1. Exposure Evaluation and Control
Industrial Hygiene
Exposure Evaluation and Control

2. 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

3. Industrial Hygiene - Exposure Evaluation and Control
Measurement techniques
Estimating exposure
Open tanks
Filling tanks
Control Techniques
Personal Protection

4. 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

5. Air Monitoring Strategies
Determine worker exposure
Variable concentration
Contamination level
Control measures
Batch operations
Air circulations patterns
Seasonal variations

6. Exposure Evaluation
Threshold Limit Value - Time Weighted Average, TLVTWA
tw is the length of shift
tn is 8 hours

7. 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

8. Here we assume the effects are additive
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

9. Estimating Exposure from Open Tanks
Author derives relationships assuming no toxicants in ventilation air. I will present of more complete analysis.

10. Mass Balance on Room for Toxic Vapor

11. 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 k1/10

12. Estimating Exposure from Open Tanks (cont)
Substituting

13. Estimating Exposure from Open Tanks (cont)
Air mass balance
Assume steady state

14. Estimating Exposure from Open Tanks (cont)
Assume ideal gas and that toxic vapor has negligible mass compared to mass of air
Set equal, so

15. Estimating Exposure from Open Tanks (cont)
Substituting
Qv,out  3000 ft3/min for out doors

16. Estimating Exposure from Open Tanks (cont)
Now estimate evaporation rate – diffusion away from the liquid surface
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
is partial pressure above surface
Worst case Psat>>>

17. Estimating Exposure from Open Tanks (cont)
Substituting
With simplifying assumptions you get
Eq 3-14

18. Estimating Exposure from Open Tanks (cont)
Correlation for mass transfer coefficients
For water M0=18 and K0=0.83cm/sec

19. Estimating Exposure from Filling Tank

20. Estimating Exposure from Filling Tank (cont)
Assume vapor space above liquid is partially saturated
With a heal left in vessel  = 1

21. Estimating Exposure from Filling Tank (cont)
displacement diffusion out of tank
Assume worst case << Psat

22. Estimating Exposure from Filling Tank (cont)
Similar to, but better, than Eq. 3-24

23. Textbook Error Note that Example 3-9 on page 68 has error
7.481gal/ft3 is correct not ft3/gal

24. Control of Chemical Hazards
Engineering Control
Administrative Control
Protective Equipment

25. Engineering Controls
Inherent Safety
Containment
Ventilation

26. 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

27. 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.

28. 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

29. Containment Principles Rotating Shaft Sealing Methods
Stuffing Box and Packing
Mechanical Seal
Double Mechanical Seal
Allows evacuation between seals
Seal Maintenance procedure required

30. Avoiding Dynamic Seals
“Seal-less” pump
Magnetic coupling
Canned rotor
Diaphragm
Bellows-Seal Valve

31. Potential Leakage Locations/Occasions
Sight glasses
Gage glasses
Sampling points
Addition points
Batch processing vessels
Loading/Unloading
Packaging
Maintenance

32. Ventilation for Control
Outdoor construction
Local Ventilation
Dilution Ventilation

33. Local Exhaust Ventilation
Removes contaminants at source
Prevents toxic material from entering the workplace air
Requires less airflow than dilution ventilation

34. Components of a Local Exhaust Ventilation System
Hood or “Elephant Trunk”
Duct system
Air cleaning system
Air mover
Outlet

35. Hood Ventilation Totally Enclosed Exterior Hood
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.

36. 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

37. 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

38. 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.

39. Duct System Design Basic fluid mechanics Publications/Recommendations
Capture velocity
Entrainment velocity
Pressure losses

40. Dilution Ventilation Air flow throughout building
High air flow required
Best used in conjunction with localized hooding
Integrated with local HVAC system

41. 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.

42. 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

43. 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

44. 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

45. 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

46. Personal Protective Equipment
Routine Equipment
Emergency

47. 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

48. Eye Protection Unvented goggles Impact resistant lenses
and side shields
Chemical splash
goggles

49. Hearing Protection Ear plugs Earmuffs Range from 17 - 25 dB
Hearing bands allow on-off use
Earmuffs
Provide wide range of protection from 19 to 30 dB

50. Respirators Dust and mist respirators Filter out particulate
Need to have ambient oxygen
Does not stop vapors or gases

51. Respirators Air-Purifying Respirators
Adsorbent removes gas, vapor, or particulate
Different cartridges for different types of vapor
Needs to
have ambient
oxygen

52. Respirators Supplied Air “Unlimited” air supply from remote site
Requires compressor
Disadvantage of possible damage to hose, limited mobility and contamination from compressor

53. Respirators Self Contained (SCBA) Avoids problems of supplied air
Limited supply
Typically used for emergency operation

54. 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.

55. Protective Clothing Gloves Boots Trousers Slickers
Full body protection

56. Chemical Engineer’s Responsibilities
Engineer leadership
Legal responsibility
Ethical responsibility

57. Safety
Safety needs to become a mindset and a way of life for a practicing engineer.

58. 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.

59. 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
Similar to Eq 3-24

60. Solution continued
Find mass transfer
Filling rate

61. Solution continued
However, So