Ali Sadeddin explores the limits of gas detection equipment and details the relevant workplace regulations, including the organisations that articulate standards and codes for gas detection.
Before launching into gas detection and its governing standards, it is important to first understand what classifies a gas as toxic or hazardous.
A gas is a material that readily disperses in air at molecular level. It is neither particles, nor radiation, nor a mist. A gas consists of molecules of definite weight and structure. The molecular weight (mw) is small, often less than 500. If the molecular weight is any higher and it is airborne, then it is most likely airborne as a particle. A mist is basically airborne liquid particles. A gas that has a molecular weight less than 29 (the average molecular weight of air) will tend to rise in air, whereas a gas with a molecular weight greater than 29 will tend to sink. Vapours are gases emanating from substances that are liquids at standard conditions. As a general rule, organic vapours are heavier than air and will sink.
A hazardous gas is of danger to humans. This will almost always be from flammability, e.g. risk of fire or explosion, or a health risk – usually inhalation. The inhalation risk may be as a direct result of the toxic effects of the gas or from the displacement of oxygen. A toxic gas will be a gas that is a significant health threat. For any gas there is a lower explosive limit (LEL) and an upper explosive limit (UEL). It is measured by PPM or PPMV – this is the concentration of units at parts per million (PPM) or parts per million by volume (PPMV).
Detection can be defined as an objective determination of the qualitative or quantitative characteristics of a gas. For toxic and hazardous gases, detection will be interpreted to be almost immediate. Toxic and hazardous gas detection, therefore, is the process of identifying potentially toxic and hazardous gases in the surrounding atmosphere.
Several organisations have published codes, standards and recommended practices relating to gas detection and alarm systems, however, new standards are also being developed. These standards all relate to components of gas detection and alarm systems such as sensors and monitors. Gas detection standards provide guidance on the selection, installation, use and maintenance of hazardous gas detectors. Detectors signal when concentrations of toxic or combustible gases reach unacceptable levels.
The USA has several organisations that establish such standards. The National Fire Protection Association (NFPA) establishes fire safety standards including standards for safe operations. It also covered several kinds of gases that are harmful to worker health. NFPA 55, NFPA 70, and NFPA 325 are examples of such standards. Occupational Safety and Health Administration (OSHA) establishes and enforces protective standards to save lives, prevent injuries, and protect the health of workers in the USA. OSHA 1910.17 and 1910.103 are examples of such standards that cover hazardous gases. Factory Mutual Research (FMR or FM) audits insured facilities in the USA for compliance to acceptable safety standards. FM Class 3600, 3611 Class I Division 2, 6310 and 6320 are examples of such codes that relate to hazardous gases detection. Underwriters Laboratories (UL) is an independent, non-profit product safety testing and certification organisation that tests products for public safety, applying more than 17 billion UL marks to products worldwide each year. UL 1950 and UL 2075 are examples of such codes that cover gas detection.
European Standards Organisation (BS EN) is a European committee that works with 35,000 technical experts from 19 European countries to publish standards for the European market. There are many standards published by BS EN that cover the detection of gases. The International Electrotechnical Commission (IEC) is the leading global organisation that prepares and publishes international standards for all electrical, electronic and related technologies. IEC 1010-1 and IEC 79 are related to hazardous gas detection and measurements. European Conformity (CE) is the official marking required by the European Community for all electric and electronic equipment that will be sold or put into service anywhere in European community. It ensures that a product fulfils all essential safety and environmental requirements as defined by the European Union (EU).
The Canadian Standards Association (CSA) is a non-profit association serving business, industry, government and consumers in Canada and the global marketplace. It works to develop standards that enhance public safety, preserve the environment and facilitate trade. CSA C22.2, CSA C22.2 No 142 and 152, CSA C22.2 No 30 class I and CSA C22.2 No 157 are standards that deal with gas detection systems.
The UK’s Health and Safety Executive (HSE) is the independent group and regulator that sets standards to save lives, prevent injuries, and protect the health of workers in the UK. It publishes many standards that are often taken as guidelines for the selection of toxic and hazardous gas detection systems.
