Application of Capacitive Level Gauges in Chemical Production

With the continuous advancement of science and technology, the application of automation systems in the field of chemical production has become increasingly widespread. In the process of achieving automation in chemical production, the application of various intelligent instruments has played a significant role. Among them, intelligent level gauges serve as the "eyes" of the production process, providing timely and accurate information to safeguard industrial production.

Currently, there are various types of level gauges in use, mainly including capacitive level gauges, radar level gauges, ultrasonic level gauges, and pressure level gauges. Liquid level detection is now common in many control fields, with each method designed to meet specific needs. Their structures, ranges, and accuracies are suitable for different occasions, but most have complex structures and high manufacturing costs.

Capacitive level gauges have been widely applied in recent years due to their excellent accuracy, anti-interference capabilities, and low requirements for media and environments. This article introduces the use of capacitive level gauges and various issues that arise during production to illustrate their application in chemical production.

Ⅰ. Precautions for Using Capacitive Level Gauges

In the harsh environment of chemical production, the front and rear covers and wiring of the transducer must be completely sealed to prevent rainwater and corrosive gases from entering.

(1) During transportation, unpacking inspection, and installation, special care must be taken to prevent damage to the sensor's insulating layer.

Unpacking Inspection: After unpacking, first inspect the sensor for any damage, and then perform a power-on check. After powering on, adjust the zero-adjustment capacitor to make the output current of the instrument 4mA.DC. Holding the sensor by hand should increase the instrument's indication, indicating that the instrument is working properly. If there is no current output from the instrument after powering on, check if the power supply is connected reversely, or if the signal processor leads are loose or detached.

(2) Model and Technical Specifications:

The model and main technical specifications of the instrument are printed on the nameplate of the enclosure. Please verify them upon receipt to ensure they match the specifications ordered. When inquiring or ordering, please provide the model, factory number, and sensor probe length on the nameplate.

(3) Precautions During Transportation:

To prevent damage to the instrument, please do not open the packaging before the transducer is delivered to the user's location. Special care must be taken during transportation to prevent damage to the sensor's insulating layer.

(4) Precautions During Storage:

The instrument should be installed promptly upon arrival to avoid decreased insulation performance of the signal processor in the transducer and corrosion of metal parts. Please note the following while storage :

If possible, store the instrument without opening the packaging box.

The storage location should meet the following conditions: rainproof, moisture-proof, with minimal mechanical vibration, avoiding collision and impact, and with a temperature range of -40°C to +80°C, ideally around 25°C.

Before storing a used level transducer, carefully clean any liquid or adhered solids from the transducer sensor and housing.

If stored outdoors, the performance of the transducer may be affected. Therefore, once the transducer is moved to the installation site, it should be installed as soon as possible.

(5) Precautions During Installation:

A capacitive level gauge is a measuring instrument that uses changes in capacitance to measure the level of media in a container. The measurement process mainly relies on changes in capacitance between two electrodes. In other words, the sensitivity of a capacitive level gauge depends on the difference in dielectric constants between two media: gas and liquid. The measurement must ensure that the dielectric constants of the two media remain consistent; otherwise, changes in the dielectric constants will directly cause errors.

When installed outdoors, the probe wire cannot be exposed outside the container to prevent measurement errors due to water contact during rainy days.

The stainless steel process connection components at the lower part of the housing or junction box must be reliably connected to the outer wall of the container (grounded), with a contact resistance not exceeding 2 ohms.

During normal operation, the probe wire should not have large swings within the container, as this may cause signal instability.

When installing the probe wire, it should be kept as far away from the container's inner wall as possible, with a minimum distance not less than 100mm. When conditions limit the distance to less than 100mm, the distance between the probe wire and the container must remain relatively fixed.

For single-wire flexible probes, the excess length can be pulled out through the upper end of the process connection piece and then cut off, followed by tightening the retaining bolt. For twisted pair probes, the excess length can be coiled above the measured liquid level. It is absolutely not allowed to coil the excess length at the bottom of the container or within the effective measurement section.

When there is stirring or the liquid may produce a large number of bubbles in the container, to protect the probe wire and avoid false liquid levels caused by liquid fluctuations and bubbles, a metal or nonmetallic tube with an inner diameter greater than 80mm can be placed in the container. The lower end of the tube should have an opening for liquid inflow, and a vent hole should be left below the liquid level. When using a metal tube, ensure that the probe wire is positioned相对稳定 within the tube, and provide support to straighten it if necessary.

II. Common Fault Diagnosis

(1) During use, if the instrument has no current output, check if the signal processor "+" and "-" leads are loose or detached, and if the instrument indicator's fixed threads and pillars are loose or have poor contact. If any of these issues are found, they should be immediately reinforced.

(2) If the instrument indication is zero, use a metal tool, such as tweezers or a screwdriver, to touch the "sensor" terminal of the signal processor. The instrument indication should increase; otherwise, it indicates that the signal processor of the instrument is damaged. Replace the signal processor at this time.

(3) If the instrument indication is full-scale, disconnect the "sensor" lead of the signal processor. If the instrument indication remains full-scale, it indicates that the signal processor is damaged. If the instrument indication returns to zero, it may be due to poor insulation of the sensor. Replace the sensor immediately if insulation is poor.

(4) Method for Checking the Sensor: Disconnect the sensor lead from the signal processor and measure the resistance between the sensor lead and the metal tower wall using a 500V megger or a 500-type multimeter in the "x10K" range. The resistance should be greater than 10M ohms; otherwise, it indicates poor insulation of the sensor.

(5) Interference Identification and Elimination: If the instrument works normally in the laboratory but shows fluctuating indications or a fixed indication at a certain liquid level in the field, it can be judged that the instrument is subject to interference. Parallel-connect an electrolytic capacitor (with a capacity of about 220 microfarads and a voltage rating greater than 50 volts) across the power supply leads of the instrument to eliminate the interference.

(6) When a capacitive level gauge fluctuates, first consider whether the liquid level actually fluctuates. At this time, consult with the process personnel to resolve the issue. If the fluctuation of the level gauge is not caused by fluctuations in the liquid level, consider the impact of interference and whether the grounding is good, if there are nearby interference sources, if there is electrical welding work being performed, or if there is the influence of large electrical equipment.

(7) Generally, there is a simple diagnostic procedure for handling capacitive level gauges.

Ⅲ. Conclusion

As a relatively mature method for measuring liquid levels, capacitive level gauges are widely used in various fields such as petroleum, chemicals, metallurgy, power, papermaking, and pharmaceuticals. In chemical plants, they are used to detect the liquid levels of chemical raw materials such as polyester, spandex, acids, and alkalis; in environmental protection, they detect sewage levels; in boiler plants, they detect the liquid levels of media in pressure vessels; in hydraulic machinery, they detect lubricant levels; in the grain sector, they detect edible oil levels; and in oilfields, they detect refined oil levels. In the future, capacitive level gauges will also play a significant role in human progress and development.

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