01 August 2004

Digging up savings: Go with the flow

By Mark A. Morton and Sheldon V. Shepherd

Detecting bulk solids flow with acoustic monitoring technology.

It's important to find out if material flows are present or not throughout a bulk-processing facility; material cost savings and increased plant efficiency can offset an investment in monitoring. Most bulk solids processors can do this using low-cost acoustic emission-monitoring technology.

Noncontacting acoustic technology originally appeared on the industrial scene to detect wear on large turbines. Today the technology monitors solids flow in processing plants. It nonintrusively uses sensors to monitor high-frequency sounds or structure-born acoustics. Friction and the impact of powders, granules, and solids in motion generate the acoustics.

Also, because the sensor never contacts material, you reduce wear and tear. This noninvasive design also provides the benefit of a sensor you can install without shutting down or upsetting the process. The acoustic emission-monitoring sensor is an excellent example of how you can adapt a technology developed for one type of equipment monitoring for use in bulk solids processing.

You don't need to interrupt material flow or shut down the process to install or maintain the sensor. The unique piezocrystal only responds to signals in the 100-kilohertz range, so it ignores the low-frequency sounds that vibrations or machinery generate. The sensor can operate in a stand-alone system or interface with a facility's process control system. Typical applications include flow/no-flow or high-flow/low-flow monitoring and plugged chute detection. You can also use it as a broken filter bag alarm in a dust collection system.

Material handling applications

You can apply noninvasive detection of solids flow in several ways in the material handling industry. When the flow of solids blocks or plugs chutes, it can cause lost production time and require unnecessary maintenance. In the area of dust collection, a faulty dust collection system's release of particulate matter into the environment is not desirable for the facility and the local community.

In the past, invasive techniques were the only available ways for industry to monitor the presence or absence of material. Typical intrusive devices employed mechanical paddlewheels, tilt switches, and optoelectric, proximity, vibrating, capacitive, and inductive technologies. Unfortunately, by its very nature, any material that requires plugged-chute detection typically displays enough adhesive qualities to foul any invasive sensor, rendering it useless.

The plant engineer's ideal flow-monitoring technology doesn't contact the material you are monitoring. This eliminates the cost and hassle of cleaning the sensor on a regular basis. You also minimize wear on the sensor because it isn't subject to the abrasive properties of bulk solids flow.

Steel and cement

A challenge in the steel, cement, and other heavy processing industries is maintaining the proper fuel feed rate to the furnace-essential for ensuring the right processing temperatures. In one example, the furnace burns pulverized coal delivered through injectors. The kiln operates at 1,200°C (2,192°F). Head loss in the injectors can result in temperature loss and poor production. A sudden increase in pulverized coal load in the injectors can lead to a potentially dangerous increase in melting point temperature levels.

In this example, to ensure smooth operation and an effective way to monitor the flow of coal into the furnace, each injector has an acoustic sensor mounted on the pulverized coal in-feed conduits. The sensor monitors acoustic emissions from the movement of the coal. The sensor immediately detects any injector inflow blockage or sudden increase in pulverized coal flow. Each sensor in the process provides a 0-10 volts direct current signal, which a programmable logic controller monitors. In the event of a blockage or uneven flow, the signal generates an alarm. Operators can then take quick action to fix the problem before it upsets the process. Installed upstream from the injector, the sensor mounts outside the process for noninvasive process monitoring. You can install the unit in minutes via a clearance hole and bolt welded on a metal component of the process. The sensor has a stainless-steel housing sealed against dust and moisture. With no moving parts, it requires little or no maintenance.

Acoustic sensors continuously monitor the coal feed to the heavy-duty furnace. Operators are immediately alerted if a flow change occurs. This facilitates smooth regulation of the furnace operation to ensure maximum productivity.

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For an effective way to monitor the flow of coal into the furnace, each injector has an acoustic sensor mounted on the pulverized coal in-feed conduits.

Operation principles

Acoustic energy waves occur naturally when matter vibrates at a frequency usually between 0 hertz and 600 kilohertz. Sound is acoustic energy in the range of 20 hertz to 20 kilohertz. Lower frequency acoustic energy has a long wavelength and takes longer to attenuate. The low-frequency sound of a foghorn can travel for miles through densely attenuating fog. Higher frequency acoustic energy has a short wavelength and dissipates rapidly. Consider a low note struck on a piano, which carries for a longer time than a high note, which lasts an instant. (See the figure below.)

Originally adapted from a device used to monitor wear on large turbines, bulk solids acoustic monitoring technology uses a piezocrystal-a type of crystal that generates electrical current when it is subjected to mechanical stress. Bulk solids acoustic monitoring uses a special type of piezocrystal that only responds to a designated high-frequency bandwidth. Acoustic energy the sensor receives within a particular frequency range excites molecules in the piezocrystal, producing a continuous, measurable electrical signal it can interpret. The electrical output from the piezocrystal is in direct proportion to the level of acoustic energy the device receives. Additional amplification and processing by the sensor and associated electronics convert the piezocrystal output into an electrical signal; you can use it to indicate solids flow, 0-10 volts direct current, or 4-20 mA.

