Industrial Hygiene
Exposure Evaluation and Control
Industrial Hygiene Exposure Evaluation and Control
Industrial hygiene is defined as the
anticipation, identification, evaluation
and control of occupational conditions
which cause sickness and/or injury
Industrial Hygiene Exposure Evaluation and Control
Exposure Evaluation

Measurement techniques
Estimating exposure
Open tanks
 Filling tanks

Control Techniques
Personal Protection
Evaluation of Chemical Hazards
Detector tubes - color change for specific species
Adsorption tubes – sample air drawn through adsorbent then
released into GC
Filters – collects particulate dust and fibers
Portable monitors – hand held monitors to look for leaks or hot
spots
Real time monitors – used to determine average, maximum and
minimum concentrations.
Personal monitors – used to determine exposure of worker
Air Monitoring Strategies
Determine worker exposure
Variable concentration
Contamination level
Control measures
Batch operations
Air circulations patterns
Seasonal variations
Exposure Evaluation
Threshold Limit Value - Time Weighted Average,
TLVTWA
C TWA


tw
0
C (t )dt

tn
0
 tw

is the length of shift
tn is 8 hours
dt
Exposure Evaluation
Intermittent monitoring

tw
0
I
C (t )dt   Ci ti
i
I = number of measurements during shift
ti = is the time period over which measurement i is taken
Assume concentration is “constant” during the time period
Exposure Evaluation
Multiple Toxicants
N
CTLV TWA 
C
n 1
N
C
n 1
n
Cn
TLV TWA,n
N is total number of toxicant
Cn is the concentration relative to other toxicants
Here we assume the effects are additive
Estimating Exposure from Open
Tanks
Author derives
relationships assuming
no toxicants in
ventilation air. I will
present of more
complete analysis.
Mass Balance on Room for Toxic
Vapor
I
J
dmk
  min ,i ,k   mout , j ,k
dt
i 1
j 1
d (VC )
 Qv ,inCin  ml  Qv ,out Cout
dt
Estimating Exposure from Open
Tanks (cont.)
Assume Steady State
d (VC )
0
dt
Assume Nonideal mixing
Cout = kCmax
k=1 for perfect mixing
Table 3-11 gives values of k,
worst case scenario is k1/10
Estimating Exposure from Open
Tanks (cont)
Substituting
0  Qv,inCin  ml  Qv ,out  kCmax 
Cmax
Qv ,inCin  ml

kQv ,out
Estimating Exposure from Open
Tanks (cont)
Air mass balance
I
J
dmA
  min ,i , A   mout , j , A
dt
i 1
j 1
Assume steady state
dm A
0
dt
Estimating Exposure from Open
Tanks (cont)
Assume ideal gas and that toxic vapor has
negligible mass compared to mass of air
I
m
i 1
in ,i , A
M A PinQv ,in

RTin
J
m
j 1
out , j , A
M A PoutQv ,out

RTout
Set equal, so
Qv ,in
 Tin  Pout 


 Qv ,out
 Tout  Pin 
Estimating Exposure from Open
Tanks (cont)
Substituting
Cmax
 Tin   Pout 
 T   P  CinQv ,out  ml
out   in 


kQv ,out
Qv,out  3000 ft3/min for out doors
Estimating Exposure from Open
Tanks (cont)
Now estimate evaporation rate – diffusion away
from the liquid surface
ml 
MKA  P
sat
RTL
Ρ 
M is molecular weight
K is mass transfer coefficient (length/time)
A is surface area over which driving force exists
TL is absolute temperature of volatile liquid
Ρ is partial pressure above surface
Worst case Psat>>>
Ρ
Estimating Exposure from Open
Tanks (cont)
Substituting
Cmax
 Tin   Pout 
MKAP sat
 T   P  CinQv ,out  RT
out   in 
L


kQv ,out
With simplifying assumptions you get
Eq 3-14
Estimating Exposure from Open
Tanks (cont)
Correlation for mass transfer coefficients
1/ 3
 M0 
K  K0 

