Causes and Treatment of Large Fluctuation of Double Flange Liquid Level Transmitter

Causes and treatment of large fluctuation of double flange liquid level transmitter

This paper makes a simple record on the troubleshooting of abnormal fluctuation of plug-in double flange liquid level transmitter in a factory, hoping to be helpful to the application of double flange liquid level transmitter in winter.

A factory uses a plug-in double flange liquid level transmitter to measure the liquid level of the synthesis tower. The process medium is wax oil, the temperature is 300 ℃, the pressure is 3Mpa, and the installation distance between the upper and lower flanges is 10m. Since the beginning of winter, the ambient temperature has changed greatly, and the operators reflect that the double flange liquid level transmitter fluctuates in windy weather.

The instrument worker shall handle the relevant bills for maintenance operation before handling the fault, put forward the requirements according to the process and implement the safety measures before operation.

Fault cause analysis and judgment ideas:

The fluctuation of double flange liquid level transmitter in winter may be caused by the following reasons:

1. The type of liquid level gauge is high-temperature silicone oil, and the capillary is separately insulated, but no instrument heat tracing is added, resulting in uneven heating of the capillary.

2. The insertion depth of the measuring cylinder of the double flange liquid level transmitter is not enough, and wax accumulates in the capsule.

3. The header of double flange liquid level transmitter is not insulated, and the electronic components work abnormally under low temperature.

4. There is electromagnetic interference around.

Based on the preliminary judgment of the above faults, the instrument worker shall carry out the following fault treatment measures:

1. Effective instrument heat tracing and insulation measures:

â‘  Check the thermal insulation and heat tracing, and add steam heat tracing or electric heat tracing to the capillary to ensure uniform heating of the capillary.

② Install the capillary tube of double flange liquid level transmitter on the insulation surface of the tower wall, heat tracing by the tower wall itself (300 ℃), and heat preservation and reinforcement shall be done outside.

2. Re select the type of double flange liquid level transmitter, select the instrument with appropriate insertion depth and replace it.

3. In winter, the header of double flange liquid level transmitter shall be insulated to ensure the normal operation of electronic transmitter components.

4. Find the electromagnetic interference source and eliminate the interference.

Common faults of double flange liquid level transmitter

Fault phenomenon: the double flange liquid level transmitter has no indication

Fault cause: signal line falling off or power failure

Solution: rewire or deal with power failure

Fault phenomenon: the indication of double flange liquid level transmitter is the maximum

Fault cause: â‘  leakage of low pressure side, diaphragm, capillary or sealing liquid; â‘¡ Low pressure side (high pressure side) vent pressure valve is not opened; â‘¢ Vent and pressure plug at low pressure side (high pressure side).

Treatment method: â‘  replace the instrument; â‘¡ Open the impulse valve; â‘¢ Clean up sundries or replace the impulse valve.

Fault phenomenon: the indication of double flange liquid level transmitter is too large

Fault cause: â‘  the vent plug at low pressure side (high pressure side) leaks or the impulse valve is not opened; â‘¡ Instrument not calibrated

Treatment method: â‘  tighten the vent plug and open the pressure valve; â‘¡ Recalibrate the instrument.

Fault phenomenon: the indication of double flange liquid level transmitter has no change

Fault cause: â‘  the diaphragm or capillary at high and low pressure side is damaged at the same time; â‘¡ The circuit board is damaged.

Treatment method: â‘  replace the instrument; â‘¡ Replace the circuit board

Fault analysis of double flange liquid level transmitter

1. The liquid level responds slowly and the accuracy decreases

â‘  In a liquid level control system, a double flange transmitter is used to measure the liquid level of the container, and the transmitter is installed between the two impulse flanges. Soon after the instrument was put into use, it was found that it reacted slowly and the indication was inaccurate. A new meter was replaced, and similar phenomena still occurred

Fault analysis: the working pressure of double flange transmitter is generally above atmospheric pressure. If it needs to work below atmospheric pressure (negative pressure), the working temperature should not be too high. Skills of the transmitter: the upper limit of static pressure is 2.5MPa and the lower limit is 2.7kPa; The process temperature is - 40 120 ℃. The liquid receiving temperature of the instrument is related to the process pressure. When the temperature is 120 ℃, the minimum working pressure of the instrument is not 2.7 kPa, but above atmospheric pressure; To work below 2.7kPa, the liquid receiving temperature of the instrument can only be below 60 ℃. Since the transmitter is installed between two flanges, if it is not operated properly, it may be in vacuum state and do not meet the counting conditions of the instrument. When the transmitter works under negative pressure, the diaphragm in the flange diaphragm box bulges due to vacuum, so the pressure in the sealing system decreases, the viscosity of the filling liquid also decreases, and begins to evaporate. When gas appears in the filling liquid, the pressure transmission will slow down, so the response of the instrument is slow and the dynamic characteristics become worse.

