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A typical case of instrument failure

With the development of science andtechnology, the automation instruments are becoming increasingly important inchemical productions ranging from reality monitoring and adjustment and controlto process optimization and energy saving and consumption reduction, but theaccidents caused by instrument faults are also on the rise with its wide application.

Both instrument and electrical applianceare used for chemical processes and production, but the instruments have theunique characteristics: 1. Close combination with chemical equipment and moremutual effects. 2. It’s difficult to distinguish process error and instrumentfault. 3. The fund and manpower investments on instruments are insufficientrestricted by fixed habitats and thoughts. 4. The instruments are moreprofessional and it’s difficult to communicate with equipment and processpersonnel.

The equipment operators shall studyequipment structure, technical process and production characteristics, as wellas the trend and history of process as much as possible to help analyzeinstrument faults, since the analysis of instrument fault also covers manyother aspects according to characteristics above.

1. Doubleflange level gauge has large fluctuation

Installationplace: absorption tower of carbon dioxide.

Fault: liquid level display frequentlyfluctuates between zero position and certain fixed value.

Hazard: adjustment cannot be made normallyand gas mixture and overpressure explosion may occur easily.

Fault analysis: the liquid is lifted dueto blocking of fillers in lower equipment of negative pressure flange, whichcan make the gas-liquid two-phase or solution space at negative pressure sideoccupied frequently.

Solution: clean the blocked fillers,find out the reason and avoid the reoccurrence.

Tips: 1. Befamiliar with equipment structure, the previous and following process, 2. Befamiliar with the recent changes of production conditions. 3. Make judgment bycombining scientific analysis and data and be persuasive.

2. Theliquid level has normal display but the feed liquid pump is evacuated (it maybe fault of liquid gauge from the view of process).

Installationplace: hot water tower.

Fault: thedisplay of display level is above 50, but the liquid feed pump is evacuated andthere is small fluctuation and there are no foreign matters detected inpipeline.

Hazard: adjustmentcannot be made normally and liquid level has to be lifted to avoid evacuation,which can lead to full liquid and make the gas carry liquids.

Faultanalysis: the inlet of outlet pipe is blocked discontinuously.

Troubleshooting:check if there are pressure changes at far end (near end of equipment) of pumpinlet, be familiar with equipment structure and working process, stop themachine, open the equipment and take out the iron plate at lower part of outletpipe to solve the problem.

Tips: 1.Determine the fault with indirect method. 2. Try to get familiar with equipmentstructure and process. 3. Logical reasoning.

3. Doubleflange level gauge displays on the higher side and fluctuates in differentperiods

Installationplace: atmospheric storage tank

Fault: theinstrument has normal display in early period after the instrument is set, butthere will be major errors during changes of liquid level and the display willfluctuate in different periods when the liquid is stable.

Hazard: overflowor empty tank may happen and thus cause waste or affect production.

Faultanalysis: 1. Pressure pipe (capillary tube) may leak. 2. The media are toothick. 3. The air exhaust of pipe is not smooth.

Analysispoint: the liquid level gauge shall not be blocked; the analysis on liquidlevel gauge and media shall be preferred and the environment effects shall be considered.

Troubleshootingresult: effects from equipment and media are basically eliminated based onanalysis and it is sensitive to temperature since there is fluctuation indifferent periods, from which we can conclude that the pressure guiding mediaare abnormal and the capillary tube has gaps. The fault is relieved after thenegative pressure flange is moved downward for observation, which means thenegative pressure capillary tube has gaps. The fault disappears after the capillarytube is removed.

Tips: thisfault cannot be eliminated if the positive pressure side of tank is abnormal,unless the transmitter is replaced.

4. Towertop temperature has regular fluctuation.

Installationplace: outlet of pure alcohol atmospheric tower.

Fault: thetemperature at tower top has periodic fluctuation.

Cause analysis: 1. Poor contact ofelements. 2. Unstable gas flow rate.

Analysispoint: the whole process pipe shall be tested and inspected to find out thecause since both of them shall be considered.

Troubleshooting:it is verified that the thermometer is normal but the temperature hasfluctuations by measuring the temperature at measurement point with handheldthermometer or checking fluctuation through hand feeling. According to theinspection of process pipe, it is found that temperature fluctuation is causedby a concealed water seal tank which is set in process to keep micro-positivepressure and the gas will fluctuate when it passes the water sealdiscontinuously. The fault is relieved after liquid level of water seal isreduced to the lowest position.

Tips: 1. Understandingof process characteristics can be helpful in analysis and judgment. 2.Eliminate the cause by means of contrast and exclusion.

5. Micro-differentialpressure transmitter has serious zero drift.

Installationplace: air pipe.

Fault:there is serious zero drift in multiple micro-differential pressure transmittersand it occurs in different periods.

Cause analysis: 1. Poor quality oftransmitter. 2. Blocked pressure pipe. 3. Temperature effects. 4. Mechanicalshift.

Troubleshooting:1. In case of large stress by checking the pressure pipe, the fault ispartially relieved after stress relief. 2. The holder of transmitter inspectedis unfixed and may have mechanical shift.. The fault is eliminated afterfastening of holder.

Tips: 1.The pressure pipe for micro-differential pressure transmitter shallhave reasonable pavement and proper caliber. 2. Micro-differential pressuretransmitter must be fixed tightly since small shift will lead to largedeviation, which is different from normal differential pressure transmitters.3. Pressure pipes at two sides must be closely installed since the micro-differentialpressure transmitter is sensitive to ambient temperature.

6. Digitaltemperature (K type) meter varies with room temperature

Installationplace: compressor.

Fault:digital temperature meter (patrol inspection) varies with temperature ofcontrol room.

Causeanalysis: the compensation circuit has fault.

Troubleshooting:1. The fault of digital display meter is eliminated since it has normal displaywhen standard signals are input. 2. It is preliminarily determined thatcompensation lead fails by testing on-site cold side and temperature differencein control room. The compensation lead tested is of poor quality and fault iseliminated after the lead is replaced.

Tips: 1.This usually occurs in winter when there is large difference between indoor andoutdoor temperature. 2. It’s difficult to be detected in high-temperatureplace and will have potential impacts on production quality even there’s no obvious fault. 3. Materials must beverified and tested during instrument installation even they are famous brandsor have quality certificates.

7.Infrared analyzer has large fluctuation

Installationplace: outlet pipe of separator of methanol tower.

Fault:Infrared analyzer has large fluctuation in display.

Causeanalysis: 1. The instrument has unstable electronic circuit. 2. Instrumentdetector has fault. 3. Effects of interfering gas. 4. Heating system ofinstrument has fault. 5. Others.

Inspectionand judgment: 1. Operation inspection is made firstly by inputting standard gaswith stable flow and it is found the instrument works stably. 2. The pressureregulator and water seal of sample gas are normal. 3. The instrument has largefluctuation after standard gas with changing flow is input and the air supplypressure required by faulty instrument is high, from which it is concluded thatthe outlet pipe of analyzer has large resistance and the fault is eliminatedafter the excessive part is removed according to actual conditions.

Tips: 1. Fullunderstanding of principle and characteristics of instrument. 2. The instrumentstructure shall be simplified as much as possible when process inspection issatisfactory. 3. Observation shall be careful.