Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones
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•
Remote location
of
installation
and
unreliable elec-
trical supply
•
Fuel (natural gas, diesel oil) available
at
prices
competitive with electricity.
Factors that
may
cause rejection
of
engine drives
include
•
Concern over noise emissions
•
Concern over
air
pollutant emissions
•
Lack
of
adequate maintenance capability
on the
part
of the
owner
•
Small (less than
60 hp)
unit pump requirements.
To
determine whether
an
engine drive
is
more eco-
nomical than
an
electric motor,
the
best approach
is to
base
the
cost comparison
on a
unit
of
energy (such
as
kilowatt-hours
or
British thermal units).
The
costs
must
be
modified
to
include
the
efficiency
of the
pump-driver
combination.
Do not
forget
to
take into
account
obscure items such
as
transformation
and
motor
efficiencies
and
slip, heat losses
for
motor
drives,
the
cost
of
compressing
fuel
gas, gear losses,
and
power-absorbing accessories
for
engine drives.
The
costs
for the
various
forms
of
energy depend
on
many
factors,
including location and, sometimes, time
of
day of
consumption (particularly
for
electricity).
These factors should
be
explored
carefully
and
then
base prices should
be
determined. Once this
has
been
done,
the
following
multipliers
for
U.S. customary
units
can be
used
to
obtain
the
cost
per
British thermal
unit:
•
Biogas
—
free,
except
for the
cost
of
compression
and
transportation
and
standby
fuel,
if any
•
Crude
0//—
dollars
per
barrel:
1
9,000
Btu/lb,
7.5
Ib/gal,
42
gal/bbl
•
Diesel
fuel
—
dollars
per
gallon:
19,000
Btu/lb,
7.5
Ib/gal
•
Electricity
—
dollars
per
kilowatt-hour:
3413
Btu/kW
• h
•
LPG-
dollars
per
gallon:
2316
Btu/ft
3
,
36.5
ft
3
/gal
•
Natural
gas
—
dollars
per
therm: 1000
Btu/ft
3
,
100
ft
3
/therm.
Once
the
difference
in
energy costs
has
been calcu-
lated,
it can be
combined with adjustments
in
mainte-
nance costs
and a
comparison
can be
made against
capital
cost
difference
to
arrive
at an
initial assessment
of
economic viability.
After
an
engine drive
has
been
selected,
a
multitude
of
decisions
is
required.
The
pri-
mary
categories
for
decisions include
•
Duty cycle
•
Fuel
•
Aspiration
•
Type
of
engine.
Secondary categories include
•
Starting method
•
Cooling method
•
Controls
•
Governors
•
Accessories
•
Combustion
air
•
Exhaust silencing
•
Pollution control
•
Vibration isolation.
Finally,
the
following peripheral systems must
be
designed:
•
Lubrication
oil
storage
and
supply system
•
Fuel
oil or
fuel
gas
storage
and
supply system
•
Service piping
•
Building envelope
•
Ventilation systems.
Most
of the
decisions listed
in the
primary
and
sec-
ondary categories dictate
the
equipment
to be
supplied
as
a
part
of the
basic equipment contract. Engine
drives
are
complex because
of the
number
of
compo-
nents that must
be
coordinated properly.
A
unit
responsibility provision, with
a
single manufacturer
responsible
for
providing
the
driven equipment,
the
engine,
all
engine-related
accessories,
any
gear reduc-
ers or
increasers,
shafting,
and
equipment supports
is
a
recommended approach
for
such systems. (See Sec-
tion 16-1
and
Appendix
C,
Section 1.02B
for
unit
responsibility).
The
third group
of
issues
is
designer oriented
and
relates
to
systems
to be
designed
by the
pumping sta-
tion engineer
and to be
specified
and
supplied inde-
pendently.
In
addition,
be
aware
of
code requirements
governing stationary
engine
installations,
particularly
NFPA
37 and
local
fire
codes.
14-2.
Duty
Cycle
The
term
"duty
cycle"
means both
the
specifics
of the
application (duty), such
as
generation, direct drive,
emergency,
or
standby,
and the
time-based utilization
of
the
equipment.
Direct
Drive
Direct-drive systems
require
the
smallest engine
to
drive
a
given load because
the
engine does
not
have
to
sustain
the
loads imposed
by
squirrel-cage induction
motors'
inrush currents.
If an
adjustment
(gear reducer
or
increaser)
is
required between
the
engine
and
pump
speeds,
specify
gears with
a
nonreversing
mechanism
to
prevent reverse rotation
on
pump shut-down.
Generator
Duty
Generator
duty
requires
a
careful
analysis
of the
applied loads
and
generator response. Inrush currents
require
the
generator
to be
sized
to 120 to
150%
or
more
of the
nominal running load.
The
best approach
is to
apply loads
in
increments, thereby limiting
the
amount
of the
starting load
to be
sustained
by the
engine-generator
at any one
time.
A
careful
analysis
of
load increments (type, size)
is
required
to
ensure
that
engine capability
is not
exceeded.
A
complete
description
of the
conditions under which load will
be
applied
to the
standby generator must
be
furnished
to
the
engine-generator
set
manufacturer.
