Chapter 7.pdf

Western Canada Mine Rescue Manual Chapter 7 Gas Detection Instruments 7-1 OBJECTIVES Rescue teams can determine the gases and vapours present in an atmosphere using a few different methods and tools. Upon completion of this chapter, the trainee shall be able to demonstrate competency in:  What gase...

Western Canada Mine Rescue Manual Chapter 7 Gas Detection Instruments 7-1 OBJECTIVES Rescue teams can determine the gases and vapours present in an atmosphere using a few different methods and tools. Upon completion of this chapter, the trainee shall be able to demonstrate competency in:  What gases could be encountered during an emergency response  Selecting monitoring equipment and methods suitable for the incident  Practical skills for an effective gas detection program Introduction There are four categories of hazardous atmospheres:  Toxic  Oxygen deficiency/Asphyxiating gases  Explosive/ flammable gases or vapours  Smoke, aerosols, fumes (particulate contaminants) Intrinsic Safety is a design applied to electrical equipment and wiring for hazardous locations. The technique is based on limiting energy, both electrical and thermal, to a level below that required to ignite a specific hazardous atmospheric mixture. All personal protective equipment must be considered before any and all gas testing. Always check that any monitoring equipment or other electrical devices are intrinsically safe. SELECTING GAS DETECTION EQUIPMENT It is important to select gas detection equipment that fits the specific needs of the incident. Mine rescue gas detection involves the use of direct-reading instruments (DRI). These instruments provide information at the time of sampling, thus enabling rapid decision-making. All equipment must meet relevant health and safety legislation, standards, and regulations. When selecting gas detection equipment, the user should:  Check for conditions that could interfere with the equipment o Cross Sensitivity: Sensor’s reaction to an interfering gas. The response of a sensor to a gas that is not the target of the sensor. o Some gases, such as acetylene, can interfere with the instrument sensor and mask the presence of sulphur dioxide (SO2). o Other common interferences: Electromagnetic fields, humidity, atmospheric pressure/altitude, low temperatures, saturation and high concentrations.  Consider performance criteria/specifications of the instrument o Response Time: Ability to react to its specific gas in the time specified (Ex. 90% of reading in 30 seconds). o Noise/Drift: How much readings fluctuate when the quantity or concentration of a substance stays the same. o Limit of Detection (LOD): The lowest quantity or concentration of a substance that the instrument can register within a margin of confidence. o Accuracy: The degree to which the measurement of a quantity of a substance matches up with that quantity’s actual value. 7-2 o o o Precision: The degree to which repeated measurements under unchanged conditions show the same results. Dynamic Range: The ratio between the largest and smallest possible signals. The smallest is the LOD and the largest is sensor saturation. Note: Follow all manufacturer’s specifications for application and use. GAS DETECTOR TYPES Colorimetric (Tube-style) indicators measure more than 200 organic and inorganic gases and vapours in the air. The sealed glass tubes are filled with a granular material coated with a chemical that changes colour when it reacts to a particular gas or vapour.  Before Use: Refer to manufacturer’s instructions for the particular tube type. o Perform a pump leak test. o Ensure the direction arrow is oriented toward the pump.  Operation: A portable pump draws a known volume of air through a detector tube designed to measure the concentration. The colour Drager (L) and Gastec (R) colorimetric tubes change is then read on a scale printed on the tube.  Considerations: Measurement accuracy, limits of detection, interferences, temperature/humidity, shelf life, time period for which the colour stain is stable after sampling. Readings from a short-term indicator tube should be compared to the appropriate short-term exposure limits, such as TLV-STEL and TLV-C.    Tube Storage and Shelf Life: o Tubes have a shelf life. These expiration dates are printed on the box. o Store properly by avoiding excessively low or high temperatures and direct sunlight. Advantages: o Operation with one hand. o Low weight and simple operation. o Always ready for use (no batteries). o Tubes for more than 200 different gases and vapours. o Printed measuring scale on the tubes provide immediate reading of the result. o Low maintenance. Limitations: o Tubes and pumps are manufacturer-specific. o No alarm system, therefore not recommended for continuous monitoring o Dependent upon operator’s interpretation of results o Application can require longer periods of time in atmospheres for results to appear (up to several minutes) o Limited shelf-life o Only accurate within +/- 20% o Many cross-sensitivities 7-3  o Range of application is dependent on humidity and temperature o Requires proper storage, handling, and disposal Common types of detectors include Drager, Gastec Drager CMS This variety of gas tube tester follows the same chemical reaction principles. The major differences are:  Small tubes are contained within a plastic chip with a bar code identifier  Battery operated pump and tube reader  Display screen indicating gas being tested, range, sampling duration status and actual reading The chip has 10 individual sample tubes. Each tube can only be used once. The tester scans the chip when inserted into the reader and indicates the gas and range for which the chip is designed. Electronic gas detectors An electronic gas detector is a complex system that includes a sensor(s) surrounded by sensitive electronics, alarms (visual, audible, vibrating), a battery and a display. All electronic gas detectors must meet regulated manufacturer standards. Electronic gas detectors are available as single, multi-gas, stationary (non-portable), and specialized units.  