cbb2093 (3b) industrial hygiene

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CHAPTER 3Industrial Hygiene:

Evaluation

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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

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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

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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

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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 )(

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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.

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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

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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

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Exposure (hr) Conc. (ppm)2 1102 3304 90

hrsTCTCTCTWA nn

8......2211

ppmTWA 1558

)4(90)2(330)2(110

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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

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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.

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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)

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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

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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)

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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

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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.

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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

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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

cbb2093 (3b) industrial hygiene

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