Solid level measurement ABAID ULLAH 2007-CHEM-47

Level measurement Level measurement is defined as the determination of position of an interface between the two media. These media may be solids, fluids or the combination of solid and fluid. Many methods are available for the measurement of level with its own advantages and disadvantages.

Importance of level measurement The measurement of level is of great importance in the industrial field, particularly when control is also necessary. Most of the industrial process require the amount of material in a tank or closed vessels. In many situations it is not possible to view the interior of equipments directly.

Solids level measurement difficulty. The reasons are given below Bulk solids may be of micron size. Bulk solids may be large with sharp corners. They may produce large amount of dust. With the passage of time bulk density changed during storage

The coarse solids are more likely to clump and pile up. The solids don’t have a flat surface usually like the liquids. Solids surface have angle of repose which vary with filling, discharging. Few solid materials are hygroscopic and absorb moisture and moisture combines with solids to clump or cake making material flow difficult which in turn creates problems for level measuring devices.

Practical considerations for selecting a suitable instrument which type of measurement you want continuous and intermittent? which types of variables you want to find out except level like mass, volume? what are the dimensions of vessel and construction? what is the temperature and pressure? what is cost for installing this sensor? where the sensor is to be installed?

what kind of display you want? How angle of repose changes during filling and emptying? what are the filling and emptying rates? what are the flow characteristics of material? Most important parameter is accuracy. Which value of accuracy you want?

METHODS FOR SOLID LEVEL MEASUREMENT: Ultrasonic method Tuning/Vibrating Fork Level Measurement Switches Weight and cable Guided wave radar Laser. Thru wave radar Load cells. Strain gauges

Ultrasonic level transmitter: Ultrasonic level measurement technique is used to measure the level of solid without contacting the solid. Ultrasonic level transmitter sends sound waves into the vessel where solid is placed and is reflected back to the ultrasonic level transmitter. Software enhances the signal.

Uses: It is used in the following industries: Waste water, Open channel flow, pump stations,, dam level. Mining: Crushers, surge bins, storage silos. Raw coal bunkers.

Ultrasonic level transmitter.

Advantages: It is non-contact sensor. It compensates the temperature changes automatically. Can be used for temperature less than 3000F. It can be used for less than 260 feet. Response time is less. Installation is easy.

Disadvantages: It is used for pressure less than 40psi. Reading is changed due to the change in the angle of repose. It is not suitable for light weight material due to the absorbing of sound waves signal. Heavy dust affects the performance. Uneven shape of solids affects the performance.

Tuning fork level switch: Tuning fork level sensor is used where level limit detection is required. Piezo ceramic elements vibrates tuning fork at its resonance frequency. When the solid material reaches the tuning fork then the vibration is disturbed and is sensed electrically.

Uses: Used for very fine materials. Used for those materials which have very less density. In sugar industry. In cement and plastic industry. It is used to measure the level of granular solids which are suspended in low viscosity liquid.

Weight and cable level sensor Weight and cable level sensors have been used for many years to measure the level of the solids in storage vessels. The instrument lowers a weight on the cable and touches the solid surface and rises to the sensor. The distance it travels is measured and converted into level of the solid.

Weight and cable level sensor

Advantages: For high temperature system. It is very easy to install. Less cost. It is not affected by the angle of repose. It can be used for dusty particles. Not affected by the process condition. It can be used to measure the level up to 150 feet.

Disadvantages: Response time is high. For dusty application regular maintenance is required. It is used for pressure less than 30psi. Accuracy is moderate for greater than 30 feet. It has moving parts so parts may wear and require maintenance.

Guided Wave Radar It is a Continuous level measurement system. Guided wave radar is used to measure the level of bulk solids and powders. Micro-pulses at the speed of light are transmitted and reach the material surface and reflect back to the GWR. Time is calculated and it is converted into level of the solid surface.

Advantages: Response is instantaneous. Reading is not affected by the dust. Accuracy is very high. It is used to measure the level up to 115 feet. Can be used for high temperature and high pressure. Installation is very easy.

Disadvantages: Purchasing cost is high. Pulses can be damaged by the large particles. Not used for sanitary applications.

Guided wave radar sensor

Laser level system: It is used to measure the level for very difficult applications. Laser is a narrow beam that does not scatter on reflection. It is used for long ranges.

Advantages: No calibration is required. It can measure level up to 250 feet. Reading is not affected by the angle of repose. No effect of process conditions (temperature, pressure). Installation is easy. Response is instantaneous. High accuracy.

Disadvantages: Purchasing cost is very high. It is not suitable for high dust application. Laser level sensor

Thru-air radar sensor It operates on the same principle of GWR (Guided Wave radar) sensor. But it more desirable due to following advantages over GWR. It is non-intrusive. It can operate at higher temperature and pressure than guided wave radar level sensor. It is used to measure the level up to 130feet. Response is instantaneous than GWR

Disadvantages: It is used for those type of materials having dielectric constant greater than 1.8. Cleaning is required for dusty applications. Purchasing cost is high. Proper installation and set up is required.

Thru-air radar sensor

Load cells Load cells operate on the principle that when loads are applied on them than they change their resistance, capacitance and inductance. But mostly are designed to change their resistance.

Advantages: It is not intrusive system. Can be used for extreme conditions. Un-affected by the dust. It can be used for dangerous materials. Highly accurate. Ranges from 100 lbs to 1,000,000 lbs. .

Disadvantages: Purchasing cost is high. Calibration is required. Require extra attention to get accuracy.

Load level sensor

Strain gauge level sensor: When force is applied on strain gage then it changes its electrical resistance. Non-intrusive system. Installation is easy. Reading is not affected by the dust.

Disadvantages: Purchasing cost is high. Accuracy is less. Accuracy requires greater attention. Strain gauges are used more frequently than load cells due to less cost and more accuracy.

Strain gauge level sensor