Advantages: o Multi-gas detectors use separate sensors for oxygen and combustible atmospheres. o They can register multiple toxic gases in the same hand-held monitor. o Many models have interchangeable-sensor capabilities. o Will provide low-level and high-level alarms. o Accidental power-off protection: requires the power-off button to be held continuously for at least 3–5 seconds. o Many are capable of internal data-logging o Portable field docking stations are also available for several models o Can have an internal motorized pump or a diffusion monitor with attachable pump that allow the instrument to be used in a variety of applications including confined space entry or to measure from greater distance or height o More accurate than colorimetric tubes – accurate within +/- 10%  Limitations: o Sensor sensitivity and response to gas will degrade over time; Limited shelf life o Gases recognized are sensor-specific o Requires time to perform a pre-use procedure o Sensors and batteries have a finite life o Environmental conditions such as temperature, humidity, dust, dirt and rough handling all contribute to premature sensor degradation BW GasAlert Quattro Industrial Scientific Ventis MX4 Drager MX 7-4 Typical Life Span of Various Sensor Types Oxygen sensors 1–5 years Catalytic bead combustible gas sensors 3–5 years Electrochemical toxic gas sensors 1–4 years (depending on type) Infrared gas sensors 5–10 years Photoionization gas sensors 2–4 years PRACTICAL SKILLS FOR GAS DETECTION Atmospheric sampling Relative Density The operator performing the sample tests is responsible for:  Following all manufacturer’s specifications and guidelines  Making sure that proper sampling techniques are performed due to the stratification of gases that may be present  Proper sampling: the operator must sample prior to entry, prior to re-entry, and continuously while work is being completed.  During underground exploration, gas testing should be performed at intersections or where conditions change.  Sampling at the top, middle, and bottom of a space must be done in accordance with manufacturer specifications. o Sample every 1–2 metres (3–7 ft) due to the weights of gases. Some are lighter than air (methane), some are slightly lighter (carbon monoxide), and some are heavier than air (chlorine). Bump Testing and Calibration The most important elements of gas detector maintenance are function (“bump”) testing and calibration. These tests can be done either automatically via a docking station or manually. Functional (“bump”) testing involves a brief exposure of the monitor to a known concentration of gas(es) for the purpose of verifying sensor and alarm operation.  Manufacturers recommend that a bump test be performed on every instrument prior to each day’s use. Docking stations used for bump tests  Bump tests are not measures of the instrument’s accuracy.  Any time an instrument fails to respond properly during a bump test, a full calibration should be completed successfully prior to use. 7-5 Steps for Gas Detector Operation 1. Visually inspect for damage and contamination 2. Turn on instrument in good atmosphere and check battery level 3. Zero the instrument 4. Bump test (functionally check) instrument 5. Clear the peaks if applicable Bump Test Procedure This check is performed by applying a known concentration of gas to verify sensor performance and alarm operation. Following the simple steps below will aid you in performing your bump check. 1. Turn the instrument on and warm-up 2. Zero the instrument 3. Apply calibration gas 4. Allow sensors to respond to calibration gas 5. Verify proper alarm function 6. Remove gas and allow the monitor to clear 7. If any of the sensors fail to respond, remove from service Calibration ensures that detectors perform properly. The process uses a calibration gas with a specific concentration.  Refer to manufacturer’s recommended protocols for calibration.  It is recommended to perform full instrument calibration using calibration gas(es) monthly to ensure maximum accuracy.  Establishes a point of measurement accuracy  Provides insight into the condition of the sensors in the instrument  Adjusts the readings to account for changes due to sensor degradation Calibration Gases are certified and traceable mixtures of gas concentration. Calibration gas cylinders are identified by:  Gases and Concentrations  Part Number  Expiration Date Follow manufacturer’s recommendations when using regulators for disposable gas cylinder and disposing cylinders. 7-6