Abstract: In the process of using a gas chromatograph, it is inevitable that you will encounter a variety of faults. This article introduces common troubleshooting methods for chromatographs from several aspects such as gas circuit system, detection system, temperature control system, etc. Reference for the maintenance and use of gas chromatograph. Keywords: gas chromatograph failure analysis failure handling
introduction
In recent years, gas chromatography analyzers have been widely used in the petrochemical industry because of their high separation efficiency, fast analysis speed, small sample consumption, and multi-component measurement. They have a very important position in chemical analysis. However, due to inadequate maintenance of the staff, imperfect sample pretreatment system and defects in the instrument itself, various failures occurred during the use of the instrument, which affected the normal production order. Therefore, it is very important to be able to analyze and eliminate faults in a timely and accurate manner.
Composition and working principle of gas chromatograph
The general gas chromatograph is composed of six basic systems, namely: carrier gas system, sampling system, separation system, temperature control system, detection system and recording system.
The gas chromatograph uses physical separation technology to separate multiple components in the chromatographic column, and then enters the detector for detection after separation. In order to avoid the process medium containing harmful components or some components that do not need to be detected and to shorten the analysis cycle, the chromatograph often cooperates with the column cutting technology to cut off the components that do not need to be detected, and then the microprocessor The amount of signal generated by the component entering the detector automatically calculates the component content value.
Common faults and troubleshooting methods of gas chromatograph
3.1 Failure of the gas circuit system
The gas path system of the gas chromatograph is a system in which the carrier gas runs continuously and the pipeline is closed. The gas tightness of the gas circuit system, the stability of the carrier gas flow rate, and the accuracy of the flow rate all have an impact on the gas chromatography detection results.
The failure of the gas circuit system mainly shows that the flow rate cannot be adjusted to a predetermined value steadily. The possible causes of the analysis are: (1) the gas circuit system is leaking or blocked; (2) the pressure reducing valve or the regulator valve is faulty; The source pressure is insufficient or fluctuating; (4) The flow control valve is contaminated or damaged.
The treatment methods for the above reasons are as follows:
In the gas path, the specific fault location is found in accordance with the sequence of the gas trend to eliminate or clear the blockage.
Replace the pressure reducing valve or regulator valve.
Adjust the pressure of the air source to an appropriate range and have a stable output.
Clean valve parts and replace if necessary.
3.2 Detector failure
Thermal Conductivity Detector (TCD) The thermal conductivity detector uses the difference in thermal conductivity between the measured gas and the carrier gas and between the components of the measured gas to generate an unbalanced voltage in the measuring bridge, thereby measuring the concentration of the component.
Common faults of the thermal conductivity detector: a. The bridge current cannot be adjusted to a predetermined value. The causes of such faults are: (1) the thermal conductivity unit is not connected correctly; (2) the hot wire is disconnected or the lead is open; (3) ) The bridge power supply is faulty; (4) The bridge configuration circuit is disconnected; (5) The ammeter is faulty.
The cause of the fault that the baseline of the detector cannot be zero-adjusted: (1) the resistance of the hot wire is asymmetric or the lead is wrong; (2) the hot wire hits the wall or the pollution is serious; (3) the lead of the zero-adjusting potentiometer is open; (4) the recorder is open Or no response; (5) The difference between the measured gas circuit and the reference gas circuit is too large. 3.2.2 Hydrogen Flame Ionization Detector (FID) The hydrogen flame ionization detector is based on the burning of carbon-containing organic matter in a hydrogen flame to generate fragment ions, which form an ion current under the action of an electric field. The detection is based on the intensity of the electrical signal generated by the ion current. Components separated by the chromatography column.
Common faults of hydrogen flame ionization detector
The detector does not catch fire
Causes of failure: (1) The ignition coil of the detector is disconnected; (2) The flow rate of hydrogen, air and carrier gas in the gas circuit is not properly matched; (3) The polarization voltage is unstable; (4) The nozzle is blocked.
Solution: replace the ignition coil
Re-adjust the flow ratio of hydrogen, air and carrier gas.
Provide a stable voltage source and eliminate wiring faults.
Clean the nozzle
Baseline produces noise
Causes of failure: (1) Impurity contamination in hydrogen, air, and carrier gas; (2) Improper flow ratio of hydrogen, air, and carrier gas in the gas circuit; (3) Poor grounding and poor shielding of the electrical unit; (3 ) The nozzle is contaminated; (5) There is a leak in the gas circuit system.
Solution
Replace the gas source or regenerate hydrogen and air filters.
Re-adjust the flow of hydrogen, air and carrier gas.
Check whether the ground wire is connected well, and whether there is external electric field interference.
Clean the nozzle.
Eliminate air leakage.
3.3 Temperature control system failure
The failure of the temperature control system is mainly manifested in that the column thermostat does not heat up. The possible causes are:
There is a problem with the aging of the temperature control components of the instrument or its own quality.
The use temperature is relatively high, it is easy to cause the heating wire and platinum resistance to be damaged after a long time.
The voltage used by the instrument is unstable, which makes the temperature control components work abnormally.
The instrument is struck by lightning and the circuit is damaged, so the instrument must be well grounded.
Peak out fault
Common deformed peaks are: a. Front extension peak
Causes of failure: (1) The sample is condensed in the system; (2) The sample is condensed in the system; (3) The carrier gas flow rate is too low; (4) The vaporization temperature of the inlet is too low; (5) Two peaks appear at the same time; (6) The injection volume is too large, causing the chromatographic column to overload.
Treatment methods: (1) Properly increase the temperature of the vaporization chamber, chromatographic column and detector; (2) Repeat sample injection to improve the sampling technique; (3) Properly increase the carrier gas flow rate; (4) Increase the inlet Temperature to shorten the vaporization time; (5) optimize the chromatographic conditions, replace the chromatographic column if necessary; (6) change the quantitative tube. b. Trailing peak.
The cause of the failure is solid debris in the column; (2) improper use of the column or degradation of column performance, interaction between the sample and the carrier; (3) the column temperature is too low; (4) the sampling gas pipe is dirty.
Treatment methods: (1) Aging the column; (2) Reselecting the chromatographic column and changing to a more polar packing; (3) Appropriately increasing the column temperature; (4) Cleaning or replacing the sampling gas tube. C. Flat peak
Causes of failure: (1) The sliding wire resistance of the recorder or the mechanical part driving the stylus is faulty; (2) The measuring range of the recorder is exceeded; (3) The sample injection volume is too large.
Treatment methods: (1) repair the recorder; (2) change the range of the recorder or reduce the injection volume; (3) reduce the injection volume.
Troubleshooting strategy
When a fault occurs during the gas chromatography analysis, you should be good at using logical reasoning to find out the problem, check the performance of the fault, look for clues, and analyze the symptoms through the comparison of the front and back of the fault to find the possible causes, and then in turn Change the conditions and steps, replace suspicious parts with good parts, and gradually analyze and solve the problem.
Do failure prevention
The easiest way to maintain and troubleshoot chromatographic systems is not to completely passively cope with various possible problems. The key to the problem is that various preventive maintenance measures should be taken at ordinary times, so that the incidence of failures can be reduced.
Conclusion
The above introduces several common faults of gas chromatograph and its troubleshooting methods from several aspects. In daily work, if these matters can be taken seriously and problems can be resolved in a timely manner, the normal use period of the instrument will be greatly extended, and the performance of the instrument will be maximized.
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