Hood Operability

[CHRIS KUECHLE 937-5276 <KUECHLE.CHRIS@a1.abbott.com>]

Our plant maintenance staff performs an annual PM on all of our chemical 
fume hoods.  The PM includes velocity measurements at a number of points
(usually 20 to 30) at the face of the open hood.  The readings are averaged
to give a single value, which is then compared against the spec range, and
the appropriate actions (if any required) are taken.

On occasion, a large piece of equipment (such as a water bath) is placed
inside the hood, and as you might expect, the flow directly in front of the
object is significantly reduced.  If the object is large enough, a 
sufficient number of test points are obstructed such that the single "average"
value falls below the spec range, and the hood fails.  Since there is finite
amount of make-up air supplied to the room, no amount of HVAC damper tweaking
will ever get the flow at the "unobstructed" points of the hood face high
enough to push the overall average back into the spec range.

Direct radiation, contamination and/or inhalation hazards in this situation
are negligible, as the work typically involves uCi quantities of H-3 and C-14
incorporated into biological molecules.  In virtually all cases, the hood is
merely a nice, enclosed area to do work.

Moving or removing the offending object is not feasible, as most items are
quite heavy, and department managers are typically unwilling to pay for 4
hours of rigger's time (2 persons, 1 hour minimum charge, two trips) for a
PM that takes less than 30 minutes.

How are situations such as this handled at your facility?  Have you developed
criteria for hood operability in "variance" cases such as this?