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Gas Hazards & Areas 

- Flammable Gas Hazards
- Flammable Limit 
- Flammable Gas Properties
- Flammable Gases Data 
- Toxic Gas Hazards 
- Hygiene Monitoring
- Toxic Exposure Limits 
- Toxic Gases Data 
- Asphyxiant (Oxygen Deficiency) Hazard 
- Oxygen Enrichment 
- Typical Areas that Require Gas Detection 

There are three main types of gas hazard - Flammable, Toxic and Asphyxiant.  
Risk of fire and / or explosion e.g. Methane, Butane, Propane

Risk of Poisoning e.g. Carbon Monoxide, Hydrogen, Carbon Dioxide,

Risk of suffocation e.g. Oxygen deficiency. Oxygen can be consumed or displaced
by another gas

Flammable Gas Hazards
Combustion is a fairly simple chemical reaction in which Oxygen is combined rapidly with another substance resulting in the release of energy. This energy appears mainly as heat – sometimes in the form of flames. The igniting substance is normally, but not always, a Hydrocarbon compound and can be solid, liquid, vapour or gas. However, only gases and vapours are considered in this publication.

(N.B. The terms ‘flammable’, ‘explosive’, and ‘combustible’ are, for the purpose of this publication, interchangeable).

The process of combustion can be represented by the well known
fire triangle.

Three factors are always needed to cause combustion:

1.  A source of ignition
2.  Oxygen
3.  Fuel in the form of a gas or vapour

In any fire protection system, therefore, the aim is to always remove at least one of these three potentially hazardous items.

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Flammable Limit
There is only a limited band of gas/air concentration which will produce a combustible mixture. This band is specific for each gas and vapour and is bounded by an upper level, known as the Upper Explosive Limit (or the UEL) and a lower level, called the Lower Explosive Limit (LEL).

At levels below the LEL, there is insufficient gas to produce an explosion (i.e. the mixture is too ‘lean’), whilst above the UEL, the mixture has insufficient Oxygen (i.e. the mixture is too ‘rich’). The flammable range therefore falls between the limits of the LEL and UEL for each individual gas or mixture of gases. Outside these limits, the mixture is not capable of combustion. The Flammable Gases Data in section 2.4 indicates the limiting values for some of the better-known combustible gases and compounds. The data is given for gases and vapours at normal conditions of pressure and temperature. An increase in pressure, temperature or Oxygen content will generally broaden the flammability range.

In the average industrial plant, there would normally be no gases leaking into the surrounding area or, at worst, only a low background level of gas present. Therefore the detecting and early warning system will only be required to detect levels from zero percent of gas up to the lower explosive limit. By the time this concentration is reached, shut-down procedures or site clearance should have been put into operation. In fact this will typically take place at a concentration of less than 50 percent of the LEL value, so that an adequate safety margin is provided.

However, it should always be remembered that in enclosed or unventilated areas, a concentration in excess of the UEL can sometimes occur. At times of inspection, therefore, special care needs to be taken when operating hatches or doors, since the ingress of air from outside can dilute the gases to a hazardous, combustible mixture.

(N.B LEL/LFL and UEL/UFL are, for the purpose of this publication, interchangeable).

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Flammable Gas Properties

Ignition Temperature
Flammable gases also have a temperature where ignition will take place, even without an external ignition source such as a spark or flame. This temperature is called the Ignition Temperature. Apparatus for use in a hazardous area must not have a surface temperature that exceeds the ignition temperature. Apparatus is therefore marked with a maximum surface temperature or T rating.

Flash Point (F.P. °C)
The flash point of a flammable liquid is the lowest temperature at which the surface of the liquid emits sufficient vapour to be ignited by a small flame. Don’t confuse with Ignition Temperature as the two can be very different: 

To convert a Celsius temperature into degrees Fahrenheit:
Tf = ((9/5)*Tc)+32.

E.g. to convert -20 Celsius into degrees Fahrenheit, first multiply the Celsius temperature reading by nine-fifths to get -36. Then add 32 to get -4°F.

Vapour Density
Helps determine sensor placement
The density of a gas / vapour is compared with air when air = 1.0
Vapour density < 1.0 will rise
Vapour density > 1.0 will fall

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Flammable Gases Data
Data may change by country and date, always refer to local up-to-date regulations. 

References: BS EN 61779-1:2000 Electrical apparatus for the detection and measurement of flammable gases-Part 1: General requirements and test methods.

NIST Chemistry Web Book June 2005 release. Aldrich Handbook of Fine Chemicals and Laboratory Equipment 2003-2004.

