Referenced Learned Bodies Include:

• Center for Chemical Process Safety – Continuous Monitoring for Hazardous Releases
• UK HSE – Review of Alarm Setting for Toxic Gas and Oxygen Detectors (RR973)
• ISA-TR84.00.07-2018 – Guidance on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness

Purpose of Fixed Detectors in a Fire & Gas System (FGS): Fixed Gas Detectors are typically an input to a Fire & Gas System (i.e. control system) and its primary function is to activate the alarm and then the Fire & Gas System initiates executive action to mitigate the effects of the gas release (e.g. annunciate, evacuate, isolate, ventilate, system shut-down, etc.). As per ISA TR84, “Fixed Detectors are the primary means of safety to alert personnel who either are not in the area at the time or are within the area but not immediately exposed to a hazardous release. The goal is to either prevent personnel from entering the area or evacuating personnel from the area, depending on their initial location”.

Fixed Detectors are not the same as Personal Monitors: Personal Monitors typically have Alarm Thresholds set at the lowest of the Time-Weighted Exposure Limits and the purpose is to get personnel to evacuate at first exposure. As per ISA TR84, “Personal Monitors are the primary means of safety once a worker is in an area containing and/or near to equipment containing the toxic gas. [Within High Hazard Rank applications], Fixed Detectors should not be the primary means of safety at these locations, because a very large number of detectors would be required to protect every possible exposure”.

Set Alarm Thresholds As Low As Reasonably Practical (ALARP): Fixed Gas Detection shall provide, within practical limits, the earliest detection of presence of a hazardous release or accumulation of a flammable or toxic gas. The UK HSE’s suggests that “It is desirable that this should only occur when there is a serious leak“.

But, Alarm Thresholds are to be set to Minimize Spurious/Nuisance/False Alarms: Lowering the Alarm Threshold may introduce unintended spurious/nuisance/false alarms caused by the process, operators, environment, or detector. If spurious/nuisance/false alarms are not managed properly, this can lead to excessive alarm activation which can result in operators turning off the alarms, sometimes with dangerous consequences. As per UK HSE, [Alarm Thresholds for Fixed Detection], “may be set higher than those for a portable detector. However, these action levels should still enable any workers present in the area, to safely exit the area and initiate effective mitigation”.

#2 – Maximum Suggested Alarm Thresholds:

To help find the balance between setting the Alarm Threshold “As Low As Reasonably Practical” (ALARP) while taking into consideration the need to avoid false alarms, below is a found “Recognized And Generally Accepted Good Engineering Practices” (RAGAGEP) that provides the Maximum Suggested Alarm Thresholds for both Toxic + Flammable Gases.

As per the Center of Chemical Process Safety, Gas Detection Alarm Thresholds must be based on the hazard properties of the hazardous gas in question. For Toxics, use Immediately Dangerous to Life and Health (IDLH), which has a 30-minute exposure consequence which allows time to don Respiratory Protection Equipment (RPE) and/or escape. For Flammable gases, use the well defined Lower Flammable/Explosive Limit – Meter (LFL-m/LEL-m) for Open Path Gas Detection. If both are applicable, use the lower of the two.

In summary, here are the suggested Maximum Alarm Threshold Concentrations:

• The Maximum Low-Level (i.e. Hi) Alarm:
  • For Toxics, it shall be set based on half of the IDLH (i.e. 50% of IDLH).
  • For Flammables, use 20% of LFL-m/LEL-m.
• The Maximum High-Level (i.e. Hi-Hi) Alarm:
  • For Toxics, it shall be set at the ILDH (i.e. 100% of IDLH).
  • For Flammables, use 40% of LFL-m/LEL-m.

For Open Path Toxic Gas Detection, we’ll need to use the industry based assumptions for plume sizes based upon the confinement of the area:

• Within Enclosed or Congested Areas, it is assumed that the plume could be between 5-10 m (15-30 ft) in diameter by the time it passing through the Active Measurement Path.
• Within Open Areas, it is assumed that the plume could be between 10-20 m (30-60 ft) in diameter by the time it passing through the Active Measurement Path.

To help provide more clarity to the table above, the following terms have been further defined:

Minimal Detectable Limit (MDL): This is the smallest concentration value that is reliably and repeatedly detected when exposed to a known gas concentration. In a comparable example, think of the Minimal Detectable Limit (MDL) in similar terms to a Signal-to-Noise Ratio where the MDL is the smallest signal that can be detected through the noise floor.

Non-Detectable Range: Any ppm-m concentration below the Minimal Detectable Limit (MDL), regardless the value of the R2 Confidence Factor, should be strictly treated as noise.

