The life & times of an HVAC Engineer











{November 8, 2010}   Is this thing switched on?

In the facility I am currently working on, part of the process is to spray the product with a fine mist of 70% Isopropyl Alcohol, ‘IPA’. As you can imagine, that poses something of a hazard. To paint a picture of how much of a hazard, here are a couple of facts:

Lower Explosive Limit of IPA = 2%
[i.e. only 2% of the air volume needs to be IPA for it to still be flammable]

Flash Point of IPA = 12oC
[i.e. the room temperature only needs to be 12oC for the gas to vaporise & be ignitable]

Image credit: bruce7 from istockphoto

So, as it’s critical to spray the product with this hazardous substance, how do you go about making sure the operators don’t get blown up? Well there are a variety of different ways, so to name just a few;

  • Minimise the amount of spray used
  • Ensure all equipment within the hazardous zone created is safe for that environment (i.e. it is non-sparking / intrinsically safe / ATEX rated)
  • Provide extract ventilation to keep the amount of IPA in the room below the lower explosive limit

Well as a building services engineer, and thus a designer of ventilation systems the latter is the most relevant to me. So off I went & designed the ventilation to remove the IPA and protect the operators. Brilliant, Chloe saves the day…just one problem though…how do we know it’s working? And if it’s not working, how do we stop the machine from continuing to spray IPA into the room? Aah. Yes. Well…best do something about that hadn’t we.

So to make sure the machine doing the spraying knows that it’s safe to spray, we’ve included a flow sensor in the extract duct. The machine receives a signal from the sensor to say there is air flow, and then it can safely spray the product with IPA. We can all breath (an IPA free) sigh of relief. But no…what if the sensor is broken?! Okay guys…we’re getting into double jeopardy here, but as it’s for safety then the more the merrier, what do you suggest?

A couple of process engineers later and to ensure we have a double layer of protection to check the ventilation is working we are installing a sensor on the fan motor – that way we know it’s running. If the fan motor isn’t running then you know it’s not safe to spray the IPA.

I can’t help but thinking though, just because the fan motor is running doesn’t mean that there is extract ventilation…the fan or drive shaft could be broken. A little bit of me thinks that a few ribbons (perhaps that’s giving way to my girly side though) around the ventilation intake would be a visible indicator of the extract working that could never give a false signal. It would be reliant on the operators stopping the machine from spraying though, as ribbons can’t give a signal directly to the machine!

Image credit: The Seattle Times

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