Training Module Describe Flammable 100% 90% C 80% No Combustion 70% 60% 50% 40% 30% 20% UEL B Combustion No Combustion 10% 0% LEL A Human Development
HDC Human Development All rights reserved. No part of this publication may be copied, reproduced, stored in a computer or retrieval system, published, distributed, or transmitted in any form or by any means whatsoever, including without limitation by mechanical means, photo copying, recording, digital or electronic media, the Internet, or otherwise, without the express prior written permission of HDC Human Development (HDC). HDC grants to the purchaser of a Single User License (as defined in the agreement with such purchaser) a limited license to store the electronic file(s) on one computer only and to make a single paper copy of this publication. HDC grants to the purchaser of a Site License (as defined in the agreement with such purchaser) a limited license to store the electronic file(s) on one local area network server accessible by individual users' computers at one site or location only and to make paper copies of this publication for a company s employees at the same site or location only. Each site or location must purchase a separate Site License for employees at that site. HDC grants the purchaser of a Corporate License (as defined in the agreement with such purchaser) a limited license to store the electronic file(s) on its intranet and on computers at company sites or locations and to make paper copies for any or all employees. Nothing in the foregoing restricts, amends or abrogates the provisions of the agreement between HDC and the purchaser of the applicable license. Any copying or use other than pursuant to such a license is illegal. For further information, please consult the applicable license agreement. This publication is designed to provide general information regarding the subject matter covered. Care has been taken to ensure the accuracy of the information and that the instructions contained in this publication are clear and reflect sound practice. The user understands that HDC is not providing engineering services. The user understands that any procedures (task steps) that are published or referenced may have to be modified to comply with specific equipment, work conditions, company standards, company policies and practices, legislation, and user qualifications. HDC does not make any representations, guarantees, or warranties of any kind whatsoever with respect to the content hereof and the results to be achieved by implementing the procedures (task steps) herein. To the maximum extent permitted by applicable law, in no event shall HDC be liable for any damages whatsoever (including without limitation, direct or indirect damages for personal injury, damages to the environment, damages to business property, loss of business profit, or any other pecuniary loss). The use of the information and procedures (task steps) herein is undertaken at the sole risk of the user. ISBN 1-55338-000-2 Canadian Cataloguing in Publication Data 1. Gases Flammability. 2. Gas-detectors. I. HDC Human Development Consultants. TH9271.D48 2000 665.7 028 7 C00-900571-4 This training kit consists of the following parts: Training Module and Self-Check Blank Answer Sheet Knowledge Check and Answer Key Published by HDC Human Development Published in Canada HDC Human Development Website: www.hdc.ca E-mail: marketing@hdc.ca Phone: (780) 463-3909 April, 2004
Contents Training Objectives 1 1 Introduction 1 2 Combustion 3 3 Measuring the Concentration of a Flammable Gas 6 3.1 LEL (Lower Explosive Limit) 7 3.2 UEL (Upper Explosive Limit) 13 4 Portable Gas Detectors 14 4.1 Flammable Gas Detector Technology 15 4.2 Limitations of Flammable Gas Technology 17 5 General Operating Conditions 19 6 Self-Check 21 7 Self-Check Answers 23 April, 2004 Page i of i
Training Objectives Upon completion of this training kit, you will be able to: Explain the purpose and importance of testing for the presence of flammable gases Describe the conditions necessary for a fire or explosion to occur Explain the concepts of LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) Calculate % LEL of a flammable gas in the atmosphere, given the percentage concentration by volume of that flammable gas Calculate the percentage concentration by volume of a flammable gas in the atmosphere, given the % LEL of that flammable gas Explain the general operating principles of flammable gas detectors Describe the main factors affecting the accuracy of flammable gas detector readings Identify safety precautions which must be followed whenever taking flammable gas readings 1 Introduction Many of the products and chemicals used in industry have the potential to burn or explode. This potential for fire poses a critical safety hazard during the transportation, processing, and storage of the products, during the maintenance of related equipment, during underground construction, and during entry into confined spaces where flammable products may have accumulated. At most industrial facilities, specific measures have been taken to minimize the potential for fire or explosion by employing various strategies, such as: identifying hazardous areas implementing specific policies relating to vehicle entry, confined space entry, smoking, safe work agreements, and work procedures to minimize fire and explosion hazards when work is being performed in hazardous areas April, 2004 Page 1 of 23
