I125 sampling -Reply

[FRAMEP@ORAU.GOV]

>We need to measure (air sample) radioactive I125 gaseous effluent
>releases. We know that ANSI 13.1 discusses this but we only have the
>1969 version and really can't wait for ANSI to send us a new one
>(although we have ordered the catalogue to find the most current
>version and order it). The question is: At what distance from any
>points of turbulence (i.e. bends, fans, etc) should a
>sampling port be postioned. 

As luck would have it, I'm on the committee revising N 13.1 - perhaps
the least knowledgeable member of the committee but thats another
issue. I've been on it since 1986 or so. Believe me, the committee 
can't wait for the new one either. We've got a first draft. If I was
a betting man: it might hit the streets in 5 years. 

Technically the current (yes, 1969) version doesn't pertain to stack
/duct sampling because the appendices that do talk about these topics
are not really part of the standard. Thats why regulators
specifically cite the appendices. The new N13.1 will deal exclusively
with stacks and ducts. Workplace air sampling will be taken care of
in another standard. Environmental air sampling will be dealt with in
the new N13.9 which is currently dead in the water (I'm on that one
too).

The new N13.1 will downplay the issue of isokinetic sampling.
Certainly it won't recommend it. You'll possibly see that single
point extraction is preferred to multiple nozzles and that an
emphasis is placed on evaluating how well mixed the effluent is at
the sampling point and evaluating losses of particulates in the
sampling lines.

Nevertheless, we have to live with the version cited by the powers
that be: the 1969 version.

Although you are dealing with what is probably a gas, you will
probably be sampling particulates as well and isokinetic will
probably be required. Although there is no strict rule here, the
following commonly cited guidance tends to keep people happy: you
want to have the sampling point at least 8 duct/stack diameters
downstream of any disturbances to airflow and at least 2 diameters
upstream. It could be 10 downstream and 50 upstream, the key point is
that the effluent be well mixed. Obviously, the farther downstream
the better. Note that the flow is always turbulent, no matter where
you are. Laminar flow will only exist in extremely unusual
situations. 

You want to be downstream of the filters and fan (yes some people
have to be told this). Try to avoid sampling on a horizontal run and
if there is reason to suspect higher concentrations on one side,
sample in that area. Avoid areas of cyclonic flow. 40CFR 60 App A
Method 1 describes how to identify unacceptable levels of cyclonic
flow. Flow straighteners might be necessary.

When these conditions can't be met, you might have to make
measurements at different points across the effluent to demonstrate
how well mixed the stuff is at the compromise location. Multiple
withdrawal points might be necessary to keep people happy. 

If anything, go overboard characterizing the flow conditions prior to
sampling. Feel free to call, theres plenty else to discuss.

Paul Frame 
Oak Ridge Associated Universities