Respirators are routinely found in chemical laboratories and plants. Respirators should be used only
on a temporary basis, until regular control methods can be implemented;
as emergency equipment, to ensure worker safety in the event of an accident;
as a last resort, in the event that environmental control techniques are unable to provide satisfactory protection.
Respirators always compromise worker ability. A worker with a respirator is unable to perform or respond as well as a worker without one. Various types of respirators are listed in Table 3-11.
Respirators can be used improperly and/or can be damaged to the extent that they do not provide the needed protection. OSHA and NIOSH have developed standards for using respi- rators,l"ncluding fit testing (to ensure that the device does not leak excessively), periodic in-
13NIOSH Respirator Decision Logic, DHHS-NIOSH Publication 87-1-8 (Washington, DC: US Depart- ment of Health and Human Services, May 1987).
3-4 Industrial Hygiene: Control 97
Table 3-1 1 Respirators Useful to Chemical Industry Example of
TY Pe commercial brand Limitations
Mouth and nose MSA Dustfoe@ 88' 0, > 19.5%; single use; PEL > 0.05 mg/m3 dust mask
Mouth and nose with MSA Comfo Classic 0, > 19.5%; GMA cartridge (black) for concen- chemical cartridge cartridge trations less than the IDLH concentration for
organic vapors; GMC cartridge (orange) for concentrations less than the IDLH concentra- tion for Cl,, HC1, and SO,
Full face mask with MSA Industrial 0, > 19.5%; type N canister for concentrations chemical canister Canister, Gas Mask3 less than 100 times PEL and less than the IDLH
concentration for acid gases, CO, ammonia, and organic vapors; escape concentrations of 2% for acid gases, CO, and organic vapors and 3% for ammonia; escape capacity less than 6 min Self-contained breathing MSA MMR XtremeB Good for toxic and noxious gases with concen- apparatus (SCBA) Air Mask4 trations below and above the IDLH concentra-
tion. Capacity between 30 and 60 min per specifications
'MSA Home Page 2000, Air-purifying Respirators, Conventionally Maintained, DustfoeB Respirator (Pittsburgh, PA:
MSA International).
2MSA Home Page 2000, Air-purifying Respirators, Conventionally Maintained, Comfo Classic.
'MSA Home Page 2000, Air-purifying Respirators, Conventionally Maintained, Replacement Canisters for Gas Masks.
4MSA Home Page 2000, Supplied Air Respirators, Self-Contained Breathing Apparatus.
spections (to ensure that the equipment works properly), specified use applications (to ensure that the equipment is used for the correct job), training (to ensure that it is used properly), and record keeping (to ensure that the program is operating efficiently). All industrial users of res- pirators are legally bound to understand and fulfill these OSHA requirements.
Ventilation
For environmental control of airborne toxic material the most common method of choice is ventilation, for the following reasons:
Ventilation can quickly remove dangerous concentrations of flammable and toxic materials.
Ventilation can be highly localized, reducing the quantity of air moved and the equip- ment size.
Ventilation equipment is readily available and can be easily installed.
Ventilation equipment can be added to an existing facility.
98 Chapter 3 Industrial Hygiene
The major disadvantage of ventilation is the operating cost. Substantial electrical energy may be needed to drive the potentially large fans, and the cost to heat or cool the large quan- tities of fresh air can be large. These operating costs need to be considered when evaluating alternatives.
Ventilation is based on two principles: (1) dilute the contaminant below the target con- centration, and (2) remove the contaminant before workers are exposed.
Ventilation systems are composed of fans and ducts. The fans produce a small pressure drop (less than 0.1 psi) that moves the air. The best system is a negative pressure system, with the fans located at the exhaust end of the system, pulling air out. This ensures that leaks in the system draw air in from the workplace rather than expel contaminated air from the ducts into the workplace. This is shown in Figure 3-5.
There are two types of ventilation techniques: local and dilution ventilation.
E x h a u s t
2 n d F l o o r L e a k a g e O u t o f D u c t s
B l o w e r I s t F l o o r / H o o d I n t a k e
P o s i t i v e P r e s s u r e V e n t i l a t i o n
E x h a u s t
A
N e g a t i v e P r e s s u r e V e n t i l a t i o n
----
2 n d F l o o r --z----
1 s t F l o o r
Figure 3-5 The difference between a positive and a negative pressure ventilation system. The negative pressure system ensures that contaminants do not leak into workplace environments.
