Raw
sewage
is
usually pumped
at
heads
of
less than
30
m
(100
ft) by
single-stage pumps with open impellers.
Several
(at
least two) units
are
installed
to
provide
back-up
and to
handle
the
widely
fluctuating flow
range.
In
motor sizes below
75 kW
(100
hp),
line start-
ing
is
almost universal. Reduced-voltage
or
part-wind-
ing
starting
may be
required
in
pumping stations
located several miles
or so
from
main electrical substa-
tions.
Reduced-
voltage
starting
may
also
be
desirable
to
limit voltage dip.
In
adjustable-speed systems,
raw
sewage pumps
usually
require only
a
moderate
speed
range, which simplifies
the
speed-control equipment.
Features that should
be
considered when writing
the
specifications
are the
same
as
those listed
for
water
pumping,
but
with special emphasis
on the
ability
of
the
unit
to be
returned
to
service quickly
after
flooding.
13-14.
Need
for
Engine-Generators
The
need
for
standby power
from
engine-generators
is
not
only site-specific
but
depends
on the
need
for
absolute reliability
and
what
the
owner
is
willing
to
pay.
Diesel engines
are
usually preferred, because
they
are
smaller than
gas
engines
and
hence less
expensive. However, storing diesel
fuel
on
site
requires double-wall storage tanks
and
deterioration
is
a
problem.
On the
other hand,
if
natural
gas can be
piped
to the
site,
no
storage
is
required.
Gas may not
be
available
at
some sites,
and in
earthquake-prone
areas,
it is not
completely
reliable.
Pollutant
emissions
from
gas
engines
is
minimal, whereas
it is
more
diffi-
cult
to
control pollutant emissions
from
diesel
engines. Furthermore, regulations tend
to
become
more stringent with time (see Section
14-14).
Alto-
gether,
the
need
for and
choice
of
standby power gen-
eration requires
the
most
careful
consideration.
In
the field of
electric
power supply, substations
supplied
from
main transmission power
lines
are the
most reliable. Substations rarely
fail
even when
the
power lines
or
even
the
substations themselves
are
struck
by
lightning. Generally, only
one or two
out-
ages
per
year occur.
The
next most reliable component
is
the
distribution substation, which operates
at
lower
voltage.
One to six
outages
per
year might
be
consid-
ered average.
The
least reliable
are
long power lines
to
isolated communities where outages
are
more
frequent
and
last
for
longer periods. These
lower-
voltage
power
lines
suffer
more
frequent
power outages because
of
accidents, lightning, construction,
and so on.
Outages usually involve only
a
small area (for
example,
a
quadrant
of a
small town),
but
occasionally
an
entire city
or
even
several
states
are
affected.
On
July
2,
1996,
a flashover
between
a
tree
and a
345
,000-
volt
transmission line triggered
a
chain
of
events that,
due to
record-setting demands
for
power,
led to
outages that lasted
up to six
hours
in
some areas
throughout
15
states
and
affected
approximately
two
million customers.
The
blackout
of
1977
left
New
York
City
and
Long Island without
electric
power
for
several hours.
The
blackout
of
November
9 and 10,
1965,
left
most
of the
northeastern United States
and
two
Canadian provinces without electric power
for
most
of two
days. Natural disasters such
as
hurri-
canes, tornadoes,
and
earthquakes tend
to
create many
hours
or
days
of
outages. Following
the San
Francisco
(actually Loma Prieta) earthquake
of
October
17,
1989, power
was not
restored
to all
parts
of the
area
for
several days.
The
Oakland
fire of
1991
burned
the
distribution power lines
to the
only water booster
pumping
station that supplied water
to
northeast Oak-
land,
so
there
was no
effective
way to fight a
grass
fire
that destroyed
a
large residential district.
Some pumping plants
can be
without power
for as
much
as an
hour
(or
even more) before
flooding
occurs,
and
some utilities have concluded that,
for
their situation, this grace
period
makes standby power
unnecessary (see Section
24-11).
On the
other hand,
the
public
has
become more intolerant
of
utility
fail-
ures.
This attitude, coupled with
the
deregulation
of
power generation
(a
move that
can
only result
in
trim-
ming costs
by
every producer
to
remain competitive)
is
likely
to
result
in
less
reliable
power
in the
future.
Even though power
in any
given area
has a
history
of
good reliability, there
is no
guarantee that such reli-
ability will continue
in the
future.
No
blanket rules
can be
given. Investigate each
project separately.
To
arrive
at the
most rational deci-
sion, examine
the
history
of
power failures,
the
conse-
quences
of
down time,
and the
costs
—
both
monetary
and
subjective
—
such
as
political concerns, customer
exasperation,
and so on.
13-15.
Design
Checklist
The
following checklist contains aspects
of the
char-
acteristics desirable
in
motors used
for
pumping ser-
vice.
The
typical pumping station service
is
severe
and
warrants
the
expense
of the
more rugged motor
designs. Such motors, manufactured
in
NEMA stan-
dard
frames
from
size
286
through
445 (up to
about
150 kW or 200 hp at
1800 rev/min), typically have
cast-iron
frames,
whereas smaller motors with rolled
steel
or
aluminum
frames
may be
available
from
the
same makers.
1.
Motor
frame
(cast iron
or
[above
200 to 400 hp]
fabricated
steel)
2.
Enclosure
to
suit application