<< Click to Display Table of Contents >> Navigation:  Rules / PPE Requirements > Gas Detection

Placement

The responsibility for gas testing is normally assigned to the Client’s Representative on site.  If appropriate gas detection has not been completed by this representative, Power Tech Industries Ltd. employees shall ensure that it is completed (as required / as appropriate) before they start the work at hand.

Multi-cell gas detectors will be provided and shall be used to determine gas levels before critical work such as confined space entry or hot work is performed.

Employees who are traveling from site to site and performing instrumentation calibrations or any other form of routine hot work will carry / wear a multi-cell gas detector with %LEL Flammable Gas and H2S cells, as a minimum.

Employees working in sour facilities / areas shall be provided with and shall wear personal multi-cell monitors.

Care / Calibration

All gas detection equipment shall be calibrated according to the manufacturer’s requirements.  Calibration records shall be maintained and shall indicate the history of service and calibrations for each gas detectors (by serial number), within preventative maintenance records.  H2S detection equipment must also be bump checked with calibration gas to ensure it is still within the allowable limits.  This bump check must also be completed according to the manufacturer’s recommendations.  Bump checks can be done by employees, if they are provided with the proper training, calibration kits and bottles.

Gas monitor bump testing must be done daily, and calibration routines must be performed monthly.

Training

All employees will be provided with training on the use and care of portable gas detection equipment.  It is the Management’s responsibility to assess the need for this training and provide training as required.  It is recommended that at least one person from each operating area has attended the PITS Detection and Control of Flammable Substances training course.

Equipment Types

Three types of gases are commonly tested in the oil and gas industry; namely Oxygen, Flammable Gas and Toxic Gas.

OXYGEN

Testing is usually done when a confined space needs to be entered.  The normal level of oxygen in breathing air is 20.9% by volume.  Allowable limits for oxygen concentration:

A maximum of 23% by volume;

A minimum 19.5% by volume.

If these limits cannot be maintained, supplied air breathing apparatus shall be worn.

All gases are capable of displacing oxygen in a confined space.  Although a gas may not be considered toxic (such as nitrogen), it may be able to effectively displace oxygen to the point where a worker will suffocate.

Oxygen detection equipment usually depends on a wet catalytic cell that is subject to freezing; detectors should not be left in extremely cold environments for prolonged periods of time.  This will also help to maximize the life expectancy of the batteries.

Reduced oxygen will result in asphyxiation below approximately 15% by volume. Increased oxygen concentration is both toxic after a period of time, but more importantly is the increased fire hazard that it produces.

FLAMMABLE GAS

Detection is performed to identify the presence of a flammable gas prior to the performance of hot work or confined space entry.

Flammable gas is usually measured in %LEL, which stands for the lower explosive limit of the gas used to calibrate the instrument (usually methane).  A reading of 100% LEL is the lowest concentration of methane gas in air that will support combustion (allowing a fire or explosion to occur).  Although detectors are calibrated to methane (for example), they will also detect other flammable gases.  As all flammable gases have different lower explosive limits, a reading of 100% LEL may not be the lowest point at which combustion will be supported when measuring another gas.  This is why most gas detectors will alarm at 10% LEL (allowing for a safety factor).

Allowable limits for flammable gas:

A maximum of 10% LEL for any work.  (If the work area has 10% LEL, workers must leave the area until it is purged with an inert gas).

A maximum of 0% LEL for hot work or confined space entry.

All flammable substances must be in a gas state and mixed with air in order for the gas detector to produce a reading.  Consider the following:

When taking an LEL reading inside of process equipment that has been purged (or is under purge) with an inert gas such as nitrogen, the LEL detector will not provide an accurate reading.  The catalyst head on the detector requires the flammable gas to be proportionately mixed with oxygen to provide an accurate reading.  The same is true if there is only gas inside of a piece of process equipment (with no oxygen).

If an LEL test is taken early in the day and the temperature is cool, flammable gas entrapped in oil may not be released in sufficient quantity to give an LEL reading.  As heat is applied through the day or by a work process (such as welding) the liquid may start to flash and release gas.

In either of the two above scenarios, monitor equipment frequently or continuously.

Do not allow the sensor to get wet as this will destroy the cell and the reading will not be accurate.

TOXIC GAS

Detection of toxic gases is usually performed using Colorimetric tubes (Dreager or Gastec are common detector names).  Some multimeter gas detectors include some forms of toxic gas detection, such as Carbon Monoxide, Carbon Dioxide and Hydrogen Sulphide.

All detector tubes came with instructions on how to use the specific tubes.  Refer to this information before taking a test.

•The Colorimetric tubes are filled with a catalyst that reacts with the substance being measured and is dependent on the volume of gas being drawn through the tube.  If the volume being drawn through the tube is not as required by the instructions, the reading provided will be inaccurate.  Ensure that full pump strokes and the required number of pump strokes are performed as required.

Most tubes provide a reading in parts per million (PPM), there are 10,000 PPM in 1% by volume.  Consult with the applicable material safety data sheet (MSDS) for the allowable level / concentration, known as the Occupational Exposure Limit (OEL).

Colorimetric tubes are usually only accurate to within 25%.

All tubes have an expiry date stamped on the box.  Inspect for / check these expiry dates and replace as required.

Inspect the pump before each use to ensure it is not leaking and that it is functioning properly.  Refer to the manufacturer’s instructions on maintenance and testing of this equipment.

Testing Methods / Technique

•Testing shall be performed by a competent and trained worker prior to the performance of hot work, confined space entry, or any work where workers may be exposed to harmful gases.

All gas test results shall be recorded on the safe work permit or pre-job hazard assessment form.

Ensure that the electronic gas detectors are turned on and allowed to run through their calibration cycle (in a known gas free environment) before they are used in a potentially gaseous environment.  Some detectors calibrate to zero as part of start-up sequence.  Check the calibration of the gas detector with a calibration bump test kit.  Do not test it on a process gas source or a gas tank of a vehicle; as this often throws the detector out of calibration.

Potential sources of gas in the area should be tested first such as dead spaces below grade, sewers, sumps, excavations, open sources of hydrocarbon, sample points, drains, etc.  

Always test for heavier and lighter than air gases in air traps.  If a small source is located close to the job, attempt to isolate it and monitor it periodically in case it becomes worse.

Be sure not to draw water into the detector.  If this occurs, the gas detector reading will usually wander uncontrollably - have it re-calibrated by a certified service company.

Periodically monitor hot work and confined space entry.  A general rule is to re-test every 2 - 3 hours.