--.
L e a k a g e i n t o D u c t s H o d I n t a k e -s--3-4 Industrial Hygiene: Control
Local Ventilation
The most common example of local ventilation is the hood. A hood is a device that ei- ther completely encloses the source of contaminant and/or moves the air in such a fashion as to carry the contaminant to an exhaust device. There are several types of hoods:
An enclosed hood completely contains the source of contaminant.
An exterior hood continuously draws contaminants into an exhaust from some distance away.
A receiving hood is an exterior hood that uses the discharge motion of the contaminant for collection.
A push-pull hood uses a stream of air from a supply to push contaminants toward an ex- haust system.
The most common example of an enclosed hood is the laboratory hood. A standard labo- ratory utility hood is shown in Figure 3-6. Fresh air is drawn through the window area of the hood and is removed out the top through a duct. The airflow profiles within the hood are highly dependent on the location of the window sash. It is important to keep the sash open a few inches, minimally, to ensure adequate fresh air. Likewise, the sash should never be fully opened be- cause contaminants might escape. The baffle at the rear of the hood ensures that contaminants are removed from the working surface and the rear lower corner.
Another type of laboratory hood is the bypass hood, shown in Figure 3-7. For this design bypass air is supplied through a grill at the top of the hood. This ensures the availability of fresh
Figure 3-6 Standard utility laboratory hood. Airflow patterns and control velocity are depend- ent on sash height. Source: N. Irving Sax, Dangerous Properties of lndustrial Materials, 4th ed.
(New York: Van Nostrand Reinhold, 1975), p. 74. Reprinted by permisdon of John Wiley &
Sons, Inc.
100 Chapter 3 Industrial Hygiene
Figure 3-7 Standard bypass laboratory hood. The bypass air is controlled by the height of the sash. Source: N. Irving Sax, Dangerous Properties of Industrial Materials, 4th ed. (New York:
Van Nostrand Reinhold, 1975), p. 75. Reprinted by permission of John Wiley & Sons, Inc.
air to sweep out contaminants in the hood. The bypass air supply is reduced as the hood sash is opened.
The advantages of enclosed hoods are that they completely eliminate exposure to workers, require minimal airflow,
provide a containment device in the event of fire or explosion, and provide a shield to the worker by means of a sliding door on the hood.
The disadvantages of hoods are that they limit workspace and
can be used only for small, bench-scale or pilot plant equipment.
Most hood calculations assume plug flow. For a duct of cross-sectional area A and aver- age air velocity E (distanceltime), the volume of air moved per unit time Q, is computed from
For a rectangular duct of width Wand length L, Q, is determined using the equation
Q, =
LWU.
(3-26)3-4 Industrial Hygiene: Control 101
E x h a u s t
Q, = Volumetric Flow Rote, ~ o l u m e / T i r n e
Figure 3-8 Determining the total
L = Length volumetric air flow rate for a box-
W = Width type hood. For general operation
a control velocity of between 80 and
i = R e q u i r e d C o n t r o l V e l o c i t y 120 feet per minute (fpm) is desired.
Consider the simple box-type enclosed hood shown in Figure 3-8. The design strategy is to pro- vide a fixed velocity of air at the opening of the hood. This face or control velocity (referring to the face of the hood) ensures that contaminants do not exit from the hood.
The required control velocity depends on the toxicity of the material, the depth of the hood, and the evolution rate of the contaminant. Shallower hoods need higher control veloci- ties to prevent contaminants from exiting the front. However, experience has shown that higher velocities can lead to the formation of a turbulent eddy from the bottom of the sash; backflow of contaminated air is possible. For general operation a control velocity between 80 and 120 feet per minute (fpm) is suggested.
Instruments are available for measuring the airflow velocity at specific points of the hood window opening. Testing is an OSHA requirement.
The airflow velocity is a function of the sash height and the blower speed. Arrows are fre- quently used to indicate the proper sash height to ensure a specified face velocity.
Design equations are available for a wide variety of hood and duct shapes.14
141ndustrial Ventilation: A Manual of Recommended Practice, 19th ed. (Cincinnati: American Conference of Governmental Industrial Hygienists, 1986).
102 Chapter 3
.
