Process Weighing
Tank Weighment System
Process
In many process industries, handling and mixing a wide variety of Raw materials in a specified proportion, has been an important production process. For proper batching of raw materials the tank in which it is contained is to be accurately measured. In this regard, tank weighment system is very essential.
Measurement of tank while it is stable is of no difficult by using normal electronic weigh indicators. But no ordinary type electronic weight indicators will be useful for weighing reactor vessels / tanks especially if the reactors are going to be ON (agitating) when the weighment has to be done.
Mechanical vibration generated by the reactors causes huge distortion in the load cells output which further will be amplified by these ordinary type weight indicators. As such, such weight indicators cannot be employed for accurate weighment at all.
Solution
Manual and semi-automatic production processes result in in-accurate weighment, Costly labour and long time, Automatic Tank Weighing System provides very accurate and automatic weighing for this purpose.
The tank weighment system consists of tank / reactor or a silo, which contains the ingredients to be measured. The tank has a stirrer to mix up the materials in it. Due to the agitating motion or mechanical vibration of the tank, measurement cannot be done through normal weight indicators. To ensure accurate weighing by eliminating vibrations, Hardy based weight indicators are used.
The level of the ingredients in the tank is sensed by the level sensors place at the low and high level so that it senses both the positions accurately. Tank is provided with pneumatically operated doors at the bottom, which opens according to the sensing of the sensors. The temperature of the tank is maintained by water circulating around the tank by using tubes. These temperature controlling is maintained by the Allen Bradley PLCs. The weight of the tank is displayed locally by using displays. This would be useful for the operator. The mimic present in control centre provides complete details of the tank weighing system.
Hardy Weight Indicator
Applications of Hardy Instruments are most popular in India and worldwide. They are unique in design and cannot be compared to any other indicators available in the world. The Model HI-2151 Weight Controller uses patented WAVER SAVERÒ, Technology that eliminates plant / process vibration, so that accurate weighment can be done always. Further this product has internal resolution of over 10 lakh internal counts and 9.85 lakh display counts. Because of that, it can tolerate large dead load, over-sizing of the load cells and still have a huge resolution to work with. The internal resolution offered by Hardy equipment is 25-50 times more than the typical ordinary weight controller resolution, offered by others in the market.
A hardy weight indicator comes with standard two set point output options and RS232C option. Various option cards can be added on in the future. Hardy equipment employs a six-digit alphanumeric Red LED Display and has an accuracy of 0.0015% (non-linearity).
Double-ended shear beam load cell
A load cell is used to measure the weight of material, placed over it. In tank weighing system the load cells are connected to the tank to measure the tare and gross weight of the tank and thereby measuring the net weight of the material present inside the tank. To ensure accurate weighment double-ended shear beam load cell is used.
The Double-ended shear beam is very simple in design and very easy to install and maintain. It has several advantages over the performance of Compression type of load cells and single ended shear beam load cell. Some such advantages are furnished below.
| • |
Double ended Shear Beam load cells offer better output (3 mV/V) than compression type (typically between 1 mV/V to 2 mV/V) |
| • |
They are insensitive to axial loads and bending moments |
| • |
They offer better stability to the tank structure |
| • |
They do not require constrainer and such mounting accessories at all. Such arrangement is necessary for compression type |
| • |
GE Fanuc India |
| • |
Siemens India |
| • |
Sensortonics India |
Usually Sensortronics make of double-ended Shear Beam load cells also have some specific features like
| • |
Sealed to IP68 standards |
• |
Low profile |
| • |
Offer fast return to zero |
• |
Low creep |
Because of the design of the double ended shear beam load cell, they does not require any additional mechanical supports such as constrainers, Tie-rods, bearings etc which will effect the performance and thereby increase the maintenance resulting in more downtime.
Effectively using the HARDY Waver saver with double-ended shear beam loadcell, the system can be guaranteed for a least count of 1 Kg and accurate/stable
Salient features
| • |
Eight based level sensing hence more accurate. |
• |
Local weight display. |
| • |
Use of double ended shear beam load cells - Absence of side loads. |
• |
Temperature control by Allen Bradley PLCs |
|