using intrinsically safe electrical equipment and wiring to prevent sparks installing fixed monitoring equipment to detect the presence of flammable gases The development and implementation of safety measures to minimize the potential for fires are based on the principles of combustion. Combustion is defined as the process of burning. Chemically, combustion is the process of oxidization, in which oxygen rapidly combines with a material, giving off heat and often light. Materials which oxidize readily, that is, materials which can easily ignite, are called flammable materials. In order for combustion to take place and be sustained, three elements must be present: oxygen a source of ignition (source of high temperature) fuel TEMPERATURE FUEL OXYGEN Figure 1 Fire Triangle If one of the three elements is missing, combustion cannot take place. Consequently, all fire-related safety measures concentrate on eliminating one or more of the three elements which are needed for combustion. In general, employees must be able to identify the causes and conditions which create fire hazards, and take the correct action to minimize the potential of fires. This module focuses on controlling one element of the fire triangle fuel. This control is achieved by measuring the concentration of flammable gases to determine the potential for a fire or explosion and then taking corrective action. The module also provides a brief description of the use of purging to eliminate oxygen as a means to minimize the potential for fire or explosion. Employees must also be able to use a gas detector to determine levels of flammable gas in the work environment. April, 2004 Page 2 of 23
After completing this module, employees must review vendor literature for the specific portable flammable gas detector(s) in use at their facilities to be able to operate and interpret the readings and alarms produced by that specific model of detector. NOTE The concentration of a flammable gas can be measured either as a percentage by volume in the atmosphere or as a percentage LEL (Lower Explosive Limit). These units of measurement can be easily confused. The reader should pay special attention to the definitions and use of these terms when reading this module. 2 Combustion Figure 2 Percent Concentration by Volume of A Specific Flammable Gas Figure 2 shows various concentrations of a flammable gas in the atmosphere. At 0% concentration by volume, no flammable gas is present; at 100% concentration by volume, all of the atmosphere has been displaced by the flammable gas. April, 2004 Page 3 of 23
The concentration of flammable gas in the atmosphere at point A is very low and combustion cannot be sustained in the presence of an ignition source. This inability to burn can best be explained at the molecular level. A flammable gas molecule and an oxygen molecule require a specific minimum amount of heat in order to chemically react. At point A there is a great distance between flammable gas molecules (due to the low concentration of flammable gas). If one of the gas molecules were to chemically react with an adjacent oxygen molecule, heat would be given off. By the time this heat reaches the next gas molecule, the molecules in the atmosphere would have absorbed most of the heat energy. Consequently, there would not be enough heat reaching the nearest gas molecule and an adjacent oxygen molecule to initiate a chemical reaction. A continuous chemical reaction (i.e., combustion) does not take place. An example of this condition would be painting the exterior of a house with an oil-based paint. An open paint can would be giving off some flammable gases, but the concentration of the flammable gases would be too low to cause a fire if a source of ignition (e.g. a cigarette) were present. An additional safeguard is to eliminate any source of ignition (e.g. the cigarette). The concentration of flammable gas in the atmosphere at point B is at a level which supports continuous combustion. The gas molecules are close enough that the heat of combustion of one molecule will reach the next gas molecule before the atmosphere can absorb all of the heat energy. This heat then provides the energy for the nearest gas molecule to react with an adjacent oxygen molecule, producing additional heat. The process continues from one gas molecule to the next, resulting in sustained combustion. At point C, the concentration of flammable gas in the atmosphere is at such a high level that combustion cannot be sustained in the presence of an ignition source. Because of the high concentration of flammable gas, there is a great distance April, 2004 Page 4 of 23