The
informa-
tion
to be
furnished
should include
•
Size
and
NEMA code letter
for
each pumping unit
or
other large motor
to be
started
•
Magnitude
of
miscellaneous loads (electrical
resis-
tance loads, lights, small motors)
•
Motor start sequence
after
the
standby generator
has
started
and
reached stable operation.
Ask
the
manufacturer
to
consider this information
and
recommend specific equipment
for the
proposed
application.
The
information thus received
can be
used
for the
preliminary allocation
of
space within
the
pumping
station.
Be
sure
to be
generous with space
requirements because manufacturers
may
change their
minds
in a
bidding situation.
If
competitive bids
are
sought,
include
all of the
information
and the
provi-
sion
for a field
test
of the
standby generator.
An
analysis
for the
required size
of
generator
and
engine
is
given
in
Example
9-10.
Continuous
Duty
Continuous-duty
engines should
be
selected
using
conservative rating factors (Section 14-6).
The
princi-
pal
objectives
are
reliability,
low
operating costs,
and
a
long service
life.
Just
as
with
electric
-motor-driven
units,
continuous
duty
applications require multiple
units
to
permit maintenance operations without
a
loss
of
capacity.
Standby
Duty
and
Standby
Generators
Standby
duty engines used
to
drive pumps
or
genera-
tors
should provide rapid starting
and be
able
to
assume
the
load soon
after
starting. Special starting
aids, such
as
jacket water
and
lube
oil
heaters, should
be
provided. Consult
the
engine manufacturer
for
rec-
ommendations. Engines
for
this service should have
a
relatively high torque versus speed relationship.
Standby
generators
are
considered
in
Chapter
9,
which
states that standby engine-generator sets need
to
meet
and
sustain
the
high starting loads imposed
by
squirrel-cage induction motors.
One
effective
tech-
nique
is to
size
the
generator
for the
worst-case condi-
tion
of
running loads
and
start-up,
to
provide
the
generator with
a field-forcing
regulator,
to
size
the
engine
for the
maximum steady-state operating loads,
and
to
equip
the
engine with
a
large
flywheel to
assist
in
sustaining
the
starting loads.
It is
important
not to
oversize diesel engines
for
standby generators. Under-
loaded
diesel
engines develop carbon deposits
in the
exhaust
gas
passages. These deposits reduce
the
reli-
ability
of the
engine. Reduced reliability
is
particu-
larly
a
problem with standby generators because
of
the
following:
•
Standby generators
are
typically selected
for
start-
ing
loads under worst-case
conditions
—
not
running
loads.
•
Standby generators
are
exercised frequently, usu-
ally
under less than peak load conditions.
In
pumping stations, solutions
may
include
the
fol-
lowing:
•
Recommended operating procedures that force
the
station
to
operate under peak loads. These might
include
(1)
shutting down
a
wastewater pumping
station
and
storing
the
wastewater
(if
possible) until
all
pumps
can be
started
and
operated under power
supplied
by the
standby generator
for
long enough
to
clear
the
carbon deposits
from
the
engine
or (2)
operating
a
water pumping station
at a
maximum
rate under standby power.
•
Provision
(in the
station motor control center
or at
the
standby generator breaker)
for
connecting
a
por-
table
electrical
load bank.
14-3.
Fuel
for
Engines
Factors that govern
the
choice
of
fuel
for an
engine
installation include
•
Duty cycle.
•
Application
of the
equipment (for example, diesel
fuel
may not be the
best selection
for a
standby gen-
erator because
of the
duty cycle. Conversely, natu-
ral gas may not be
available
on a
reliable
basis).
•
Economics.
Fuels
available
for fixed-engine
installations
are
gas-
eous: (natural gas, liquid propane gas, biogas)
and
liq-
uid
(diesel, gasoline).
Combination
fuel
systems
are
available that allow
some engines
to
function
on
more than
one
fuel,
which
improves economy
or
reliability.
These
include
the
following:
•
Bifuel
carburetion
systems.
These
permit
the
utili-
zation
of
more than
one
gaseous
fuel.
As a
rule,
these operate
on an
either/or basis. Blending
is not
possible. Each
fuel
has its own
pressure regulator
and
carburetor. Switchover
to the
standby
fuel
occurs
on low
pressure
of the
preferred
fuel.
•
Blended
fuel
systems.
These
are
external
to the
engine
and
require mixing equipment
to
blend
two
gaseous
fuels
together
to
meet engine
fuel
demand.
Typically,
air is
used
to
reduce
the
fuel
value
of a
standby,
higher-
value
fuel
to
that
of a
lower-
value
primary
fuel.
The
advantage
is
that
the
blending
system
can be
used
to
maximize utilization
of the
primary
fuel
when engine demands
are
greater than
its
availability. Fuel blending systems
can be
costly.
However, once
set up
they have proven
to be
reli-
able
and
economical
to
operate. Blended
fuel
sys-
tems
can be
used, with some limitations,
on low
exhaust
emission applications.
•
Dual
fuel
engines. This category
is
limited
to
rela-
tively
large [550
kW
(750
hp) and
larger], slow-
speed (1200 rev/min
or
less), diesel engines.
