Continuous Emission Monitoring System (CEMS) shelters provide a controlled environment for analyzer operation. CEMS shelters isolate instrumentation from the adverse conditions in the environment to ensure the accuracy and precision of sample readings. In this article, we review how a CEMS shelter works, its construction, and the benefits of using this system in any facility.
How a CEMS Shelter Works
The function of a CEMS shelter is to continuously monitor gaseous emissions from a processing facility. For example, refineries, waste incinerators, and power plants utilize CEMS shelters. Although the operation of a CEMS varies according to a client’s needs, its basic function is to access and document the emissions from a plant’s operations.
To achieve this, a typical system uses a probe-heated tube bundle and sample pump to extract a representative sample of the stack flue gas. Then, the pump transmits the sample via a sample line to the gas analyzers in the CEMS shelter. As per requirements, the sample passes through a conditioning system before and after analysis by the analyzer.
Examples of analyzers include O2, SO2, NOx, CO, and CO2. These analyzers operate continuously, feeding data of the gas analysis to a Data Acquisition System (DAS). A DAS collects, stores, and calculates pollutant emissions, with results available to the facility operators and regulatory agencies. Also, DAS validates the measurement data through certification tests. Through this process, facility operators are able to make any necessary changes to ensure that there is compliance with environmental rules and regulations.
Construction of CEMS Shelters
Generally, constructing a CEMS shelter does not follow a specific procedure because manufacturers customize solutions to meet project needs. For example, most shelters are installed in Class 1 Div 2 areas, but this may vary depending on the client’s factory layout.
After all specifications have been agreed upon, the construction ideally occurs in a controlled shop environment rather than on-site for cost-effectiveness and quality control. The first area of focus is the configuration of the shelter. When locating each unit within the CEMS, consideration should be given to other equipment the unit will interface with within the shelter and on the factory floor. Another important factor is the size of the shelter, which is a function of the number of units within. Depending on the client’s needs, a CMS shelter could contain:
- Analyzers
- Marshaling panels
- PLC cabinets
- HMI cabinets
- Power Distribution Unit
- Sample chillers
- Gas monitors and alarms
The amount of samples for testing per time and the number of units within the shelter influence its size and configuration. In addition, the size and configuration influence power, HVAC, and lighting requirements. Estimating these requirements should be thorough to ensure that all equipment within the shelter operates at optimum power and temperature. Also, it is important to give consideration to future developments by making allowances for expansion of the testing system.
Prior to shelter arrival on site, all units are pre-piped and pre-wired with compatible interfaces for connecting with what is available at its installation point. Then, the fully assembled unit is tested with test gasses before hauling to the client’s base for installation. Generally, it is preferable to install the shelter in close proximity to the sample probe, thus minimizing exposure to the sample line.
Benefits of a CEMS Shelter
CEMS shelter is an extractive analyzer which offers a different emissions monitoring option to in-situ analyzers. In-situ analyzers are mounted directly in the stack flue, so no sample is let out of the stack.
The in-situ design is cheaper than building an entire shelter for analyzers; however, there is a limit to the number of analyzers that can fit on the stack. This limits the components that can be analyzed using the in-situ option. A CEMS shelter not only can accommodate a wide range of analyzers, it should offer room for expansion if necessary.
Other benefits of using a CEMS shelter include:
- Long service life, high availability, and minimal maintenance requirements of all units within the shelter. This is because the shelter is robust and protects all units from adverse ambient conditions.
- Because the shelter houses other equipment, such as PLC cabinets and marshaling panels, the analyzers can perform more sophisticated functions.
- There is a high level of quality control as all units and piping are pre-fabricated in the shop rather than on-site. Also, each unit is tested before being transported to site, so the installation is quick. Thus, it provides a turnkey solution.
- The risk of damage to the measurement and analysis units during transportation is minimized due to the robustness of the CEMS shelter.
- Typically, CEMS shelters have CSC certification, which enables low-cost transportation on shelter ships. Also, they do not require seaworthy packaging when on shipping vessels.
- Shelters can be purpose-built with special dimensions, earthquake protection, explosion and fire protection.