Zero Gas Noise: Typically, Zero Gas Noise will produce gas concentrations that are below our Minimal Detectable Limit (MDL) with low and high R2 Confidence Factors  and can be generated from spectroscopic, mechanical, or atmospheric sources. However, Zero Gas Noise can produce gas concentrations that also exceed our Minimal Detectable Limit (MDL) with “good” R2 Confidence Factors to two (2) times our published Minimal Detectable Limit (2x MDL).

Non-Actionable Range: While measurements within the Non-Actionable Range (i.e. below 2x MDL) can be both above our Minimal Detectable Limit (MDL) and have “good” R2 Confidence Factor values, it is difficult to determine if the gas concentrations measurements are from Target Gas within the Active Measurement Path or just Zero Gas Noise. It recommended that Actionable Concentrations (i.e. Alarm Thresholds) not be set within the Non-Actionable Range and/or below 2x MDL.

Actionable Range: This is where the results are repeatable, accurate, and linear. It is within the Actionable Range where end-users can confidently set Alarm Thresholds that are “As Low As Reasonably Practical” (ALARP) while taking into consideration the need to avoid false alarms.

Lowest Actionable Concentration (LAC): As the name states, this is the Lowest Concentration that should be considered for use in setting an Alarm Threshold (i.e. 2x MDL). False Alarms between 2x MDL and 5x MDL while rare could be still be possible.

Lowest Recommended Alarm Threshold (LRAT):To provide an additional margin of safety, Alarm Thresholds that are greater than five (5) times the Minimal Detectable Limit (5x MDL) will provide acceptable protection from less-common and/or circumstantial spurious/nuisance/false alarms. Concentrations above 5x MDL are to be considered valid.

Sensitivity: We define Sensitivity as the smallest incremental change in concentration that is reliably and repeatability detected above the Minimal Detectable Limit (MDL). In a comparable example, think of our Sensitivity as Shot-to-Shot Noise. As a general rule-rule-of-thumb for all gases, the Sensitivity (i.e. Shot-to-Shot Noise) is a quarter of the Minimal Detectable Limit (e.g. 0.25 x MDL).

Highest Actionable Concentration (HAC): This is the end of the Actionable Range and Alarm Thresholds should not exceed this Full Scale value.

Non-Linear Range: Concentration measurements will continue to display beyond the Non-Linear Range. The measurements will be repeatable, but they will no longer be accurate. Typically, concentration measurements beyond the Non-Linear Range will display lower than actual.

IN-SITU MONITORING

Path Integrated Concentration: Parts Per Million – Meter (ppm-m)

The units of measure for Open Path Gas Detectors is Parts Per Million – Meter (ppm-m), which is a Path Integrated Concentration.

It is important to note that the Path Integrated Concentration is completely independent from the physical path length (i.e. distance between the Open Path (OPX) Head and the Retro-Reflector).Think of Path Integrated Concentration as each of the target gas molecules within the Active Measurement Path being counted and added together to provide the total concentration. Basically, this measurement methodology can be summed up as this being a molecule counter.

To help visualize what Path Integrated Concentration is, lets assume a background concentration of 10 ppm. The image below shows how the Path Integrated Concentration increases with path length even though the background concentration remains the same. Atmospheric gases like Methane (CH4) and Carbon Dioxide (CO2) will exhibit a similar result.

Path Average Concentrations: Parts Per Million

The Path Integrated Concentration (ppm-m) can be converted to a Path Average Concentration (ppm) by dividing your ppm-m concentration by the physical path length (m) of the Active Measurement Path. The path length can be programmed into the GasFinder to automatically convert the outputted concentration in a Path Average Concentration (ppm).

Path Average Concentrations are typically used in short path length applications where the plume is either being fully measured or where the concentration assumed to be more or less homogeneous.

It is typical for Path Average Concentrations to be used with the following Measurement Heads: Remote Point (RPX) Probe, Stack/Duct (SDX) Probe, In-Line (ILX) Probe, Insertable (IPX) Probe and Extractive Measurement (EMX) Probe.

Acetylene (C2H2):

Path Integrated Concentration: Parts Per Million – Meter (ppm-m)

The table below visualizes the Minimal Detectable Limit (MDL), Lowest Actionable Concentration (LAC), Lowest Recommended Alarm Threshold (LRAT), and Highest Actionable Concentration (HAC) with Path Integrated Concentrations (ppm-m) that are independent of path length.

Path Average Concentrations: Parts Per Million

The table below visualizes the how the Path Average Concentrations (PAC) for the Minimal Detectable Limit (MDL), Lowest Actionable Concentration (LAC), Lowest Recommended Alarm Threshold (LRAT), and Highest Actionable Concentration (HAC) are effected by path length. As a quick reminder, to obtain the Path Average Concentrations, simply divide the Path Integrated Concentrations (ppm-m) by the Path Length (m).

To help provide more clarity to the table above, the following terms have been further defined: