Gas path failure analysis of laboratory gas chromatography instruments

For the gas path part, the phenomenon of faults that are easy to occur can be divided into three categories:

(1) Flow regulation failure;

(2) Gas path leakage failure;

(3) Air circuit blockage and pollution failure.

A large part of the various faults in the gas chromatograph are related to the gas path. Therefore, it is necessary to understand and familiarize with the gas path failure.

First, the flow adjustment

1, the flow can not be adjusted

(1) Visual inspection: First check the instrument system for obvious air leakage. When there is a large leak in the gas path of the instrument system, it is likely that the flow rate will not be adjusted. If you can't hear the leak, turn it to (3).

(2) Checking the leak: After hearing the leaking sound, it can be gradually positioned according to the direction in which the sound is emitted. At this point, the application of the soap can be used to further determine where the leak occurs. Stop the leak in time after finding the cause.

(3) Pre-column pressure observation: Observe the numerical value of the column front pressure indication table, which can quickly determine whether it is caused by the gas source or the internal airway blockage and damage of the instrument. If the pre-column pressure is too low (precisely lower than the predetermined pressure value during normal flow operation), then the gas source needs to be inspected; if the pre-column pressure is normal, the instrument's internal gas path needs to be checked.

(4) High pressure inspection of the cylinder: After opening the cylinder valve, observe the indication of the high pressure gauge, the pressure should be between 1 and 15 MPa. If the pressure is below 1 MPa, the cylinder is deactivated and the air is ventilated; if the pressure is within the proper range, the cylinder pressure is normal.

(5) Low-pressure output check on the pressure-reducing valve: Adjust the pressure-reducing valve to see if the indication of the low-pressure gauge on the cylinder can be adjusted to between 0.25 and 0.6 MPa. If it is normal, it may be suspected that the pneumatic filter joint is clogged or there is a problem with the stability valve on the instrument. In this case, it should be carried out according to (6); if the low pressure value is not normal, the pressure reducing valve has a problem and needs to be carried out (7) Repair.

(6) Filter blockage and pressure regulator check: Slowly unscrew the connector exiting the filter outlet to the instrument air source to see if there is a strong airflow running out of the joint. If so, the filter is not blocked and the regulator valve may be defective. After confirming that the regulator valve is not out of gas, valve disassembly and cleaning can be performed, which may be caused by blockage between the valve needle and the valve seat in the regulator valve. If the valve still does not work properly after cleaning, it is better to change a new valve; if there is no strong airflow flowing out of the unscrewing joint in the above test, it is necessary to check the possible blockage before and after the filter inlet; of course, the blockage of the intermediate pipeline is also Possible, but the incidence is very small.

(7) Repair of the pressure reducing valve: After the structure of the pressure reducing valve is known, the pressure reducing valve can be removed and repaired. Since there is a high pressure on the inlet side of the pressure reducing valve, it is best not to disassemble blindly if there is no repair experience. If conditions permit, it is recommended to switch to a new valve; when changing the valve, it must be noted that the hydrogen meter or oxygen meter should be used separately from the pressure reducing valve used in other air source meters. The special name of the gas source should be indicated on the pressure reducing valve.

(8) Deactivation, ventilation: When the pressure of the cylinder is too small, it should be immediately deactivated, replaced with a new one or inflated. Under too small a pressure, not only the gas source output is unstable, but also the impurity concentration in the gas source will increase significantly, which is particularly disadvantageous for high sensitivity analysis. Another problem that must be noted is that the residual gas in the cylinder, especially the residual gas in the hydrogen cylinder, cannot be discharged casually.

(9) Remove the column inlet gas path: Remove the gas joint at the inlet of the column and observe whether the rotor in the flow meter can rise to the uppermost end. If it can be raised to the uppermost end, the gas path in front of the column is normal, and it is transferred to (10) for further inspection. If the rotor does not reach the uppermost end, the gas path in front of the column is blocked, and (13) inspection is required.

(10) Remove the column end: After connecting the column inlet back to the original gas path, remove the column outlet side joint, and observe whether the rotor in the flow meter can be adjusted to a predetermined value. If it is possible, it will judge that the pipeline and detector behind the column are blocked, and it needs to be treated according to (11); if the rotor is still not adjusted, it can be considered that the column is too tightly packed and should be carried out according to (12).

(11) Blockage inspection and elimination: Queue and clean when it is judged that the pipeline or detector is blocked after the column.