The
engine
is
typically started
as a
diesel.
Once
in
oper-
ation,
up to 90%
(heating value)
of the
fuel
is
pro-
vided
in the
form
of gas
(natural
gas or
biogas) with
the
remainder provided
as
diesel
oil.
Ignition
is by
compression, rather than
by
spark.
Natural
Gas
Commercially available natural gas,
with
a net
higher
heating
value (HHV)
of
4.45
x
10
4
J/L
or
12.4
kW
-
h/m
3
(1200
Btu/ft
3
),
is a
nearly ideal engine
fuel
for the
fol-
lowing
reasons:
•
Ignition takes place
uniformly
over time
and
avoids
high
momentary loads
on
engine parts.
• Gas has a
nearly
uniform
fuel
value
of
predictable
quality.
• Gas is
clean
(no
water vapor
or
contaminants).
• Gas is
much more reliable than electricity (also,
a
limited
fuel
supply
can be
stored).
• Gas is
usually available
at a
wide range
of
pressures.
•
Engines
are
available
from
20 kW (30 hp) to
3700
kW
(5000
hp).
•
Higher compression ratios
for
improved
fuel
con-
sumption
may be
used.
•
Clean-burning engine designs that meet
air
pollu-
tion code restrictions
and
that have substantially
improved
efficiencies
are
available.
Disadvantages include
the
following:
• Gas may be
more expensive than alternative
fuels,
especially
if a
long transmission
pipe
is
needed.
•
Natural
gas may not be
available under emergency
conditions
if it is not
stored
on
site.
Thoroughly
explore
its
availability with
the gas
utility.
• Gas may not be
available with adequate pressure
for
new-technology clean-burning engines,
and gas
compressors
may be
required.
•
Regulatory agencies
may
prohibit
the use of gas for
an
engine
fuel.
•
Gas,
in the
right concentration
in
air,
can
create
an
explosive mixture.
Propane
Propane, available
as a
liquid under pressure (LPG),
is
useful
as a
reserve
fuel
for
spark-ignited engines.
The
advantages
of
propane include
the
following:
•
Propane
has a
relatively high
fuel
value
of
9.32
x
10
4
J/L
or
25.8
kW
-
h/m
3
(2500
Btu/ft
3
).
• A
large quantity
of
fuel
can be
stored
in a
relatively
small volume
7.1
1
kW
-
h/L
(91,900
Btu/gal).
•
Propane
can be
stored
for
long periods without
deterioration.
•
Propane
has a low
liquid
specific
gravity
(0.51).
•
Propane vaporizes
at
-43
0
C
(-42
0
F)
and is,
there-
fore,
useful
in
cold
climates.
The
disadvantages
of
propane include
the
following:
•
Unless
the
rate
of
fuel
consumption
is
relatively
low,
liquid must
be
vaporized with
an
external heat
source prior
to
transport
to the
engine carburetor.
• The
high gaseous
specific
gravity (1.52 relative
to
air)
is
hazardous
at low
points
in
structures
if
leaks occur.
• The
sharp peak
in
ignition characteristics
may
increase maintenance,
and it
limits
the
compression
ratio
to
8.0:1.
•
Propane cannot
be
used
in a
compression-ignition
engine.
Propane
may be too
expensive
for
continuous use,
but given
the
advantages listed here,
it can be an
ideal
reserve
fuel
or
fuel
for a
standby generator.
Butane
Butane
is
somewhat similar
to
propane,
but
because
its
combustion characteristics
are
erratic,
it is not
acceptable
for
most modern engines. Butane
does
not
vaporize below
-0.5
0
C
(31
0
F),
which
is a
problem.
Biogas
Biogas,
a
product
of
anaerobic decomposition
of
organic materials,
may be
readily available
for use as
an
engine
fuel
at
wastewater treatment
plants
and at
sanitary
landfills.
As a
rule,
the gas
consists
of 40 to
65%
methane,
35 to 65%
carbon dioxide,
and
small
amounts
of
hydrogen
sulfide,
nitrogen,
and
haloge-
nated
hydrocarbons,
and it
leaves
the
digester
or
land-
fill
saturated with moisture.
It may
also contain
contaminants
such
as
siloxane, arsenic, vanadium,
chromium,
and
other substances.
The
moisture and,
perhaps (depending upon
the
engine manufacturer's
requirements),
the
contaminants must
be
removed
before
it is
introduced into
the
fuel
system.
The
fuel
value
varies
from
about
15,000
to
24,000
J/L
or
4.1
to
6.7
kW/m
3
(400
to 650
Btu/ft
3
).
The
advantages
of
using this
gas for a
fuel
are as
follows:
• The
combustible constituent (methane) makes
it a
good
fuel
for
internal combustion engines.
• The gas is a
wasted byproduct
of
most wastewater
treatment plants
and
sanitary landfills and, thus,
is
free
except
for the low
capital investment required
for
the
collection system.
•
With
the
proper selection
of
engines
and
engine
cooling systems
and an
appropriate
gas
system
design,
the
only treatment required
to
make
the gas
suitable
as a
fuel
is
trapping
and
draining
the
con-
densate.
