CHAPTER 6 VENTILATION REQUIREMENTS

Transcription

1 CHAPTER 6 VENTILATION REQUIREMENTS A. General When there is no air movement, a flammable gas or vapor released to the atmosphere will spread in all directions. With air movement, the flammable gas or vapor will move in one particular direction, thereby covering a large distance when forced by air currents in this direction. As the flammable gas or vapor travels away from the point of release, its ignitable concentrations will diminish because of dilution and eventually it will reach a concentration below the lower explosive limit (LEL). The presence of sufficient ventilation will influence the distance and the time at which a flammable gas or vapor will enter its explosion range and reach a non hazardous concentration. Before the flammable gas or vapor enters its explosion range, the gas will be too rich and the vapor too lean to form an ignitable mixture. The point at which the flammable gas or vapor will enter its explosion range comes closer to the point of release if sufficient ventilation is applied. Consequently, the point at which a non hazardous concentration is reached also comes closer to the point of release if sufficient ventilation is present. How much these points will move closer depends on the quantity of the moving air, its velocity and the vapor density of the flammable product and its respective range of flammability (UEL LEL). However, even under optimum ventilating conditions, a potential for ignition of the flammable substance may exist when it is released to the atmosphere. This is true only close to the source of hazard. The purpose of ventilation is to reduce the danger in a hazardous location to a lower level or to prevent a location in a hazardous area from becoming hazardous. There are two types of ventilation which normally are used for reducing the danger level in a location; natural ventilation and mechanical ventilation. Both types of ventilation must have sufficient capacity to dilute a flammable gas or vapor in the air to acceptable low concentrations. The selection between the two different types of ventilation generally depends on whether the ventilation is applied indoors or outdoors. The general practice is to use natural ventilation for outdoor locations, and to use mechanical ventilation for indoor

2 locations. There are also two different types of mechanical ventilation that can be used to reduce the danger level in a location. One type is a "pressure fan" which pressurizes an enclosed space. The other type is an "exhaust or suction fan" which causes a negative air flow in the location. The pressure fan is normally used for a roofed location with four walls without a source of hazard which is required to have a nonhazardous classification. Locations which have three walls cannot be made nonhazardous. The suction fan is normally used for a location with three or four walls which contains a source of hazard and which is required to be classified Div. 2. Sometimes, the pressure fan may also be used for a roofed space with a source of hazard if the space has only three walls. The vapor density of the flammable product normally plays an important role in the choice between a pressure and a suction fan when 3-wall spaces are involved. In providing natural or mechanical ventilation, it is important to consider the presence of obstructions. To compensate for obstructions in an indoor location, the mechanical ventilation output must be increased and for an outdoor location the size of the hazardous area should be made larger. B. Natural Ventilation The general concept for natural ventilation is that natural ventilation is considered capable of diluting flammable gases or vapors in the air to safe concentrations. Based on this favorable feature, most outdoor hazardous location can be classified Div. 2. Wind conditions have a great impact on the traveling distance of a flammable gas or vapor in the air. The impact depends entirely upon the wind velocity. The traveling distance of a flammable gas or vapor (i.e., the horizontal distance between the point at which the flammable material is airborne and the point at which the flammable material will reach a safe concentration) is inversely proportional to wind velocity. The lower the wind velocity, the greater the traveling distance and the longer it takes before an ignitable concentration of gases or vapors is diluted to safe concentrations. The higher the wind velocity, the shorter the traveling distance and the faster the dilution. In between both conditions the distance of the traveling gas or vapor in the air will vary from small to large and this in turn will influence the point at which the flammable substance will reach non hazardous concentrations. Although the actual traveling distance of a flammable gas or vapor in the air can be calculated, a quick solution cannot be obtained due to the complexity of the calculation. Factors such as emission rates, wind velocities, crosswinds, and the terrain conditions must be known before the calculation can be applied. The emission rate is a function of pressure in the system and the size of the rupture

3 opening. Emission rates therefore must be assumed. Crosswinds may or may not exist and their existence and speeds are difficult to predict. Calculations of the gas or vapor traveling distance based on these unknown factors therefore become far from simple. Wind conditions, on the other hand, are more realistic in the application provided their behaviors are known. But wind conditions alone are not sufficient to determine the traveling distance of a flammable gas or vapor in the air. Some of the wind behaviors are shown in table 1-8. For given quantities of flammable gases or vapors in the air, "moderate" wind conditions tend to shorten the traveling distance of a flammable substance in the air. "Light" and "very light" wind conditions on the other hand will allow the traveling distance to be longer for the same given quantity of flammable gases or vapors in the air. Between "light" and "moderate" wind conditions, the traveling distance tends to be more dependent on the emission rate and the quantity of the flammable gas or vapor in the air. Under ideal conditions, when winds are "light" or "moderate" and are blowing over flat terrain without crosswinds, it is possible to determine the traveling distance of a flammable gas or vapor in the air fairly accurately. Unfortunately, the actual conditions in outdoor locations are usually not ideal because of the existence of crosswinds and wind changes. If only a steady wind velocity should exist, a flammable gas or vapor in the air will travel a certain distance before reaching safe concentrations. The traveling distance will be shorter if wind changes direction and crosswinds will shorten the distance even more. As a result of these conditions, safe concentrations will be reached at a much shorter distance, i.e., closer to the source of hazard. The length of the traveling distance, therefore, is dictated by these three conditions. Since the changes in wind direction and the existence of crosswinds are generally unknown, wind velocities alone are normally used in combination with other factors to determine the traveling distance of a flammable gas or vapor in the air. These other factors are: 1) the size of the source of hazard, 2) the system pressure, 3) the operating mode, and 4)( most important), whether the quantity of the flammable gas or vapor released to the atmosphere is small or large. This type of information will greatly simplify the determination of the traveling distance which in turn will simplify the selection of a suitable boundary size. The application of the additional factors is explained in the following example. Assume a small closed source of hazard breaks down when it is processing a flammable substance under low pressure. What is the traveling distance of the escaping vapor expressed in terms of "short" or "long" when wind conditions in that area are unknown? And, what is the required size of the hazardous boundary?

4 First determine the possible size of the rupture opening in terms of "small" and "large". The rupture opening cannot be large but must be considered to be small because of the following reasons. The source of hazard is small and the pressure in the system is low. The rupture opening, therefore, most likely is small. Because of the small opening and low pressure, the emission rate is considered low also. In view of these considerations, it is concluded that small quantities of flammable vapors are released into the atmosphere. Next, consider two wind conditions from table 1-8 Since actual wind conditions are unknown, consider only the worst wind condition without wind changes and crosswinds. Select two conditions, light winds with 4 to 7 miles per hour, and moderate winds with 13 to 18 miles per hour. As stated before, light winds will allow longer traveling distances than moderate winds. Consider of the two, the worst possible wind condition which is the light wind. Light winds allow a flammable gas or vapor to travel a longer distance. This longer distance, however, is defeated by the low emission rate and the quantity of the flammable vapors in the air which are expected to be small. A low emission rate and small quantities of flammable vapors in the air allow faster dilution than when the emission rate is high and the quantity of the flammable vapors in the air is large. Faster dilution means that the point of safe concentration is much closer to the point of release than initially was anticipated. As a result of this, the actual traveling distance of the flammable vapor is short. To complete the evaluation, another factor must also be known. That is, what type of source of hazard is involved? Is the source of hazard of the static or dynamic type? The type plays an important role in establishing the size of the hazardous area because it will influence the extent of the hazardous area. Table 1-8. Wind Conditions Type Miles of per Wind Hour Wind Effects Observed on Land Very Light 1-3 Direction of wind shown by smoke drift, but not by wind vanes. Light 4-7 Leaves rustle, ordinary vane moved by wind. Gentle 8-12 Leaves and small twigs in motion, wind extends light flag. Moderate Raises dust, loose paper, small branches. Fresh Small trees in leaf begin to sway, crested wavelets form on inland waters. Compiled by U. S. Weather Bureau

5 For example, if the source of hazard should consist of a storage container which leaks flammable liquid, the vapor from the liquid may travel less than 3 feet before reaching safe concentrations. A rotary equipment on the other hand which operates under pressure may leak a flammable vapor that travels more than 3 feet before reaching safe concentrations. Under the first condition, the boundary size needs to be at least 3 feet and under the second condition it needs to be at least 5 feet or even 10 feet. Fortunately, it is not important to know what the actual traveling distance is. In evaluating a particular situation it is only important to determine the type and size of the source of hazard, its operating mode, pressure in the system, flammability class and vapor density. It is not necessary to determine vapor traveling distance or wind velocities. All that is required for determining the size of the boundary for a particular situation are the features mentioned above and whether ventilation is present in sufficient quantity or not. With this information the required boundary size can be obtained directly from Table 1-4. C. Mechanical Ventilation 1. Pressure Fans A pressure fan is normally required for a location that is totally enclosed and does not contain a source of hazard. If the location is totally enclosed, the pressure in the enclosed space will prevent flammable gases or vapors from entering the space. Such a location is allowed to be classified nonhazardous if provided with a suitable safeguard. A pressure fan can only be applied if the air intake for the totally enclosed space is located in a nonhazardous area. The air pressure is considered "sufficient" if flammable vapors or gases are prevented from entering the enclosed space. This is accomplished when the pressure fan for the totally enclosed space produces an even pressure of not less than 0.1 inch of water above atmospheric pressure but not more than 0.25 inch of water with all openings closed. A pressure greater than 0.25 inch of water may make it difficult to open doors. To make the pressurized system reliable, safeguards must be provided in addition to the pressure fan. The normal practice is to use pressure fans for control rooms and switch rooms, or other 4-wall locations where a nonhazardous classification must be maintained. Normally, locations containing sources of hazard are not classified nonhazardous. On the other hand, enclosed locations containing sources of hazard are allowed to be classified nonhazardous if the sources of hazard are applied with fume hoods. Enclosed locations containing small sources of hazard which have a probability factor of below 10 are allowed to be classified partially non hazardous. (For probability factors see Fig. 1-9.)

