Level Measurement for Bulk Material
- 30 Aug, 2023
Selecting the level measurement in the process control industry are based on characteristics of the process material, such as density, capacitance, temperature, etc. It is important to understand the fundamental mechanics of the measurement technologies in order to correctly specify the appropriate method for the application.
For a start, a good way to narrow down the number of level measurement options is to categorize the level measure into contact type and non-contact type.
When determining contact and non-contact type approach, it is important to identify the process material being measured (generally classified as liquids, granular, solids, slurry and interfaces), as well the size and shape of the tank, the pressure and temperature that the process require, amount of agitation, available power etc. One also must consider whether the material is corrosive and could possibly cause damage to the measuring device, whether it is volatile and a contact sensor might create a safety hazard, whether the agitation, temperature, or pressure of the process material could affect the reading of or damage the contact sensor, and any of the troublesome possibility. On the other hand, non-contact solution may be more costly and may be outside of the budget constraint. These factors influence the types of level sensors that can be used.
Tuning Fork Level Switch
A tuning fork level switch comprises a two fork which is driven to oscillation at its natural frequency, usually by a piezo-crystal assembly. When in air, the forks vibrate at their natural frequency which is monitored by a detector circuit. When process material covers the forks, The frequency of operation switch is chosen to avoid interference from normal plant vibrator which may cause false switching.
- It is unaffected by flow, bubbles, foam, vibration, solids content coating, properties of the liquid and Product variation.
- It is very reliable in both high and low applications when advance checks and diagnostics are conducted.
- It is not suitable for very viscous material.
- Material build up between forks may cause level detection disturbances.
Single Bar Vibrating Probe Level Switch
The single bar/rod vibrating probe level switch shares similar operation principles of the tuning fork. It is easy to mount and can be mounted at almost any angle. Once base calibration is done upon installation, the device functioning is reliable and requires minimum monitoring. This device can withstand tough lateral loads and static electricity. Furthermore, it eliminates the application issues associated with jamming and clogging between tines on a tuning fork device.
- Conveyor material flow level measurement
- Solid and liquid material level measurement
- Powder and particle level measurement
- Dust collector and ash level measurement
Rotary Paddle Level Switch
The purpose of the rotary paddle switch is to detect the presence of solid/powdery material in most kinds of tanks, bins and containers. The unit is usually located through a bin wall at the top, middle or low level of a bin. During normal operation, when no material is present, the corresponding motor rotates the paddle at RPM. When the paddle is impeded by the process material, the frequency of oscillation drops and is detected by the switch electronics, which in turn changes the output state of the level switch to operate an alarm, pump or valve.
- Plastic and Synthetics
- Grain, feed and wood chips
- Cement, sand, gravel and clinker
RF Admittance Level Sensor
RF admittance level sensor uses pulsed radio frequency to determine when material or liquid is approaching the end of the probe. Since this type of sensor uses the change in dielectric to determine the level of a liquid or granular solid material, it is also called a capacitance-level sensor.
It consists of the main sensing probe and the ground reference probe, the two probes are strategically located, one for ground reference and the sensing probe for hi level determination of the process material in the vessel. When the vessel is empty, the capacitance is measured by taking measurement between the main sensing probe and the ground reference probe. As the target material (level in the tank) begins to increase, the capacitance changes proportionally. RF admittance level sensors use the capacitance between the main sensing probe and the ground reference probe to calculate the level of material in a vessel.
RF admittance level sensor is easy to install and to debug, It is unaffected by material attachment to the probe, is stable, has heat resistance up to 550°C, and options of anti corrosion and explosion proof are available. It is mainly used in asphalt plant, paper factory, glass factory, beverage factory, cement factory, paint factory, pharmaceutical factory, coal mine plant, food production factory, sewage treatment plant, etc.
Liquid Level Transmitter
Liquid level transmitters are engineered for detecting levels of liquids and are for detecting interfaces between two different liquids such as oil and water. These transmitters measure the level by measuring the head pressure of liquid. They should be resistant to corrosion, resistant to heat, and come with overload protection. They are widely used in petrochemical, tar, heavy oil processing, storage and transportation, water conservancy, pulp paper making, pharmaceutical industry, municipal engineering, civil engineering sewage treatment plants etc.
Radar Level Transmitter
The radar level transmitter antenna sends a very narrow microwave that is continuous. The continuous wave travels in space at the speed of light, and when it hits the surface of the tested medium, its energy is then reflected back and received by the same antenna. The time interval between sending microwave and receiving microwave is proportional to the distance between antenna and the surface of the medium. The level measurement by a radar level transmitter is achieved by calculating the antenna to the measured distance on the surface of the material medium. An important factor while using radar type level transmitters is the dielectric constant of the material. For measurement of interference, there should be at least a difference of 5-8 dielectric constant. Otherwise, the radar will not create an echo.
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Non-contact radar level transmitters are ideal for highly corrosive liquids.
- Radar level transmitters are suitable for measuring liquid, solid, slurry, or interface levels.
- Since there are no moving parts, maintenance is very low.
- The measurement is unaffected by liquid properties like density changes, temperature changes, and pressure changes. The only dielectric change affects the measurement.
- Radar-type level transmitters have a good response and accuracy as compared to other level measurement techniques.