Chemical Engineering April 2023 - 30

Feed
Cooling
water
Light
product
Heavy
product
Cooling
water
FIGURE 2. A typical heat-pump system on a distillation tower is shown
Electric motors
Several facilities have had positive
results from replacing steamturbine
drivers with electric motors.
Improved energy efficiency, reduced
emissions, improved reliability and
lower water consumption have been
observed, and some sites have
avoided additional steam-boiler investment.
The project can impact
the power infrastructure, steam, fuelgas
and cooling water systems, as
well as potentially impacting process
controls and relief systems.
If installing a new pump or compressor,
consider if an electric motor
is a viable option, even if the previous
or typical driver for the service was a
steam turbine.
If a facility decides to replace an
existing steam turbine on a compressor
or pump with an electric motor
and there is adequate plot space
available, installation of a new pump
or compressor along with the replacement
motor may not cost more
when factoring in the cost savings
of doing construction outside of the
turnaround. Installing a new pump or
compressor also replaces an older
pump or compressor that can be
optimized for current operations and
may result in reduced maintenance
and operating costs.
used to
Steam-turbine drivers are often
drive
large compressors.
Steam-turbine drivers have an inherent
ability to change speeds, which
can provide more flexibility and operability
for the compressor to manage
changes in gas molecular-weight and
flow requirements. When considering
switching an existing steam turbine
driver to an electric motor, a decision
will need to be made whether
30
a variable-speed
driver is required or if
a fixed-speed driver
can be used. If a
fixed-speed driver
is used, the impact
on the compressor
operation will need
to be considered. If
the gas molecular
weight varies significantly,
the compressor
capacity and discharge
pressure can
vary at a fixed speed,
potentially causing
process and equipment
design pressure issues. Relief
scenarios and flare loads may also
be impacted. Figure 3 and Table 2 illustrate
some of the process impacts
of motor speed.
Energy consumption may increase
in some cases due to excess flow
or pressure now produced by the
motor-driven compressor. The suction
throttle valve and spillback valve
may need to be modified or replaced
to provide the flexibility that was
previously provided by varying the
turbine speed.
quired,
If a variable-speed driver is reconsider
installing
a variable-speed
motor or installing a
fixed speed motor with a variable
speed gear box. Installing a variablespeed
motor may be an option, but
may have a higher cost and larger
space requirement for large motors.
A better option may be to install a
variable-speed gearbox to convert
a fixed-speed motor to different
speeds as required to optimize the
compressor operation.
A knowledgeable engineering
team should be consulted to study
the impacts of electrification and
propose mitigations.
Example: Replacing a steam turbine
with a fixed-speed motor.
There are two main types of steam
turbines - condensing and backpressure.
There are also condensing
turbines that extract steam at one or
more intermediate pressures.
Condensing turbines exhaust steam
at a very low pressure - well below
atmospheric pressure. The exhaust
steam is then condensed, usually in
cooling-water
exchangers,
although
air coolers are sometimes used.
Backpressure turbines take a relahigh-pressure
steam, as an
example, 600 psig, and exhaust it at
a lower pressure, for instance, 150
psig. Since backpressure turbines
extract less energy from the steam
compared to a condensing turbine,
more steam is required for a backpressure
turbine compared to a
condensing turbine producing the
same driver load. Table 3 shows an
example of how steam demand can
be reduced with electrification of an
existing steam turbine.
tively
ELECTRIFICATION
SYSTEM IMPACTS
For any project, capital cost is an
important factor - and often the
deciding factor on the feasibility of a
project. But there are other factors
that should be considered to fully
and properly evaluate the impact of
an electrification project to the facility.
For an electrification project where
steam-supplied energy is being replaced
by electricity, the obvious
impacts are to the steam system
and power supply. Any electrification
project
is also likely
to have
secondary impacts to other utilities
and systems.
Electrical infrastructure
Electrification will directly impact the
electrical system. The existing electrical
infrastructure may indeed have
the necessary spare capacity to supply
the additional power load. However,
the reality is that most facilities
have limited or no spare capacity,
thus requiring some new infrastructure,
such as new substations or upgrades
to the existing infrastructure.
Electrification loads can range
from 5,000 to 20,000 hp, and can
be even larger in some cases. Motors
of this size have a significant
impact on the
electrical
system,
which needs to be studied carefully,
as most facilities were not originally
designed to support these large additional
loads. The following are three
key electrical-system studies to be
performed to assess the existing
electrical infrastructure:
* Load flow - to check capacity
of existing equipment to support the
new loads
* Short circuit - to check if existing
equipment is rated to support
additional motor short circuit
contribution
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APRIL 2023
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Chemical Engineering April 2023

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