6 2. Suction Fans While the pressure fan will prevent flammable gases or vapors from entering an enclosed space, the purpose of the suction fan is to dilute and remove the flammable gases and vapors from the space. The discharge from the location shall be to a safe exterior location without recirculation of the exhaust air. In addition, the location of the suction fan shall provide air movement across the floor to prevent accumulation of flammable gases or vapors. A suction fan is considered to give adequate ventilation if it provides sufficient air movement and its operation is continuous regardless of whether the flammable gases or vapors are confined or present in the air. The fan capacity must be such that the moving air is capable of diluting the flammable gases or vapors in the air to sufficiently low levels as follows. 1) For locations containing Class I flammable liquid operating at temperatures above their flash point, suction ventilation shall be considered sufficient if vapor air mixtures are diluted to concentrations below 1/4 of the LEL of the flammable product not exceeding a distance of 5 feet from the source of hazard. 2) The 25% safety margin is not required for Class II and Class III liquids. For Class II and III liquids with temperatures above flash point the dilution shall be slightly below the LEL unless the temperature is substantially above flash point. When the temperature is below flash point, dilution is allowed to be within the explosion range. Ventilation requirements for Class II and III liquid are different for storage and dispensing areas. (See "Storage and Dispensing of Flammable Liquid" in Chapter 3.) A practical substitute is to apply a minimum ventilation rate of 1 cfm per square foot of floor area. The question may arise as to what difference exists between a dilution slightly below the LEL and a dilution 25% below the LEL as far as explosion hazard is concerned. The only reason for requiring a dilution of flammable gases or vapors to below 1/4 of the LEL is that the 25% margin is capable of compensating for unfavorable environmental conditions such as some loss in air flow and reduction in dilution. Under favorable environmental conditions, a fan built with a 25% safety margin, i.e., a fan with 4 times greater output, will produce a dilution of flammable gases or vapors in the air to below 1/4 of the LEL. Under unfavorable environmental conditions such a fan will still be capable of producing sufficient dilution, that is, between the LEL and 1/4 of the LEL, but not below 1/4 of the LEL. This performance cannot be expected from a fan which is not provided with at 25% safety margin. When a 25% safety margin is not applied, the fan is build to produce a dilution slightly below the LEL. However, such a fan may not be capable of diluting the flammable gas or vapor to slightly below the LEL when the conditions are not favorable. For example, when there are too many obstructions in the path between the fan and the air intake opening or when the path is too far away from the source of hazard or when the fan is not properly

7 located with respect to the location of the air intake opening, some loss of air flow will occur. Fans with 25% built-in safety margins are more costly, but their higher cost is, for reasons explained above, well worth spending when Class I flammable products are involved. Suction ventilation does not always have to dilute a flammable gas or vapor to below the LEL if the vapor density of the gas or vapor is less than A gas or vapor with a density below 0.75 will rise quickly by itself when airborne. Economically, it might be desirable to have just enough ventilation that accelerates the upward flow of the gas or vapor when airborne. This is particularly true when fume hoods are involved. Factors such as atmospheric pressure and elevated temperatures will also have an impact on the dilution of a flammable gas or vapor. For example, at elevated temperatures, the LEL of the flammable product decreases. This decrease makes the LEL of the flammable product smaller. As a result of this a fan built without a safety margin will not be capable of diluting a flammable gas or vapor below the LEL because the point of safe dilution moves to above the LEL whereas a fan with a 25% safety margin is still capable of diluting the flammable gases or vapors to below the LEL. Since elevated temperatures will lower the LEL, it is necessary to correct the volume of air. The temperature borderline for the correction is 25O 0 F. Any temperature above 25O 0 F requires a correction of the volume of air produced. For example, if the required volume of air to reach a non hazardous concentration slightly below the LEL is 1.0 Pu CFM for temperatures of 25O 0 F or less, a correction factor of 0.7 must be applied for any temperature above 25O 0 F. The required volume of air is then, 1.0 * 0.7 = 1.43 CFM and not 1.0 CFM. The density of air also plays an important role in calculating the required volume of air. If the density is lowered, a correction factor must be applied. The weight of 1.0 cubic foot of air is lbs at 7O 0 F. If the temperature is in excess of 7O 0 F, the density of air changes and a correction factor must be applied to the volume of air in addition to the 0.7 correction factor. For example, if the actual temperature is 35O 0 F, the weight of air per cubic foot is less and needs to be corrected. If the weight of air per cubic foot at 7O 0 F is 1.0 Pu, then at 35O 0 F temperature it is Pu. The correction factor required is then 1 * = This correction factor can also be calculated by the following equation: = The volume of air required for diluting a particular flammable gas or vapor is calculated as follows. For example, if the danger level of a location containing a Class I flammable product must be reduced from Div. 1 to Div. 2, the volume

