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Analysis of the cause of ignition failure of calorific value analyzer

浏览次数:4506 返回列表 Release time: 2016-08-13 Browse times: 4506 Return to list

Analysis of the cause of ignition failure of calorific value analyzer
With the continuous discovery and development of large natural gas and light hydrocarbon gas fields in China, natural gas and light hydrocarbons are widely used as an economic, clean and efficient energy fuel in various industries. We know that different substances have different heats of combustion, and natural gas and light hydrocarbons contain multiple components and the proportions of the components are different. Therefore, the heat released by them will change. The cracking furnace of ethylene plant in our factory uses light hydrocarbon as fuel. The temperature control of the cracking furnace directly affects the stable operation of the ethylene plant. Therefore, our factory uses a FLO-CAL 5000 series calorific value analyzer to analyze the calorific value of the fuel of the cracking furnace online.

2 Instrument measurement principle
FLO-CAL 5000 series calorific value analyzer is a fast response instrument. The sample continuously enters the burner for combustion, and the rate of change of the heat released by combustion is fed back to the nozzle baffle control system through the thermal expansion tube. The control system regulates the flow of cold air into the combustion chamber through the air orifice. The change of air flow keeps the temperature of combustion products constant. Therefore, the air flow rate changes with the heat released by the gas. Measuring the air flow is equivalent to directly measuring the heating value of the fuel gas.
The sample gas continuously flows into the precision pressure regulator of the analyzer, and the regulator controls the pressure of the sample gas flowing into the sample gas orifice of the burner. The air provided by the instrument air source is sent to the air control valve. The air control valve regulates the air flow through the air orifice to the combustion chamber. A part of the air is supplied to the combustion chamber and mixed with the combustion gas, and the other part of the air acts as a heat exchange medium surrounding the combustion chamber.
When a low air inlet pressure and / or a low sample gas inlet pressure occurs, the interlock protection device acts to prevent the airflow from entering the combustion chamber. The air and sample gas pressure switches are normally open switches. When the air and gas reach a small pressure, the pressure switch is closed, and then the solenoid valve is opened to allow the sample gas flow into the combustion chamber.
The mixed gas baffle on the top of the burner mixes the heat exchange air and the combustion products, and the mixed heat is transferred to the thermal expansion tube, and the length of the thermal expansion tube is proportional to the temperature change. A free-splicing rod is installed by a spring and a thermal expansion tube, and a baffle is connected to them at the bottom, as shown in Figure 1. The air supplied to the instrument is fed back to the baffle through the nozzle through the restrictor orifice. A small change in the baffle will cause a change in the air flow rate from the nozzle. This change causes the load in the control valve to change causing back pressure. An increase in the load on the diaphragm of the air control valve will cause an increase in air flow through the valve. The change in air flow maintains a constant temperature on the thermal expansion tube. The compensation pipe adjusts the temperature change of the incoming air. Its net effect is constant air temperature difference.
The pressure through the air orifice is measured by a differential pressure transmitter. The differential pressure through the air orifice reflects the change in air velocity. In the calorific value of the calorimeter, it is proportional to the calorific value measured.
3 Cause analysis and countermeasures of instrument ignition failure
The common faults of the calorific value analyzer in actual use are: meter verification fault, differential pressure transmitter cannot output or output unchanged, ignition fault, etc., and ignition fault is a common fault of this table. Knowing from the principle, in order to measure the calorific value of fuel gas, these fuel gases need to enter the working area of the instrument to burn, which has some potential dangers to the safety of the instrument equipment itself and maintenance personnel. To this end, the table has designed an ignition interlock control system specifically for the ignition problem.
The analysis shows that when the instrument is powered, the instrument cannot ignite immediately, but wait for the instrument air source and sample gas pressure to be adjusted to the set value specified by the instrument.After the sample gas and air pressure switches are closed, 220VAC can be supplied to the ignition unit and Sample gas inlet solenoid valve. At this time, the sample gas enters and the ignition unit starts to ignite. Therefore, in the first step of ignition, the sample gas and the inlet air of the instrument should be adjusted to the specified value, and then the instrument should be powered. If the above conditions are met, there is no ignition of the ignition device electrode. At this time, the voltage between the terminals 9 and 5 in the interlock diagram should be measured. At this time, the voltage should be 220 VAC. If there is no voltage, it means that 200 VAC is not supplied here. Turn off the instrument, and use a multimeter to check whether the sample and air pressure switch are closed and the line is intact. Any pressure switch not closed or line problems will cause a circuit break. The power supply cannot be supplied to terminals 9 and 5. Replace the corresponding equipment.
At the same time, due to the long-term use of the instrument or frequent opening and stopping, the loosening of the joint can also cause sample gas or air leakage, resulting in the corresponding pressure switch cannot be closed, so it can not ignite. If 220 VAC has been supplied to terminals 9 and 5, check the voltage on terminals 11 and 13, if there is no voltage, check whether the flameout pressure switch is closed, if not, try to adjust the nozzle shutter mechanism Incorrect, it will cause the back pressure of the flameout pressure switch to be too large. If it still cannot be closed, replace it. If the flameout pressure switch is also normal, check whether the coil and contacts of the ignition relay are normal, and replace the relay if abnormal. If there is voltage on terminals 11 and 13, but the ignition electrode does not ignite, indicating that there is a problem with the circuit of the ignition unit, replace or repair it. If the ignition electrode can ignite, but not ignite, it means that the ignition energy is not enough. Remove the electrode and sharpen the tip or replace it with a new electrode. If the instrument ignites after a period of time, and then the fire goes out, you should first check whether the settings of each instrument meet the conditions.If the conditions are met, you should check the condition of the flame arrester at this time. The gas can only be discharged through the flame arrester from the vent on the top of the instrument.
Long-term combustion, especially when the combustion air ratio is not suitable, a large amount of black ash will accumulate in the flame arrester, resulting in serious exhaustion of the flame arrester, and the exhaust gas such as CO2 generated by the combustion cannot be discharged outside the instrument, causing the internal pressure of the instrument to stall. Phenomenon, for this reason, you must adjust the air ratio knob after the fire, so that the flame appears blue instead of yellow. At the same time, regularly cleaning the dust deposits in the flame arrester is also an effective method.
4 Conclusion
In short, there are other reasons for the failure of ignition, such as air control valve, precision gas pressure regulator and other components failure, may cause ignition failure. At the same time, the explosion-proof design of the FLO-CAL 5000 series calorific value analyzer is explosion-proof.In actual operation, the explosion-proof cover of the instrument may often be opened. important.

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