There are no standards, regulations or specific formulas to calculate exactly how many fixed gas detection sensors are required to monitor and protect a certain area. Each application is different and should be evaluated on an individual basis. When assessing an application, however, there are a few guidelines that should always be considered.
What Do You Want To Monitor?
Areas to consider include point detection, area monitoring or perimeter monitoring.
Point detection involves monitoring a specific object such as a valve, fitting or vent that could potentially leak harmful gases. In these applications, the gas detection sensor should be placed 18 to 24 inches away from the potential leak source and in an area where it will not get sprayed with high pressure liquid or mist. Placing the sensor too close to a high pressure source could allow gas to be dispersed past the sensor before it can be detected. Placing it too far away from the source could result in the gas concentration being diluted before it reaches the sensor. Always ensure that the sensor is placed in such a manner that high pressure gas and liquids do not directly hit the sensor.
Area monitoring applications focus on detecting gases that accumulate in a specified area. This can include gas that accumulates in a mechanical room, research laboratory or other area as a result of gas leaks, a byproduct of combustion or leaks in a process or system such as a refrigerant leak.
Perimeter monitoring is used for observing established boundaries of an area and to alarm when a gas leak is leaving or entering the specified areas. Chemical plants typically have perimeter monitoring systems around their facilities to ensure that if a leak occurs, operators know if it has reached the fence line or traveled onto adjacent properties. Many chemical plants also place monitors on the outside of office buildings to ensure that leaks are detected prior to harmful gases entering occupied office space.
Know the Properties of Monitored Gases
Every gas has specific properties that must be considered for effective monitoring. Understanding the physical characteristics of these gases helps in determining the placement height of sensors. Gases that are lighter than air accumulate toward the ceilings of confined areas or rooms. Gases that are heavier than air will accumulate in lower areas closer to the floor or in pits, drains or stairwells. Gases with a similar density to air are best monitored in the “breathing zone.”
When placing sensors either toward the ceiling or floors, caution should be taken in the placement and location. For example, if there is a layer of high temperature air or gas that accumulates near the ceiling, placing the sensor at the highest location may not be the best choice. Positioning it 3 to 4 feet lower than the ceiling will keep the sensor below this thermal barrier and allow it to more effectively respond to gas. In the same respect, installing sensors directly at floor level also may not be ideal. Sensors could be damaged if there is a spill or flood, or by being exposed to cleaning agents. These sensors should be placed 12 to 18 inches above the ground.
When monitoring for personal protection, placing sensors in the breathing zone will give a good indication of the gas concentrations personnel are inhaling. Other key factors to consider are the temperature of the gas being monitored and the ambient air temperature of the area. Hot gases usually rise while cold gases gather closer to the floor.
Know the Environment You Are Monitoring
In simple terms, gases tend to flow like smoke. For gases lighter than air, the “smoke” goes up; for those gases heavier than air, the “smoke” goes down. Other factors that affect how gas flows in an area include airflow and movement within the area. If large fans are pulling in fresh air, that airflow will push the gas in that direction. If large exhaust fans are pulling air out, the gas also will be pulled toward these exhaust points. It is important to understand the airflow in the environment for the most effective and accurate monitoring. If unsure, conduct a smoke test and see where the airflow pulls the smoke sample. Remember that sensors placed directly in high airflow paths may not provide the best solution since gas could move too quickly past the sensors to be detected.
Certain sensors have cross interferences to other gases. Always be sure to identify any cross-interfering gases for the sensors being used. Verifying that these gases are not present in the area being monitored will ensure that there are no false readings or alarms.
Finally, be sure to consider the classification of the area in which the units are being installed. If the area is classified as a hazardous location, then only products that have been certified for use in hazardous locations should be used.
Coverage Area of a Single Sensor
The standard rule of thumb is that a single sensor can cover approximately a 40-foot radius. For noncritical applications such as a piping runs, the coverage area may be expanded. For applications that are life critical, more sensors may be required and the coverage area should be reduced.
Alarm thresholds should be set according to the application. For areas where health and safety are a concern, alarm thresholds should be set lower. Understanding the time weighted average (TWA) and immediately dangerous to life and health (IDLH) concentrations for toxic gases such as ammonia are critical for setting lower alarm thresholds. NIOSH, OSHA and IIAR are excellent resources for this type of information.
Machinery and plant safety alarm thresholds tend to be higher and are used to protect against catastrophic failures. Keep in mind that gas concentrations diffuse as the leak disperses, so setting thresholds lower will alert to potentially hazardous conditions sooner.
After the sensors are installed, the most critical aspect of the system — general system maintenance — often is the most overlooked. All sensors and gas detection systems require some periodic maintenance, which can include cleaning dirt and debris off the equipment, replacing sensor filters and sampling pumps, or cleaning sampling probes and filters.
The only way to ensure that sensors are working properly is to expose them to gas. Gas should be delivered directly to the sensor via the normal operating sampling path. This means if a sensor operates by diffusion principle, the gas is allowed to diffuse through the sensor filters. If the sensor is part of an aspirated sampling system, then gas should be delivered through the entire sample system, including any filtering. Testing the sensors in this manner will indicate if there are any clogged filters, blocked tubing or damage to the gas path.
There are two types of gas testing: bump testing and calibration verification. A bump test simply exposes the sensor to a concentration of gas above the alarm threshold value to ensure that the sensor or instrument alarms accordingly.
A calibration verification test consists of exposing the sensor to a known concentration of gas and ensuring that the sensor or instrument is reading within ±10 percent of that applied gas concentration. If a sensor fails either a bump test or calibration verification, a complete calibration should be performed.
Testing of the sensors depends on the application but a general rule of thumb is that sensors should be tested at least quarterly. For life-critical applications, monthly testing may be a good standard. For those applications that are not life critical, semi-annual or annual testing may be all that is needed. Testing schedules are best determined by individual company policy (based on maintenance and safety standards) and local codes.
In any event, all sensors should be calibrated on a routine basis. Calibration ensures that the sensors are responding accurately to the desired gas being monitored. Just like testing, calibration intervals depend on application. For those applications that are not life critical, calibration frequency should occur semi-annually. In applications where personnel health is critical, equipment should be calibrated at least quarterly or more frequently. Calibration policy is best determined by company policy and local codes.
When installed and maintained properly, fixed gas detection equipment is a reliable way to protect property and equipment, reduce operation costs, improve the environment and, ultimately, save lives.