8 of air required from the suction fan must reduce the flammable concentration to below 25% of the LEL, which is calculated as follows. Consider a process equipment which contains a flammable liquid operating at F. In case the process equipment should accidentally release the flammable liquid, it is assumed that the emission rate of the liquid equals 30 gallons per hour. It is also assumed that not more than 80% per gallon of liquid will evaporate in 60 minutes. The effective quantity of liquid which will evaporate reduces to 24 gallons per hour. If it is also assumed that the explosion limits of the flammable vapor in air ranges from 1.25% to 10% at 7O 0 F, a vapor-air mixture, with a ratio of 98.75% of air and 1.25% of vapor, is too lean to produce an explosion. The volume of vapor produced from 1 gallon of solvent can be calculated from the specific gravity of the liquid and the vapor density of the vapor as follows. Cubic feet of vapor from 1 gallon of liquid is: A7 Va = 8.33 x Sp. 1 Gr. per, hour x VD x LEL x C where Va is vapor released in cubic feet per gallon of solvent per hour, 8.33 is the equivalent weight of 1 gallon of water in lbs., is the weight of 1 cubic foot of air in lbs. at 7O 0 F ambient temperature, VD is vapor density of the solvent, Sp. Gr. is the specific gravity of this solvent, and C is the correction factors for the LEL of the solvent vapor at elevated temperature (C=I for temperatures up to 25O 0 F and above 25O 0 F C=0.7) The density of air at 7O 0 F is inches of mercury atmosphere pressure and 50% relative humidity. If the specific gravity of the liquid is assumed to be 0.72 and the vapor density is 8, which is the relative weight of a volume of vapor to the weight of an equal volume of air under the same conditions, the cubic feet of vapor from 1.0 gallon of solvent is: o ^o ^ Q 79 Va = * ' = 10 cubic feet of vapor per hour x 8 Since there are 24 effective gallons of liquid released per hour, 24 gallons will produce 240 cubic feet of vapor per hour or 4 cubic feet per minute. The volume of air required to dilute the vapor to slightly below the LEL is 4 x ( ) = 320 cubic feet of air per minute. To reduce the level to below 1/4 of the LEL, the volume of air required is 4 x 320 = 1,280 cubic feet of air per minute. To compensate for the higher operating temperature, the following equation for correcting the air density will apply: c = (460+ T2) (460 + Tl)

9 where Tl is 7O 0 F ambient temperature and T2 is the actual dilution air temperature. If it is assumed that the vapor will not cool off and remain at a temperature of F, the correction factor for air is: c = ( ) = 15 ( ) Since the temperature is above 25O 0 F, the correction factor for the LEL is 0.7. Total correction factor is then = 2.143, and the total volume of air required is x 1,280 = 2,742 cubic feet of air per minute. If all elements are included in one equation, the outcome is also: 8.33/60 x 0.72 x 24 x 100 x 1.5 = 2>742 cubic feet per minute 0.075x8x1.25x However, although the explosion hazard is reduced in the location by diluting the flammable gas or vapor concentration in the air to below 1/4 of the LEL, the concentration may still cause a health hazard for personnel in the location. To prevent personnel from exposure to the toxicity of a contaminant in the air, the contaminant must be even more diluted. Therefore, it may be necessary to dilute the flammable gas or vapor concentration to far below 1/4 of the LEL. Therefore, when a health hazard could exist in the location, the safety margin should not be at 1/4 of the LEL but below the threshold limit value (TLV) of the flammable product. This is shown in the following example. Assume a sudden spillage of ethyl acetate liquid in a process area operating at a temperature of 17O 0 F. This flammable product has the following characteristics: Vapor Density = 3.0 Specific Gravity = 0.9 LEL = 2.0% TLV = 400 Safety Factor = 6 Assume a spillage of 22 gallons per hour or 0.36 gallons per minute. The evaporation rate is assumed to be 100%. The quantity of vapor released to the air from 0.36 gallons of liquid is: 8.33/60 x 0.9 x 0.36 n 0 u. f, f = 0.2 cubic feet of vapor per minute x 3.0 To reduce the vapor to safe concentrations the volume of air required is 0.2 x ( ) = 10 cubic feet of air per minute.

10 To dilute the vapor concentration to below 1/4 of the LEL, the air volume must be increased by 4 x 10 = 40 CFM. To reduce the contaminant in the air to below the TLV = 400, the capacity of the ventilation system must be increased to: 833/60 x 0.9 x 036 1,000,000 x 4 = 500 CFM x 6 = 2,000 CFM x 3.0 x 400 This is the ventilation that must be applied to maintain a healthy atmosphere. D. Approximate Location of Mechanical Ventilation It is important to establish the approximate location of mechanical ventilation. The location of a pressure fan in an enclosed space is normally not critical. Generally a pressure fan can be installed at any point in an enclosed space since the function of the pressure fan is to pressurize the location. The location requirements of a suction fan is more critical. The air flow of a suction fan must not only pass over the source of hazard but also dilute and remove the contaminant in the air. To establish the proper location for a suction fan, therefore, is important and should be given great consideration, with respect to the type and location of the source of hazard. The basic requirement for establishing the location for a suction fan depends on the vapor density of the flammable product. If the flammable gas or vapor in the air in the enclosed space is heavier than air, a suction fan located in the roof of the enclosed space as shown in Fig. 1-2OA has little or no effect on diluting and removing the flammable contaminant in the air. A flammable gas of vapor lighter-than-air is also not diluted and removed if the suction fan is located in the wall 12" from the floor. Therefore, it is necessary to establish the location of the suction fan on the basis of the vapor density of the flammable product. A vapor density greater than 1.0 is defined as heavier-than-air and a vapor density of less than 0.75 is defined as lighter-than-air. However a vapor density between 0.75 and 1.0 is not necessarily lighter-than-air. It can also be considered as heavier-than-air. The reason for this is that flammable gases or vapors with vapor densities between 0.75 and 1.0 are usually unstable in the air. If released from their confinement they may not rise instantly as they would if the vapor density is smaller than Gases and vapors with densities between 0.75 and 1.0 may move over the floor first before rising. During their initial presence in the air, and when they move over the floor, the gas or vapor acts as if it is heavier than air, and when the gases or vapors begin to rise they act as lighter-than-air. This behavior might take place if the diluting air is not brushing the source of hazard. (Brushing = ventilating air moving over and alongside the source of hazard.) If