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Toxic Gas Hazards
Some gases are poisonous and can be dangerous to life at very low concentrations. Some toxic gases have strong smells like the distinctive ‘rotten eggs’ smell of H2S. The measurements most often used for the concentration of toxic gases are parts per million (ppm) and parts per billion (ppb). For example 1ppm would be equivalent to a room filled with a total of 1 million balls and 1 of those balls being red. The red ball would represent 1ppm.

More people die from toxic gas exposure than from explosions caused by the ignition of flammable gas. (It should be noted that there is a large group of gases which are both combustible and toxic, so that even detectors of toxic gases sometimes have to carry hazardous area approval). The main reason for treating flammable and toxic gases separately is that the hazards and regulations involved and the types of sensor required are different.

With toxic substances, (apart from the obvious environmental problems) the main concern is the effect on workers of exposure to even very low concentrations, which could be inhaled, ingested, or absorbed through the skin. Since adverse effects can often result from additive, long-term exposure, it is important not only to measure the concentration of gas, but also the total time of exposure. There are even some known cases of synergism, where substances can interact and produce a far worse effect when together than the separate effect of each on its own.

Concern about concentrations of toxic substances in the workplace focus on both organic and inorganic compounds, including the effects they could have on the health and safety of employees, the possible contamination of a manufactured end-product (or equipment used in its manufacture) and also the subsequent disruption of normal working activities.

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Hygiene Monitoring
The term ‘hygiene monitoring’ is generally used to cover the area of industrial health monitoring associated with the exposure of employees to hazardous conditions of gases, dust, noise etc. In other words, the aim is to ensure that levels in the workplace are below the statutory limits.  

This subject covers both area surveys (profiling of potential exposures) and personal monitoring, where instruments are worn by a worker and sampling is carried out as near to the breathing zone as possible. This ensures that the measured level of contamination is truly representative of that inhaled by the worker.

 It should be emphasised that both personal monitoring and monitoring of the workplace should be considered as important parts of an overall, integrated safety plan. They are only intended to provide the necessary information about conditions as they exist in the atmosphere. This then allows the necessary action to be taken to comply with the relevant industrial regulations and safety requirements. 

Whatever method is decided upon, it is important to take into account the nature of the toxicity of any of the gases involved. For instance, any instrument which measures only a time-weighted average, or an instrument which draws a sample for subsequent laboratory analysis, would not protect a worker against a short exposure to a lethal dose of a highly toxic substance. On the other hand, it may be quite normal to briefly exceed the average, long-term (LTEL) levels in some areas of a plant, and it need not be indicated as an alarm situation. Therefore, the optimum instrument system should be capable of monitoring both short and long term exposure levels as well as instantaneous alarm levels.

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Toxic Exposure Limits

European Occupational Exposure Limits
Occupational Exposure Limit values (OELs) are set by competent national authorities or other relevant national institutions as limits for concentrations of hazardous compounds in workplace air. OELs for hazardous substances represent an important tool for risk assessment and management and valuable information for occupational safety and health activities concerning hazardous substances.

Occupational Exposure Limits can apply both to marketed products and to waste and by-products from production processes. The limits protect workers against health effects, but do not address safety issues such as explosive risk. As limits frequently change and can vary by country, you should consult your relevant national authorities to ensure that you have the latest information.

Occupational exposure limits in the UK function under the Control of Substances Hazardous to Health Regulations (COSHH). The COSHH regulations require the employer to ensure that the employee’s exposure to substances hazardous to health is either prevented or if not practically possible, adequately controlled.

As of 6 April 2005, the regulations introduced a new, simpler Occupational Exposure Limit system. The existing requirements to follow good practice were brought together by the introduction of eight principles in the Control of Substances Hazardous to Health (Amendment) Regulations 2004.

Maximum Exposure Limits (MELs) and Occupational Exposure Standards (OESs) were replaced with a single type of limit - the Workplace Exposure Limit (WEL). All the MELs, and most of the OESs, are being transferred into the new system as WELs and will retain their previous numerical values. The OESs for approximately 100 substances were deleted as the substances are now banned, scarcely used or there is evidence to suggest adverse health effects close to the old limit value. The list of exposure limits is known as EH40 and is available from the UK Health and Safety Executive. All legally enforceable WELs in the UK are air limit values. The maximum admissible or accepted concentration varies from substance to substance according to its toxicity. The exposure times are averaged for eight hours (8-hour TWA) and 15 minutes (short-term exposure limit STEL). For some substances, a brief exposure is considered so critical that they are set only a STEL, which should not be exceeded even for a shorter time. The potency to penetrate through skin is annotated in the WEL list by remark “Skin”. Carcinogenicity, reproduction toxicity,  irritation and sensitisation potential are considered when preparing a proposal for an OEL according to the present scientific knowledge. 

US Occupational Exposure Limits
The Occupational Safety systems in the United States vary from state to state. Here, information is given on 3 major providers of the Occupational Exposure Limits in the USA - ACGIH, OSHA, and NIOSH.