(12) The column packing is too tight: the main reason for the column packing being too tight is that the carrier mesh is too large, causing excessive gas resistance. The flow rate can be adjusted to a predetermined value after appropriate use of a smaller number of carriers or by shortening the length of the column.

(13) Remove the flow meter outlet gas joint: After unscrewing the gas flow at the outlet end of the rotameter, observe whether the rotor can rise to the highest end. If it is possible, it is determined that the injection and vaporizer air circuit are blocked, and the treatment is carried out according to (14). If the rotor still cannot rise to the highest end, the flow valve may be considered to be damaged or the flowmeter inlet pipe is blocked. At this time, press (15).

(14) Plugging of the inlet: The clogging of the injector can be carried out according to the cleaning procedure of the syringe.

(15) Flow valve and pipeline blockage: It will be quickly determined whether the flow pipeline is blocked by segmentation test. If yes, follow the cleaning of the gas pipeline; if the pipeline is normal before the flowmeter, the flow control valve can be removed for cleaning.

2, the flow is too big to adjust

If the gas flow rate is always large and cannot be reduced, it can be considered as a malfunction of the pneumatic control system. There are three reasons for such failures: first, there is leakage in the gas path after the flowmeter; second, the gas path is too small; and third, the flow control valve is damaged. The inspection method is as follows: first block the gas path outlet of the detector and observe whether the rotor in the flow meter can be lowered to zero. If it cannot be reduced to zero, it is necessary to consider the inspection of the leaking gas. For the specific method, see the inspection and elimination of the gas leakage; if the rotor can be lowered to zero, the system will not leak. At this time, it should be observed whether the flow rate has a large change when the flow regulating valve rotates. If there is any change, the gas path air resistance can be appropriately increased; if there is no change, the valve member itself should be suspected to be in problem, and the cleaning part of the valve member should be treated. The treated valve parts should be reinstalled into the original gas path for control test.

Second, the gas path leak check

1. Airway drain inspection

According to the strictness of the airway tightness, the method of checking whether the gas path leaks is divided into three levels: A, B, and C.

Class A test leak:

The most cursory observation of a serious leak in the gas path. Usually, after the gas source is turned on and stabilized, you should not hear the running sound of the wire at each joint of the pipeline and the valve joint. If you hear a clear leak, the system has a big leak! The leak must be traced to the leaking sound and excluded. Common causes of large leaks in the system are: the pneumatic connection is not tightened, the pipeline in the gas path is cracked and no suitable gasket is added. Look for serious leaks in the gas path, or confirm the presence or absence of bubbles in the joints with soapy water when the flow rate of the flow path is maximized.

Class B test leak:

A check for a slight leak in the gas path. The method is to block the gas path outlet and observe the rotor in the flow meter in the gas path. If it can slowly fall to zero, it can be considered that the B-stage test leakage of this gas path is qualified. If the rotor cannot fall to zero, use soapy water to carefully observe the joints. Until the leak is found.

Class C test leak:

Check for very small air leaks in the gas path. The method is to block the gas path outlet and observe the system pressure gauge. It is not allowed to have a drop of 5 kPa (corresponding to 0.05 kgf/cm2) or more within half an hour. The system pressure should be above 0.25 MPa (equivalent to 2.5 kgf/cm2). If necessary, an external 0.5 gauge pressure gauge can be connected to the outlet of the system to read the pressure change.

When it is confirmed that there is a leak in the pneumatic system, the method of blocking or closing the air passage can be used to narrow the range of occurrence of the air leakage. Most of the air leaks occur at the air line joints, and leaks inside the air line valve parts also occur from time to time. As for the leaks in the middle of the pipeline, it is rare except for sharp turns.

2. Elimination of air leakage in the air circuit joint

When it is found that there is a leak in the joint, first check the joint used as follows:

(1) Whether the joint is matched with the gasket, annealing and no scratches;

(2) Whether the joint tightness is clean and smooth without dirt;

(3) Whether the joints are aligned with each other when the joints are assembled;

(4) Can you first tighten the joint by hand?

If there is no abnormality in the above inspection, tighten the joint with a wrench (usually two). When tightening, pay attention to the pressure. For joints with plastic, rubber and PTFE gaskets, the pressure should not be too large. Generally, it can be tightened after tightening. For joints with metal gaskets, the pressure can be increased. Large, but should also be based on airtightness.

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