Industrial HygieneTable 3-12 Nonideal Mixing Factor k for Various Dilution Ventilation Conditions1 Mixing factor:
Vapor Dust Ventilation condition
concentration concentration
( P P ~ ) (mppcf) Poor Average Good Excellent
over 500 50 117 114 113 112
101-500 20 118 115 114 113
0-100 5 1/11 118 117 116
'N. Irving Sax, Dangerous Properties, 6th ed., p. 29. The values reported here are the reciprocal of Sax's values.
Other types of local ventilation methods include "elephant trunks" and free-hanging can- opies and plenums. The elephant trunk is simply a flexible vent duct that is positioned near a source of contaminant. It is most frequently used for loading and unloading toxic materials from drums and vessels. Free-hanging canopies and plenums can be either fixed in position or attached to a flexible duct to enable movement. These methods will most likely expose work- ers to toxicants, but in diluted amounts.
Dilution Ventilation
If the contaminant cannot be placed in a hood and must be used in an open area or room, dilution ventilation is necessary. Unlike hood ventilation, where the airflow prevents worker exposure, dilution ventilation always exposes the worker but in amounts diluted by fresh air.
Dilution ventilation always requires more airflow than local ventilation; operating expenses can be substantial.
Equations 3-9,3-12, and 3-14 are used to compute the ventilation rates required. Table 3-12 lists values for k, the nonideal mixing factor used with these equations.
For exposures to multiple sources the dilution air requirement is computed for each indi- vidual source. The total dilution requirement is the sum of the individual dilution requirements.
The following restrictions should be considered before implementing dilution ventilation:
The contaminant must not be highly toxic.
The contaminant must be evolved at a uniform rate.
Workers must remain a suitable distance from the source to ensure proper dilution of the contaminant.
Scrubbing systems must not be required to treat the air before exhaust into the environment.
Example 3-10
Xylene is used as a solvent in paint. A certain painting operation evaporates an estimated 3 gal of xylene in an 8-hr shift. The ventilation quality is rated as average. Determine the quantity of dilu- tion ventilation air required to maintain the xylene concentration below 100 ppm, the TLV-TWA.
Suggested Reading 103
Also, compute the air required if the operation is carried out in an enclosed hood with an opening of 50 ft2 and a face velocity of 100 ftlmin. The temperature is 77°F and the pressure is 1 atm. The specific gravity of the xylene is 0.864, and its molecular weight is 106.
Solution
The evaporation rate of xylene is
0.1337 ft" 62.4 Ib,
Qm =
(g ) (&) (T)
(0.864)From Table 3-12, for average ventilation and a vapor concentration of 100 ppm, k = 118 = 0.125.
With Equation 3-9, we solve for Q,:
= 13,300 ft3/min required dilution air.
For a hood with an open area of 50 ft2, using Equation 3-25 and assuming a required control veloc- ity of 100 fpm, we get
The hood requires significantly less airflow than dilution ventilation and prevents worker exposure completely.
Suggested Reading
Industrial
Hygiene
Lewis J. Cralley and Lester V. Cralley, eds., Industrial Hygiene Aspects of Plant Operations, v. 1-3 (New York: Macmillan, 1984).
J. B. Olishifski, ed., Fundamentals of Industrial Hygiene, 2d ed. (Chicago: National Safety Council, 1979).
B. A. Plog, ed., Fundamentals of Industrial Hygiene, 3d ed. (Chicago: National Safety Council, 1988).
N. Irving Sax, Dangerous Properties of Industrial Materials, 6th ed. (New York: Van Nostrand Reinhold, 1984), sec. 2 and 3.
Richard A. Wadden and Peter A. Scheff, eds., Engineering Design for the Control of Workplace Hazards (New York: McGraw-Hill, 1987).
A. C. Wentz, Safety, Health, and Environmental Protection (Boston: WCBIMcGraw-Hill, 1998).
104 Chapter 3 Industrial Hygiene
Ventilation
Industrial Ventilation: A Manual of Recommended Practice, 19th ed. (Cincinnati: American Conference of Governmental Industrial Hygienists, 1986).
Wadden and Scheff, Engineering Design, ch. 5.
Problems
3-1. Determine (a) whether the following chemicals are covered under the PSM regulation (29 CFR 1910.119) and (b) their threshold quantities: acrolein, hydrogen chloride, phos- gene, propane, ethylene oxide, and methanol.
3-2. Determine (a) whether the following chemicals are covered under the PSM regulation and (b) their threshold quantities: ammonia (anhydrous), hydrogen selenide, formalde- hyde, methane, and ethanol.
