In the case of circular vents, each of the compartments under a vent can be combined into a zone since their exhaust gas will directly effect the visibility of the emissions from that vent. The actual number of probes will be determined by the geometry of the individual compartment exhaust to the vent plenum. For example, Figure 5 illustrates a four-compartment exhaust under a circular ridge vent. There are a total of 4 probes combined in this zone. However since the probes are linked in series to the triboelectric signal origination device (signal detector), the output is seen as a single signal. The baghouse PLC tracks which compartment is cleaning, and identifies the cleaning spike alarm so the information can be sent to a data file for operator reference, or the compartment can be isolated until manually inspected and returned to service by the operator. This minimum programming will optimize the number of zones needed in the BBD system.



The CRV design of exhaust configuration can be optimized in a similar manner as the circular vent arrangement. The maximum number of compartments comprising a monitoring zone can be established by the owner/operator, however the number of compartments contributing exhaust to a given length of CRV should be used to set the number of compartments per zone.

Each facility will need to be independently evaluated to meet the objectives of the owner/operator while developing a BBD system design that is effective and provides the lowest installation cost.

Number of Probes:

The geometry of the exhaust plenum in a negative pressure baghouse and the exhaust port geometry of a positive pressure compartment will establish how many actual probe detector rods are installed. Since the probe rods are connected in series, the triboelectric signal detector sees the effect of each rod as a cumulative triboelectric change. The change of each rod is added to the next rod in the series until the total change is measured by the triboelectric signal detector and transmitted as a relative 4 - 20 milli-amp signal to the Input/Output (I/O) board of the PLC.

For example, the configuration of probe rods illustrated in Figure 5 contains a total of 4 probe rods, however the PLC sees only a single probe signal (the sum of the 4 probe rods individual triboelectric changes) from the triboelectric signal detector for this zone.

Figure 6 illustrates the typical probe rod geometry in the plenum of negative pressure baghouse. The figure is a cross section of the plenum duct. The probe's length should reach to the mid-point of the duct, and be located on the centerline of the section.





Figure 6.1 illustrates the location of probe rods across the exhaust port of the clean room into the plenum beneath the CRV or circular vent. The figure is a cross section looking back into the clean room of the compartment. Exhaust gas would be flowing from the page towards the reader. Generally, the lower velocity associated with the larger cross section of the exhaust port requires several probe rods located across the port. The probe rods would be linked in series, and as noted previously their triboelectric change is seen as a common signal.

These illustrations are not hard design parameters, but are presented to act as guidelines that can be used for the selection of the probe rod locations. Probe rod lengths are generally limited to 6 feet when using standard 316 stainless steel. Probe rods of greater length require special construction and materials to insure that the rod does not flex under the air loading and its own weight. Alternative detector designs using other than probe rods are being developed by some manufacturers.

Temperature Effects Upon the Probe Rods and Detector Locations

The probe rods are generally not effected by temperature extremes. The ambient temperature range listed for the probe rods is -60°F to 400°F. Most baghouses have off gas temperatures below 270°F, the upper temperature limit of polyester bags.

The triboelectric detector on the other hand is much more sensitive to temperature extremes. The operating range for the hardware of the detector is 20°F to 160°F. Since the coaxial signal length from the last probe rod location to the detector is limited to 150 feet, the detector is not able to be located at ground level on larger baghouses. Generally the detector equipment is located at the penthouse or bag maintenance level of the baghouse. The detectors for multiple zones can be located in a central cabinet that can be either heated or cooled as necessary to maintain the equipment within the specification temperature range.

Location of the detectors on the sheet metal walls of the compartments, although a typical location, can present a temperature problem during summer in the warmer latitudes. An off gas temperature of +200°F can conduct a significant amount of heat through the sheet metal wall, especially when the ambient outside temperatures can exceed 100°F heat index on a daily basis. Cooling of the cabinet containing the detectors can present more of a challenge than heating. A small radiant heater or l00 watt light bulb in an insulated cabinet can provide sufficient heat during cold months. While chilled air will be necessary to cool the cabinet in seasonally high temperatures.

It is very important to identify the manufacturer's temperature specification for the detector hardware, and plan to protect the equipment from the extremes in temperature that might be present in the baghouse environment. The output signal from the detector will not be reliable when it is operating outside of its temperature envelope. Figure 7 illustrates a triboelectric signal that has been effected by heat interference. The spikes in the illustration do not correspond to cleaning spikes.