Unfortunately regulations, standards and codes only cover a small percentage of all applications in need of gas monitoring. In most cases, other criteria must also be used to decide which gases and equipment components to monitor. The most important consideration is the actual hazard posed by the gases. These hazards are driven by toxicity or explosion risk.
There are several principal regulations that are concerned, either solely or in part, with preventing, controlling or mitigating the effects of exposure to toxic gases or vapours. Gas detectors and their associated alarms play a role in controlling exposure. Gas detection instruments are used to detect the presence of toxic and combustible gases, as well as oxygen deficiency or in the case of fire and explosion hazards, oxygen enrichment. Workers cannot rely on their sense of smell to alert them to danger in the case of odourless hazards, necessitating the use of gas detectors whenever a worker enters an area with potential atmospheric hazards.
Before purchasing a detection system, the user must decide which gases the system should monitor. Legal requirements provide a good starting point for making this decision. Local and federal regulations, fire and building codes, and industry safety standards specify the use of gas detectors in certain types of facilities and for certain types of toxic and combustible gases. The primary hazard associated with handling flammable or combustible gases is the risk of explosion that could be caused by a leak or spill. As a rule of thumb, the lower the flashpoint or lower explosive limit (LEL) of a gas, the more important it is to monitor. System users should install monitoring devices to detect leaks of any combustible and flammable substances whose flashpoint is below ambient temperatures, since these substances immediately give off vapours that may be sufficient to form an ignitable mixture. Note that certain combustible gases are also toxic, with permissible exposure limits under their LELs.
Some publicly available guidance on setting alarm levels is proffered by gas detection manufacturers, although typically, the levels used in practice for specific applications are the result of discussions between the manufacturer and the user. Suggested values for individual gases are based on OSHA, ACGIH and NIOSH eight hour time weighted averages (TWA), short term exposure limits (STEL) and ceiling limits, as summarised in Table 1. They conclude, however, that there is no simple answer to alarm setting, as individual applications will vary. Also, in the USA, while OSHA standards have the force of law, the other learned institutions may disagree on allowable levels.
In determining the required alarm levels for fixed gas detection systems, the following should be taken into account:
• Any industry standards and recommendations
• The LEL of the gas or vapour
• The size of the potential leak and the time to reach a hazardous situation
• Whether the area is occupied
• The time required to respond to the alarm
• The actions to be taken following the alarm
• The toxicity of the gas or vapour
In all cases alarm levels should be set with sufficient safety factors included to account for poor mixing of gases. This should ensure that a hazardous atmosphere does not exist anywhere in the area being monitored.
Fixed detector systems can be designed to trigger automatic shutdown of plant and equipment or to increase mechanical ventilation rates. The response time should also be considered in conjunction with the alarm level. A longer response time may be acceptable, for example, if the system alarms to evacuate at 10% LEL rather than 25% LEL, for the same gas leakage rate and detector position.
Gas monitoring instruments are designed to protect personnel from unseen hazards that may exist in workplace environments, including confined spaces. Exposure to excessive levels of toxic gas or an oxygen-deficient environment can cause workers serious illness and even death. Gas explosions are often catastrophic, injuring or killing personnel and destroying property. The International Safety Equipment Association (ISEA), founded in 1933, is a trade association for manufacturers of protective equipment, including environmental monitoring instruments. The ISEA recommends, at a minimum, verification of sensor accuracy before each day’s use. The only way to guarantee that an instrument will detect gas accurately and reliably is to test it with a known concentration of gas. Exposing the instrument to a known concentration of test gas will show whether the sensors respond accurately and whether the instrument’s alarm functions properly.
Toxic gas detectors are commonly used throughout the workplace to warn of potentially harmful exposure to dangerous gas leaks. Various factors relating to the characteristics of a toxic gas release and the detector itself should be considered when setting alarm levels on toxic gas detectors, to minimise risk of potentially dangerous exposure levels to personnel in a particular area of the workplace.
