SCIENCE
IN THE G L A S S
INDUSTRY
METERING OF GLASS CHARGE RAW MATERIAL COMPONENTS
(REVIEW)
V. E. Manevich, K. Yu. Subbotin, and D. L. Benyukhis
UDC 666.1.002
The intensification of glass production and the more stringent requirements being imposed on product quality necessitate upgrading of glass charge preparation technology and equipment. The central problem that arises in this connection is that of improving metering efficiencv. The metering system should assure high metering accuracy combined with high throughput, reliability, and flexibility {the capability of metering several components). Batcher size should allow either vertical or horizontal batching/mixing lines, with raw material discharge onto a conveyor belt or directly into the mixer. The engineering implementation of a glass-charge raw material metering system is therefore quite complex. Permissible charge chemical composition deviations from specified levels amount to 0.05-0.30% for different types of structural and technical glasses [i, 2]. Deviations exceeding these limits lead to instability of glass and glass-product founding, working, and physicochemical properties. Glass blending in the furnace requires expenditure of additional fuel to intensify convection and diffusion processes. Up to 60% of the rejection level in glass production is a consequence of nonuniform charge chemical composition. Use of batchers for several components greatly reduces capital and operating costs in small-scale production of multicomponent glasses. Capital costs are reduced bv high hatcher throughput at large plants, e.g., those manufacturing plate glass. This conflicts with metering accuracy requirements. The metering process can be vibratory or discrete. The principal characteristics of continuous metering are high throughput and low accuracy (2-3% of specified batch size). Discrete metering is therefore used to prepare glass charges. Figures 1 and 2 show block and operating diagrams of a discrete metering material is moved by feeder 2 from storage bunker 1 to batcher hopper 3, from metered batch is transferred to conveyor 5 by offloader 4. The actual masses ing and outgoing batches are established by measuring system 6. This sort of scheme is characteristic of virtually all types of discrete botchers.
system. The which the of the incommetering flow-
Until recently, development of metering systems took three major directions: improvement of measurement and control instruments, of control algorithms, and of feeder design. Discrete metering systems are equipped with various types of feeders: vibratory, drum, screw, and gravity. Vibratory feeders have relatively high throughput and eliminate the possibility of material adhesion, but they also have significant drawbacks (the possibility of accidental material spillage from the feeder chute, high-frequency alternating loads, and high noise levels), which limit the throughput of automatic metering systems and reduce metering accuracy. Rotary feeders are equipped with compartments designed in such fashion that each of them corresponds to the permissible metering accuracy and, when the feeder is stopped, drive system inertia does not cause any impermissible deviation of batch mass from the specified level. Metering systems with rotary feeders consume less energy than those with vibratory feeders and give a lower metering error (0.3%) [3]. The drawbacks of rotary feeders are blade wear and possible jamming when lumpy material accidentally enters the system. A constant material transport speed must be maintained in a screw feeder, in order to obtain a continuous uniform flow and reduce the dynamic metering error component. Flow nonuniformity arises as a consequence of the presence of voids in the material and its adhesion to the feeder wall. These shortcomings are overcome by equipping the feeder with additional State Scientific-Research Institute of Glass. Veda Industrial Association. from Steklo i Keramika, No. 7, pp. 10-12, July, 1990.
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0361-7610/90/0708-0250512.50
.~ 1991 Plenum Publishing Corporation
Iranslated
2
5 () Fig.
i.
Block diagram of metering system.
