03_hazardous_substances_and_other_chemicals_engineering_controls_and_personal_protective_equipment_20140117153253 Flashcards
General ventilation or ‘dilution’ ventilation is a term used to define the flow of air into and out of a working area, so that any contaminants are diluted by adding some fresh air. It can be provided by:
Natural ventilation which relies on wind pressure and temperature differences to move fresh air through a building and is usually not fully controllable. ‘Forced’ or mechanical ventilation which uses mechanical supply and/or extraction to provide fresh air and is controllable.
LEV might not be the right control solution when:
There are a large number of widely-spaced sources The source is large and LEV is impossible to apply over the entire source The source position is not fixed The source emits relatively small amounts of contaminant (which is not toxic) The contaminant is offensive but not harmful.
For LEV to work effectively the hood has to be carefully matched to the contaminated air source that needs to be controlled. Consideration needs to be given to the following:
Particle size of contaminant cloudsVisibility of particle cloudsMovement of particles in airAbrasive or corrosive particlesSticky dust, mist and condensateFlammable or combustible substances
The basic components of an LEV system are:
An inlet, for example: a hood or enclosure, to collect and contain the contaminant close to its source. Ductwork, to convey the contaminant away from the source. A filter / air-cleaner to remove the contaminant from the extracted air-stream (Note: the filter should normally be located before the fan). A fan or other air-moving device to provide the necessary airflow. Further ductwork to and outlet or exhaust which discharges the cleaned air to the outside atmosphere at a suitable point.
Types of hood
Enclosing hoodsReceiving hoodsCapturing hoods
LEV systems work effectively when the airborne contaminant cloud is contained, received or captured by the hood. The effectiveness of LEV can be judged by:
How much the hood constrains the contaminant cloud. How well the LEV induced airflow carries the contaminant cloud into the system. How little of the contaminant cloud enters the process operator’s breathing zone.
Types of LEV Filter / air cleaner
Particle collectors [Fabric filters, Cyclones, Electrostatic precipitators]Scrubbers [Venturi scrubbers, Self-induced spray collectors, Wet cyclone scrubbers, Packed tower scrubbers]
The fan is the most common air mover. It draws air and contaminant from the hood, through ductwork to discharge. There are five general categories of fan:
Propeller e.g. general or dilution ventilation Axial not suitable for dusts Centrifugal are the most commonly used fans for LEV systems Turbo exhauster Compressed-air-driven air mover.
The following factors can reduce the effectiveness of LEV:
Poor design: inappropriate inlet for type and size of contaminant cloud, or underpowered fan unable to capture contaminated air. Poor use: system not switched on when needed, or inappropriate positioning of moveable hood. Unauthorised modification: can imbalance a system and adversely affect air flows. Inadequate maintenance: damaged ducting, congested filters and damaged fan blades will compromise the effectiveness of the LEV. Changes of work activity: generating more contaminant than the LEV was designed to cope with.
How often the LEV system should be checked depends on how complicated the system is, how likely it is to fail, and the consequences if it does. Regular maintenance inspections should be made of the following areas:
moving parts that may wear, such as fan bearings or filter shakers non-moving parts, such as hoods, ductwork and seals (which can suffer physical or chemical damage and wear) parts that deteriorate with use, such as filters or flexible ducting items that need regular attention, such as filters that need replacing, or removing sludge from a wet scrubber. This may involve daily operator checks and periodic checks by managers.
A variety of qualitative and quantitative methods can be used to assess the performance of LEV. Qualitative assessment techniques
The ‘dust lamp’ or ‘Tyndall illumination’ makes fine particles visible. The ‘Tyndall effect’ is the forward scattering of light. This is commonly seen when a shaft of sunlight entering a building shines through mist, dust or fume in the air. The ‘dust lamp’ reproduces this effect by producing a powerful parallel beam of light (see Figure 3.7). It shows the density and movement of particle clouds in its path. The lamp should be moved to illuminate different parts of the cloud and indicate the full cloud size and behaviour.Other forms of observational assessment can be made using smoke from pellets, smoke tubes or smoke generators. These can: show the size, velocity and behaviour of airborne contaminant clouds identify capture zones and boundaries confirm containment within a hood identify draughts, their direction and size show the general movement of air.
A variety of qualitative and quantitative methods can be used to assess the performance of LEV. Quantitative assessment techniques
Inlet Face or capture velocity. See Figure 3.10. Ducts Transport (duct) velocity is measured in a straight section of duct, well downstream of bends and other turbulence sources. The duct velocity must be sufficient to keep the particles airborne (otherwise) particles may settle out in the ductwork and affect the overall efficiency of the system. The actual dust velocity required is determined by considering the nature of the particles that the system is handling such as size and density. An anemometer e.g. thermal anenometer or hot wire, velometer or a pitot-static tube could be used. The choice of instrumentation is governed by the air velocity in the duct (a thermal anemometer is more suitable if this is less than 3ms-1). An anemometer responds to the cooling effect of airflow and the pitot-static tube measuring, via a pressure gauge, velocity pressure which can then be converted to velocity. Fan/air mover Measures can be taken of: fan direction and speed; the pressure difference across the fan; and the volume flow rate. A big difference in pressure across the fan (i.e. resistance) indicates that the system will not be as efficient. Air cleaner Filters: pressure differential across the filter. Wet scrubber: pressure difference at the inlet and outlet, and the water pH if relevant to the scrubbing performance. Air sampling Additionally, air sampling may be carried out as proof of the efficiency of a LEV system and effective control of the contaminant where the hazard and potential risk is great. (See element B4). Air sampling may also include testing emission levels from the exhaust e.g. volatile organic compounds (VOCs). When carrying out testing all instruments should be calibrated; and where there may be flammable atmospheres intrinsically safe instruments should be used.
A thorough examination and testing of LEV can be considered to comprise three stages:
Stage 1: a thorough visual examination to verify the LEV is in efficient working order, in good repair and in a clean condition. Stage 2: measure and examine the technical performance to check conformity with commissioning or other sources of relevant information. Stage 3: assess whether the control of worker exposure is adequate.
PPE should be regarded as the last resort to protect against risks to health and safety. Engineering controls and safe systems of work should be considered first. This is because:
PPE protects only the person wearing it, whereas measures controlling the risk at source can protect everyone at the workplace. Maximum levels of protection are seldom achieved with PPE in practice and the actual level of protection is difficult to assess. If RPE is used incorrectly, or is badly maintained, the wearer is unlikely to receive adequate protection. RPE is uncomfortable to wear and is an intrusion into normal activities. RPE may interfere with work.
Type and level of protection required The risk assessment will indicate the type of protection required from:
type of substance and its nature e.g. corrosive, toxic etc. form of the substance route of entry into the body concentration of the substance.
There are two broad categories of RPE:
respirators and breathing apparatus (BA)
The key factors in selecting appropriate RPE are:
The level of oxygen present in the air to be breathed The presence of toxic chemicals that may pose an imminent risk to life.
Respirators work by
filtering contaminants out of the air so that the wearer breathes clean air
BA works by
delivering a supply of breathable air from an uncontaminated source