Determining the exact level and filing speed of bulk solids can be a challenge, especially in small vessels. With very high filling speeds of approx. 200-400 mm/s, depending on the pump used, small vessels require fast and precise level measurement to prevent overfilling.
In the past, capacitance level meters have been used to achieved level measurement of bulk solids. However, these meters are dependent on medium dielectric constants and a specific probe calibration in tanks, rendering them inadequate for bulk solids within small vessels.
Guided Radar Level Transmitters are ideal for various types of processing and storage applications of bulk solids, delivering exceptional performance in liquids with low dielectric constants (low reflectivity) such as oils and hydrocarbons.
Known as TDR (Time Domain Reflectometry), also known as guided Microwave or Guided Radar Level, work by using low-energy, high frequency electromagnetic impulses, generated by the sensors circuitry.
This is propagated along the probe, which is immersed in the liquid to be measured. When these impulses hit the surface of the liquid, part of the impulse energy is reflected back up the probe to the circuitry, which then calculates the fluid level from the time difference between the impulses sent and the impulses reflected.
A sensor is then used to output the analysed level as a continuous measurement reading through an analogue output, or it can convert the values into a freely adjustable switching output signal. Guided Radar Level Transmitters are ideal for the following applications:
- Unstable process conditions such as changes in viscosity, density, or acidity
- Agitated surfaces: Boiling surfaces, dust, foam, and vapour do not affect device performance
- TDR also works with re-circulating fluids, propeller mixers, and aeration tanks
Achieving continuous level measurement of bulk solids in silos or hoppers
Determining accurate measurement of bulk solids in silos and hoppers can be hampered by the humidity and conductivity of the product. Continuous level measurement can also be interrupted by issues of motor stress and breakdown of a level instrument when measuring heavy bulk solids, such as coal. Level instruments need to be flexible and robust to ensure the maximum and the minimum level is controlled reliably.
Rotating Vane Level Monitors with a double microswitch are ideal for measuring bulks and solids in silos and hoppers, including coal. Regardless of weight, humidity, and conductivity of the product, the maximum or minimum level is controlled reliably.
Rotating vane level monitors are often flexible and available with different vanes and process connections to match the media to be measured and the size of the tank. A synchronous motor drives a rotating vane that is extended into the tank by means of a shaft.
As soon as the bulk reaches the rotating vane, its rotation is blocked. The restoring force moves the pivoted motor away from its original position. Hereby, a micro switch is actuated, which gives out an alarm signal. A second microswitch turns off the motor.
If the filling level decreases, the rotating vane is released again, and the force of a spring pulls the motor back to its original position. The motor gets turned on again and the working contact is switched back. The switching sensitivity can be adjusted in three steps by changing the spring position.
Following issues with motor stress and breakdown of an instrument used on a coal hopper, a Kobold Rotating Vane Level Switch for Bulk solids was used. This device was manufactured with a double microswitch to ensure the motor was able to turn off when the media (coal) was pressing on it. This alleviated any motor stress or breakdown during filling.
Applications for Rotating Vane Level Switches with double microswitches are:
- Cereal
- Flour
- Granulated plastics
- Cement
- Sand
- Cacao
- Sugar
- Various bulk materials
