The
final
rule
does
not
distinguish
between
the
replacement
of
components
that
are
expected
to
be
replaced
frequently
or
periodically
and
the
replacement
of
components
that
may
occur
on
a
less
frequent
or
one­
time
basis.
It
likewise
does
not
distinguish
between
the
replacement
of
larger
and
smaller
components,
instead
requiring
greater
scrutiny
if
the
replacement
in
question
is
part
of
a
project
that
exceeds
20%
of
the
replacement
value
of
the
process
unit.

Our
decisions
on
these
points
are
derived
from
reflection
on
the
function
of
the
exclusion
in
the
context
of
the
Act.
As
explained
above,
and
as
described
more
fully
in
our
legal
analysis
set
forth
below,
we
do
not
believe
that
application
of
the
major
NSR
program
to
"
modified"
plants
is
designed
to
require
existing
plants
that
are
continuing
to
operate
in
a
manner
consistent
with
their
original
design
to
curtail
their
rate
of
production
or
hours
of
operation
beyond
limitations
set
forth
in
their
existing
permits.
We
likewise
do
not
believe
that
the
program
is
designed
to
discourage
plants
from
replacing
parts
or
components
so
as
to
preserve
their
ability
to
produce
at
that
rate.
Rather,
we
believe
Title
I
of
the
Clean
Air
largely
leaves
to
State
and
local
permitting
authorities
whether
to
require
adjustments
in
the
operations
of
those
plants
in
order
to
reduce
emissions
to
the
degree
needed
to
attain
or
maintain
national
air
quality
standards,
and
how
to
weigh
the
trade­
offs
such
adjustments
may
produce
in
terms
of
potential
economic
impacts
and
loss
of
productivity.
Instead,
we
believe
the
central
function
of
the
application
of
major
NSR
permitting
requirements
to
"
modifications"
is
to
assure
that
plants
install
state­
of­

theart
pollution
controls.

We
recognize
that
on
these
points,
the
approach
taken
by
our
final
rule
thereby
differs
in
some
respects
from
the
multifactor
case­
by­
case
approach
we
have
been
using
in
identifying
RMRR,
and
particularly
from
some
of
our
applications
of
that
test
to
certain
equipment
replacements.
We
believe,
however,
that
this
adjustment
in
our
approach
is
fully
warranted
for
the
reasons
outlined
above,
and
described
more
fully
in
our
legal
analysis
below.
