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Optical Scintillation Flow Monitors
Temperature causes turbulence in gases, which affects light transmission. This effect is called optical scintillation (OS). Common examples include the twinkling of stars, which is caused by the atmosphere, or heat distortion from hot pavement.
OS can be measured and used to determine air velocity. This technology, which has been used for some time to measure wind shear at airports, has recently become available for stack monitors.
The advantages of OS are that the two heads are mounted straight across from each other on the stack, so no extra platforms are required (see Figure 4). The OS technology also works at high temperatures, including those on the hottest glass stacks. While this is a new technology compared to other types of flow measurement, it has been proven in over a dozen glass furnace stack installations.
Data Acquisition and Handling System
As the main human/machine interface, the DAHS is the heart of the CEMS system. It is what the operator uses to determine compliance and to identify and address problems. The DAHS, which consists of a computer and may include a data logger or PLC, typically performs the following functions:
- Record raw data from the monitors
- Use the raw data to calculate values such as flows, emission rates and averages
- Create alarms when there is a monitor problem or an exceedance
- Create emission monitoring reports
Provide graphs and trends of emissions
Desirable DAHS features include:
- Simple alarms that alert operators when they need to take action
- Remote access and control of the system via the Internet
- An email system for alarm notification and regular report distribution
- Direct access to monitor functions and operating parameters; when combined with remote Internet access, this feature enables remote diagnostics and service
The DAHS is a software system that is written and created by the programmer, and it is customized for each application. Various systems feature significant differences in their capabilities, ease of use, and look and feel. Special attention should be given to evaluating and selecting the DAHS to ensure that it fits the plant’s specific needs.
CEMS Best Practices
The goal of any monitoring program is to provide maximum data capture (system uptime) and the tools that the operator needs to stay in compliance—and to do so with a minimal workload to the plant operating personnel. The CEMS should be designed so that all of the daily checks are done at the DAHS in the control room. The facility’s quality assurance/quality control (QA/QC) plan should list the daily checks required to ensure that the system is operating properly. These checks may include sample flow rates, system pressures, vacuum pressures, calibrations and calibration gas pressures, and system alarms.
Many systems require the operator to leave their workstation and perform these checks on the instruments themselves. The CEMS may be located some distance from the control room, which means the operator must stop monitoring the furnaces to do the checks.
A better approach is to place sensors on all of the critical parameters and tie them into the DAHS. This practice allows the required daily checks to be done in the control room without the operator leaving their station.
Alarms need to be clearly displayed and not subject to interpretation, and should tell the operator if action is needed immediately. Alarm responses should also be very simple. Furnace operators are trained to operate furnaces, not CEMS, and should be expected to respond to emissions problems by managing the furnace.
An outside service should be available for managing CEMS hardware or software problems. The service should include a 24/7 service number, an email alert system and an Internet connection to the system. The Internet connection allows the service provider to immediately connect to the system and start troubleshooting.
If the system has been designed with adequate connectivity, the service provider can perform a detailed diagnosis and often fix the problem remotely. If the problem can’t be corrected online, then the service provider can determine if the problem can wait until the next scheduled maintenance visit or if a technician needs to be sent to the site immediately. In all cases, decisions about the CEMS should be made with the help of a trained CEMS professional.
Periodic preventive maintenance (PM) should be required no more than quarterly. In addition to PM, some CEMS systems require monthly or weekly checks that need to be done by plant personnel. This requires the plant personnel to be trained to perform and document the PM checks. Failure to perform or document a PM could result in a reportable deviation.
Instead, the system should be designed so that PM is performed once a quarter. Most sites have a CEMS service vendor on-site once each quarter to take care of required cylinder gas audits (CGAs) and relative accuracy test audits (RATAs). While on site, they can also perform the required equipment PM, ensuring that the maintenance is done by trained technicians and minimizing the plant’s workload.
The plant must have spare parts available to repair the CEMS. The equipment manufacturer determines the required spares and quantities, and these should be listed in the site’s QA/QC plan. Most plants buy a set of spares when they purchase the CEMS and keep them in their general storeroom. The spares can be expensive, sometimes costing $20,000 or more, and many of them are delicate electronic components (boards, power supplies, sensors, etc.) that can be damaged if not handled and stored properly.
An alternative is for the site to keep only consumable spares (those that are replaced during the quarterly PMs) on-site, and to have the CEMS vendor stock the other spares. The vendor must agree to 24-hr delivery of the parts to prevent excessive monitor downtime. In this arrangement, the vendor ensures that the parts are properly stored and maintained, and the plant is only charged for the parts when they are used.
For additional information, contact Air Tox Environmental Co., Inc. at 479 Tolland Turnpike, Willington, CT 06279; call (860) 487-5606; fax (860) 487-5607; or visitwww.airtoxenviro.com.