11 the ventilating air is not brushing the source of hazard, a flammable gas or vapor with a vapor density between 0.75 and 1.0 must be considered as heavier-thanair. If, on the other hand, the air flow is brushing the source of hazard, the gas or vapor is considered lighter-than-air because in this case the gas or vapor will be caught instantly and sucked away by the moving air. As a result of these conditions, a flammable gas of vapor with a vapor density between 0.75 and 1.0 released into the air, which is not instantly caught by the moving ventilating air, will require a larger hazardous area. However, acetylene gas is the only known Class I flammable material with a vapor density between 0.75 and 1.0. All other Class I flammable gases or vapors have densities less than 0.75 or greater than The range between 0.75 and 1.0, therefore, can be deleted for any Class I gas or vapor other than acetylene. Consequently, the division between "lighter"- and "heavier"-than-air can be conveniently drawn at Any Class I gas or vapor with a density greater than 0.75, therefore, must be considered as heavier-than-air and with a vapor density below 0.75 as lighter-than-air. Since only two ranges of densities are left, the suction fan should be located in compliance with these two ranges. A suction fan, therefore, must be located in the roof or in the wall directly underneath the ceiling as shown in Fig. 1-2OA and B if the suction fan is to dilute and remove a Class I flammable material with a density of less than If, on the other hand, the fan is used for a flammable material with a density greater than 0.75 the fan must be located in the center of the wall or in the wall 12" from the floor as shown in Fig C and D. The choice between the two locations is a matter of whether the source of hazard will be brushed by the ventilating air or not. If, by placing the suction fan in the center of the wall, the source of hazard is not brushed as shown in Fig. 1-2OC, then the fan should be placed 12" from the floor where ventilating air will brush the source of hazard as shown in Fig. 1-2OD. However, if in spite of this, the fan should be located in the center of the wall where the source of hazard is not brushed by ventilating air, airborne flammable material will travel a long distance, covering a large floor area before reaching safe concentrations as shown in Fig. 1-2OC. Under such a condition a suitable hazardous boundary cannot be selected from Table 1-4 because the boundary is too small. The listed boundaries in Table 1-4 are developed on the basis of optimum dilution and removal of airborne flammable gases and vapors. In a case where the boundary selected from Table 1-4 is too small, because ventilation is not brushing the source of hazard, it is necessary to compensate for the too small size. A larger boundary size than listed in Table 1-4 must be applied. Larger boundary sizes are 5, 10, 15, 20, and 25 feet respectively for 3, 5, 10, 15 and 20 feet standard sizes listed in Table 1-4. Although a source of hazard which is brushed by ventilating air may produce a gas or vapor that also travels a long distance as in Fig. 1-2OC, the gas or vapor will usually reach safe

12 concentration much closer to the source of hazard as shown in Fig. 1-2OD and E. In these cases, the hazardous boundary in Table 1-4 is not too small. The selected locations for the suction fans may not always be the best solution if the fans are obstructed by ducts, pipes, or other unavoidable items. In these cases, it is necessary to use judgment with consideration given to all factors discussed herein. The correct location for a suction fan may also not be the total answer for solving air flow problems if the location needs to be cooled or when mini sources of hazard are involved. For locations containing mini sources of hazard an intrinsically safe or explosion-proof gas detector could be used instead of a continuously operating suction fan. The gas detector should be set at below 25% of the LEL and must activate a standby suction fan and an alarm only when airborne gas is detected. However, where normally a sufficiently ventilated area containing mini sources of hazard may be classified Div. 2, if operation of the fan must depend on the gas detector, it is necessary that part of the location, in particular an area surrounding the mini sources of hazard, be classified Div. 1. It may not always be possible to locate the suction fan in the wall of a 4- wall building because of adjacent rooms or an external building attached to the walls. In these cases, where a suction fan is used with a flammable material having a vapor density greater than 0.75 which cannot be located in any of the four walls, the suction fan should be located in the roof as an alternate solution. This suction fan must be connected to a duct system with an air inlet opening that allows incoming ventilating air to brush the source of hazard as shown in Fig. 1-2OE. In a 3-wall building where a suction fan with or without a duct system cannot be located in the wall opposite the open perimeter of the 3-wall building, a roof mounted pressure fan may be used as an alternate solution instead of a suction fan as shown in Fig. 1-2OF. It must be born in mind that the mechanical ventilation as shown in the illustrations in Section II are not intended to show the required location of the mechanical fan. These locations merely indicate the presence of forced ventilation. They do not have any bearing on their actual location. The actual location for the mechanical ventilation in the illustrations should be obtained from Fig and as explained herein. For example in Fig. A-8 in Section II, sources of hazard are located in a 4- wall building. According to Fig. A-8, the vapor density of the flammable product is heavier-than-air, which means that the vapor density is greater than For the enclosed location to be classified Div. 2 with sizes as shown in Fig. A-8, forced ventilation is necessary. This is shown in Fig. A-8 by means of a mechanical fan in the roof. Because the 4-wall building contains sources of hazard and because the vapor density is greater than 0.75 the mechanical fan must be of the suction type and its location should be in the wall as indicated in Fig.