Most plant vibration is in the low-frequency range, well below 100 hertz. The sensor monitors a relatively narrow bandwidth, detecting acoustic energy between 75 kilohertz and 175 kilohertz, nearly a thousand times the frequency of normal plant noise. Typical sources in an industrial environment for high-frequency acoustic emission are friction or the impact of particulate matter against metal.

Another important property to consider is propagation, the transfer of acoustic energy at a molecular level. Acoustic energy easily propagates through dense materials such as metal, whereas it propagates poorly through less dense materials, such as air. This enables the technology to be immune to high-frequency extraneous noise anywhere other than the designated monitoring area. The sensor detects the high-frequency energy waves generated by the impact of solids against a metal surface, such as a chute, pipe, or pneumatic line.

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The signal from the crystal is converted to a typical sensor output.

Dust reclamation

In a dust reclamation system, cement dust forces itself through a line into a cyclone, which separates the air from the dust. Operators then add reclaimed cement dust to finished cement. If the dust in the chute to the rotary valve bridges and becomes an obstruction, you should immediately detect it to maintain the process.

In the event of a dust bridge, there is no way of knowing whether or not the rotary valve is starved until the system shuts down. The process is difficult to inspect during operation, and it isn't possible to have an inspection port at the discharge of the system for safety reasons. Shutting down for manual inspection means time, effort, and material loss.

The acoustic sensor detects changes in high-frequency acoustic emissions from machinery or bulk materials in motion. It reacts to flow changes-warning of a blockage, product absence, or equipment failure. Installation of an extended-temperature sensor is easy. It mounts outside the process, and can be bolted or bonded in place in minutes without process shutdown. A control unit measures the output.

The acoustic sensor has proven to be a cost-effective investment in productivity. Operators can now maintain the system efficiently, because the acoustic sensor provides constant flow-monitoring data. Operators are immediately aware of a plugged chute, blower shutdown, or rotary airlock feeder failure, and can take quick action to correct the problem. This frees staff from frequent manual inspections of the blower, the rotary feeder, and the line.

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In dust reclamation, the sensor monitors flow through the conveying line and indicates that the system is operating properly. The optimum installation point is at the bend in the pneumatic line approximately 1 meter from the cyclone.

Behind the byline

Mark A. Morton is a product marketing manager at Siemens Energy & Automation in Spring House, Pa. Sheldon V. Shepherd is an account manager at Siemens Energy & Automation in Grand Prairie, Texas.

South African mine goes for platinum idea

By Ellen Fussell

Impala Platinum is a group metal producer as well as a miner, refiner, and marketer of precious group metals. Based in South Africa, the company produces about 1.7 million ounces of platinum each year. That's the equivalent of 25% of global production. It is investing in initiatives to enhance capacity in concentrating smelting and refining. Near base metal refineries in Johannesburg, the company concentrated on improving performance in the plant. It wanted to improve scope and delivery of plantwide information to key personnel.

The first step was to identify and develop applications for key improvement areas in increased uptime, throughput, overall equipment and extraction efficiencies, and reduced waste and cost. Over the course of a year, Impala engineers worked with Citect engineers on a suite of industrial information management modules, including metrics, production quality, downtime, and cost maintenance and tracking.

The answer was a suite of modules connecting to multiple plant and business systems, which transformed data and presented it as real-time information for productivity analysis, data mining, querying, and reporting. The revamp gave decision makers the ability to identify production bottlenecks, so managers can take action sooner.

"We wanted to tap into the disparate plant system we currently have-production, financial systems, management-as well as maintenance systems," said Tim Spandiel, manager of Impala's base metal refinery. "We wanted to make all information available in the same data depository and accessible by Internet clients and personnel within the refinery. And we wanted information to be available in real time. Our first priorities were to implement production and quality modules and do process analyses on data-and to focus on our extraction efficiencies."

The modules gave the team the ability to authenticate security and streamline information into a common format, reducing miscommunication. Cross-analysis capabilities gave the team the ability to compare production in multiple plants. Other features included a single run-time platform, interoperability of modules, drill-down capabilities, and customizable favorites, reports, and views. Stressing that the real focus was on downtime, Spandiel said, "It's important to make sure we run at full capacity."

"We also recognized we had islands of data we were trying to bring together, so we didn't have that disparate source of data," said Martyn Fox, manager of technical Impala refineries. "And we wanted to spend more time analyzing the data rather than collecting it."

Some advantages Fox saw were the integration of various modules that "allowed us to take elements from a particular module, use it in conjunction with elements from another module, and do things like mass balancing and metallurgical accounting easier," he said. "Furthermore, because the system is stable and there is this integration between modules, it allows us to roll out further modules easily. We can now minimize the amount of further training," he said. "Personally, the fact is the Web-based system allows you to customize views to those elements you want to see. I can get a good analytical picture off-site of what's happening at the refinery. One of the areas we've made our objective is with the downtime modules. That's allowed us to put more production through the plant."

Fox said one benefit was in multiple input. "In the past we've had a number of people-up to four or five-enter data from similar type systems into spreadsheets, and those spreadsheets gravitated up the system until they eventually got to the manager a week down the line," he said. "Now it's all electronic, and we don't have that paper trail." Fox said the overall recovery of metals was another huge benefit. "Because we're able to speedily get results from systems that tell us what's in our waste streams, we're able to take action much quicker, and that enables us to minimize our losses to our waste streams."