M 
For water M0=18 and K0=0.83cm/sec
Estimating Exposure from Filling
Tank
Estimating Exposure from Filling
Tank (cont)
Assume vapor space above liquid is partially
saturated
Pv   P
sat
0  1
With a heal left in vessel  = 1
Estimating Exposure from Filling
Tank (cont)
 M  P  MK ( P  Ρ )
ml  QL 

RTL
 RTL 
sat
displacement
sat
diffusion out of tank
Assume worst case Ρ << Psat
Estimating Exposure from Filling
Tank (cont)
M P
ml 
RTL
C ppm
sat
Q
L
 KA
sat
ml RT

P
T
6
6

 10 
Q

KA

10


L
kQv ,out PM
kQv ,out PTL
Similar to, but better, than Eq. 3-24
Textbook Error
Note that Example 3-9 on page 68 has error
7.481gal/ft3 is correct not 7.481 ft3/gal
Control of Chemical Hazards
Engineering Control
Administrative Control
Protective Equipment
Engineering Controls
Inherent Safety
Containment
Ventilation
Inherent Safety Aspects
Substitution

Use chemicals and equipment which are less hazardous
Attenuation

Use chemicals under conditions which make them less
hazardous
Isolation

Isolate equipment and/or sources of hazard
Intensification

Reduce quantity of chemical
Containment Principles
“Containment” refers to keeping the process
materials contained within the processing
equipment
Design for internal deflagration
Vent to containment or control equipment
Use rupture disks or safety valves to vent
excessive pressure spikes
 Venting to containment vessel or flare, etc.

Containment Principles
Sealing Points and Leak Protection
Static Seals
Welds
 Flanges
 Covers/Heads
Welds are better than
flanges

Dynamic Seals
Relative motion
between seal parts
 Rotating Shafts
 Valve stems

Containment Principles
Rotating Shaft Sealing Methods
Stuffing Box and Packing
Mechanical Seal
Double Mechanical Seal

Allows evacuation between seals
Seal Maintenance procedure required
Avoiding Dynamic Seals
“Seal-less” pump
Magnetic coupling
 Canned rotor
 Diaphragm

Bellows-Seal Valve
Potential Leakage
Locations/Occasions
Sight glasses
Gage glasses
Sampling points
Addition points
Batch processing vessels
Loading/Unloading
Packaging
Maintenance
Ventilation for Control
Outdoor construction
Local Ventilation
Dilution Ventilation
Local Exhaust Ventilation
Removes contaminants at source
Prevents toxic material from entering the
workplace air
Requires less airflow than dilution
ventilation
Components of a Local Exhaust
Ventilation System
Hood or “Elephant Trunk”
Duct system
Air cleaning system
Air mover
Outlet
Hood Ventilation
Totally Enclosed

Enclosed structure around processing
equipment with limited (No) access. Emissions
taken to be treated
Exterior Hood

Also called “Elephant Trunk”. Duct inlets
located close to source. Often flexible duct that
can be moved some, i.e. elephant trunk.
Hood Ventilation - Booth
Booth Hood


Standard “fume hood”
seen in laboratories
Need to keep the
window always
slightly opened to
ensure there is some
are flow
Hood Ventilation - Booth
Booth Hood

Bypass laboratory
hood ensures that there
is always a positive
flow through the hood
and minimizes the
circulation patterns that
might allow fumes to
be released
Negative Ventilation Systems
Need to keep exhaust
system under negative
pressure so that any
leakage will be from
the rooms into the
exhaust system and
not vice versa.
Duct System Design
Basic fluid mechanics
Publications/Recommendations
Capture velocity
 Entrainment velocity
 Pressure losses