13 SUCTION FAN 4 WALL BUILDING (VD = GREATER THAN 0.75) SOURCE OF HAZARD NOT BRUSHING SUCTION FAN SUCTION FAN SOURCE OF HAZARD BRUSHING 4 WALL BUILDING (VD = GREATER THAN 0.75) SOURCE OF HAZARD NOT BRUSHING SAFE CONCENTRATION REACHED HERE FIG APPROXIMATE LOCATION OF MECHANICAL VENTILATION

14 4 WALL BUILDING (VD = GREATER THAN 0.75) SOURCE OF HAZARD AIR FLOW SUCTION FAN BRUSHING SAFE CONCENTRATION REACHEDHERE SOURCE OF HAZARD AIR FLOW 4 WALL BUILDING (VD = GREATER THAN 0.75) DUCT SUCTION FAN BRUSHING SAFE CONCENTRATION REACHED HERE 3 WALL BUILDING (VD = GREATER THAN 0.75) PRESSURE FAN SOURCE OF HAZARD FIG APPROXIMATE LOCATION OF MECHANICAL VENTILATION

15 1-2OD or E and not as shown in Fig. A-8. The replacement air which enters the location must be at least equal to the volume of air sucked out of the location. Multiple inlet points are usually the best way to provide uniformity of make up air. During winter, when the warmer air is removed from the location, it is best to mix the cooler make up air with the warmer air in the location. E. Demarcation Line A demarcation line is an imaginary line which is exclusively used for mini sources of hazard associated with lighter-than-air gases. The demarcation line should only be applied for small process plants associated with Class I flammable gases which are processed, handled, and/or transmitted. For small process plants the gas is normally stored in a single container with a gas content of not more than 400 cf (cf = cubic feet of gas at psia and 7O 0 F). The gas in the container normally consists of hydrogen or process gas with more than 30% hydrogen or it consists of a lighter-than-air gas of equivalent hazard. Safety dictates that the container be located in a free ventilated area. Process equipment is normally located indoors. Because the process area is small the piping system connected to the container required for transporting the gas to the process plant has a size not greater than 1/4" or 1/2". Because of this small size the components in the piping system are of the mini type and may consist of one or more valves, manifolds, screwed fittings, pressure reducers, gauges, etc. Only components with non-seal type pipe connections are considered the actual source of hazard, not the piping itself. The demarcation line is normally drawn horizontally directly below the mini sources of hazard as shown in Fig Its purpose is to divide the hazardous area for each of the individual sources of hazard into two zones which are different in size. One zone is small and located below the demarcation line. The other zone is large and located above the demarcation line. The smaller zone needs to have a radius of 3 or 5 feet. The larger zone needs to have a size of at least 15 feet or more. The reason for the different sizes is because hydrogen gas will instantly rise once it is released into the atmosphere. The gas below the demarcation line, therefore, needs a small hazardous area while above the demarcation line it needs a much larger hazardous area. If the source of hazard is brushed by the ventilating air, the air flow will oppose the flow of gas if it escapes underneath a source of hazard. It is therefore virtually impossible for a gas escaping from underneath a source of hazard to be ignited by an ignition source if the ignition source is 3 feet away from the source of hazard. If brushing does not occur the gas underneath the source of hazard will not be opposed and may, therefore, contaminate the area beyond the 3 feet safe distance. Since contamination may

16 occur the distance of 3 feet should be extended to 5 feet. Pressure in the system also plays an important role in the size of the safe distance, in particular when the gas escapes from underneath the source of hazard. At low or moderate pressure with the source of hazard being brushed by ventilating air, the safe distance below the source of hazard should be 3 feet. If the source of hazard is not brushed by ventilating air, or if the system pressure is high, the safe distance must be 5 feet instead of 3 feet. (Brushing = air movement over and along side the source of hazard.) Above the demarcation line the danger zone is a vertical cone, which is required to have a minimum safe distance of 15 feet. The width of the cone is normally much smaller than 15 feet. It is a function of the speed at which the gas is forced upwards. Sometimes it is rather difficult to determine the width of the cone and whether an electrical equipment above the demarcation line is within the cone. In these cases where it is difficult to determine whether an electrical equipment will fall within the cone or not, it is safer to consider the electrical equipment within the cone. Equipment "A" in Fig must be suitable for a Div. 2 location because the electrical equipment is within the cone. Equipment "B" is located outside the cone but since it is close to the cone it is safer to consider it within the cone. Equipment "C" and "D" below the demarcation line may be of the general purpose type since they are below the 3 or 5 feet boundary. Equipment "E" must be explosion-proof because it is within the 3 feet danger zone. Any electrical equipment of the heat producing type located within a distance of 3 feet from the source of hazard must be of the explosion-proof type. Equipment "F" which is too far from the hazardous cone may also be of the general purpose type. For lighter-than-air Class I flammable gases which are processed, handled and/or transmitted, it is mandatory that an electric exhaust fan be used in the roof. A roof opening in lieu of a electric suction fan is not permitted unless a fumehood with forced ventilation is used directly above the source of hazard in which case a roof opening must also be applied. Since hydrogen gas, if airborne, will rise quickly, it is not necessary to dilute the gas to below 1/4 of the LEL. This is true for fume hoods above the source of hazard. Some suction air is necessary to accelerate the rising hydrogen gas to the fume hood. A fume hood located directly above the source of hazard in a high bay area may need only a low velocity of air to assist the upward movement of the airborne hydrogen gas. Above the fume hood it is sufficient to have a ventilating opening in the building ceiling and roof. Indoor locations with low ceilings not using fume hoods may be provided with an electrically operated fan which only purpose is to accelerate upward movement of airborne gases. Gas detection for activating the operation of an exhaust fan shall not be used in lieu of a permanently operating electrical exhaust fan.