Troubleshooting: install the instrument under the two flanges.

â‘¡ The indication action of a double flange liquid level gauge of a device is slow

Fault inspection and analysis: after checking that the table body and signal circuit are normal, after removing the lower flange, it is found that there is sand material flowing out at the flange opening, and the sand material remains at the lower flange. The change of liquid level cannot act directly on the flange surface rapidly, resulting in slow change of instrument indication.

Troubleshooting: remove the dirt and reinstall the flange after cleaning, and the instrument returns to normal.

2. Fluctuation of liquid level indication

â‘  In summer, when the temperature is high and the sun is sunny, the liquid level indication fluctuates and returns to normal at night

Fault inspection and analysis: since the double flange is installed on the south side of an outdoor device, the sunlight can directly irradiate the flange capillary without thermal insulation system, resulting in a sharp rise in the capillary temperature. In addition, due to the poor quality of the instrument when leaving the factory, the internal air is not cleaned before filling the silicone oil into the capsule (or the expansion coefficient of the filled silicone oil is too large), resulting in the thermal expansion of the air, the pressure in the capsule increases and the instrument indication fails.

Troubleshooting: temporarily install insulation layer on the flange, capillary and other parts in direct sunlight to prevent direct sunlight. In case of overhaul, replace a qualified double flange after meeting the maintenance conditions.

â‘¡ The differential pressure transmitter is used to measure the liquid level of a vessel, and the indication fluctuates continuously during operation

Fault inspection and analysis: one is the process itself, that is, the change of liquid level caused by the fluctuation of measuring medium. It is confirmed that this is not the reason. The second is the internal leakage of the balance valve in the three valve group. Open the high-pressure valve, close the balance valve and low-pressure valve, and open the blowdown valve at the low-pressure side. The liquid cannot be discharged. This situation is eliminated. Third, the transmitter damping is improperly adjusted. It is still fluctuating after observation. It still fluctuates after adjustment. Disassemble the transmitter and recheck to see if the diaphragm is damaged. After verification, everything is normal. After reinstalling the transmitter, it is found that the flushing water valve vibrates. After investigation, it is found that the flushing water pressure is unstable and the temperature is high, which will cause a large number of water vapor columns in the impulse pipe, make the impulse pipe unstable, and cause the output of the transmitter to fluctuate.

Treatment: the flushing water system was reformed, and the transmitter indication was normal after treatment.

3. Inaccurate liquid level indication

â‘  The indication of a double flange instrument for measuring the liquid level of a gasifier in a synthetic ammonia is always out of alignment with the on-site flap instrument

Fault inspection and analysis: after analysis, there may be three reasons: local indication fault of the flap instrument, blockage at the pressure tapping of the double flange instrument and damage to the diaphragm of the double flange instrument. Through mechanical inspection, the possibility of failure of the flap is eliminated, and the pressure tapping of the instrument is backwashed to ensure that there is no blockage at the pressure tapping of the instrument. Then the double flange of the instrument is removed to check the diaphragm, and it is found that the diaphragm has been damaged and leaked.

Troubleshooting: replace the instrument and solve the problem.

â‘¡ The double flange liquid level transmitter is used to measure the liquid level of the primary syngas decarbonization tower. The process reflects that the indication is high, the actual liquid level drops to almost zero, and the indication changes only slightly

Fault inspection and analysis: the transmitter was inspected on site and there was no problem. The analysis may be that the medium crystallization caused the high indication on the capsule. Because the alkali liquor is easy to crystallize and there is a small amount of carbon black in the gas, it is mixed and stuck together, very hard and pressed on the capsule, that is, the liquid level drops, and the capsule is still subject to a constant force. Therefore, the actual liquid level drops to zero, The indication remains unchanged. Remove the double flange, and sure enough, there is crystalline black material on the capsule.

