Many industrial processes, from wastewater treatment to power generation, produce exhaust containing traces of combustible or toxic fumes. Whether this exhaust will be redirected to another location in the facility or ejected to the environment, it is necessary to measure for potential concentrations of gas in the exhaust air stream to ensure compliance with applicable safety and regulatory standards.
This topic is explored in an Emerson white paper entitled “Improving Safety by Analyzing Gas Streams in Ducts”. As the title suggests, leaks of combustible gases to ducts pose a significant threat to the well being of an organization’s personnel and property. In a power plant, for example, seep ages of natural gas in turbine installations may lead to fires and subsequent destruction of equipment if the gas is ignited.
In situ measurement of the gas stream can be beneficial because it yields quick response times. However, in situ monitoring fails in many applications due to the harsh conditions native to duct effluent streams. In a power plant, high temperatures and the presence of corrosive agents often result in the fouling of in situ sensors, decreasing both their effective lifetime and the accuracy of the results obtained during this time. In the case that sensor performance has fallen below accepted standards or the sensor fails outright, a likely occurrence in harsh duct environments, maintenance must be performed to replace the sensors. Such maintenance often requires operational shutdown. This downtime is especially problematic in applications, such as gas turbines, where continuous operation is essential to overall process efficiency and profitability.
In addition to its susceptibility to degradation by caustic gas streams, traditional in situ analysis fails to capture the full picture of the duct composition. Usually, the contents of a gas stream are not uniform across the duct cross section due to stratification and flow profiles that create uneven mixing. In situ methods fall victim to this in-homogeneity since they sample only a small fraction of the total gas within the duct.
External measurements provide an alternative means to monitor the contents of a gas stream. Rather than placing the sensor in the duct, a sample gas stream is drawn from the duct and sent to a remotely-mounted instrument. Since the sensor is not exposed to high heat or humidity, the sensor lifetime is greatly extended. Likewise, if an issue does arise, the external instrument can be repaired without necessitating a process shutdown.
A variety of systems exist to pull a gas sample from a duct. To be effective, the system must deliver a sample representative of the gas flowing through the entire duct while remaining unclogged by debris. One such system is the Emerson Gas & Smoke Aspirator which is an air-driven sampling instrument that uses a Self-Compensating Duct Probe (SCDP) to obtain a sample which can be sent to an external instrument for analysis.
The compact form factor and mechanical simplicity of the SCDP eases installation and minimizes maintenance issues due to a reduction in tubing connections, which also decreases potential leak sources. In the unlikely event of a leak, Emerson aspirators feature failsafe operation and have fault indications to clearly communicate the problem to operators.
Typically, three SCDPs are placed across each ventilation exhaust outlet perpendicular to the air flow, thereby achieving better coverage of the duct and readings that are representative of its contents overall. In this three-probe system, an alarm is configured to sound when two-out-of-three channels detect a dangerous concentration of target gas.
A well-chosen and properly installed aspirator system mitigates the risk of harm to personnel and property as well as reduces the chance of costly unplanned shutdowns. You can find out more details on this topic by reading the full white paper here.