Continuous Emission Monitoring. James A. Jahnke

Чтение книги онлайн.

СКАЧАТЬ the coarse filter are the best methods for performing this check (Reynolds 1989). If the system is not pressurized by the calibration gas, this method can be used to determine if leaks, blockage, or gas adsorption are occurring in the system.

      The system should also be capable of checking the calibration directly at the analyzer. A comparison between instrument readings from a probe calibration check and an analyzer calibration check is frequently useful in system troubleshooting.

      The calibration gases are usually injected automatically every 24 hours although some operators prefer to perform calibration checks manually so that the system can be watched more closely.

EXTRACTIVE SYSTEM COMPONENTS AND ACCESSORIES

      The design of an extractive system involves considering more than piecing together a probe, pump, and a conditioning or dilution system. The task is more complex since decisions must be made on at least the following minimum components:

      1 Sample probe: construction and composition

      2 Probe blowback: design and frequency

      3 Sample line: composition, length, and diameter

      4 Valves and fittings: construction and composition

      5 Pressure and vacuum meters: quality

      6 Moisture conditioning system: refrigerated, dilution, capacity, design, and construction

      7 Filters: coarse, fine, or coalescing

      8 Pumps: capacity, type, and quality

      9 CEM shelters or cabinets: location, shelter HVAC requirements, enclosed space safety considerations, temperature stability

      10 System controller: programmable logic controllers, datalogger, or microprocessor to sequence and control automatic functions

      11 Electrical support: fuses, circuit breakers, regulating equipment, and uninterruptible power supplies (UPS)

      12 Calibration gases: location, injection point, tubing requirements, regulators, and manifold, certified gases as appropriate, cylinder gas cabinet or weather overhang, cabinet heater/air conditioning as appropriate

      13 Automatic calibration system: valves, injection sequencing, interconnections with the DAS and/or DCS

      Issues associated with many of these components are discussed in further detail by McNulty et al. (1974) and Podlenski et al. (1984). The resistance of different materials to acid gases, flow rate, condensation requirements is particularly addressed.

      Extractive CEM systems are most commonly housed in a temperature‐controlled room or a modular shelter. This adds significantly to the expense of a CEM system, but it does provide a centralized location to conduct system operations. To reduce costs, system manufacturers can also incorporate systems in temperature‐controlled National Electrical Manufacturers Association (NEMA) enclosures, a common practice in process monitoring applications. Alternatively, by close‐coupling the system to the stack, costs associated with sampling line and shelter are essentially eliminated.

      To construct a working extractive system that delivers a representative sample to the gas analyzers is not something that can be done without experience, requiring an understanding of the interaction of gases, temperature, and materials. This experience is most often gained through trial and error and may require time to acquire.

MINI‐SYSTEMS

Compact miniature CEM systems advertised as mini‐systems or microsystems can be obtained at costs less than one quarter of the cost of traditional CEM systems that use discrete analyzers mounted in an instrument rack located in a temperature‐controlled shelter. Although these systems are often designed for relatively clean or specialized applications, such as gas turbine NO_x monitoring, there are few technical barriers limiting system miniaturization for wider use. Increasing miniaturization in sensor design and microprocessor systems enables the compact construction of entire CEM systems that include extractive components, conditioning systems, and analytical systems (Josseau 2009; Koch et al. 2007). A single microprocessor can provide both system and analyzer control for multiple gas sensors. RS232‐RS485 outputs allow direct input into the plant distributive control system or environmental manager's computer.

MODULAR SAMPLING SYSTEMS

      Miniaturization in extractive sampling system design has gone a step further through the development of modular, miniaturized sampling systems. Sponsored by the Center for Process Analysis and Control (CPAC), at the University of Washington, a consortium of equipment manufacturers and end‐users was formed for this purpose. Intended primarily for the process industries, refineries, and chemical companies, the “New Sampling Sensor Initiative” or “NeSSI” established the ISA standard, ISA 76.00.02‐002, for the design of modular sampling platforms (ANSI/ISA 2002). The goals of the program were to simplify and standardize sampling system design by using modular flow components that could be arranged like building blocks.

      Derived from modular concepts developed in the semiconductor industry, functional components such as valves, gauges, regulators, sensors, and analyzers are mounted on interconnected building blocks. The standard specifies that each building block is to have a “footprint” of about one and a half square inches. The building blocks are machined with holes and passages in configurations that allow the functional components to be interconnected. Instead of stainless steel or Teflon tubing with nut and ferrule fittings, common in traditional extractive systems, the functional components (valves, gauges, regulators, and so on) are interconnected by the use of annular o‐ring slip‐fit pressure connectors or connecting tubes having specially designed o‐ring boss end fittings.

Parker‐Hannifin, Swagelok, and Circor have designed ISA 76–compliant sampling systems; however, each company uses their own approach to interconnecting the blocks (Figure 3‐28a–c).

       In the Parker‐Hannifin system (Dudley and Cost 2012), the modular blocks are arranged on a pegboard, or other support, where they are interconnected by annular slip‐fit pressure connectors. The flow channels of the Parker‐Hannifin system are all in one plane, allowing for straightforward topology.

       In the Circor system (Sherman 2005), the building blocks interlock, creating their own structure, which then may be mounted on a plate using a stand‐off support. The system utilizes a single building block in conjunction with various types of tube sets (“standard flow tube sets”). Flanges on the blocks allow them to be connected together as a “stick,” with the tube sets connected under the blocks to route the gases through the blocks to the functional components.

       In the Swagelok design (Wawroski 2004), interconnecting tubes called “basic flow components” (similar to the Circor tube sets) are inserted in channels machined into a platform called a “substrate layer.” The building blocks are mounted on top of the substrate layer and are interconnected to the flow tubes from the bottom. A separate manifold layer, beneath the substrate layer platform, is used to connect the flows between the substrate channels. The building blocks joined with the valves, regulators, gauges, sensors, and other system components constitute the “surface‐mount” layer.

СКАЧАТЬ