The
following
are the
disadvantages
of
biogas:
•
Only modest
fuel
pressures
are
available without
resorting
to gas
pressurization systems.
• The low
fuel
value requires special
carburetion
systems.
• The
saturated
gas is
warm
as it
leaves
a
landfill
or a
digester.
As it
cools,
condensate
forms
and
must
be
trapped
and
drained
safely
from
all low
points
in
the
piping system.
•
Contaminants such
as
particulates
and
hydrogen
sulfide
may
cause corrosion and/or clogging
of
appurtenances.
•
Most
landfill
gases contain
a
wide spectrum
of
cor-
rosive
constituents. Maintenance
can be
quite
costly.
• The gas flow
from
wastewater treatment plants
fluc-
tuates
greatly
(3:1
peak
to
average
or
greater). This
fluctuation
nearly
always mandates
the
provision
of
an
alternate source
of
fuel
(usually natural
gas or
diesel
fuel,
although propane
is
sometimes used).
Fuel
supply systems must
be
designed
and
engines
must
be
specified
to
accommodate
the
special
charac-
teristics
of
biogas. Some engineers provide scrubbing
systems
and
fuel
filters to
protect biogas
fuel
engines
against corrosion
and
contaminants. Some engine
manufacturers
have
been
able
to
apply naturally aspi-
rated
and
some
turbocharged
engines
to
biogas
fuels
without
conditioning
the
fuel
to
remove contaminants
other than condensate. Turbocharged engines with
draw-through
(low
fuel
inlet pressure) systems have
a
good record.
The
only protection
found
to be
neces-
sary
has
been high-temperature
cooling
systems
to
guard
against corrosion. Other engines, however,
require extensive
and
costly contaminant removal sys-
tems. Engine manufacturers should
be
consulted
and
required
to
furnish
guarantees regarding requirements
for
fuel
quality before considering them
for
candi-
dates
for
projects where biogas will
be the
fuel.
Diesel
Fuel
Diesel
oil is an
excellent
choice
for
standby systems.
At
current prices
it is too
expensive
for
continuous
operation.
Its
advantages are:
• The
high
flash
point
and low
volatility makes
diesel
oil
safer
to
store than most
fuels.
•
Higher compression
ratios
compared
to gas
engines
permit greater horsepower
per
cylinder
and
improved
efficiency.
• It is
common
and
easily obtainable.
The
disadvantages
of
diesel
fuel
include
the
following:
•
Diesel
oil
deteriorates
in
storage:
at 6- to
12-month
intervals,
the
tank must
be
drained
and
refilled.
An
alternative
is to
plan operating cycles
and
limit
fuel
tank
size
to
ensure
a
full
turnover
of
fuel
inventory
every
6
months.
• The
fuel
supply systems
are
more complex than most.
•
Engines
are
harder
to
start than other types.
•
Engine maintenance costs
are
high.
•
Engines
are
costly (but
no
more
so
than
gas
engines).
Gasoline
Gasoline
is
often
used
for
engines powering small-
to
medium-sized portable pumps,
but the
disadvantages
for
stationary gasoline engines
are
overwhelming.
•
Gasoline
is a fire and
explosion
hazard
due to its
low
flash
point
and
high vaporization rate.
•
Gasoline engines generally
are
more temperamental
than
other types.
•
Gasoline engines
are
relatively high-speed designs,
and
they
are
limited
to
approximately
375 kW
(500 hp).
• The
allowable storage period
for
gasoline
is
very
short.
14-4. Aspiration
Engines
may be
either naturally aspirated
or
pressure-
charged.
Pressure charging
is
typically accomplished
by an
exhaust
gas-driven
turbocharger
or an
engine accessory
train-driven
blower.
For
most engines,
the
combustion
air is
pressurized
and the
fuel
is
then added
at the
com-
bustion
chamber. This method
of
operation requires
significant
[140
to 420
kPa
(20 to 60
lb/in.
2
)]
pressure
for
the
introduction
of
fuel
into
the
combustion cham-
ber. Diesel
fuel
is
usually injected with
an
engine
accessory train-driven pump.
A
variety
of
means,
including
fuel
injection systems,
are
used
for
gas-
fueled
engines.
A
simpler system, available
from
some
manufacturers,
uses
an
arrangement wherein
the
fuel/
air
mixture
is
established
at
atmospheric pressure
through
a
carburetion
system, then compressed
and
delivered
to the
combustion chambers. This latter
arrangement
has
proven particularly attractive
for
dirt-
ier
fuels
and for
control
of
exhaust emissions. Draw-
through
designs have also demonstrated much higher
efficiency
than
other
pressure-charged
gas-engine
designs.
Turbocharged
engines
can
more easily achieve
exhaust
emission requirements than naturally aspirated
engines.
The
fuel
gas
passages
and
metering devices
for
naturally aspirated engines
are large
—
an
advantage
when
using
biogas
or
other dirty
gas
fuels.
Naturally
aspirated engines
may not be
capable
of
meeting
air
pollution
code restrictions
in
degraded
air
basins, even
with
currently available control technology.