between oxygen molecules. If one of the gas molecules chemically reacts with an adjacent oxygen molecule, heat is given off. By the time this heat reaches the next oxygen molecule, the flammable gas molecules have absorbed most of the heat energy. Consequently, there is not enough heat reaching the nearest oxygen molecule and an adjacent gas molecule to initiate a chemical reaction. A continuous chemical reaction (combustion) does not take place. A practical way of thinking of this concept is that combustion cannot take place because of the lack of oxygen. One application of this concept is at oil battery storage tanks. When oil tanks are being emptied, air is not allowed to enter the tanks to equalize the pressure since the presence of air creates the potential for combustion. Instead, make-up gas is piped into the tanks to maintain a high concentration of gas above the oil, ensuring an oxygen-free condition. The concentration of flammable gas can be controlled in two ways to minimize the risk of fire: 1. Ensure that no flammable gas (or a very low concentration of flammable gas) is present ideally a concentration of 0% gas by volume (no flammable gas present). 2. Ensure the concentration of flammable gas is very high, eliminating the oxygen ideally a concentration of 100% by volume of flammable gas (no oxygen present). To ensure the workplace is safe from potential fires or explosions, employees use specific equipment to identify the presence of a flammable gas and measure the gas s potential for igniting. For low concentrations of flammable gases, employees use a portable flammable gas detector to monitor and measure the gases. At high concentrations of flammable gases, employees use an oxygen analyzer to measure the concentration of the oxygen. Identifying and measuring the concentration of a flammable gas is only the first step in ensuring a safe workplace. Ultimately, the responsibility lies with employees to: 1. Determine the potential risk of fire for specific flammable gas readings taken in various work conditions, and then 2. Take action to eliminate any risk. April, 2004 Page 5 of 23
3 Measuring the Concentration of a Flammable Gas Figure 3 Measuring the Potential for Combustion of a Specific Flammable Gas At very low concentrations of a flammable gas in the atmosphere, say 0.25% by volume for most flammable gases, combustion cannot take place when a source of ignition is present. As the concentration increases, a point will be reached where combustion will occur if there is a source of ignition. Refer to Figure 3. This level of concentration of the flammable gas is known as LEL (Lower Explosive Limit). Some flammable gas detector vendor manuals may use the term Lower Flammable Limit (LFL) to mean LEL. Concentrations of a flammable gas below its LEL will not burn; concentrations of a flammable gas at or above its LEL will potentially burn or explode if a source of ignition is present. April, 2004 Page 6 of 23
If the concentration of the flammable gas is increased above its LEL, a point will be reached where burning will cease. This point is known as the UEL (Upper Explosive Limit). All concentrations of the flammable gas above its UEL will not burn or explode in the presence of a source of ignition because there is not enough oxygen to support combustion. All concentrations of the flammable gas between its LEL and UEL will burn or explode if a source of ignition is present. 3.1 LEL (Lower Explosive Limit) Each type of flammable gas has a unique LEL (and UEL). The following table lists the LEL and UEL (as approximate percent concentration by volume) of flammable gases common to industry. Figure 4 LEL and UEL of Selected Flammable Gases Gas LEL (Percentage Concentration by Volume) methane 5.0% 15% ethane 2.9% 13% UEL (Percentage Concentration by Volume) propane 2.1% 9.5% isobutane 1.8% 8.4% n-butane 1.8% 8.4% isopentane 1.4% 8.3% n-pentane 1.4% 8.3% acetylene 2.4% 80% to 100%* carbon monoxide 12.5% 74.2% ethylene glycol 3.2% 15.3% gasoline (motor) 1.3% 6% hydrogen sulfide 4.3% 45.5% * UEL of acetylene is difficult to measure and test results may vary from 80% to 100%. April, 2004 Page 7 of 23
