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 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

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

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

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

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

Estimating Exposure from Open Tanks (cont) Substituting

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

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

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

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>>>

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

Estimating Exposure from Open Tanks (cont) Correlation for mass transfer coefficients 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 With a heal left in vessel  = 1

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

Estimating Exposure from Filling Tank (cont) 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 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.

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 Earmuffs 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 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 Similar to Eq 3-24

Solution continued Find mass transfer Filling rate

Solution continued However, So