Pressure charging
is
accomplished
by an
exhaust
gas-driven
turbocharger
or by an
engine accessory
train-driven
blower. With pressure charging,
fuel
gas
pressures
are
high
[from
140 to 550 kPa (20 to 80
Ib/
in.
2
)],
engine
efficiency
is
increased
by 10% or
more,
and
cooling requirements increase because more heat
is
wasted.
Pressure-charged designs that incorporate clean-
burn
technology
are
capable
of
achieving
air
pollution
code restrictions
without
additional emission control
technology.
14-5. Types
of
Engines
Engines
useful
in
pumping stations
can be
categorized
by
ignition, cycle,
and
configuration.
Ignition
Based
on
ignition, engines
can be
subdivided into
those using spark ignition
and
those using compres-
sion ignition.
With
spark ignition,
the
fuel
charge
is
ignited,
as in
most
automobile engines,
by a
spark arcing between
two
direct current
electrodes.
Medium-
or
slow-speed
spark-ignited
engines
should
be
specified with
break-
erless,
electronic capacitor discharge-type ignition
systems.
If
available, dual spark plugs should
be
spec-
ified
for
larger-bore engines.
Compression ignition engines employ
the
heat
generated
by the
compression
of the
fuel-air
mixture
to
ignite
the
fuel
charge. Compression ignition
engines include both straight
diesel
and
diesel-gas
(dual-fuel)
engines.
Cycle
Stationary engines
are
available
in
both two-
and
four-
stroke cycle designs. Two-stroke engines require
a
separate scavenging blower (usually powered
from
the
engine accessory train)
to
remove exhaust gases
from
the
cylinder
and
introduce combustion
air
into
the
cyl-
inder.
In
general,
two-
stroke
cycle
engines
are
limited
to
compression ignition designs
and are
less
efficient,
more complex, lighter
in
weight, less expensive,
and
noisier than four-stroke cycle engines.
Four-stroke cycle engines
are
available
as
both
spark
ignition
and
compression ignition types.
Configuration
Stationary engines
are
available
in
either in-line
or
vee-block
configurations. Compared with in-line
engines
on the
same power
basis,
vee
engines
are
gen-
erally more complex, more expensive
to
maintain,
smoother operating, more compact,
and
require less
floor space.
14-6.
Application
Criteria
Application
criteria,
the
rules
by
which
a
designer
establishes
how
engine manufacturers
may
apply their
products
to a
given power requirement, include
the
following:
•
Brake mean
effective
pressure
•
Piston speed
•
Rotational
speed.
Recognize
that
not all
engine manufacturers
use
the
same criteria
for
furnishing
the
buyer with output
power ratings
for
their products. Some manufactur-
ers' ratings
are
very conservative, whereas others
tend
to
overstress
their products. Instead
of
accept-
ing
manufacturers' ratings, provide
a set of
criteria
applicable
to all
manufacturers
as a
means
of
estab-
lishing
a
fair
basis
for
bidding.
The
following recom-
mended application criteria will produce
a
consistent
approach
to the
selection
of
internal combustion
engines.
Brake
Mean
Effective
Pressure
Brake mean effective pressure (BMEP)
is a
widely
used empirical measure
of the
load imposed
on an
engine. Although
not
universally accepted
by all
engine designers,
it
does provide
a
useful tool
for
controlling
the
size
of the
equipment
to be
used
for
a
particular application.
In SI
units, BMEP
is
given
by
BMEP
=
bkW
=
60
bkW
LAn LAN
where
the
BMEP
is in
kilopascals,
bkW is the
brake
kilowatts required
at the
output
shaft,
L is
length
of
piston
stroke
in
meters,
A is net
piston area
in
square
meters,
n is
number
of
power strokes
per
second,
and
N
is
number
of
power
strokes
per
minute.
In
U.S.
cus-
tomary
units
BMEP
=
33,000
bkW
(14
_
№)
where BMEP
is in
pounds
per
square inch,
bhp is the
brake horsepower required
at the
output
shaft,
L is
pis-
ton
stroke
in
feet,
A is
piston area
in
square inches,
and
Af
is
number
of
power strokes
per
minute. Note
that
the
coefficient
33,000
in
Equation
14-
Ib
reduces
to
1.00
in
Equation
14-
Ia.
When
applied
to
two-cycle engines, Equation
14-
Ia
reduces
to
BMEP
=
60,000
MW
VN
where
V is the
displacement
in
liters
and N is
number
of
power strokes
(or
engine revolutions)
per
minute.
In
U.S.
customary units, Equation
14-
Ib
reduces
to
BMEP
=396,000
bhp
VN
where
V is the
displacement
in
cubic inches.
For
four-
cycle
engines,
the
right
side
of
Equation 14-2 must
be
doubled.
A
suggested
set of
BMEP limits
for
four-
stroke cycle engines
and
various types
of
engine
applications
is
given
in
Table
14-1.
Another
approach advocated
by
some authorities
is
to
limit BMEP
to a
percentage
of the
BMEP
at the
manufacturer's
listed maximum power rating.