Note in the table that LEL is measured as percent concentration by volume of the flammable gas in the atmosphere. Using the percentage concentration by volume of a flammable gas in the atmosphere as a means for determining the potential for a fire or explosion is not practical. An employee would have to determine the type of gas present, measure the concentration of the gas (as a percentage by volume) and then compare the measurement to the specific LEL for that gas. To make measurement taking and decision making easier, flammable gas detectors measure the potential for combustion or explosion as a % LEL (percentage of LEL). This approach requires that the LEL for all gases be set at 100%. Figure 5 Example of 100% LEL The following table, comparing the percentage concentration by volume of three gases at 100% LEL, illustrates that each flammable gas has a unique LEL. April, 2004 Page 8 of 23
Figure 6 Defining the LEL of Three Flammable Gases as 100% LEL Gas LEL as a % Concentration by Volume in the Atmosphere Flammable Gas Detector Reading methane 5.0% 100% LEL acetylene 2.4% 100% LEL gasoline 1.3% 100% LEL The following table refers to two different atmospheres, each containing a different flammable gas. The table compares concentrations of two flammable gases in the atmosphere at the same percentage LEL measured by a flammable gas detector. Figure 7 Comparing the Concentration of Two Different Flammable Gases with Gas Detector Readings Concentration of Methane Gas in the Atmosphere Concentration of Propane Gas in the Atmosphere Flammable Gas Detector Readings 5% methane (95% air) 2.1% propane (97.65% air) 100% LEL 4% methane (96% air) 1.68% propane (98.12% air) 80% LEL 3% methane (97% air) 1.26% propane (98.59% air) 60% LEL 2% methane (98% air) 0.84% propane (99.06% air) 40% LEL 1% methane (99% air) 0.42% propane (99.53% air) 20% LEL The flammable gas detector measures the potential for combustion in terms of percent LEL, and is therefore able to detect and take LEL measurements of any flammable gas. This method of measurement is very useful for employees because employees can use the flammable gas detector reading to directly determine the degree of risk of a fire occurring. The % LEL scale permits operators of flammable gas testing equipment to immediately determine how close the flammable gas concentration is to the LEL. A reading of 100% LEL indicates that there is enough flammable gas present to result in combustion, while a reading of 50% LEL indicates that the concentration of flammable gas is only half of the required amount for combustion to take place. The % LEL scale therefore serves as an indication of the relative hazard posed by concentrations of flammable gas: low LEL percentages indicate safer working conditions, while percentages close to 100% indicate an extremely dangerous work condition. April, 2004 Page 9 of 23
This method for measuring the potential for combustion has a secondary benefit only a single gas, in this case methane, is required as a standard to calibrate flammable gas detectors. However, when measuring the LEL of other types of flammable gases, a correction factor supplied by the manufacturer may have to be used to obtain an accurate reading. Sometimes employees have difficulties in understanding the difference and relationship between percent concentration of a flammable gas in the atmosphere and percentage LEL. Employees must not confuse these concepts since the results could expose employees to a high risk condition. To ensure there is no misunderstanding, employees must be able to convert: 1. Percentage concentration by volume of a known flammable gas in the atmosphere to percentage LEL 2. Percentage LEL of a known flammable gas to percent concentration by volume in the atmosphere Converting Percentage Concentration by Volume of a Flammable Gas in the Atmosphere to Percentage LEL Example 1 A flammable gas has an LEL of 4% by volume (i.e., 4% concentration by volume = 100% LEL). If the sample atmosphere has a concentration of 1% by volume of this gas, what is the percentage LEL of the gas? y = % LEL 4% concentration by volume = 100% LEL Step 1: set up the equation y 100% LEL = 1% concentration 4% concentration Step 2: solve for y y = 1% concentration 4% concentration x 100% LEL y = 25% LEL The flammable gas is at 25% LEL. April, 2004 Page 10 of 23
Example 2 A flammable gas has an LEL (Lower Explosive Limit) of 2.5% by volume (i.e., 2.5% concentration by volume = 100% LEL). If the sample atmosphere has a concentration of 0.5% by volume of this gas, what is the percentage LEL of the gas? y = % LEL 2.5 % concentration by volume = 100% LEL Step 1: set up the equation End of Sample A full licensed copy of this kit includes: Training Module and Self-Check Knowledge Check and Answer Key Blank Answer Sheet April, 2004 Page 11 of 23