P
P~
Po _
Fig. 2. Operating diagram of metering system. tl, ts) Intake feeder start and stop; t 2) intake feeder stop command; t~) end of material loading; t4...t 5) determination of masses; t 5, t s) offloader start and stop; t 6) offloader speed switching; t 7) offloader stop command; t 9) end of material offloading; tg...tl0) establishment of "zero" mass; P~, Po) actual masses of loaded and offloaded batches; Pb) specified batch mass. mixers, preferably a single mixer that delivers material directly to the screw [4]. Dualand triple-speed charging protocols are employed to increase the throughput of metering systems with screw feeders while maintaining metering accuracy [5]. Gravity feeders are most reliable and have the highest throughput. Metering systems with such feeders are widely used in the glass industry, but only for very loose materials. The thrust of measurement system upgrading has been toward a fundamental change in mass measurement methods and the manner in which the signal indicating metered material mass is transformed. Lever-type systems, in which the mechanical displacement associated with a change in the mass of the material delivered to the hatcher bucket was transmitted through a lever system to the pointer of a gauge (directly or through an intervening electromagnetic device), were initiallv employed. The angle of pointer rotation corresponded to the mass of the material. This sort of metering system had low hatching accuracy and could not be adapted for remote control. Such metering systems were later equipped with photoelectric systems for converting the pointer rotation angle to a digital code. Remote control of the metering process became possible. A signal in the form of a digital code is more noise-tolerant. However, the complex kinematics of the lever system were retained. A qualitative leap was made with the appearance of sensors that convert information on the mass of a body to a continuous electrical signal. This made it possible to avoid levertype systems and to expand the use of electronics in mass metering, where resistance strain gauges are now dominant [6, 7]. However, practice has shown that weighing devices with resistance strain gauges do not always achieve a resolution of even i000 scale steps. Only 251
the leading firms have been able to improve this to 2000-3000 steps. A further increase in precision (the theoretical resolution of strain gauges can reach 6000 scale steps) is quite complicated in technological terms and will raise instrument cost [8]. The accuracy of raw material hatching for glass charge preparation at leading foreign firms is 0.1% when strain gauges with an accuracy rating of 0.03-0.05 are used. Recent years have seen increasing use of optical methods for measuring mechanical parameters, including mass and force [g]. The development of mechanical stresses, flexures, and microfiexures in an optical fiber affects the parameters of the signal propagating in it [i0]. This can be manifested in a phase shift or a change in optical power loss, polarization, or transmission speed. Optical measurement methods are making it possible to improve the accuracy, speed, and noise tolerance of instruments for mass and force determination. Foreign firms have developed a number of balances and metering systems in which interference-type optical sensors and optical strain gauges consisting of one or more fibers are used to measure mass and force [8]. The most significant advances in metering hardware have been associated with progress in control systems. These systems were originally based on logic elements that provided compactness, reliability, and ease of repair [ii]. Mediumscale integrated circuits subsequently came into use. There are Soviet-developed systems for highly reliable control of glass charge metering/mixing lines, incorporating stabilized equipment operations timing and automatic fault diagnosis [12, 13]. A new hardware base is being provided by further reduction of control panel size, lowering of energy consumption, and improvement of reliability. Control algorithms are implemented by logic modules. Thyristor modules provide the basis for power circuit automation. The appearance of specialized microprocessors and their use to control industrial processes have resulted in considerable progress in glass charge preparation technology. Microprocessor systems allow specification and implementation of control programs, while permitting interactive metering system control. This mode is implemented in the form of prompts with keyboard input [14-15]. Microprocessor systems monitor metering conditions, effect self-diagnosis and a u t o matic system correction, and communicate with the top-level computer that controls the metering/mixing line. The microprocessor handles documentation, data processing for automated material composition analysis, assessment of metering dynamics, variation of specified batch mass, and computation of technical-economic indices. Microprocessor control makes the overall metering/mixing liable and less expensive, while simplifying maintenance.
line control system more re-
An important trend in strain measurement is toward combining the sensor with a microprocessor (intelligent sensor). This makes it possible to compensate for systematic sensor errors (hysteresis, zero drift, and thermal errors). Research has been conducted over the last few years at the State Scientific Research Institute of Glass, the Veda Commercial Association, and the Bor Glass Plant in order to develop a fully automated metering/mixing line. This has resulted in design of a fundamentally new line of high-accuracy batchers utilizing the partial metering method. This method permits algorithmic implementation of high metering accuracy in reliable small batchers of simple design when precision strain gauges and microprocessors are available. The main advantage of the partial metering method is the fact that total batch mass is made up with an error produced solely by the last partial batch. All other partial batches are measured under static conditions and summed by the microprocessor. Batching in the last (n-th) cycle is specified by the equation n--l
3l--~mi, where M is the total batch mass and m i is the partial batch mass. Metering cycles.
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in the last cycle takes place with a reduced delivery rate, unlike previous
The error of the new metering method does not exceed 0oi%. There is a decrease in batcher size, and it is possible both to simplify the metering/mixing line flowscheme and to carry out preliminary mixing of the material as it is loaded into the mixer. Further improvement of metering efficiency will be due to upgrading of diagnosis and control algorithms and signal conversion and transmission methods. LITERATURE CITED i.
2. 3. 4. 5. 6.
7. 8. 9. i0o ii. 12. 13. 14. 15.
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