17 The extent of the danger zone for a single gas container located in a freely ventilated location is not the same as for the components in the piping system indoors. Because of the high pressure in the container, 2000 psi and more, a Div. 2 zone of 5-feet radius instead of 3-feet radius is required below the demarcation line and also a Div. 2 cone of 15-feet minimum length is required above the demarcation line. Dilution of lighter-than-air gases by suction ventilation shall be as follows: as a general rule for Class I flammable gases or vapors: below 1/4 of the LEL. However, for a single gas container with a 1/4" or 1/2" piping system and mini sources of hazard, small quantities of the lighter-than-air gases need not be diluted to below 1/4 of the LEL. With a roof exhaust fan and a source of hazard that is brushed by ventilating air, air quantities are sufficient if the gas is slightly diluted to below the LEL. If a fume hood is used, a low draft of ventilating air is considered sufficient if it assists the upward flow of the gas and all airborne gas particles are caught by the fume hood. The gas must exit in a nonhazardous area via a roof mounted vent stack of at least 7 feet high. The application of ventilating air is not required if all components in the piping system are provided with high integrity fittings and are pressure tested. F. Safeguards Safeguards are required because mechanical ventilation can break down. If the mechanical ventilation breaks down, the location becomes instantly hazardous. Therefore, safeguards are required to either prevent the failure of the ventilating system or to warn against the failure of the ventilation system. Failure of safeguards are not considered because of their low wear and because they normally are powered from different circuits than the ventilating system. Safeguards are normally necessary for enclosed spaces which are required to be classified non hazardous and which do not contain a source of hazard. Normally, no safeguards are necessary for locations which contain sources of hazard. However, if a greater sense of security is required, safeguards are applied for some dispensing areas and for locations which are classified partially non hazardous. Generally, only the upper part of the partially classified location is classified non hazardous. Safeguards are also necessary for spaces which do require fume hoods in small process areas and spaces which are located above or below a hazardous area. Safeguards are generally required if the space has four walls and is provided with sufficient ventilation to obtain a non hazardous environment in the space. There are two types of safeguards available: type "A" and type "B". The type "A" safeguard is a redundant ventilating system which operates on loss of ventilation. This type is, or may be, provided with an audible and visible alarm system. The type "B" safeguard is only an audible and visible alarm system which

18 SOURCE OF HAZARD DIV. 2 CONE DEMARCATION LINE INLET R 1 = 3 FEET RADIUS R 2 = 5 FEET RADIUS M = ELECTRICAL EQUIPMENT 0 OF THE HEAT PRODUCING TYPE FIG DEMARCATION LINE X-X

19 operates on loss of ventilation. The loss of air-flow must be detected by an air vane or by a differential air pressure control sensor which must operate the required safeguard. A type "A" safeguard is required if a nonhazardous space is located in a Div. 1 area. The basis for this requirement is the continuous or frequent presence of flammable gases or vapors in the surrounding Div. 1 area. A type "B" safeguard is required if the nonhazardous space is located in a Div. 2 area. The basis for this requirements is the occasional presence of flammable gases or vapors in the Div. 2 area as a result of failure or rupture of the source of hazard in the Div. 2 area. G. Wiring Diagrams for Safeguards Two simplified wiring diagrams are shown in Fig. 1-22A, one for type "A" safeguard, and one for type "B" safeguard. The wiring diagram for the type "B" safeguard is simple and straight forward. When switch "Sl" is closed to energize fan "Fl", the alarm will go off momentarily. The alarm in circuit #1 is controlled by an air switch "AS", an air vane normally located in the air duct. Once the fan "Fl" operates, the air flow will force the contacts of the air switch "AS" to open. If a failure of the fan "Fl" occurs, switch "AS" will close its contacts and set off the alarm. The alarm can be silenced by opening switch "Sl". For the alarm to operate on loss of power, each circuit breaker can best be connected to its own power source. The wiring diagram for type "A" safeguard is more complex. As shown in the wiring diagram of circuit #1, fan "Fl" is powered by one power source while the redundant fan "F2" in circuit #2 is powered from another source. Both fans must operate under the following conditions. (1) The circuits must allow each fan to operate independently. (2) Under air flow failure of fan "Fl", the alarm must go off and start fan "F2". (3) Under loss of power in circuit #1, the alarm must also go off and start fan "F2". The following is a detailed description of the operation of fans "Fl" and "F2" for safeguard "A" as shown in Fig. 1-22A. Condition 1 a. Manual Starting To start fan "Fl" or fan "F2" independently depress the associated start button in circuit #1 or circuit #2 respectively. At the same moment that fan "F2" is being manually started, relay "R4" will disable the alarm and circuit #1 so that

