specification, otherwise it will affect the safe operation of the equipment.

05 Case five

On April 1, 2017, coal feeding machine is switched on about 3 min after reclaimer system automatically jump stop, touch screen display fault for: rake, walking motor, shunting, reclaimer limit (overrun and ultralimit a total of four limit for the travel switch normally closed point, 24 v dc input signal switch input module and PLC) air switch and control cable, power cable air switch trip. In fact, the above 7 signals in the PLC input module of the indicator light has been on, proving that there is no tripping phenomenon. At first, it was suspected that the 24V power supply or N line shared by 7 signals had loosened, thus causing the false impression. However, after checking all 24V power supply and N line, no loosening was found.

Continuous test several times, each time appears the same situation, that is, run about 3min and then jump stop, suspected that there is a problem with the program. Power off the CPU, remove the memory card and reinstall it. After energizing the CPU, reinstall the program. The fault still exists after startup. Field inspection of the running track of the rake found that the hydraulic pressure detection line of the hydraulic coupler of the rake was loose, and the 24V power supply line was exposed outside the junction box. When the rake ran to this position, it touched the line head, causing the 24V power supply short circuit to the ground. Reclaimer has two 24 v regulated power supply, the above seven signal combined with hydraulic coupling stress testing, a total of eight switch signals and Shared a 24 v power supply, switch input module and PLC as long as there are 8 ~ 15 v voltage indicator is on, at the same time due to short circuit time is short, so difficult to find and deal with the hydraulic pressure test line is expected to rake hydraulic coupler, eliminate malfunction phenomenon.

06 Case six

On April 10, 2017, the kiln tail and the system in the kiln stopped continuously. After each stop, the failure phenomenon was that the equipment was not ready. The inspection showed that the main power switch in the PLC cabinet tripped, and it started up normally after the reset. Began to suspect that the quality of the air switch has a problem or the capacity of the air switch is not large enough. After replacing the air switch, the air switch still trips after a period of time. There is a certain regularity in the stopping time interval of the air switch, so we think that the air gun of the kiln end system and the flow control valve under the raw material homogenization warehouse are automatic circulation control, and the time interval has a certain regularity. Therefore, the flow control valve under the raw material homogenization warehouse and the air gun control circuit at the kiln end are emphatic checked. Eventually found raw meal homogenizing library under a flow control valve has a problem, when the control program loop to the valve automatically opens, the open limit switch quantity signal lines will meet with broken wire K1 line of DCS system, causing K1 line to ground short circuit, resulting in the PLC cabinet of total power trip, cause burning all the equipment in the preheater kiln and jump stop, seriously affect the safe operation of the equipment.

The solution: we have to deal with insulation damage, also to the whole control loop is improved, namely according to the division of equipment group started, segmenting the different control equipped with independent air switch, so, when a group of equipment after the air switch trip, won’t cause other group jump stop devices and even the entire system.

The above two cases of fault phenomenon tells us that although the fault has a certain concealment, but also has a certain regularity, as long as you are familiar with the process and control principle, you can find the law, and eventually find a solution to the problem.

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