Chemical Hazard Controls: Ventilation & Water Sprays

• The session will cover engineering controls like push-pull ventilation systems, high-pressure low-volume systems, and water spray systems for chemical health risk assessment (CHRA).

Learning Objectives

• Identify and state the components of a push-pull ventilation system.
• Evaluate the whole system and advantages of a push-pull ventilation system.
• Identify and state the components, advantages, and disadvantages of an LVHV system.
• Understand general principles and applications of wet methods to control worker exposure to CHH (Chemical Health Hazards).

Scope

• Inherent Safer Design Strategies.
• Components, design criteria, and advantages of a Push-Pull System.
• Principles, advantages, and disadvantages of LVHV.
• Main components, inspections, and testing of a water spray system.

Push-Pull Ventilation System

• A Local Exhaust Ventilation system exhausts contaminated air before diffusing into the environment.
• Ventilation provides an air jet that intercepts contaminated air and carries it to the exhaust hood for better control.
• The system consists of two components: a push hood and a pull hood.
• A push hood provides an air jet to bring contaminants to the vicinity of the exhaust hood.
• A pull hood is an exhaust hood connected to the ducting system, like in an LEV (Local Exhaust Ventilation).
• Purpose includes spot ventilation, uniform flow ventilation, and shield systems.
• Spot ventilation has an ideal ratio of 70:30; see diagram for setup.
• Examples of uniform flow ventilation and shield systems, also with diagrams.
• Two parameters to consider when using a push-pull system are push air velocity and capture velocity on the capturing surface.
• The average velocity of push air must reach at least 0.5 m/s, with Vmax and Vmin within ±20% of the average velocity.
• Opening of the push hood is divided into 16 to 64 equal areas, with the distance between centers approximately 155mm.
• Measurement is done using a hot-wire anemometer.
• The capturing surface must be decided where the contaminant is located or at 70% of the distance from the push hood.
• Measurement is taken on an imaginary surface approximately 155mm between each measurement point.
• Capture velocity must reach at least 0.2 m/s
• Smoke tubes can also show capture distance and characteristic of the air flow visually.
• The K factor is a unitless factor that relates flow rate of pull and push systems, with the equation K = Qpull/Qpush
• Air volume of the pull hood must be larger than the air volume of the push hood
• K is usually between 1.5 to 2.0
• K factor is used to design the push-pull system; engineers decide the volume of push air and use the K factor to determine the volume of pull air.
• A pull-push system is recommended if the pull system is insufficient, chemical exposure monitoring is above the Permissible Exposure Limit, or the source of airborne containment is very wide.

Design Criteria

• The flow produced from the system must be uniform; velocity difference between measurement points (push hood) must be ± 20%.
• Diffusion angle must be between 15° to 20°.
• Vpush ≥ 0.5 m/s
• Vcapture ≥ 0.2 m/s
• K factor 1.5 ~ 2.0
• In push-pull systems, the pull hood pulls the turbulence flow into the hood, reducing contaminant scattering.
• A pull hood is usually bigger than a push hood.

Advantages

• Exhaust velocity decreases rapidly with increasing distance from the hood, which limits the effective capture range.
• Push air maintains velocity over a large distance and transports the contaminant closer to the exhaust hood, extending the range.

Low Volume-High Velocity (LVHV) System

• A unique exhaust application uses small air volumes at high velocities to control dust from portable hand tools and machining.
• Control is achieved by exhausting air directly at the point of contaminant generation using a custom-made hood.
• Capture velocity is relatively high, but exhaust volume is low.
• Components include a close-fitting hood mounted directly at the tool, a flexible lightweight plastic hose attached to the hood, and also to a flexible extension hose to connect the flexible hose to the air cleaner and the exhauster.
• Additional components include the air cleaner and the exhauster.

Advantages and Disadvantages

• Small diameter hose results in very high velocities, which allows the application of LEV to portable tools.
• Contaminant is exhausted directly at the point of generation.
• There is a reduction of replacement air as the volume exhausted is low.
• Some disadvantages include the fact that some small parts can be sucked and higher noise levels.
• Coolants used in hand tools can be disturbed.
• Recommendation is justified if there is a chemical exposure monitoring result from the workplace before high-pressure low volume being introduced, airborne contaminant at the workplace, poor housekeeping, airborne contaminant exposure to the workers, and/or physical health effects being indicated by the dermal contact to the workers.

Wet Methods/Water Spray System

• To reduce the ability of dusty materials to become airborne.
• Humidity may be added to create agglomeration among small particles and to add weight.
• Contaminants will settle due to additional weight.
• Industries that may utilize this include Quarry, Manufacturing, and Mining.
• Factories and Machinery Act 1967 includes legal requirements but may be repealed (see document).
• Regulation 16 on Mineral Dust Regulations, water spray shall be evenly distributed as possible and cover the entire processing area with optimally sized water droplets to ensure maximum removal of airborne dust.
• Water supply shall be free from particles > 0.5mm diameter and as far as practicable acid free.

