Posted By holmezy Hi All, last week I had a query re face velocity, and since then things have moved on a bit. There is a currently a HSE target area regarding silica dust. My company process's stone and also sells equipment and we have had a visit recently from the HSE, and basically were ok. However, many of our customers have also had visits and "allegedly" have been told that their dust extraction system (wet cabinet) is either, depending on location; a) absolutely fantastic, does a great job (down south) or b)totally inadeaquate, (up North) The reason for the "inadequate" comment has been that it doesnt have enough air flow. Some customers have had a requirement for 2m cubed per second verbally quoted to them, but no-one seems to know where this figure comes from. Also, I cant understand how someone can say this without actually testing the air flow. Our sales chaps are getting lots of flak and hassle from existing customers and potential new ones regarding the performance and have passed it on to me. All I can find re recommended airflow rates is from COSHH essentials ST3 which states airspeed 1 - 1.5 m per sec. Whats the correlation between air speed and cubic meters per second? Where do I find more info? I've ordered the LEV guidance from HSE books but it wont be here for a few days. Also had a quote from a customer saying that the comment he had from the inspector was that "absolutely no dust is allowed" so why the minimum level of 0.1mg/m3? I've measured the air speed of one of said cabinets and it is approximately 3m/sec over an area of approx 1sq/m so what does that give me? I'm starting to feel a little bewildered!! Holmezy

Posted By Chris Packham A good starting point could be HS/G37 "An introduction to local exhaust ventilation". Since this is an HSE publication, can they then argue? Chris

Posted By holmezy Derek, I agree with you wholeheartedly re the "no dust" statement. Calcs just back up what I thought. Chris, got that on order plus another re LEV etc but wont be here til next week. Paul, I agree that all sorts of factors can influence the flowrates etc which is why I can't understand how an inspector can say that its not adequate, or for that matter, that its ecxcellent, just by looking at it. So far, none of our customers have had anything in writing from the HSE, just told that its not good enough and it should be 2m/sec??. I cant find any reference to 2m/s (and if my calcs are correct our machines give 3m/sec) but there are quite an angry bunch of customers at the moment. I thought the limit was 0.1mg/m3 for RCS? Holmezy

Posted By carol hanks The 2m/s you are being quoted is the generally accepted capture velocity for fine dusts, i.e. the velocity required at the point of dust generation for effective capture. This is not the same as the face velocity of a hood or inlet. As Paul says, you probably need advice from an occupational hygienist as the important point to note is the exposure limit relates to respirable crystalline silica, not to "dust" generated by the process.

Posted By Paul Leadbetter Holmezy HSG37 tabulates a range of capture velocities so I hope your copy arrives soon! Paul

Posted By Paul Leadbetter Chris If my memory serves me well (and it often doesn't!), those are transport (or duct) velocities, aren't they? Paul