3-3. Determine whether the following chemicals (a) are covered under the RMP (40 CFR 68.130) and (b) are listed as toxic or flammable. If they are listed, (c) what are their threshold quantities? The chemicals are acrolein, hydrogen chloride, phosgene, propane, ethylene oxide, and methanol.
3-4. Determine whether the following chemicals (a) are covered under the RMP and (b) are listed as toxic or flammable. If they are listed, (c) what are their threshold quantities? The chemicals are ammonia (anhydrous), hydrogen selenide, formaldehyde, methane, and ethanol.
3-5. In reviewing the results of Problems 3-1 to 3-4, describe why the threshold quantities are lower for the PSM-regulated chemicals than for the RMP-regulated chemicals.
3-6. Review the details of the RMP (40 CFR 68), and describe the three program categories that are used for consequence modeling.
3-7. Review the details of the RMP (40 CFR 68), and describe the endpoint parameters for consequence analyses for the worst-case scenarios.
3-8. Review the details of the RMP (40 CFR 68), and describe the endpoint parameters for consequence analyses for the alternative case scenarios.
3-9. Review the RMP (40 CFR 68) to determine the conditions that need to be used for dis- persion modeling for the worst-case scenarios.
3-10. Review the RMP (40 CFR 68) to determine the conditions that need to be used for dis- persion modeling for the alternative case scenarios.
3-11. Describe several typical alternative case scenarios for an RMP study.
3-12. A process plant inventories the following chemicals: vinyl chloride, methyl ethyl ketone, ethylene oxide, styrene, and cyclohexane. Determine the hazards associated with these chemicals. What additional information might you request to perform an appropriate as- sessment of the risk associated with these chemicals?
3-13. The TLV-TWA for a substance is 150 ppm. A worker begins a work shift at 8 A.M. and completes the shift at 5 P.M. A one-hour lunch break is included between 12 noon and 1 P.M., where it can be assumed that no exposure to the chemical occurs.
Problems 105
The data were taken in the work area at the times indicated. Has the worker ex- ceeded the TLV specification?
Concentration Time ( P P ~ )
8:10 A.M. 110
9:05 A.M. 130
10:07 A.M. 143
11:20 A.M. 162
1212 P.M. 142
1:17 P.M. 157
2:03 P.M. 159
3:13 P.M. 165
4:01 P.M. 153
5:00 P.M. 130
3-14. Air contains 4 ppm of carbon tetrachloride and 25 ppm of 1,l-dichloroethane. Compute the mixture TLV, and determine whether this value has been exceeded.
3-15. A substance has a TLV-TWA of 200 ppm, a TLV-STEL of 250 ppm, and a TLV-C of 300 ppm. The data in the following table were taken in a work area:
Concentration Time ( P P ~ ) 8:01 A.M.
9:17 A.M.
10:05 A.M.
11:22 A.M.
12:08 P.M.
1:06 P.M.
2:05 P.M.
3:09 P.M.
400 P.M.
5:05 P.M.
A worker on an 8-hour shift is exposed to this toxic vapor. Is the exposure within com- pliance? If not, what are the violations? Assume that the worker is at lunch between the hours of 12 noon to 1 P.M. and is not exposed to the chemical during that time.
3-16. Sax" provided the following working equation for determining the dilution air require- ments resulting from evaporation of a solvent:
(3.87 X 108)(lb,of liquid evaporatedlmin) CFM =
(molecular weight)(TLV)(k) ,
where CFM is the ft3/min of dilution air required. Show that this equation is the same as Equation 3-9. What assumptions are inherent in this equation?
"N. I. Sax, Dangerous Properties of Industrial Materials, 6th ed. (New York: Van Nostrand Reinhold, 1984), p. 28.
Chapter 3 Industrial Hygiene
Problems 3-17 through 3-22 apply to toluene and benzene. The following data are wail- able for these materials:
Benzene (C6H6) Toluene (C,H,)
Molecular weight 78.11 92.13
Specific gravity 0.8794 0.866
TLV ( P P ~ ) 10 50
Saturation vapor pressures:
where Pmt is the saturation vapor pressure in mm Hg, T is the temperature in K, and A, B, and C a r e the constants, given by the following:
Benzene 15.9008 2788.51 -52.36 Toluene 16.0137 3096.52 -53.67
3-17. Compute the concentration (in ppm) of the saturated vapor with air above a solution of pure toluene. Compute the concentration (inppm) of the equilibriumvaporwith air above a solution of 50 mol % toluene and benzene. The temperature is 80°F and the total pres- sure is 1 atm.