Dilution Ventilation
Air flow throughout building
High air flow required

Best used in conjunction with localized hooding
Integrated with local HVAC system
Ventilation Exhaust May Require
Cleaning
Absorption
Adsorption
Flare or Incineration
Stack to prevent re-entry
Best to treat localized exhaust system,
prohibitive to treat a dilution ventilation
system.
Administrative Control
Techniques
Work Rules to Limit Exposure Time and/or
limit accessibility to areas with high
concentrations.
Good Housekeeping
Functional Operating and Maintenance
Procedures
Education and Training of all personnel
Good Housekeeping
Keeps toxics and dusts contained
Use dikes around tanks and pumps
Provide water and steam connections for
area washing
Provide lines for flushing and cleaning
Provide well-designed sewer system with
emergency containment
Elements of PPE Training Program
Standard and regulatory requirements
Hazard characterization in the workplace
Implementation of engineering and management
controls
Description of need, capabilities and limitations of
PPE
Demonstration of proper use, fit, care,
maintenance and repair of PPE
Explanation of PPE written policy, regulations and
enforcement
Discussion of record-keeping requirements
Personal Protective Equipment
Engineering and Management controls can
reduce or even eliminate many occupational
safety hazards. However, it may be
impractical or impossible to keep the work
area completely free of contaminants or to
keep all workers away from dangerous
locations.
PPE is the last line of defense
Personal Protective Equipment
Routine Equipment
Emergency
Protection of the Head
Hard hats should be
able to withstand the
impact of a 8 lb iron
ball dropped from 5
feet
Should be non
conducting
Eye Protection
Unvented goggles
Impact resistant lenses
and side shields
Chemical splash
goggles
Hearing Protection
Ear plugs


Range from 17 - 25 dB
Hearing bands allow
on-off use
Earmuffs

Provide wide range of
protection from 19 to
30 dB
Respirators
Dust and mist respirators

Filter out particulate
Need to have ambient oxygen
Does not stop vapors or gases
Respirators
Air-Purifying Respirators
Adsorbent removes gas, vapor, or
particulate
Different cartridges for different types of
vapor
Needs to
have ambient
oxygen
Respirators
Supplied Air
“Unlimited” air supply
from remote site
Requires compressor
Disadvantage of
possible damage to
hose, limited mobility
and contamination
from compressor
Respirators
Self Contained (SCBA)
Avoids problems of
supplied air
Limited supply
Typically used for
emergency operation
Respirators
All respirators need to be fit properly and
tested routinely to ensure that they function.
Emergency respirators need to be serviced
routinely to ensure that they function when
needed.
Protective Clothing
Gloves
Boots
Trousers
Slickers
Full body protection
Chemical Engineer’s
Responsibilities
Engineer leadership
Legal responsibility
Ethical responsibility
Safety
Safety needs to become a mindset and a way
of life for a practicing engineer.
In Class Problem
As a homework team solve the following problem
Fifty-five gallon drums are being filled with 2butoxyethanol. 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 vapor concentration (in ppm) if the
ventilation rate is 3000 ft3/min. The molecular weight
of 2-butoxyethanol is 118 and the vapor pressure is 0.6
mm Hg at these conditions.
Solution
Area=8 cm2
Filling rate 30drum/hr
Φ=1.0 (splash filling)
V=55gal
M=118 lbm/lbmol
Po=0.6 mmHg
Qv=3000 ft3/min
TL=Ta
Design Equation
Cppm 
 PsatT
kQv ,out PTL
Similar to Eq 3-24
Q
L
 KA  10
6
Solution continued
Find mass transfer
 M0 
K  K0 

M


1
3
 18 
cm
 0.83
s  118 
1
3
 0.4435 cm
s
Filling rate
3
30drums 55gallons
ft 3
hr
ft
QL 



 3.676
min
hr
drum
7.481gal 60 min
Solution continued

kCppm 

atm
3

ft
 
2  60 s  
6
 760mmHg  3.676 ft 3

0.4435
cm
8
cm

10










min
 3000 ft 3
 1atm
 min   30.48cm  





min 

1 0.6mmHg  
kCppm  0.9695  dimensionless
However,
So
0.1  k  0.5
9.695  Cppm  1.939