Application of Ultrasonic Level Transmitter for Level Measurement of Limestone Slurry

Ultrasonic level transmitter is a non-contact liquid level meter with simple structure. It is easy for installation and maintenance and can be used for real-time level monitoring at control center or onsite. It is widely used in level measurement for various liquids. The transducer (probe) of the ultrasonic level transmitter sends out high frequency ultrasonic pulses. When the pulse hits the medium surface, it is partially reflected back and received by the transducer and converted into an electronic signal. The medium level or the distance from the transducer to the medium surface is calculated according to the time difference between the transmission pulse and reception pulse and velocity of sound wave. In practice, sometimes the ultrasonic level transmitter cannot operate normally depending on where the ultrasonic level transmitter is installed and how it is installed. To clarify this issue, we take the application of ultrasonic level transmitter for limestone slurry as an example.

1.   Application background:

A large state-owned chemical company in northern China purchased a batch of Jiwei Uson-11 Ultrasonic Level Transmitter to measure and monitor the level of wastewater and limestone slurry in May last year. The feedback from the customer was that the Uson-11 for wastewater has been working accurately and reliably as specified since the installation, but two Uson-11s for measuring the level of limestone slurry were not working well and showing inaccurate level measurement. Sometimes measurement data varied and false liquid level alarm happened.

Jiwei sent its experienced engineer immediately to investigate the situation onsite and provide technical support when the customer´s feedback was received. The engineer found the problem was repeatedly happening onsite. After investigating and analyzing the problem, he recognized that it was not due to the product quality but because of onsite application conditions.

2.   Problem analysis

Through onsite investigation and analysis, the engineer from Jiwei found failure of the instrument in level measurement of limestone slurry had three main causes:

1)  There was a mixing device in the vessel.

There was a stirrer or a mixer in the vessel in this application and the liquid surface was quite fluctuates. Because the liquid level was dynamically waving with uncertainty, the signal received by the transducer was disturbed and the measurement accuracy of the instrument was affected.

2)  The Uson-11 was installed very close to the wall of the vessel.

The beam angle of the installed Uson-11 is 8～12º. If the instrument is installed too close to the wall of the vessel, there will be more false echo signals received by the transducer because of beam transmitting angle and multiple reflections. The measuring accuracy will be reduced as a result.

3)  The Uson-11 with problems was installed without a shelter.

For an ultrasonic level transmitter, liquid level is calculated by the formula S=C×T/2 which is based on the relationship between distance S and sound wave transmitting speed C and travel time of the wave T. From the formula, we can see that we need to find the ultrasonic wave travel speed C in the air. The accuracy of ultrasonic wave travel speed C will greatly affect the level measurement accuracy of the instrument. Ultrasonic wave travel speed C is changing with temperature and the relationship between C and temperature is expressed as: C＝（331.45＋0.61t/）, t is in℃and C is in m/s. The temperature sensor is arranged inside the probe of Uson-11. It is protected and packaged in the probe with sealing adhesive. From the formula above, we know temperature is a main factor affecting ultrasonic sound wave speed. If the instrument is installed outdoor under the sun without shelter, the temperature of the electronic housing and the probe will increase continuously while the ambient temperature does not change much. So the measured temperature is very different with the ambient temperature and therefore it results incorrect ultrasonic sound travel speed and inaccuracy level measurement. That is exactly what was happening in this application then.

In northern China, usually it is hot and dry during the summer and extremely cold in winter. The ultrasonic level transmitter installed outdoor is susceptible to the changing temperature. It will introduce error in ultrasonic sound travel speed and results inaccurate level measurement. In this situation, install an extra sheltering box for the Uson-11 will avoid big changes of the temperature. At the same time, it would be better and necessary to install a temperature sensor where reflects process temperature and use its reading to calculate sound wave travel speed.

3.   Solutions to solve the problem in this application:

In order to solve the problems of inaccurate and unstable level measurement in this application for limestone slurry based on analysis above, a few detailed suggestions were proposed:

1)  Installation position for the Uson-11 should be 60cm apart from the wall of the vessel.

Because the Uson-11 customer selected was with 10m measuring range and 10ºbeam emitting angle, put all of these into consideration, we recommended the customer to install the Uson-11 60cm apart from the wall of the vessel.

2)  Install a protection cover.

Because the instrument was installed outdoor, extra protection cover could shelter the instrument from sun and rain and avoid the error in sound speed calculation introduced by incorrect temperature measurement to achieve accurate level measurement.

Fig.1 Uson-11 onsite installation

3)  Install a stilling tube

Installing a stilling tube can keep the ultrasonic pulse emitted from the transducer of the Uson-11 away from the stirring range, so the emitted pulses aim to a smaller area on relatively still liquid surface. It reduces the disturbance of the medium surface to the minimum level and ensure accurate measurement.

In real application, the instrument installation can be two different ways as shown in Fig.2: install it on a bypass tube outside of the vessel or install it on a stilling tube inside of the vessel. The diameter of the stilling tube shall be greater than 100mm.

Fig.2

After all the recommendations above were followed and appropriate measures were taken, the problems in this application were successfully solved. Two Uson-11s from Jiwei have been working well since they were reinstalled and problems happened before have never happened again.

4.   Technical features of Uson-11 from Jiwei

Comparison with common ultrasonic level transmitters, Jiwei Uson-11 has following advantages based on its innovative design and modifications:

1)   With updated software and hardware.

Dramatically waving medium surface is very common in industrial applications. Jiwei updated the software and hardware for Uson-11 to improve its robust and stability. A time extended averaging algorithm is added into the program and improves the adaptability of the instrument for complicated working conditions. In hardware, effective and rigorous electronic filter circuit is added to filter out the secondary wave signal or clutter signal.

2)   With temperature compensation function, adaptable to the working process with greater temperature difference, strong adaptability and high measuring accuracy.

3)   Ingress protection level of Uson-11 reaches to IP66/IP67.

4)   With RS-485 interface.

5)   Special echo signal processing algorithm is used, can effectively avoid false echo waves.

Uson-11 Ultrasonic Level Transmitter Jiwei developed is a highly reliable and accurate instrument. It is widely used in various industrial process, smart city management and wastewater treatment. It is highly evaluated by our customers.