Produktionshygiene Production hygiene Wirtschaftliche Hygiene
CBB2093 (3b) Industrial Hygiene
-
Upload
nazrul-amin-muhammad -
Category
Documents
-
view
47 -
download
8
Transcript of CBB2093 (3b) Industrial Hygiene
1
CHAPTER 3Industrial Hygiene:
Evaluation
CAB2093 2
Learning Objectives
After completing this chapter, students should be able to : Estimate TWA and TLV-TWA Evaluate worker exposure to toxicants and noise Assess worker exposure to toxic vapors Determine the vaporization rate of a liquid
2
CAB2093 3
Evaluation phase determines the extent and degree of employee exposure to toxicants and physical hazards in the workplace
Need to consider likelihood of small and large leaks• Sudden exposures to high concentrations through large
leaks may lead to acute effects (ex: burning eyes, unconsciousness, etc)
• Repeated exposures to low concentrations may cause chronic effects (permanent and serious impairment)
Existing control system to protect the workers from the exposure must be checked regularly to ensure its effectiveness
Exposure levels must be sampled regularly and benchmarked against the acceptable standard (ex: TLVs,PELs or IDLH)
Introduction
CAB2093 4
Evaluating Exposures to Volatile Toxicants By Monitoring
Determination of worker exposures can be done by monitoring the concentrations of toxicants in air continuously
For continuous concentration C(t), the TWA (time-weight average)concentration is computed:
wt
dttCTWA0
)(81
C(t) is the concentration of toxicant in the air in ppm or mg/m3
tw is the worker shift time in hours
TWA is for 8 hrs exposureTLV-TWA for 12 hrs exposure
hrsTCTCTCTWA nn
8......2211
3
CAB2093 5
Evaluating Exposures to Volatile Toxicants By Monitoring
If more than one chemical is present, the effects of toxicants can be assumed to be additive.
The combination of exposures :
Wheren is the total number of toxicants,Ci is the concentration of chemical i(TLV-TWA)i is theTLV-TWA for chem. Species i
If the sum exceeds 1 , the workers are overexposed
n
i i
i
TWATLVC
1 )(
CAB2093 6
Evaluating Exposures to Volatile Toxicants By Monitoring
The mixture TLV-TWA can be computed from:
n
i i
i
n
ii
mix
TWATLVC
CTWATLV
1
1
)(
)(
If the sum of the concentration of the toxicants in the mixtureexceeds this amount, the workers are overexposed.
For mixtures of toxicants with different effect (acid vapor mixed with lead fume), the TLV-TWA cannot be assumed to be additive.
4
CAB2093 7
Evaluating Exposures to Volatile Toxicants EXAMPLE 3-2
Air contains 5 ppm of diethylamine (TLV-TWA of 10 ppm), 20 ppm of cyclohexanol (TLV-TWA of 50 ppm), and 10 ppm of propylene oxide (TLV-TWA of 20 ppm).What is the mixture TLV-TWA and has this level been exceeded.
Solution
n
i i
i
n
ii
mix
TWATLVC
CTWATLV
1
1
)(
)(ppmTWATLV mix 25
2010
5020
105
10205)(
The total mixture conc. Is 5 + 20 + 10 = 35 ppm. The workers are overexposed
CAB2093 8
Evaluating Exposures to Volatile Toxicants EXAMPLE 3-2
Alternatively :
4.12010
5020
105
The quantity is greater than 1, the TLV-TWA has been exceeded
n
i i
i
TWATLVC
1 )(
The total mixture conc. Is 5 + 20 + 10 = 35 ppm. The workers are overexposed
5
CAB2093 9
Evaluating Exposures to Volatile Toxicants EXAMPLE 3-3
Exposure (hr) Conc. (ppm)2 1102 3304 90
hrsTCTCTCTWA nn
8......2211
ppmTWA 1558
)4(90)2(330)2(110
CAB2093 10
Evaluating Exposures to Noise
Noise levels are measured in decibels (dB)
010log10
IINoise intensity (dB)
Source of noise Sound intensity Maximum exposure level (dBA) (hr)
An absolute sound scale (in dBA), the hearing threshold is set at 0 dBA.