Flammable gas detectors, which are often used in conjunction with toxic gas detectors, provide a safety warning based on the lower flammable limit. The use of toxic gas detectors including their alarms for workplace safety is subject either directly, or as is more often the case, indirectly, to: legislation augmented by codes of practice; British, European or worldwide standards; and industry guidance that can be produced by industry trade associations or expert groups and individual companies.
Additionally, two vital requirements also need to be fulfilled:
• Equipment maintenance including function checks/bump tests and calibration
• Worker training in how to use the detector correctly and act appropriately in the event of alarm activation
There are a number of methods used to detect the presence of various gas compounds. Typically, the most universally accepted methods are: electrochemical, catalytic bead, infrared, and paper tape.
Electrochemical Electrochemical gas sensors contain various components designed to react with a specific toxic gas. The reaction generates a current, which is measured by the instrument and translated into a concentration value (PPM or PPB).
Catalytic bead Catalytic sensors ‘burn’ combustible gases on a small extremely hot bead; the instrument measures the resulting increase in resistance and translates it into a percentage of the LEL.
Infrared Infrared instruments shine a ‘tuned’ beam of light through the gas sample. If the target gas is present, a portion of the beam’s light spectrum is absorbed in proportion to the concentration of the gas. Infrared-based instruments are generally more expensive to purchase but provide low, long-term ownership costs, as they require less maintenance and do not require span calibration.
Paper tape Paper tape instruments use chemically impregnated tape for very accurate and specific detection of toxic gases. Much like a piece of litmus paper, the tape changes colour when exposed to a given gas. The colour change is detected by a photocell, analysed and translated into a concentration value.
Calibrate for accuracy
Calibration refers to an instrument’s measuring accuracy relative to a known concentration of gas. Gas detectors measure the concentration of a gas in an air sample by comparing the sensor’s response to the response generated by a calibration gas of a known concentration. The instrument’s response to the calibration gas serves as the measurement scale or reference point. The responsiveness of electrochemical sensors will vary with environmental conditions. Sensor response will be different depending on the actual environmental conditions. As much as possible, therefore, the monitors should be calibrated at environmental conditions that are the same as, or similar to, actual field conditions. Calibration at locations where the equipment is to be used is always preferable.
Most instruments are equipped with two levels of alarms: warning and danger. The warning alarm alerts the user that the environment has a detectable concentration of gas and is therefore potentially hazardous. The danger alarm indicates that the gas concentration exceeds the programmed hazard threshold, and the area is approaching a hazardous level. Whether an instrument warns and/or alarms at the proper time depends on its detection abilities and its ability to translate its findings into an accurate reading.
If the instrument’s reference point has shifted, the reading will shift accordingly and be unreliable. This is called ‘calibration drift’ and it happens to all detectors over time. An instrument that experiences calibration drift can still measure the quantity of gas present, but it cannot convert this information into an accurate numerical reading. Regular calibration with a certified standard gas concentration will update the instrument’s reference point, ensuring that the instrument will produce continued, accurate readings.
Calibrate for safety
The primary reason for proper, regular instrument calibration is to prevent inaccurate gas concentration readings that could lead to injury or death. It is vital to worker safety that these instruments are maintained and calibrated properly. Instrument inaccuracy, due to improper or irregular calibration, can lead to serious accidents. Correctly calibrating an instrument helps to ensure that the instrument will accurately respond to the gases it is designed to detect, warning users of hazardous conditions before they reach dangerous levels. In addition to detecting and correcting for calibration drift, regular calibration assures the user that the instrument is functional.
Gas detection instruments are often subjected to harsh operating and storage conditions in which they can be damaged. Both of these factors can affect instrument performance, leading to inaccurate readings or even instrument failure. While from a visual inspection a unit may appear to be fit for purpose, it could actually be damaged internally.
Regular calibration is the only way to be certain that a detector is fully functional. Moreover, a standing policy for regular calibration sets the tone for a safety-conscious work environment and indicates to workers that safety is a priority. As a result, workers may be more likely to keep safety principles in mind throughout the workday.
Published: 07th May 2015 in Health and Safety Middle East