Main components

• Water sources, a pump and engine, water filter, piping system, fittings, and a sprinkler head.
• Water sources: Streams, rivers, wells and city water. -Streams discharge quite ample in the rainy season but dry season discharges are smaller. -Rivers may be normally present throughout the year. -Wells are classifies as shallow wells, deep wells and tube wells and contain groundwater.
• Types of Mechanical pumps are what you can you use, which you can use water for, or you can use fluids to higher elevation or pressure.
• Water pressure is one of the most important components, and water pressure is the energy to make the water spray job.
• Pump Horse formula: Pump Horse Power = Q*H / 76.1 * Ep
• System design flow rate (liter/second). H stands for total pressure loss (m) including friction loss and static loss. A pump efficiency is also included, in addition to the engine horse power.

Example Calculation:

• Given design flow rate for one of the water spray system is 3.0 liter/second.
• Total pressure loss for the system is 17.0 m and pump efficiency is 65%.
• Determine: Minimum required pump horse power for the system
• Determine: Minimum engine horse power for this system Minimum Pump Horse Power = Q*H / 76.1 * Ep , = 3 X 17 / 76.1 X 0.65 = 1.03 HP Minimum engine horse power for this system= 1.3 X pump horse power , = 1.3 X 1.03 = 1.34HP

Types of losses

• Suction loss, elevation loss, nozzle operating pressure, riser loss, fitting loss, losses in mainline and lateral pipes.

Fittings

• Flow meter- determine the volume of supply
• Pressure relieve valve - prevent system failure due to over pressure
• Non-return valve - Prevent back flow when pump stops pumping
• Flow control valve - controls water flows in the system
• Pressure regulating valve - to regulate the pressure in the system
• Pressure gauge- measure pressure at the several points in the system

Filters

• Filtration- to prevent solids and to prevent solids, which are too large from entering the system
• Helps - Extend the life and lower the maintenance of water spray system -If unwanted materials entering the system can cause Clogging, Formation of dry spot and Wear in equipment
• Types of Filters- Media filter, disk filter, screen filter, cartridge filter, centrifugal filter -Choosing the Filters factors to be considered involves Inlet and outlet water quality, Availability of filter and parts, and Price
• Filters size equivalent, selection have to consider trade off between Level of filtration, and Clogging problem -Various Microns: 800 / 500 / 300 / 250 / 200 / 100 , with there respective MM: 0.8 / 0.5 / 0.3 / 0.25 / 0.2 / 0.1 -There respective Mesh: 20 / 30 / 50 / 60 / 75 / 155

Piping system

• Types of commonly used pipe: PVC- Polyvinyl chloride and PE- Polyethylene pipe;

Types of commonly used pipe

• PVC – Polyvinyl chloride
• PE – Polyethylene pipe
• PVC white or gray – semi-rigid
• PE black and flexible
• Generally PE pipe has better resistance to sunlight than PVC pipe.
• If used, PVC pipes have to be buried in the ground to prevent failure, and PE pipe not transparent to sunlight can prevent the formation of algae.

Lateral Pressure Loss

• Lateral Pipe- name given to the pipe section between the control valves and the sprinkler head.
• Guide – Maximum lateral pressure loss < 20% sprinkler head operating pressure

Reasons

• For limiting variation in performance between the sprinkler head controlled by the same valve.
• Otherwise, pressure and water flow in the first sprinkler head become too great and last sprinkle head head become too low - Flooding and dry spot.
• The two types of sprinkler heads is divide into two types based on method they use to distribute water

Spray heads- fix spray heads

• Small head
• Fan shaped pattern of water
• Pattern and radius of water is determined by interchangeable nozzle installed (1/2 circle, full circle, etc.)
• Need between 20-30 PSI of water pressure to operate properly
• Spaced up to 18 ft apart

Rotors- Operate by rotating streams

• Space between 18-55 ft apart, some above 55ft
• Requires a lot of waters to operate than spray heads
• Water pressure in the rotor head must be be greater than distance travel

Selection of Sprinkler Heads

• Factors to be considered
• Spacing- rotors spaced farther apart
• Cost- cost per sprinkler head, rotors more expensive
• Area to cover- < 18 ft - fix head
• Design pressure < 40 Psi. rotor unsuitable

Inspection

What to look for

• Condition of motor
• Availability of trained person
• Availability of equipment manual
• Availability of safe operating procedure
• Condition of Pipeline.
• Condition of Pump House
• Condition of valves
• Air Monitoring Record - if any
• Calibration Sticker / Record - if Any
• Basic Health and Safety Aspects

Testing

• -Machine Guarding, Dangerous Slope
• What to measure and Test: - Water Flow Rate - Flow Meter
• Pressure Across the system- pressure gauge
• Testing the Acidity - pH Paper , the Test Pressure Relive Valve, and Testing of the Sprinkler Head

Justification on the Recommendation for Water Spray

• Assessors need to suggest the Water Spray System if any of the following matters appear
• There is chemical exposure resulting from the workplace before the water spray system implemented/introduced.
• The is sighting of airborne contaminants in the workplace
• There is poor house keeping at the workplace due to the air airborne contaminates
• There is a duration of airborne contamination exposure to the workers
• There are any physical signs of health/effects due to the dermal contact to the workers.