The American Conference of Governmental Industrial Hygienists (ACGIH) publishes Maximum Allowable Concentrations (MAC), which were later renamed to “Threshold Limit Values” (TLVs).  

Threshold Limit Values are defined as an exposure limit “to which it is believed nearly all workers can be exposed day after day for a working lifetime without ill effect”. The ACGIH is a professional organisation of occupational hygienists from universities or governmental institutions. Occupational hygienists from private industry can join as associate members. Once a year, the different committees propose new threshold limits or best working practice guides. The list of TLVs includes more than 700 chemical substances and physical agents, as well as dozens of Biological Exposure Indices for selected chemicals.

The ACGIH defines different TLV-Types as:

Threshold Limit Value – Time-Weighted Average (TLV-TWA):  the time-weighted average concentration for a conventional 8-hour workday and a 40-hour workweek, to which it is believed that nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Threshold Limit Value – Short-Term Exposure Limit (TLV-STEL):  the concentration to which it is believed that workers can be exposed continuously for a short period of time without suffering from irritation, chronic or irreversible tissue damage, or narcosis. STEL is defined as a 15-minute TWA exposure, which should not be exceeded at any time during a workday.

Threshold Limit Value - Ceiling (TLV-C):  the concentration that should not be exceeded during any part of the working exposure.

There is a general excursion limit recommendation that applies to those TLV-TWAs that do not have STELs. Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded.

ACGIH-TLVs do not have a legal force in the USA, they are only recommendations. OSHA defines regulatory limits. However, ACGIH-TLVs and the criteria documents are a very common base for setting TLVs in the USA and in many other countries. ACGIH exposure limits are in many cases more protective than OSHA’s. Many US companies use the current ACGIH levels or other internal and more protective limits.

The Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor publishes Permissible Exposure Limits (PEL). PELs are regulatory limits on the amount or concentration of a substance in the air, and they are enforceable. The initial set of limits from 1971 was based on the ACGIH TLVs. OSHA currently has around 500 PELs for various forms of approximately 300 chemical substances, many of which are widely used in industrial settings. Existing PELs are contained in a document called “29 CFR 1910.1000”, the air contaminants standard. OSHA uses in a similar way as the ACGIH the following types of OELs: TWAs, Action Levels, Ceiling Limits, STELs, Excursion Limits and in some cases Biological Exposure Indices (BEIs).

The National Institute for Occupational Safety and Health (NIOSH) has the statutory responsibility for recommending exposure levels that are protective to workers. NIOSH has identified Recommended Exposure Levels (RELs) for around 700 hazardous substances. These limits have no legal force. NIOSH recommends their limits via criteria documents to OSHA and other OEL setting institutions. Types of RELs are TWA, STEL, Ceiling and BEIs. The recommendations and the criteria are published in several different document types, such as Current Intelligent Bulletins (CIB), Alerts, Special Hazard Reviews, Occupational Hazard Assessments and Technical Guidelines.

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Toxic Gases Data
The toxic gases listed below can be detected using equipment supplied by Honeywell Analytics. Gas data is supplied where known.

As product development is ongoing, contact Honeywell Analytics if the gas you require is not listed.

Data may change by country and date, always refer to local up-to-date regulations.

Ref: EH40/2005 Workplace exposure limits, OSHA Standard 29 CFR 1910.1000 tables Z-1 and Z-2 and ACGIH Threshold Limit Valves and Biological Exposure Indices Book 2005. 

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Asphyxiant (Oxygen Deficiency) Hazard
We all need to breathe the Oxygen (O2) in air to live. Air is made up of several different gases including Oxygen. Normal ambient air contains an Oxygen concentration of 20.9% v/v. When the Oxygen level dips below 19.5% v/v, the air is considered Oxygen-deficient. Oxygen concentrations below 16% v/v are considered unsafe for humans. 

Oxygen depletion can be caused by:
•  Displacement
•  Combustion
•  Oxidation
•  Chemical reaction 

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Oxygen Enrichment
It is often forgotten that Oxygen enrichment can also cause a risk. At increased O2 levels the flammability of materials and gases increases. At levels of 24% items such as clothing can spontaneously combust.  

Oxyacetylene welding equipment combines Oxygen and Acetylene gas to produce an extremely high temperature. Other areas where hazards may arise from Oxygen enriched atmospheres include areas manufacturing or storing rocket propulsion systems, products used for bleaching in the pulp and paper industry and clean water treatment facilities.   

Sensors have to be specially certified for use in O2 enriched atmospheres.

