Engineering Controls Flashcards
General Approaches to Control of Health Risks
ELIMINATION/SUBSTITUTION: most effective form of prevention control, simply either eliminate use of hazardous agent, or process its used
ISOLATION: processes or operations should be completely enclosed, with operators outside enclosure
SEGREGATION: high risk separated from lower risk
Engineering Controls
Remote operations with seperate enclosed air conditioned control rooms
Exhaust ventilated booths for spray painting
Robotic welding with built-in LEV system
Local extraction system attached to grinding machines
Ventilation
Process capable of producing exposures to hazardous substances, commonly controlled by mechanical air handling methods
Reasons for Use:
- control odours
- thermal comfort
- humidity
- dilute flammable vapours below LEL
- dilute workplace airborne concentrations of low toxicity gaseous contaminants
Local Exhaust Ventilation (LEV)
Removing a sufficient amount of air to also remove the hazard
Supply Systems
Used to create a comfortable environment (by controlling e.g. temperature, humidity), and replacing air exhausted from the workplace
Dilution Ventilation
Uses fresh air to reduce the concentration of airborne contaminants to well below the OES in worker’s breathing zone
- doesn’t affect the amount of hazardous gas released
- used for low toxicity gases or vapours that are released uniformly throughout the workplace from a number of small sources at a continuous rates
Dilution Ventilation (Appropriate When)
Air contaminant has low toxicity
Are multiple sources
Emission is continuous
Concentrations close to/lower than OEL
Volume of air needed is manageable
Contaminant can be sufficiently diluted before inhalation
Limitations:
- hazardous substances may be moved towards the worker
- amount and toxicity of contaminants must be low
Air Changes/hour
Air changes/hour = (Qx3600)/V
V = room volume in m3 Q = air flow rate in m3/s 3600 = conversion for seconds to hours
Pressure
For air to flow there must be a pressure difference and air will flow from higher pressure to lower pressure
Considered to have 2 forms:
- static pressure
- velocity pressure
Static Pressure
The pressure exerted in all directions by a fluid that is stationary
Velocity Pressure
The pressure requires to accelerate air from 0 velocity to required velocity and it proportional to the kinetic energy of the air stream
Velocity pressure = p x (v2/2)
p = density of air (kg/m3) V = air velocity (m/s)
Capture Velocity
The air velocity required at the source of emission so as to cause the contaminant to move towards the capture device and be removed
LEV Design Principles
Hood design and hood exhaust air flowrate are the two most important LEV design parameters
- HOOD: located close to or enclosing the contaminant source
- DUCTWORK: adequate for application and sized for duct velocity
- FAN: located outside the facility to keep all interior ductwork under negative pressure
- EXHAUST STACK DISCHARGE: vertical up-blast and well clear of fresh air inlets, skylights, etc.
- Make up fresh air to replace the amount of air exhausted from the building
Types of Hoods
Total enclosure
Partial enclosure
Capture hods
Receptor hoods
Total Enclosures
Advantages: - worker outside enclosure - minimise air extracted Disadvantages: - inhibit access - high exposure on entry
