Chemical Engineering November 2012 - 38

NOMENCLATURE
Feature Report
about them, but this knowledge does
not need to be extensive. This is fortunate
because some of the information
received on the electrical aspects
is seemingly inconsistent and is assumed
to be explained by the different
recommendations and design practices
of the various manufacturers, owners
and engineering firms. A firm's electrical
engineers will handle the electrical
design, including the VFD selection, in
accordance with the applicable design
practices. Process engineers need only
perform preliminary economic evaluations
accordingly and be aware of the
issues. The knowledge of the issues
will allow them to understand any interaction
with the process design, to
communicate effectively with the rest
of the project team, and to prevent the
unnecessary repetition of work. It will
also help in applying judgment to the
results of evaluation methods.
Maintenance. VFDs are solid state
devices that require very little maintenance.
Above a certain size, they require
a fan for cooling, which is the only
moving part. In addition to requiring
periodic cleaning, the fan is the most
likely component to fail. An approximate
mean-time-between-failures for
VFDs has been given as 10 years.
Motors. Not all electric motors are
capable of operating on an inverter,
but the severe-duty motors that are
typically specified for CPI centrifugal
pumps are likely to be adequate. The
potential problem is that a fan that is
directly connected to the shaft of the
motor slows down when the speed of
the motor slows down and therefore
may not provide adequate cooling in
that situation. A large turndown may
require an auxiliary fan motor to keep
the fan turning adequately while the
speed of the main motor is reduced.
Centrifugal pumps generally require
an approximate 4:1 turndown to cover
their continuous operating range,
and require a greater turndown only
briefly at startup. A severe-duty motor
will normally meet this requirement.
Some pumps, such as positive displacement
pumps, can require a
greater turndown to cover their continuous
operating range. Many firms
now specify that motors for centrifugal
pumps be capable of operating
on inverters whether or not they are
a.c.
Alternating current
API American Petroleum Institute
C1-C6 Constants
d Inside pipe diameter
EMC Electromagnetic compatibility
f
Friction factor
H Head
HAZOP Hazards and operability
analysis
L
n
Equivalent length
Rotational speed
contemplating using VFDs at the
time. Inverter-capable motors are an
important, but minor, consideration
for new installations, although they
could be a major cost in retrofits. The
existing motors should be checked
early in these projects.
Cable types. Motors operating on
inverters require cables with better
insulation and shielding than if they
were operating directly on a.c. circuits
[7, 8]. An approximate cost for this upgrade
of the cables to go with a midsized
low-voltage VFD is $3/m ($1/
ft). Some sources have said that they
would use heavier cables with VFDs,
but a published reference was not
readily found. The cost of the cables
is a minor issue for new installations,
but the suitability of existing cables
could be a significant issue for retrofits
and should be checked early. For
new applications, some firms design
the cables to be capable of handling inverters
whether or not VFDs are being
considered at the time.
Cable length. With a VFD, the length
of the cable to the motor must be limited.
For certain types of motors, base
maximum lengths as low as 50 m (150
ft) have been mentioned. Some installation
manuals [9-11] give more typical
maximum lengths as 100-300 m
(300-1,000 ft) depending on several
parameters including switching frequency,
electromagnetic compatibility
(EMC) limits, and size. These lengths
may be extended with external filters
at additional cost, but there are still
limits that cannot be exceeded.
Starter. A VFD has the capability
to be a starter for the motor and
does not require that the motor have
a separate starter. The VFD has a
mode that starts the motor at the
minimum speed and increases it at a
predetermined rate until the set speed
is reached. This is called a soft-start
and is a desirable characteristic that
reduces the impact on the pump and
the motor. Thus, the VFD not only has
starting capability, it has the best type
32 CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2012
PFD Process flow diagram
Q
Pump capacity, volume rate of
flow
VFD Variable frequency drive
VSD Variable speed drive
W Power
∆P Differential pressure
εM Motor efficiency
εP Pump efficiency
εV VFD efficiency

Density
of starting capability and there is no
reason to have a separate starter. In
an evaluation of the economics, a case
with a VFD has a credit for not having
to pay for a starter.
Location. In the CPI, VFDs are usually
located in buildings, which reduces
the costs for the VFDs because the
VFDs can have minimal enclosures.
Some designs have the VFDs and other
electrical controls for the motor in a
single enclosure with slots for the various
components. In other designs the
VFDs are separate or even in separate
buildings. The choice of the design may
depend on the owner's or the engineering
firm's standards. It may also be influenced
by maximum cable lengths.
Installation. There is no installation
cost, or a low installation cost, for a
VFD. This is easy to see if all of the
motor controls (including the starter
or the VFD) are in a single enclosure.
The connections to the enclosure would
be the same whether it contained a
starter or a VFD. If the VFD is separate,
additional connections would be
required, but this is a minor cost. The
cost for the connection of a control signal
to a VFD would be the same as that
to a control valve. If the cost estimator
uses a Guthrie-type factor on the
equipment cost of the pump and motor
to calculate the installation cost, the
cost of the VFD must be excluded from
the equipment cost or the factor must
be adjusted to compensate. If some of
the items mentioned previously, such
as more expensive cables, will not be
the same for cases with and without
VFDs, the cost differences may need to
be considered.
Loss. Instead of stating an efficiency
for a VFD, the manufacturers state a
loss. For each size, the manufacturer
gives a constant loss, which is split
into an internal loss in the VFD and
an induced loss in the rest of the circuit.
Although they vary, the total
losses are about 1.8% of the nominal
power of the drives.
Motor efficiency. Reducing the speed
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Chemical Engineering November 2012

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