3-18. Compute the density of pure air and the density of air contaminated with 100 ppm ben- zene. Do the densities of these two gases differ enough to ensure a higher concentration on floors and other low spots? The temperature is 70°F and the pressure is 1 atm.
3-19. Equations 3-12 and 3-14 represent the evaporation of a pure liquid. Modify these equa- tions to represent the evaporation of a mixture of ideal miscible liquids.
3-20. Benzene and toluene form an ideal liquid mixture. A mixture composed of 50 mol % ben- zene is used in a chemical plant. The temperature is 80°F, and the pressure is 1 atm.
a. Determine the mixture TLV
b. Determine the evaporation rate per unit area for this mixture.
c. A drum with a 2-in-diameter bung is used to contain the mixture. Determine the ven- tilation rate required to maintain the vapor concentration below the TLV. The venti- lation quality within the vicinity of this operation is average.
3-21. A drum contains 42 gal of toluene. If the lid of the drum is left open (lid diameter = 3 ft), determine the time required to evaporate all the toluene in the drum. The temperature is 85°F. Estimate the concentration of toluene (in ppm) near the drum if the local venti- lation rate is 1000 ft3/min. The pressure is 1 atm.
3-22. A certain plant operation evaporates 2 pintlhr of toluene and 1 pint18-hr shift of ben- zene. Determine the ventilation rate required to maintain the vapor concentration below the TLV. The temperature is 80°F, and the pressure is 1 atm.
Problems 107
3-23. Equations 3-12 and 3-14 can be applied to nonenclosed exposures by using an effective ventilation rate. The effective ventilation rate for outside exposures has been estimated at 3000 ft3/min.16
A worker is standing near an open passageway of a tank containing 2-butoxyethanol (molecular weight = 118). The passageway area is 7 ft2. Estimate the concentration (in ppm) of the vapor near the passageway opening. The vapor pressure of the 2-butoxy- ethanol is 0.6 mm Hg.
3-24. Fifty-five-gallon drums are being filled with 2-butoxyethanol. The drums are being splash- filled at the rate of 30 drums per hour. The bung opening through which the drums are being filled has an area of 8 cm2. Estimate the ambient vapor concentration if the ventila- tion rate is 3000 ft3/min. The vapor pressure of 2-butoxyethanol is 0.6 mm Hg under these conditions.
3-25. A gasoline tank in a standard automobile contains about 14 gal of gasoline and can be filled in about 3 min. The molecular weight of gasoline is approximately 94, and its vapor pressure at 77'F is 4.6 psi. Estimate the concentration (in ppm) of gasoline vapor as a re- sult of this filling operation. Assume a ventilation rate of 3000 ft3/min. The TLV for gaso- line is 300 ppm.
3-26. A 6-in-diameter elephant trunk is used to remove contaminants near the open bung of a drum during a filling operation. The air velocity required at the end of the elephant trunk is 100 ftlmin. Compute the volumetric flow rate of air required.
3-27. To reduce air pollution, gasoline filling stations are installing scavenger systems to remove the gasoline vapors ejected from the automobile tank during the filling operation. This is accomplished by an elephant trunk ventilation system installed as part of the filler hose.
Assume an average automobile tank size of 14 gal. If the vapor in the tank is satu- rated with gasoline vapor at a vapor pressure of 4.6 psi at these conditions, how many gal- lons of gasoline are recovered free for the station owner with each fill-up? For 10,000 gal of delivered gasoline. how many gallons are recovered? The molecular weight of gasoline is about 94, and its liquid specific gravity is 0.7.
3-28. Normal air contains about 21% oxygen by volume. The human body is sensitive to re- ductions in oxygen concentration; concentrations below 19.5% are dangerous, and con- centrations below 16% can cause distress. Respiratory equipment without self-contained air supplies must never be used in atmospheres below 19.5% oxygen.
A storage tank of 1000 ft3 capacity must be cleaned before reuse. Proper proce- dures must be used to ensure that the oxygen concentration of the air within the tank is adequate.
Compute the cubic feet of additional nitrogen at 77OF and 1 atm that will reduce the oxygen concentration within the tank to (a) 19.5% and (b) 16%. Oxygen concentra- tions within tanks and enclosures can be reduced significantly by small amounts of inert elements!
I6Matthieson, "Estimating Chemical Exposure," p. 33.