Last reviewed 19 November 2013
Nigel Bryson looks at the conclusions from a recent research report on air movement inside asbestos enclosures.
Regulation 16 of the Control of Asbestos Regulations 2012 (CAR) requires employers to: “prevent or, where this is not reasonably practicable, reduce to the lowest level reasonably practicable the spread of asbestos from any place where work under the employer’s control is carried out”.
In licensed asbestos work and removal, the main method of preventing or reducing the spread of asbestos fibres is by erecting an enclosure under negative pressure around the work area. By using ventilation units and filters, air is moved around a sealed enclosure so that any fibres that are released in the work activity are filtered out of the air, but kept within the enclosure. By maintaining a negative pressure, if the enclosure was punctured, air would be sucked into the enclosure rather than escape out.
For non-licensable work, this may mean dust-suppression techniques and workers using personal protective equipment (PPE), which is likely to include respirators. Depending on the work being carried out, an enclosure may be used to help reduce the risk of asbestos fibres spreading. However, such work will only be over a short period, so powered ventilation units are not used.
A recent Research Report published by the Health and Safety Executive (HSE), RR988 Ventilation of Enclosures for Removal of Asbestos-containing Materials, considered the issue of air movement within asbestos enclosures. This article will review some of the key findings, and highlight any information that may be useful to managers when licensed contractors are using enclosures in a building.
Maintaining air flow
The researchers tested air flows and other factors that they suspected may have an impact on maintaining an effective air flow within an enclosure. Three test enclosures were used: one small enclosure, one large one, and one connected to a ceiling void, to test what happens when a ceiling tile is removed. Various changes were made, such as the positioning of the Negative Pressure Unit (NPU), puncturing the enclosure, and opening airlocks. The effect this would have on air flow was then analysed.
Not all the results would be of interest to facilities managers, although they may be of interest to licensed contractors. The main findings were as follows.
Regardless of the extraction unit position, a ventilation rate of eight air changes per hour (ach) in the small enclosure produced a poor vertical air mixing. This meant that, at high level, the noted ventilation rate was less than 10% of the calculated rate (ie less than 0.8 ach). The result was that the air at the top of the enclosure was poorly mixed with the rest of the air in the enclosure. In effect, it would mean that any airborne fibres could potentially remain suspended in the air for longer than the standard one-hour ventilation overrun.
The location of the extract position had some effect upon the horizontal mixing within the small enclosure at the lower ventilation rate of 8 ach (384 m3h-1).
Having two airlocks open in the small enclosure increased horizontal mixing.
In the larger enclosure, a ventilation rate of 8 ach produced a well-mixed atmosphere, both horizontally and vertically, regardless of the NPU position. From this, the researchers indicated that, when considering air mixing, volume flow rate is the determining factor, not the air change rate.
Smoke testing confirmed the findings, clearly showing the layers visually within the small enclosure at 8 ach. This is significant when considering how much time is needed to clear the enclosure. Both the degree of mixing and the ventilation rate need to be considered. The researchers confirmed the value of a smoke test, which is required before enclosures are put into operation.
It is common practice to use the formula n over 1/n to determine smoke clearance. For 8 ach, this would mean the enclosure should be clear of contaminants within 1/8 hour: 7.5 minutes. However, the researchers found that, in the small enclosure, 4–5 air changes were needed to clear the smoke.
The researchers indicated that a large volume flow rate is more important in achieving a well-mixed atmosphere than NPU position or number of airlocks.
If the volume flow rates were in excess of 1900 m3h-1 for standard airlock chambers, it caused the airlock doors to “impinge on the far side of the chamber, making it difficult to work inside the chamber”.
The pressure difference between the inside of the enclosure and the surrounding area was only affected by large, unplanned openings in the enclosure walls. This was demonstrated by cutting vertical slits and using tracer gas. Only large vertical slits greater than 2m and holes with an area of around 100 cm2 had a significant effect.
The position of the airlock doors within the enclosure may be a more suitable indicator of containment integrity, rather than the pressure difference inside the enclosure and the surrounding area.
Adding flexible ducting to the NPU with 90° bends reduced the volume flow rate by approximately 1% per metre of duct and 2% per 90° bend. The researchers recommended that these reductions should be taken into account when calculating required ventilation rates for the 355mm diameter ducting they tested.
Removing ceiling tiles
The volume of the void above the false ceiling determined the effect of removing ceiling tiles on air flow.
The results of various tests showed that, once a ceiling tile is removed, the void becomes part of the enclosure and contaminated air can move freely between the spaces. Hence, on completion of work, the whole area should be considered contaminated and be cleaned.
What does it mean?
The research shows that the air flow rates are key to maintaining the integrity of the enclosure: that is keeping fibres within the confines of the enclosure. The research challenges the notion that the air change per hour formula gives an accurate indicator of how long it would take to “clear” an enclosure of smoke particles. Licensed contractors may need to reconsider how long they leave the air running at the end of a job to ensure all the airborne fibres are removed on completion of the work, depending on the size and layout of the enclosure.
Where the enclosure was deliberately punctured, unless the holes were large, there was no noticeable leakage to the outside. This confirmed that the negative pressure inside the enclosure did not allow the escape of material into the area surrounding the enclosure. The researchers used a tracer gas to do the tests.
When a ceiling tile was removed, the air from the enclosure mixed with air in the void. The researchers were able to identify the effects depending on the volume of the void above the enclosure. They indicated the voids may become contaminated when tiles are removed, and the void may need cleaning when the asbestos work is completed. Managers may need to discuss this with contractors when asbestos ceiling tiles are due to be removed.
It is likely that the findings of this research will be incorporated into the current review of the licensed contractors’ guide (HSG247).