The
fol-
lowing
are
suggested limits
to the
engine's maximum
power requirement.
•
Continuous duty, relatively constant power
demand
—
70%
•
Continuous duty, variable power
demand
—
75%
•
Standby duty, incremental application
of
load
—
85
%
•
Standby
duty,
full-load
application
after
starting
—
80%.
Derating
for
Altitude
Some engines, particularly turbocharged designs,
may
not
be
capable
of
achieving acceptable performance
if
the
criteria
in
this section
(14-6)
are
used. Note
that
the
BMEP limitations should
be
adjusted downward
for
increasing altitude
and
increasing ambient temper-
ature
—
1%
for
every
15Om
above
460 m
elevation
(every
500 ft
above 1500
ft
elevation)
and 1% for
every
5
0
C
above
32
0
C
(every
1O
0
F
above
9O
0
F).
The
above rules
are
simplified.
A
better
approach
is to use all of the
application criteria
in
this
section.
Table
14-1.
Recommended
BMEP
for
Four-Cycle
Engines
3
kPa
Ib/in.
2
Turbocharged
Turbocharged
Naturally
Naturally
Duty
aspirated
Gas
Diesel
aspirated
Gas
Diesel
Standby
660
1100 1600
95 160 170
Continuous
Constant
load
550 790 900 80 115 130
Variable
load
590 860 930 85 125 135
a
Sea
level
pressure
and air
temperature
below
32
0
C
(9O
0
F)
Piston
Speed
Piston speed, which
is
actually
the
average piston
speed,
is
calculated
as
J
N
v
=
2LN
=
|g
(14-3a)
where
v is
piston speed
in
meters
per
second
and L,
n,
and
W
are as
defined
for
Equation
14-la.
In
U.S. cus-
tomary
units
v
=
^
(14-3b)
6
where
v is
piston speed
in
feet
per
minute,
L is
stroke
in
inches,
and N is
number
of
power strokes
per
minute.
Piston speed
is an
indicator
of (1) the
rate
of
wear
on
cylinder walls
and
piston rings,
(2) the
magnitude
of
inertial
forces
at the top and
bottom
of
stroke,
and
(3)
the
class
of
engine
design
—
that
is,
"low,"
"medium,"
or
"high"
speed. Item
3
refers
to
rotating
speed only indirectly.
The
important
factor
is the
length
of
piston stroke
at a
given
speed.
The
following
limitations
are
suggested
for
average piston speed:
•
Continuous
duty
—
6.1
to 7.1 m/s
(1200
to
1400
ft/min)
•
Standby
duty—
6.6
to
8.1
m/s
(1300
to
1600
ft/min).
Rotative
Speed
Using rotative speed
to
compare competing engine
designs
can be
somewhat misleading. Rotative speed
does relate, however,
to the
frequency
of
piston rever-
sals and, hence,
to the
rate
of
maximum stress
occurrences.
The
suggested limits
for
rotative speed
are as
follows:
•
Continuous
duty,
1
15
kW
(150
hp) or
less
—
20
Hz =
1200rev/min
•
Continuous
duty,
greater than
115
kW
(150
hp)
—
15Hz
=
900rev/min
•
Standby
duty,
up to 115 kW
(150
hp)—
30
Hz =
1800rev/min
•
Standby
duty,
up to 375 kW
(500
hp)—
20
Hz =
1200rev/min
•
Standby
duty,
375 kW
(500
hp) or
more—
15
Hz =
900
rev/min.
Other
Criteria
Other
generalizations that
may be
useful
for
compar-
ing
competitive designs include
the
exhaust valve port
area
(a
larger area requires less valve maintenance),
the
number
of
pistons
(fewer
pistons mean
less
com-
plexity),
and the
experiences
of
other users, particu-
larly
in
similar applications.
1
4-7.
Starting
Methods
Stationary
engines
may be
started using
one of the
fol-
lowing stored-energy systems: compressed air, electricity
(direct current
from
storage batteries),
or
hydraulic
fluid.
When selecting
a
starting method,
place
substantial
importance
on
storing
sufficient
reserve energy
for
a
series
of
starting attempts without resorting
to
com-
mercial power.
A
list
of
advantages
and
disadvantages
of
each starting system
is
given
in
Table 14-2.
If
either
electric
or
hydraulic starting
is
chosen,
the
engine
manufacturer
usually supplies
all of the
starting
equipment.
The
design
of
compressor, receiver,
and
piping systems
for
compressed
air
starting
is
usually
the
responsibility
of the
pumping station designer,
however. Suggested design criteria
for the
various
starting methods
are
given
in
Table 14-3.
14-8.
Cooling
Methods
Because
the
cooling requirements
of
each engine
design
are
unique,
the
equipment should
be
furnished
by
the
engine manufacturer. Cooling methods suitable
for
use
with stationary engines include radiators, heat
exchangers,
and
ebullient (boiling water) cooling.
Air
cooling, which
is
available
on
some smaller engines,
is
not a
viable system
for the
engines used
in
most
pumping stations.
The
advantages
and
disadvantages
of
each method
are
given
in
Table 14-4
and the
details
of
radiator, heat
exchanger,
and
ebullient cooling
are
shown
in
Figures
14-1 through 14-3.