Figure 7 Heat Interference Effect on Triboelectric Signal

Signal Output Monitoring

The triboelectric signal is generated on a real time basis, typically 2 measurements per second are generated from the detector. This quantity of data is quite large, and real time tracking is generally set at intervals that are based upon fractions of a minute. The detector output varies and can be set to only monitor for an alarm level, or a real time tracking of a 4 - 20 milli-amp signal. The signal can be received by a PLC or directly to a PC using proprietary software. A Human Machine Interface (HMI) that uses the PLC or a PC can interface with the baghouse PLC to track compartment cleaning.

An interface between the BBD system and baghouse PLC can be set up to provide automatic visible emission protection by directing isolation of a compartment that exceeds a preset alarm level. This compartment would only be returned to service after the operator had made the necessary bag repairs and manually returned the compartment to service.

Determine Normal Signals and Scale Factor

As noted previously, each facility will have a unique triboelectric signal related to a number of variables. The identification of the normal triboelectric signal pattern can be established within a few hours of operation. The scale factor used to generate the real time tracking and historic trend recording will be the same, and is set to produce a signal pattern that has sufficient amplitude so that an operator observing the signals can readily identify cleaning spikes from the normal signal pattern. The normal cleaning spikes (those associated with no bag damage) should not exceed the 50% to 60% range of the scale. This will provide for sufficient visual amplitude while still keeping the signals on the scale. The scale factor applied to the signals will determine how this data is visually displayed.

The scale factor will also determine the amplitude of the normal on line operating signal for each zone of the baghouse. The normal signal will have variability, but should be set so that the signal is at least 10% of the scale. It is not useful to have the scale factor set so high that the normal signal is at 0% of the scale. Once the scale factor has been set to identify the normal operating triboelectric signature of the facility, the alarm levels can be established.

Even a system that provides only an alarm signal to the operator, and does not track or record real time data, must establish the scale factor for normal operations.

Establish Alarm Levels - Prevent Permit Condition Violations

Each facility construction or operating permit will establish some level of visual emissions (percent opacity) that cannot be exceeded without causing a violation of the permit. For a steel making EAF, the opacity level is set as low as 3%, and other sources can have the opacity set as high as 20%. This opacity level standard is a six-minute average of visual observations by a trained observer or measurements made by a COM located on a discharge stack. Work done by the authors, and published previously, determined that the BBD systems using triboelectric signals were able to detect changes in particulate emissions at levels well below visibility as detected by an observer or a COM . As such, a BBD system is able to provide an operator with data that allows preventative maintenance to take place long before bag leaks develop into visible emission violations.

Each operator can select the type and level of alarms used in his system. The alarm levels used by IPSCO at its two facilities include a Cleaning Spike Alarm, a Caution Level, and an Alarm Level. Each of these signal levels were established as a result of regression analysis done on surrogate particle introduction testing performed at the respective facility. The visible emission observations and COM data were correlated to the triboelectric signals and the resulting regression functions were used to set protective alarm levels during normal operations. These normal operating alarm levels track the real time data during on line operations of the baghouse, and log the events to a data file for operator reference.

- The Caution and Alarm Levels: The percent of scale ranges established by the regression function was set with the Caution Level at 25% to 29% and the Alarm Level at > 30%. The duration of the signal at the respective level was set at one (1) minute as the basis for triggering the alarm. Periodic signals that reached these levels for less than a minute were ignored by the system. The rolling time period allowed for noise signals (an operator walking on the plenum, etc.) to be generated without creating false alarms.

- Cleaning Spike Alarm: This alarm level was developed by evaluating the amplitude of the normal cleaning spikes associated with the return of recently cleaned bag to online service. During the initial return to service, there is a brief interval of small particle passage (invisible to the COM or human eye) while the filtering cake layer is reestablished on the surface of the bag. By tracking the duration and amplitude of this spike, it was observed that weakened or partially penetrated bags could be detected well in advance of visible emission problems. This cleaning spike alarm level was created in the software, and was set at 80% of scale. This refinement has been very helpful in maintaining a proactive program for early identification of broken bags. Figure 8 illustrates a triboelectric screen signal, with the alarm levels indicated for reference.

Figure 8 BBD System Alarm Levels

The signal screen in Figure 8 is from a negative pressure, pulse-jet baghouse that is cleaning on line. Depending upon the individual operator's permit conditions, and level of opacity maximum that determines a violation, surrogate testing can be done to establish protective alarm levels below the violation standard, however that is not always necessary. Alarm levels can be set based upon observation of the normal patterns and events that do result in exceedances.