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Typical Areas that Require Gas Detection
There are many different applications for flammable, toxic and Oxygen gas detection. Industrial processes increasingly involve the use and manufacture of highly dangerous substances, particularly toxic and combustible gases. Inevitably, occasional escapes of gas occur, which create a potential hazard to the industrial plant, its employees and people living nearby. Worldwide incidents involving asphyxiation, explosions and loss of life, are a constant reminder of this problem. 

Oil & Gas
The oil and gas industry covers a large number of upstream activities from the on and offshore exploration and production of oil and gas to its transportation, storage and refining. The large amount of highly flammable Hydrocarbon gases involved are a serious explosive risk and additionally toxic gases such as Hydrogen Sulphide are often present. 

Typical Applications:
•  Exploration drillings rigs
•  Production platforms
•  Onshore oil and gas terminals
•  Refineries

Typical Gases:
Flammable: Hydrocarbon gases
Toxic: Hydrogen Sulphide, Carbon Monoxide 

Semiconductor Manufacturing
Manufacturing semiconductor materials involves the use of highly toxic substances and flammable gas. Phosphorus, Arsenic, Boron and Gallium are commonly used as doping agents. Hydrogen is used both as a reactant and a reducing atmosphere carrier gas. Etching and cleaning gases include NF3 and other perfluorocompounds. 

Typical Applications:
•  Wafer reactor
•  Wafer dryers
•  Gas cabinets
•  Chemical Vapour Deposition 

Typical Gases:
Flammable: Hydrogen, Isopropyl Alcohol, Methane
Toxic: HCl, AsH3, BCl3, PH3, CO, HF, O3, H2Cl2Si, TEOS, C4F6, C5F8, GeH4, NH3, NO2 and O2 Deficiency
Pyrophoric: Silane 

Chemical Plants
Probably one of the largest users of gas detection equipment are Chemical Plants. They often use a wide range of both flammable and toxic gases in their manufacturing processes or create them as by-products of the processes. 

Typical Applications:
•  Raw material storage
•  Process areas
•  Laboratories
•  Pump rows
•  Compressor stations
•  Loading/unloading areas 

Typical Gases:
Flammable: General Hydrocarbons
Toxic: Various including Hydrogen Sulphide, Hydrogen Fluoride and Ammonia 

Power Stations
Traditionally coal and oil have been used as the main fuel for Power Stations. In Europe and the US most are being converted to natural gas.

 Typical Applications:
•  Around the boiler pipework and burners
•  In and around turbine packages
•  In coal silos and conveyor belts in older coal/oilfired stations

Typical Gases:
Flammable: Natural Gas, Hydrogen
Toxic: Carbon Monoxide, SOx, NOx and Oxygen deficiency

In most industries, one of the key parts of the safety plan for reducing the risks to personnel and plant is the use of early warning devices such as gas detectors. These can help to provide more time in which to take remedial or protective action. They can also be used as part of a total integrated monitoring and safety system for an industrial plant. 

Waste Water Treatment Plants
Waste Water Treatment Plants are a familiar site around many cities and towns.
Sewage naturally gives off both Methane and H2S. The ‘rotten eggs’ smell of H2S can often be noticed as the nose can detect it at less than 0.1ppm.

Typical Applications:
•  Digesters
•  Plant sumps
•  H2S scrubbers
•  Pumps

Typical Gases:
Flammable: Methane, Solvent vapours
Toxic: Hydrogen Sulphide, Carbon Dioxide, Chlorine, Sulphur Dioxide, Ozone

Boiler Rooms
Boiler Rooms come in all shapes and sizes. Small buildings may have a single boiler whereas larger buildings often have large boiler rooms housing several large boilers.

Typical Applications:
•  Flammable gas leaks from the incoming gas main
•  Leaks from the boiler and surrounding gas piping
•  Carbon Monoxide given off badly maintained boiler 

Typical Gases:
Flammable: Methane
Toxic: Carbon Monoxide

Hospitals may use many different flammable and toxic substances, particularly in their laboratories. Additionally, many are very large and have onsite utility supplies and back up power stations.

Typical Applications:
•  Laboratories
•  Refrigeration plants
•  Boiler rooms

Typical Gases:
Flammable: Methane, Hydrogen
Toxic: Carbon Monoxide, Chlorine, Ammonia, Ethylene Oxide and Oxygen deficiency

Tunnels/Parking Structures
Car Tunnels and enclosed parking structures, loading bays and other vehicular areas  need to be monitored for the toxic gases from exhaust fumes. Modern tunnels and parking structures use this monitoring to control the ventilation fans. Tunnels may also need to be monitored for the buildup of natural gas.

Typical Applications:
•  Parking structures 

•  Underground and enclosed car parks
•  Access tunnels
•  Ventilation control

Typical Gases:
Flammable: Methane (natural gas), LPG, LNG, Petrol Vapor
Toxic: Carbon Monoxide, Nitrogen Dioxide

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