14-9. Controls
The
engine manufacturer should
be
required
to
furnish
all of the
controls associated with
the
routine start-up
and
shut-down
and
emergency shut-down
of the
equipment.
If
remote
start-stop
initiation
is
needed,
require interfacing relays
for field
connection
to
manu-
facturer-furnished
logic circuits. Other important fea-
tures
of the
control system include
the
following:
• Use
direct current (24,
48, or 125 V) for all
engine-
related controls.
Do not use
alternating current
because
of
power failures
and
loss
of
reliability.
• Do not
locate
the
control panel
on or
near
the
engine
foundation
because
the
vibration causes
control system maintenance problems.
•
Engine controls must
be
coordinated with controls
for
the
driven machine.
For
example,
if it is an
engine-driven pump with power-operated valves,
the
valve controls must
be
coordinated with engine
start-up/shut-down
controls
to
avoid sudden surges
or
reverse engine rotation.
•
Specify
dust-tight construction
for the
control
enclosure
to
protect
the
equipment.
• The
recommended control
and
monitoring system
features
are
listed
in
Table
14-5.
1
4-1
0.
Governors
for
Engine
Control
Two
basic types
of
governors
may be
employed
for
engine control:
(1)
isochronous: (steady speed regard-
less
of
load)
and (2)
droop (speed varies slightly with
engine load).
Isochronous governors
are
necessary where
two or
more
units
are
operated
in
parallel
and
precise regula-
tion
and
load sharing
are
required, such
as
when par-
alleling
on an
electrical bus. Droop mode
is
usually
satisfactory
for
standby generators.
All
governors
use
some type
of
detector
(to
moni-
tor the
engine speed)
and a
means
of
developing
suffi-
cient
force
to
activate
the
throttle. Hydraulic
governors
are
driven
from
the
engine
and use
either
a
self-contained
oil
system
or
engine lubricating oil.
Electric governors depend entirely
on
electronic/elec-
tric actuators.
Pneumatic/hydraulic
or
electronic/
hydraulic
governors
are the
most suitable
for
variable-
speed service.
The
recommended governor types
for
various
applications
are
listed
in
Table 14-6.
14-11.
Accessories
for
Engines
The
recommended accessories
for
engines include
the
following:
• An
electric
or air
motor-driven lubrication-oil prim-
ing
pump mounted
on the
engine subbase.
The
pump
should
be
timer controlled
to
operate hourly
and
prior
to
automatic engine start.
•
Dual
oil filters
with quick
shift-over
valves
for
larger, continuous-duty engines.
• A
lubrication-oil-level
regulator
(in all
spark igni-
tion
and
some diesel engines)
to
replace
the oil
con-
sumed during operation automatically.
Method
Electric
DC
starting
motor
with rectifier
batteries
Compressed
air
Air
motor with
compressors
and
receivers
Direct injection with
compressors
and
receivers
Hydraulic
Hydraulic motor with
electric
motor-
driven pump, hand pump,
and
nitrogen-charged
accumulators
Advantages
Lowest
cost
Simple
system
Can
be
mounted
on
engine
base
Most
reliable
Can
be
used
on all but
largest
engines
Consistent power available until
stored
air is
exhausted
Very
rapid start
Requires
less
storage
volume
Smaller
compressors
Very
rapid start
High-speed cranking
Requires least
floor
space
May
be
appropriate
for
portable
generators
Disadvantages
Effectiveness
and
amount
of
stored
energy
falls with falling
air
temperature
Batteries
require
considerable
care
Batteries
subject
to
damage
if
rectifier
fails
or
overcharges;
use float-type
charger
Limited
effectiveness
on
larger
engines
Most costly
Requires
greatest
building
space
More
complex than
electric
Requires
greatest
air-storage
volume
Larger
volume
of air
required
per
start
Higher
pressures
required
Available
on
larger
engines
only
Most
applicable
to
diesel
engines
System
is
complex
Leaks
in
hydraulic lines
can be a
problem.
System
is
costly
to
maintain
Table
14-2.
Comparison
of
Engine
Starting
Methods
•
Crankcase
explosion vent valves
(on
large
diesel
and
dual-fuel
engines).
• A
thermostatically controlled jacket water heater
(on
automatically started engines, especially diesels
and
in
colder climates).
•
Engine-driven circulating water pumps
for
external
cooling water systems (heat exchanger
and
ebul-
lient
cooling).
•
Blow-by emission absorbers
for
crankcase
vents
(specify
cartridge type).
14-12.
Combustion
Air
Small-
to
medium-sized engines
can be
furnished
with
engine-mounted combustion
air filters. If the
Table
14-3.
Suggested
Design
Criteria
for
Engine
Starting
Methods
Starting
method
Components
Electric
Rectifier
Motor
Batteries
Compressed
air
Compressor
Receivers
Starting
motor
(if
required)
Hydraulic
Pump (electric)
Pump (engine)
Pump
(hand)
Accumulators
Starting
motor
Design
criteria
Floating-charge
and
quick-recovery
features
with
automatic voltage regulation.