Factory 90 8Passing truck 100 2Punch press 110 0.5
Refer to Table 3-8 for the permissible Noise exposure
6
CAB2093 11
Evaluating Exposures to NoiseExample 3-6
Determine whether the following noise level is permissible:
Noise level Duration Max allowed(dBA) (hr) (hr)
85 3.6 no limit 95 3.0 4110 0.5 0.5
Solution
n
i i
i
TWATLVC
1 )(75.1
5.05.0
436.3
No limit
The sum exceeds 1.0, employees are immediately required towear ear protection. Control methods should be developed for long-term exposure.
CAB2093 12
Estimating Worker Exposures to Toxic Vapors
For design purposes estimates of vapor concentrations are requiredIn enclosed spaces: - above open containers where drums are filled,
in the area of spills.
Volatile rate OutkQvC (mass/time)
Concentration of volatile, C in enclose volume, v(mass/volume)
Ventilation rate, Qv(Volume/time)
Evolution rate of volatile, Qm(mass/time)
7
CAB2093 13
Estimating Worker Exposures to Toxic Vapors
C is the concentration of volatile vapor in the enclosure (mass/volume)V is the volume of the enclosure (volume)Qv is the ventilation rate (volume/time)k is the non ideal mixing factor (unitless), and Qm is the evolution rate of volatile material (mass/time)
Perfect mixing k = 1, for non ideal mixing, k varies from 0.1 to 0.5
Total mass of volatile in volume
Accumulation of mass of volatile dtdCV
dtVCd
)(
VC
Mass rate of volatile material from evolution mQ
Mass rate of volatile material out CkQv
CAB2093 14
Estimating Worker Exposures to Toxic Vapors
The dynamic mass balance on the volatile species:
CkQQdtdCV vm
At steady state the accumulation term is 0,
v
m
kQQC
Let m represent mass, represent density,Subscripts v and b denote the volatileand bulk gas species
66 1010
PMTR
Vm
VVC g
b
v
b
vppm
WhereRg is the ideal gas constantT is the absolute ambient temperatureP is the absolute pressure, and M is the molecular weight of the volatile species
----(3.6)
--(3.7)
---(3.8)
8
CAB2093 15
Estimating Worker Exposures to Toxic Vapors
610PMkQ
TRQC
v
gmppm
The term mv/Vb is identical to eq 3-7. Substituting Eq 3-7 into Eq 3-8:
-------(3.9)
Is used to determine the average concentration (ppm) of anyvolatile species in an enclosure system. a worker standing near a pool of volatile liquid a worker standing near an opening container or storage tank
Assumption:A steady-state condition is assume, the accumulation termIn the mass balance is zero
CAB2093 16
Estimating Worker Exposures to Toxic VaporsEXAMPLE 3-7
An open toluene container in an enclosure is weighed as a function of time, and its is determined that the average evaporation rate is 0.1 g/min.The ventilation rate is 100ft3/min. The temperature is 80°F and the pressure Is 1 atm. Estimate the concentration of toluene vapor in the enclosure.
610PMQ
TRQkC
v
gmppm
Solution: The value of k is not known directly, use as a parameter
Qm= 0.1 g/min = 2.20 × 10-4 Ibm/minRg = 0.7302 ft3 atm/lb-mol°R,T = 80°F = 540°R,M = 92 lbm/lb-mol,P = 1 atm
63
34
10)/92)(1min)(/100(
)540)(/7302.0min)(/1020.2(
mollblbatmftRRmollbatmftlbkC
m
oom
ppm
= 9.43 ppmk varies from 0.1 to 0.5, the concentration is expected to vary from 18.9 to 94.3 ppm.
9
CAB2093 17
Estimating the Vaporisation Rate of a Liquid
Liquids with high saturation vapor pressures evaporate faster.For a vaporization into stagnant air:
)( pPQ satm
satP Is the saturation vapor pressure of the pure liquid at the temperature of the liquid
p Is the partial pressure of the vapor in the bulk stagnant gas above the liquid.
For P sat ≥ p
Lg
sat
m TRMKAPQ
whereQm is the evaporation rate (mass/time)M is the molecular weight of the volatile substanceK is a mass transfer coefficient (length/time) for
an area A,Rg is the ideal gas constant, and TL is the absolute temperature of the liquid
------- (3.10)
-- (3.12)
Lg
sat
m TRpPMKAQ
CAB2093 18
Estimating the Vaporisation Rate of a Liquid
610PkQ
KAPCv
sat
ppm
The vaporization rate of a volatile in an enclosure resulting from evaporation of a liquid
The gas mass transfer coefficient is estimated :
31
MMKK o
o
--------- (3.14)
----------- (3.18)
PRACTICE EXAMPLE 3-8