Carbon Monoxide (CO), Chlorine (Cl2), Hydrogen (H2), Hydrogen Sulfide (H2S), Methane (CH4), and Oxygen (O2) Gas Detectors
Experts in Power Industry Gas Detection
Coal fines and dust are extremely flammable. “CO monitoring for the presence of Carbon Monoxide is the way to find out soonest whether there is a fire in the making. CO is an odorless, toxic gas that is liberated at the very early stages of incomplete combustion1.” Coal Bunker, Silo and Hopper CO levels are often monitored for fire detection purposes using sample draw or in-situ monitoring. Manned Coal handling areas, including tunnels and conveyor rooms, should be monitored for fire detection and worker protection from CO exposure. The OSHA PEL for CO is 50 PPM2. Methane and Oxygen deficiency sensors may also be required in these areas. The widespread adoption of Powder River Basin Coal (PRB) has exacerbated fire problems due to its remarkable ease of spontaneous combustion. Some plants inert coal storage vessels with flue gas or CO2 to prevent fires. This presents an Oxygen depletion hazard in adjoining areas, and should be considered when planning a monitoring strategy.
Platforms, walkways and areas around the Boiler are often monitored for CO and Oxygen Deficiency and sometimes H2S when high Sulfur coal is being burned.
Chlorine, other disinfectants and chemical agents are often used in cooling water to control organic growth and scaling. Chlorine, Chlorine Dioxide and Ammonia are hazardous to employees. Chemical feed and coolant mechanical equipment areas should have dedicated gas monitors for leak detection and annunciation.
Utility Electricity Generators have a pressurized direct or indirect Hydrogen cooling system. Hydrogen conducts heat 7 times better than air and its low density (0.07 versus air of 1.0) minimizes windage losses, making it an ideal coolant.
Hydrogen has a flammability range 4% to 74% by volume2 and detonates (explodes) from 18% to 59% by volume. It is very easily ignited and burns with a pale blue, nearly invisible flame. H2 can cause serious injury to personnel, as well as severe equipment damage. Hydrogen leaks typically occur at the generator bearing and shaft seals, in the seal oil supply system, H2 and CO2 cabinets, valves, fittings and regulators associated with the Hydrogen and CO2 purging systems. Hydrogen specific gas sensors should be used for leak detection to avoid interference from Methane.
Most plants have a battery room which is used for back-up power to critical systems during a power outage. Batteries in discharge produce considerable Hydrogen. It is advisable to monitor for explosive levels of Hydrogen and trigger ventilation when detected. 24 VDC gas detection and ventilation should be employed so that power is available during an outage.
Percent Sulfur levels in Coal can produce hazardous concentrations of Hydrogen Sulfide in areas near the boiler due to casing and flue work leaks. Flue gas clean-up measures such as scrubbers and bag-houses may also produce significant ambient H2S. These areas should be evaluated to determine if monitoring is required for personnel safety. H2S exposure can stop involuntary breathing, requiring immediate attention. Consult ACGIH4 or OSHA2 for acceptable exposure levels.
Methane is the major component of Natural Gas and is much of the gas volatiles in Coal. Natural Gas fired plants typically monitor Methane at the custody transfer and pressure reduction points, above the burners, and on platforms and areas near potential casing leaks in the boiler.
Some areas of a coal-fired plant, such as coal tunnels, may require Methane and Oxygen deficiency monitoring. A review of the coal “as received” analysis and tunnel air-change rate will reveal the monitoring requirements.
CO2 is heavier than air and collects in low areas presenting a substantial safety hazard to workers. In locations where Flue gas or CO2 are used for inverting coal storage vessels, Oxygen deficiency monitors are required. The alarms specified by OSHA are 19.5% and 18% by volume when two alarms are employed.
Instrument shelters and other enclosed areas with potential for Oxygen deficiency should have interior monitoring with internal and external indication and annunciation of the hazard. Remote or control room notification is required for all hazard alarms. Note: Personal monitors are not appropriate for this application because they do not sense the hazard until after entry into the shelter.
Oxygen monitoring of inverted coal pulverizes and conveying pipes is also practiced. In this case, low Oxygen is desirable in order to prevent combustion or explosion of the pulverize coal.
A WORD ABOUT PORTABLE AND PERSONAL GAS MONITORS
The advent of miniaturized electronics and gas sensors has resulted in the widespread adoption of portable or personal gas detection. However, the user should be aware that personal monitors must be functionally verified prior to each use! Verification includes confirming a charged battery, proving response to the target gases and verification of alarm operation. In high hazard locations, workers should work in pairs and carry a means of communication. For higher traffic or continuously manned areas, personal monitors may not be the most cost-effective, reliable or accountable solution.
SENSIDYNE FM APPROVED SENSALERT SMART GAS SENSORS
Sensidyne SensAlert PLUS Smart Gas Sensors bring exceptional performance and reliability to area monitoring. SensAlertPLUS sensors store all operating parameters in non-volatile memory for auto-recognition and set-up by the gas transmitter. They can be shop calibrated and are approved for hot-swapping in the plant, even in a hazardous classified areas. Cumulative exposure, TWA alarms, Predictive Sensor Failure, Intrinsically Safe Sensor Heads, and Sensor Test-On-Demand functions deliver the highest system integrity and accountability for the most demanding applications.
Integrating a Process and Personnel protection gas detection system requires that the notification and control procedures mandated by Code and Law be followed, including Insurance Company mandates.
Compliance with these issues is simplified by using a Sensidyne Controller to process the gas sensor inputs, provide annunciation and supervisory control. A 4-Channel System is illustrated below. Up to 16 gas sensors may be accommodated by one Controller.
GAS SENSOR MOUNTING CONFIGURATIONS
Gas sensors are located in the breathing zone (4 to 6 feet above floor) for gases similar in density to air, including Oxygen deficiency. For lighter than air gases, (Methane, Ammonia or Hydrogen), the sensors must be positioned within 12 inches of the ceiling or highest point of an enclosed area. Heavier than air gases like solvent vapors or Halocarbon refrigerants should have the sensors placed within 12 inches of the floor or lowest point in the enclosed area. Target gas density and potential leak sources should always be identified when configuring a gas detection system.
Gas sensors are often mounted remotely from the transmitter, which is normally kept at eye level. These situations arise due to gas density variations or difficult access locations. Tubing is run to a calibration adapter on the sensor assembly, so that routine calibrations can be performed from the transmitter. Other configurations include duct mounting, and drawing a sample from a remote or inaccessible location. The latter has a flow switch to indicate sample system integrity, as required by Code.
Careful attention to gas sensor placement, supervisory control functions and emergency response planning will produce a functional, accountable and low maintenance area monitoring system which complies with local codes and laws.