Troubleshooting: after removing the crystalline black substances on the capsule, install and open the meter to operate normally.

â‘¢ A tower uses double flanges to measure the liquid level. When the liquid level at the bottom of the tower is very high, but the double flange liquid level transmitter has no indication

Fault inspection and analysis: after inspection, the instrument maintenance personnel suspected that the indication value of the double flange liquid level transmitter was inaccurate. The method of opening the balance valve to balance the positive and negative pressure chambers of differential pressure was used in the double flange liquid level transmitter, but the indication value of the liquid level gauge was still the original indication and did not return to zero. In this way, the maintenance personnel mistakenly thought that the zero position of the instrument was inaccurate and forcibly adjusted the liquid level indication to the zero position, so the instrument had no indication, and the indication was only available when the liquid level was higher than the original indication.

Troubleshooting: this fault is caused by the misoperation of the instrument maintenance personnel. When adjusting the zero point of the double flange liquid level transmitter, it is necessary to close the upper and next valves, open the intermediate balance valve, and then drain the medium in the two double flanges. At this time, it is considered that the upper and lower flange plates are under pressure, and then the transmitter can be zeroed.

â‘£ The indication of liquid level gauge for measuring the liquid level of diethanolamine intermediate tank in ethanolamine unit is inaccurate, and the indication of process reflection is high

Fault inspection and analysis: through positive and negative discharge, it is found that it is caused by the loss of isolation liquid in the negative measuring pipe.

Troubleshooting: by filling the negative measuring tube with diethanolamine, the instrument indication is normal.

Note: in ethanolamine unit, the isolation liquid filled in the negative measuring pipe for measuring liquid level by differential pressure method is usually the medium measured by itself. When dealing with the instrument for measuring liquid level by differential pressure method, be careful not to open the balance valve easily, so as to avoid the loss of the isolation liquid filled in the negative direction, resulting in inaccurate measurement of the instrument.

⑤ A boiler drum level control system uses differential pressure transmitter to measure the level, and a glass plate level gauge is installed on the other side of the drum. During startup, the output of differential pressure transmitter is much higher than the indicated value of glass plate liquid level gauge

Fault inspection and analysis: when using differential pressure transmitter to measure the liquid level of closed container, the impulse pipe shall be filled with condensate in advance, and l00% negative migration shall be used to remove the liquid column in the negative pressure pipe more than in the positive pressure pipe, so that the positive and negative differential pressure of differential pressure transmitter △ P = H ×ρ, H is the liquid level, ρ Is the relative density of water. The range of differential pressure transmitter is h ×ρ, H is the distance between the upper and lower pressure tapping valves of the steam drum.

When adjusting the differential pressure transmitter of boiler drum water level, the water density shall be the value of boiling state during normal boiler production.

When the boiler is just started up, the temperature and pressure in the boiler do not reach the design value. At this time, the relative density of water is greater than that in normal production. Although h remains unchanged, Δ P = H ×ρ As the value increases, the output increases. Glass plate level gauge is only related to h, so its indication is normal, resulting in that the indication of differential pressure transmitter is greater than that of glass plate level gauge.

This phenomenon is temporary. When the boiler reaches the normal operating condition, the indications of the two meters can be consistent. It does not need to be handled, but it should be explained clearly to the process operator.

4. Reconstruction of liquid level measurement

There is a problem that is not conducive to maintenance in the liquid level measurement at the bottom of the air separation and air cooling tower. There is no vent valve, balance pipe and valve behind the positive and negative primary pressure taking valve. When the pressure taking valve is opened and closed in production, the instrument will be greatly overloaded, exceeding the service conditions specified in the technical measures of the double flange liquid level transmitter, which is not conducive to the normal operation of the instrument. Under the existing state, the instrument cannot be at the same static pressure or zero differential pressure, which is not conducive to judge and deal with the faults in the operation of the instrument.

Under the existing conditions, add a valve and a connecting pipe (with valve) at the short pipe behind the upper and lower pressure tapping valves (high point and low point), so as to facilitate the inspection of instruments and deal with faults.

5. Interlock failure

The liquid level control interlocking instrument lsll-202 of tower t-210 of a petrochemical unit gave an alarm interlocking signal, and the interlocking action automatically closed pdv-222 and F level heater 4 valve, resulting in the shutdown of the unit

Fault inspection and analysis: after the instrument personnel checked each link of the measurement circuit one by one, it was found that

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