20 fan "Fl" cannot be started. Fan "Fl" can only be started when relays "R4" and "TDC" are not activated. If these relays are not activated, relays "Rl," "R2," and "R3" will be energized when depressing the start button in circuit #1. Relay "R2" in circuit #1 will lock itself into its power source and start fan "Fl." Relay "R3" in circuit #2 will also lock itself into its power source and will light pilot light "L" indicating that circuit #2 is in the standby mode. (This pilot light and relay "R3" will remain in operation until the stop button in circuit #2 is depressed.) When relays "R2" and "R3" are energized, fan "F2" is prevented from automatically coming on. As a result of the operation of fan "Fl", air switch "AS" in circuit #2 will open its contacts. This switch remains open as long as there is air flow from fan "Fl" preventing the time delay relay from operating. If there is no air flow from fan "Fl", switch "AS" remains closed and eventually will activate the alarm and fan "F2". b. Manually Stopping To stop fan "Fl" in circuit #1, depress the stop button in circuit #1. Relay "R2" in circuit #1 will be de-energized causing fan "Fl" to stop. When fan "Fl" stops because relay "R2" is de-energized, contact "R2" in circuit #2 will make fan "F2" operate. To prevent operation of fan "F2", press the stop button in circuit #2 before stopping fan "Fl." Depressing the stop button in circuit #2 will deactivate relay "R3" which otherwise would allow fan "F2" to start operating. Depressing the stop button in circuit #2 will also de-energize the standby pilot light. Standby Mode a. Mechanical Failure of Fan "Fl" For circuit #2 to be in the standby mode, relay "R3" and associated pilot light must be energized. This is accomplished automatically by starting fan "Fl" by depressing the start button in circuit #1: If a mechanical failure of fan "Fl" should occur while in operation, relay "TDC" will be energized as a result of the closure of switch "AS." Switch "AS" will close due to loss of air flow. After some time delay, relay "TDC" will sound the alarm and disable circuit #1. The disabling of circuit #1 causes the closure of contact "R2" in circuit #2 which causes fan "F2" to operate. Since switch "AS" does not operate under air flow from fan "F2," the alarm will continue to operate. The alarm can be silenced by operating the start/silencer button in circuit #2 which in turn activates the flasher. The flasher is a reminder that the alarm is being disabled and fan "F2" is on. To re-energize circuit #1, it is necessary that relay "R4" in circuit #2 be deenergized first. This is accomplished by depressing the stop button in circuit #2. b. Power Failure in Circuit #1 If a loss of power should occur in circuit #1 during the operation of fan

21 "Fl/' circuit #1 will be de-energized causing the closure of contact "R2" in circuit #2. (Contact "R3" in circuit 2 was closed during startup of fan "Fl/') The closure of contact "R2" causes fan "F2" to operate. If air switch "AS" in circuit #2 has entered its rest mode, because it is without air flow from fan "Fl," it energizes relay "TDC" in circuit #2. After some time delay, the "TDC" relay will initiate the alarm. Fan "F2" remains in operation until it is manually shut down by depressing the stop button in circuit #2. If power to circuit #1 is restored, circuit #1 cannot be re-energized as long as relay "R4" and/or "TDC" is in operation. If relay "TDC" causes the alarm to sound, the alarm can be silenced with the start/silence button in circuit #2. The alarm can also be silenced by depressing the stop button in circuit #2, but this causes fan "F2" to stop and will also cause the disabling of the standby mode. If fan "F2" should remain in operation but the alarm stopped, only the silence button should be used. If a differential pressure switch "DPS" should be used instead of air vane "AS," it is necessary that circuit #2 be modified. The modification consists of an addition of one NC contact "R2" which must be wired in parallel with the "DPS" switch between points "A" and "B" in circuit #2. This contact "R2" is necessary to override the opening mode of the "DPS" switch when fan "F2" starts to operate. A "DPS" switch normally is not located in an air duct but in the room and therefore will operate at any time when fan "Fl" or fan "F2" are on. Without this additional "R2" contact, the alarm may sound only momentarily when fan "F2" is coming on. The wiring diagram in Fig. 1-22A shows both fans "Fl" and "F2" and the control circuit for one and the same voltage level. If, however, a different voltage level is required for the control circuit, or if the hp ratings of the fans are too high for a 120 volt circuit, the wiring diagram in Fig. 1-22B should be applied. In this wiring diagram, the control circuit is separated from the fan circuits. A 24 Vac voltage could be used for the control circuit, while the voltage source for the fans could be 208 or 460 volts. In this schematic, an auxiliary contact "C2" is wired in parallel with the NC contacts "Rl" in circuit #2. This "C2" contact is optional and is located in the fan "F2" starter. It prevents interruption of current flow to fan "F2" when the start button in circuit #1 should be depressed when both contacts "R4" and "TDC" in circuit #1 are in their rest mode.