July 9, 1993


MEMORANDUM

SUBJECT:  Proposal for Calculating Plume Rise for Stacks with Horizontal
          Releases or Rain Caps for Cookson Pigment, Newark, New Jersey

FROM:     Joseph A. Tikvart, Chief
          Source Receptor Analysis Branch, TSD (MD-14)

TO:       Ken Eng, Chief
          Air Compliance Branch, Region II


     In response to your request, the Model Clearinghouse has reviewed your
proposal for treating horizontal and capped stacks at Cookson Pigment so
that the model (SCREEN or ISC2) will properly treat plume rise from the
Cookson Pigment stacks.  We concur in principle with the approach, with
some relatively minor changes.

     First, the analysis provided by New Jersey Department of Environmental
Protection is technically correct.  We suggest, however, that the exit
velocity for horizontal and capped stacks be set to a lower figure than 0.1
m/s.  A 0.1 m/s exit velocity may still result in significant momentum
plume rise being calculated, even though these kinds of sources should have
zero momentum rise.  We therefore suggest setting the stack exit velocity
to a lower value, such as .001.  

     For horizontal stacks that are not capped, we suggest turning stack
tip downwash off, whether there are buildings or not.  Stack tip downwash
calculations are inappropriate for horizontal stacks.  

     For vertical stacks that are capped, turn stack tip downwash off and
reduce the stack height by three times the actual stack diameter.  The cap
will probably force stack tip downwash most of the time.  The maximum
amount of the stack tip downwash (as calculated in ISC2) is three times the
stack diameter.  Reducing the stack height by this amount, while turning
off the stack tip downwash option, causes the maximum stack tip downwash
effect.  The resulting concentrations may err slightly on the high side.
For stacks with small diameters, such as those at Cookson Pigment, the
error should be quite small.  Note, however, that this approach may not be
valid for large diameter stacks (say, several meters).

cc:  A. Colecchia
     D. Wilson