12
or 24 V dc
with Bendix release.
12
or 24 V
nickel-cadmium type;
sufficient
storage capacity
for at
least three starts
at
3O
0
F
(lower,
if
appropriate
for
project).
Two-
or
three-stage air-cooled (liquid-
cooled required
for
higher pressures).
Provide two. Size
to
recharge receivers
in 1
hr.
Set
start/stop controls
to
ensure
receivers
are
fully
charged. Allow
no
more than
10%
loss
of
pressure before
starting lead compressor alarm
at 15%
loss
of
pressure. Aftercooling
is not
necessary
if
discharge piping
is
insulated.
Provide
at
least two. Store
air at 300
lb/in.
2
(gauge)
or
greater
to
reduce size
and
provide better starting action. Provide
sufficient
storage
for at
least three
successive
starts
—
more
if
more than
one
engine. Size piping
to
engines
to
minimize pressure loss.
Use
copper
piping
to
prevent corrosion.
Provide pressure-reducing station
at
motor
to
adjust
for
motor limitations. Must
be
fitted
with
Bendix release. Provide
lubricator
and
exhaust silencer. Solenoid
valve
located
on
receiver side
of flexible
hose connection
to
engine piping system.
System
pressure
not
more than
3000
lb/in.
2
(gauge)
Piston type
with
internal relief
and
check
valves.
Double-acting piston type
with
lever
for
hand
operation.
Nitrogen-precharged
bladder type.
Sufficient
storage volume
for at
least
four
starting attempts.
Not
less than
four
accumulators.
Independent means
for
draining each accumulator with pressure
gauge
on gas
side. Filters required
to
protect
seals
and
rings
in
motors, pumps,
and
accumulators.
Multipiston
type with Bendix release.
System
Radiator,
(Figure 14-1)
Heat exchanger,
(Figure 14-2)
Ebullient
cooling,
(Figure 14-3)
Equipment
Engine
jacket water
circulating
pump
Thermostat
Lube
oil
heat exchanger
Radiator
and fan (if
located
at
engine,
fan
is
usually
engine
driven.
If
radiator
is
remote
from
engine,
fan
is
motor driven)
Engine jacket water pump
Thermostat
Lube
oil
heat exchanger
Waste
heat exchanger
External
source
of
cooling water
Engine jacket water
pump
may or may not
be
necessary
Steam separator
Exhaust
boiler
(if
exhaust
heat reclamation
is
desired)
Source
of
softened make-
up
water
Source
of
condensing
water
if
reclamation
of
waste
engine heat
desired
Source
of
cooling
water
(or
radiator) required
for
engine
oil
Operation
Water
is
circulated
through
engine,
oil
heat exchanger,
and
radiator
by
pump
Thermostat maintains
desired system water
temperature
Fan
circulates
air
through
radiator, thus cooling
the
water
Water
is
circulated
through engine,
oil
heat exchanger,
and
waste heat exchanger
by
jacket water pump
Water
from
an
external
source
is
circulated
through
waste heat
exchanger
to
cool
engine jacket water
Water
circulates through
engine
to
steam
separator (and exhaust
heat
boiler)
thermally
if
no
jacket water
pump
is
required
Jacket water
flashes to
steam.
If no
heat
reclamation, make-up
water
is
added
to
steam
separator
to
replace
lost water;
if
heat
reclamation
is
desired,
cooling
water
in
steam
condensing heat
exchanger condenses
steam; cooled water
returns
to
engine;
typical engine water
inlet temperature
is
225
0
F,
outlet
228
0
F
Advantages
Least costly system
Simple
to
operate
and
maintain
All
engine heat, including
oil,
can be
accommodated
Relatively simple system
Modest cost
Possible
to
reclaim waste
heat
for
useful
purposes
Water
or
wastewater
pumped
by
pumping
station
can be
source
of
cooling water
(through
heat
exchanger)
All
engine heat
can be
accommodated
Low
temperature
differential
across
engine reduces
thermal stresses
and
wear
Stable, simple, easy
to
maintain
Ideal
for
maintaining
high exhaust
temperatures
necessary
for
biogas
fuels
Particularly suited
for
reclamation
of
waste
engine heat
Quiet
Power consumption
nearly negligible;
system
is
safe,
impossible
to
overheat
engine
Disadvantages
Fan
requires considerable
power
System
is not
readily
adaptable
for
reclamation
of
waste
engine heat
Radiator
is
vulnerable
to
leakage (typical
pressure rating
10-15
lb/in.
2
(gauge)
Fans make considerable
noise
Difficult
to
provide high
temperature cooling
needed
when biogas
is
used
as
fuel
Thermostat
failure
can
cause engine
overheating
Two
pumped water
systems
Energy consumption
can
be
significant
System
is not
particularly
suited
for
reclamation
of
exhaust heat
Thermostat
failure
can
cause engine
overheating
Higher
first
cost
Requires
special
engine
construction
(bearings, gaskets,
and
seals)
Full advantage
of
system
realized
by
utilizing
thermal circulation
Many
engine
designs
are
not
suitable
Table
14-4.
Engine
Cooling
Systems