Enviro Safety Partnership's LEV testing team were commission by the University to review the control of hazardous substances across a number of departments with high risk processes. We carried out a detailed survey of all LEV systems in the departments to HS258 using a modified version of the University's test procedure. We identified areas for improvement in the LEV systems and the University's management of LEV systems.
In order to assess the performance of the LEV systems the following measurements methods were used
Static Pressures. Static pressures were measured using a standard pitot tube and electronic manometer. These were obtained by measurements on a duct traverse in accordance with BS 848: Part 1 Section 34. Static pressures in Pascal's (Pa) were quoted for each test point.
Velocity Pressures. Velocity pressures were measured using a standard pitot tube and electronic manometer. These were obtained by measurements on a duct traverse in accordance with BS 848: Part 1 Section 34, with measurements taken in straight lengths of ducts well away from any disturbance to airflow such as a bend, junction, or change of section. Where possible the point of measurement were at a distance equal to at least 6 duct diameters and preferably 10 duct diameters from any such disturbance. All the measurements of an individual traverse wererecorded. Velocities were quoted in Pascal's (Pa) and meter per second (m/s). Velocities were calculated using the standard equation v= 1.29 times square root of velocity pressure (Pa) at standard air density of 1.2 kg/m3. Where duct airflow temperatures are greater then +/- 100C of the standard 210C for standard air density, corrections to air density were made. When duct velocity pressures were below 20 Pa, duct velocities were measured using a thermal anemometer using a duct traverse in accordance with BS 848: Part 1 Section 34.
Duct Dimensions. The internal duct dimensions were measured using a steel rod and 3m ruled tape.
Duct Volume Flows. The volume flow at each test point was calculated by multiplying the measured velocity at the point by the cross sectional area of the duct at the test point by the time reference, normally one hour.
Temperature. The temperature of air inside ducts were measured using either a thermal anemometer or K-type thermometer.
Hood, Enclosure, Booth Measurements
Face Velocities. Measurements of face velocity were made using a rotating vane anemometer for face opening over 120mm, for smaller face openings a hot-wire anemometer was used. According to the face dimensions up to three rows of measurements were made, with measurements on each traverse to include one at each end of the traverse with the vane head at one van head diameter from the side of the opening. The number and location of measurements depended on the type of hood, size and shape. Measurements were taken in the plane of the face opening. For rectangular hoods velocity measurements were taken at three points in each of three rows parallel to the longer dimension. For smaller hoods with dimension of 200mm to 300mm two rows of measurements were taken. For hood faces with dimensions less than 200mm one row of measurements on the horizontal centre line weremade. Face velocities were measured as instantaneous values at 15 and 30 seconds with a average value determine for each point.
Face Dimensions. The face dimensions of hoods were measured using a 3m ruled tape.
Face Volume Flows. The volume flows were calculated by multiplying the measured velocity at the face by the cross sectional area of the hood face by the time reference, normally one hour.
Capture Distance. For captor hoods the maximum distance the contaminates are caught and drawn into the hood were determined using either visualisation of process dust cloud using a light beam or observation of efficiency of extraction using a smoke test. The maximum captor distance was quoted in millimetres from the centre of the hood face. A potable high powered lamp was used for dust cloud observation. Smoke test were carried out using Draeger smoke tubes.
For Canopies cloud capture distances were measured using smoke capture tests. Smoke was supplied under the canopy using a smoke generating machine and the distance of effective capture was measured. The time taken for the cloud to clear was determined to measure the cloud clearance time.