Industrial Hygiene Exposure Evaluation and Control

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

tw is the length of shift

tn is 8 hours

0

0

( )w

TWA n

t

t

C t dtC

dt

Exposure Evaluation

Intermittent monitoring

I = number of measurements during shift

= is the time period over which measurement i is taken

Assume concentration is “constant” during the time period

0( )

wIt

i ii

C t dt C t

it

Exposure Evaluation

Multiple Toxicants

N is total number of toxicant

Cn is the concentration relative to other toxicants

Here we assume the effects are additive

1

1 ,

N

nn

TLV TWA Nn

n TLV TWA n

CC

CC

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

, , , ,1 1

I Jk

in i k out j ki j

dmm m

dt

, ,

( )v in in l v out out

d VCQ C m Q C

dt

Estimating Exposure from Open Tanks (cont.)

Assume Steady State

Assume Nonideal mixing

Cout = kCmax

k=1 for perfect mixing

Table 3-11 gives values of k, worst case scenario is k1/10

( )0

d VC

dt

Estimating Exposure from Open Tanks (cont)

Substituting

, , max0 v in in l v outQ C m Q kC

,max

,

v in in l

v out

Q C mC

kQ


Air mass balance

Assume steady state

, , , ,1 1

I JA

in i A out j Ai j

dmm m

dt

0Adm

dt


Assume ideal gas and that toxic vapor has negligible mass compared to mass of air

Set equal, so

,, ,

1

IA in v in

in i Ai in

M P Qm

RT

,, ,

1

JA out v out

out j Aj out

M P Qm

RT

, ,in out

v in v outout in

T PQ Q

T P


Substituting

Qv,out 3000 ft3/min for out doors

,

max,

in outin v out l

out in

v out

T PC Q m

T PC

kQ


Now estimate evaporation rate – diffusion away from the liquid surface

M is molecular weightK is mass transfer coefficient (length/time)A is surface area over which driving force exists

TL is absolute temperature of volatile liquid is partial pressure above surface

Worst case Psat>>>

sat

lL

MKA Pm

RT

Ρ

ΡΡ


Substituting

With simplifying assumptions you get

Eq 3-14

,

max,

satin out

in v outout in L

v out

T P MKAPC Q

T P RTC

kQ


Correlation for mass transfer coefficients

For water M0=18 and K0=0.83cm/sec

1/ 3

00

MK K

M

Estimating Exposure from Filling Tank

Estimating Exposure from Filling Tank (cont)

Assume vapor space above liquid is partially saturated

With a heal left in vessel = 1

satvP P

0 1


displacement diffusion out of tank

Assume worst case << Psat

( )sat sat

l LL L

M P MK Pm Q

RT RT

Ρ

Ρ


Similar to, but better, than Eq. 3-24

sat

l LL

M Pm Q KA

RT

6 6

, ,

10 10sat

lppm L

v out v out L

m RT P TC Q KA

kQ PM kQ PT

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 PrinciplesSealing 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 PrinciplesRotating 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

AbsorptionAdsorptionFlare or IncinerationStack 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 requirementsHazard characterization in the workplaceImplementation of engineering and management controlsDescription of need, capabilities and limitations of PPEDemonstration of proper use, fit, care, maintenance and repair of PPEExplanation of PPE written policy, regulations and enforcementDiscussion 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 RespiratorsAdsorbent removes gas, vapor, or particulateDifferent cartridges for different types of vaporNeeds tohave ambientoxygen

Respirators

Supplied Air“Unlimited” air supply from remote siteRequires compressorDisadvantage 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 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 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

M=118 lbm/lbmol

Po=0.6 mmHg

Qv=3000 ft3/min

TL=Ta

Area=8 cm2

Filling rate 30drum/hr

Φ=1.0 (splash filling)

V=55gal

Design Equation

6

,

10satL

v out L

P TCppm Q KA

kQ PT

Similar to Eq 3-24

Solution continued

Find mass transfer

1 13 3

00

180.83 0.4435

118

M cm cmK K s sM

Filling rate

3 330 553.676 min7.481 60minL

drums gallons ft hr ftQhr drum gal

Solution continued

332 6

3

1 0.6760 60

3.676 0.4435 8 10min min 30.483000 1min

atmmmHg

mmHg s ftftkCppm cm cmcmft atm

0.9695 dimensionlesskCppm

0.1 0.5k However,

9.695 1.939Cppm So

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Industrial Hygiene Exposure Evaluation and Control