Photograph courtesy of Horizon Environmental Ltd.
Consider the case of a client with a boiler room, once liberally splattered with asbestos-based insulation material. It’s comparatively easy for a licenced asbestos-removal contractor (LARC) to strip out the bulk of the material with low-risk techniques, but it soon becomes a case of diminishing returns. The less asbestos there is remaining, the harder it can become to remove it, and the greater the expense.
When faced with asbestos insulation residues on semi-porous substrates like brick and concrete, removal of the final 0.1% of asbestos-containing material (ACM) is very challenging. Residual fibres can be embedded in pits, dimples and micro cracks – making the traditional, low-impact approach of hand scraping accompanied by suppression and shadow vacuuming extremely time-consuming. Often, the removal ends in an admission of failure and encapsulation – a process all too likely to be repeated by another LARC in a few years time.
Clearly this is quite an unsatisfactory position for the client. They’re spending a vast sum of money on asbestos removal, only to be presented with a residual risk that still has to be managed – and probably at the same level and cost as before.
It’s therefore easy to understand the temptation of aiming for 100% ACM removal in a boiler room. It’s broadly possible through the use of two competing techniques: blasting (using wet media), and low-vibration needle gunning. Both have their champions – two LARCs I know are very firmly in opposing camps. In the red corner we have: ‘Blasting is faster at cleaning than needle guns, even when you factor in the additional clean-up’. And in the blue corner: ‘Needle gunning is a much simpler method that creates lower exposure and is easier to manage’.
Comparing blasting and needle guns
It’s important to start with one thing that both needle guns and blasting have in common: you’re not supposed to use either technique unless you’ve already done your best to remove all significant deposits of asbestos through traditional methods. This means scraping off all but the last miniscule ACM traces manually – accompanied by sprayed surfactant and shadow-vaccing.
With the use of needle guns on the rise, I’ve also heard that HSE inspectors are coming across it more. And, quite rightly, they’re asking whether the method has been properly assessed. I’ve heard that while the HSE isn’t tremendously keen on blasting, it has questions over the vibration levels of needle guns (more on this later). Clearly this is crucial to any technique or technology: it must be properly assessed, and your team must be competent to use it.
Both techniques present their own challenges which need to be considered if you are aiming for spotless. You’ll need to balance all of the pros and cons when you complete your risk assessments.
Noise and vibration
The noise levels of blasting vary dramatically depending on several factors including the choice of media, and even the location (boiler rooms usually reverberate more, for example). Due to this uncertainty, Quill – one of the leading blasting manufacturers – is a little cautious about publishing noise figures. Essentially, it’s not possible to predict an accurate noise level unless you know the usage situation. Quill says that noise at the lance could be >110dB(A). Vibration magnitude is negligible at around 0.2m/s2. That’s 10 times lower than the EC-specified minimum level for unrestricted hours of work.
Needle guns used to be known for their huge vibration levels, but recent pneumatic variants are much improved. The one I am familiar with is the Trelawny VL303, whose manufacturer claim it has a noise level of 109.5dB(A), and vibration of 2.3m/s2. However, there does seem to be some question marks about this very low figure. Not least because normal operation is to use two hands… Clearly, unlike with blasting, whatever the HAVS (hand-arm vibration syndrome) data is, it is not negligible. If you plug 2.3m/s2 into the HSE’s HAVS calculator, you get a remarkable nine hours and 27 minutes to reach the lower exposure action value (EAV). However, worst case vibration data from the manufacturer indicates something nearer to 90 minutes or below. I understand Trelawny are conducting some independent HAVS testing and the report will be out soon.
Whether blasting or needle guns are selected, then effective hearing protection will be mandatory. Vibration needs to be looked at, and hopefully accurate data will be available soon
Waste
There’s no avoiding the issue, blasting will add waste to the project. Quill states there’ll be 0.5-1.1kg of material created per minute of use. You’ll need to consider the increased manual handling issues that this will create for the project. These may be exacerbated if you have to lug waste up from the basement – especially if there is any restricted access involved.
Fibre release
Both techniques are high-impact, high-disturbance methods that should only be used on trace residues. Both techniques use different approaches to keep dust and fibre levels down. As the name suggests, wet blasting uses water – which atomises as it hits the substrate with the blast media. This will probably be most effective when removing chrysotile residues, as amphibole fibres such as amosite or crocidolite are hydrophobic (they repel water).
Needle guns use the shadow vac technique, and come with dedicated vac cowls. The H-Type vacuum is attached at point A in the diagram, providing effective local exhaust ventilation (LEV) at the point of disturbance (B).
As with any asbestos-removal technique, you’ll need to test that exposure is as low as practicable, and investigate any elevated results.
The real problem for wet blasting comes with all that water; Quill states that you’ll be using 2-4 litres per hour. Water vapour plays havoc with air testing – whether that is standard optical or electron microscopy – occluding the filters so they can not be read. The default position seems to be that you should assume a high fibre release, and use supplied-air respirator (RAS) masks.
By contrast the needle and shadow vac technique is relatively easy to test. The results I’ve seen are favourable, with an average of 0.06f/ml (fibres per millilitre of air), highest reading of 0.12f/ml, and lowest of 0.01f/ml.
Water vapour
The high humidity of blasting creates two more issues that you need to allow for. Water does not play well with a negative pressure unit (NPU)’s HEPA filter. To counter this, Quill provides moisture vanes that work along with the standard pre-filter to protect the HEPA.
Another impact – especially in the cooler atmosphere common to basements – is that we often hit the dew point and visibility falls dramatically. Neatly, Beacon’s recirculating NPU incorporates an in-line heater to prevent this. You cannot underestimate the impact of low visibility on supervision – vision panels and especially CCTV will both become very limited, and you’ll need to identify enhanced supervision techniques to combat this. You may consider having a deputy supervisor in the enclosure to be the eyes of the lead supervisor outside.
Other considerations
There’s a rather unfortunate, nebulous bag of additional issues that you will need to factor in. As we’ve established, both blasting and needle gunning are very, very noisy, clearly requiring hearing protection. The follow on effect of that is that operatives will not be able to hear you when you try to communicate with them, whether routinely or in case of an emergency.
This is further compounded by the visibility issue discussed above – i.e. you can’t see them, and they can’t hear you. You might consider flashing beacons, activated externally, at the point of work, which will allow you to stop work quickly and easily. The extra internal supervisor would also help with this.
You also need to consider that needle guns are quite heavy, in addition, large-bore compressor hoses and metal coupling will add markedly to this. You should always step down to the narrower whip lines to minimise this manual handling issue. Generally, you should also consider fatigue as a hazard.
Another consideration with blasting is that the media obviously goes somewhere. Predominantly this will be the floor, but if operatives are working near the enclosure wall it could damage the sheeting and lead to a breach. A less obvious risk is that the media can be blasted into inaccessible voids, resulting in a spread of asbestos. Your design process needs to include careful planning of how and when to use the technique to avoid this.
Finally, while it’s not really part of the risk assessment, needle guns are much more mechanical than blasting equipment. As such they have moving parts, and need to be maintained through periodic stripping down, cleaning and oiling. They are also vulnerable to icing up where the weather outside is cold. For both cleaning and good maintenance, Trelawny recommends ISO22 low viscosity anti-freeze oil.
It’s hard to argue against a client’s wholly understandable desire for an asbestos-free boiler room, and these two techniques are the only options that get close to achieving it. As with all asbestos removal methods, however, they bring a range of issues that have to be individually and collectively assessed. It’s our job as professionals to understand the complexity, and ensure that we manage all of the risks.
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