The Caution Level is intended to give the operator an advance warning of rows or compartments that have begun to increase their signal from the normal baseline. This condition generally is an indication that a bag or bags has begun to leak at some small level. The operation program at IPSCO requires the operator to respond to the Caution Alarm as soon as possible and make the necessary investigation and corrective actions.

At the Alarm Level the baghouse PLC sends a signal an Alarm Signal to the Alarm Record file, triggers an alarm on the baghouse HMI screen, and the EAF Operator's HMI screen at the same time. Alarm Level requires immediate operator response and corrective action.

A Cleaning Spike Alarm is logged to the alarm file and announced on the baghouse operator's HMI screen. The timing to investigate the Cleaning Spike identified row or compartment is generally placed on the maintenance to do list during the next scheduled outage for the Melt Shop. This proactive investigation and corrective action for Cleaning Spike Alarms has identified problem bags long before a visible emission violation can occur.

It should also be noted that signal patterns can change over time, necessitating changes in the alarm levels. A change in the type or manufacturer of the bags used in the baghouse, and age of the bags will effect the normal signal characteristics.

Operator Training

Once the BBD system alarm and normal operating parameters have been established, the employees responsible for maintenance and operation of the baghouse system need to be trained to use the system. This will typically require both classroom and on-the-job training to provide the necessary information to establish operator confidence in the system. It is also important to assess operator feedback over time. Adjustments and improvements in the system can be made to enhance the BBD system usefulness and reliability.

Quality Control (QC) and Quality Assurance (QA) Program

A written QC/QA program needs to be developed for the BBD system at a particular facility. This program should include the following considerations, at a minimum.

- The inspection frequency and cleaning of the probe rods.

- Visual inspection of the cables/conduit of the BBD System.

- Electrical Calibration/Verification of the system components at intervals recommended by the manufacturer.

- Backup hardware for the historic signal file records.

- The period of time that historic records should be maintained. Permits can contain minimum record retetion requirements.

- Annual review of the BBD system by the manufacturer or a consultant engineer to determine whether changes need to be made in alarm levels or the system scale factor.

- The method of QC/QA record keeping.

A QC/QA program is a requirement of the standards, and specific facility permits may require specific actions. It is important to review the facility permit to insure that any such requirements are included in the written QC/QA plan for the facility.

Operation and Maintenance Program

There are several basic considerations that should be determined by the owner/operator of the baghouse before finalizing the BBD system programming and interface with the baghouse PLC.

- BBD system manufacturers provide dedicated software for application to a PC as the HMI interface. The BBD system can be effectively operated without this software, however the decision of whether to apply this software should be made before programming of the system is begun.

- Will the BBD system operate from the baghouse PLC or have a dedicated PLC. The HMI software provided by manufacturers will typically provide a signal to interface with the baghouse PLC. If the decision is made to operate the BBD system from the baghouse PLC or provide a dedicated PLC to interface with the baghouse PLC, a consultant experienced in the PLC programming of the BBD PLC should be retained to write the necessary programs.

- The owner/operator will need to identify how many alarm levels will be part of the BBD system, and how these levels will be interfaced with the baghouse PLC. The baghouse PLC can be programmed to respond to an alarm level by isolating the alarmed compartment until repairs are made.

- If the identification of leaking rows in a pulse jet baghouse is desired, it will be necessary to interface with the baghouse PLC to cause this identification to be made. This interface programming will be different for on line and off line cleaning methods, but the cleaning data from the baghouse PLC must be integrated with the BBD system signals.

- Manufacturers provide modem interface with their equipment that can be used for diagnostic analysis of the equipment. The owner/operator will need to determine whether to use this option if available.

- The data generated by the BBD system can be made available to a plant Ethernet system. A decision as to who should have access to this data internally will need to be made.

- Record keeping of the historic data is done electronically. The owner/operator should determine whether hardware backup is needed, and the method of accomplishing such backup. IPSCO has installed dual hard drives for data storage.

- The historic data can be recorded at the level of measurements (2 per second), however that amount of data is quite large and not necessary. IPSCO has set a 5 second increment for tracking this data.

Conclusions

The mandatory use of a BBD system has become a requirement for Iron & Steel Foundry operators and it has been proposed as an option for EAF steel making operators. The application of this technology can prevent permit violations and optimize the performance of baghouse emission control systems provided that the equipment is installed with a design that maximizes the technology to the type of system operated at the facility. Cost effective equipment installation and control design can be accomplished using the considerations discussed in the paper.

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