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the chemical engineer
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issue 808, october 2008
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ChemEng08
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MAKING sure you have the right
pump in the right place and that it’s
working correctly appears to be a
simple exercise. However, often the job
turns out to be more complicated than
you expected. This article points out a
few key aspects of pump selection and
evaluation to help you pick the right
equipment and get the most out of it.
pump selection
Selecting the correct pump for a
particular service can at first seem to
be a straightforward task. Mechanical
engineers tend to start with a process
data sheet where process engineering
has advised how much of a particular
liquid has to be moved and where
it needs to go. This gives fluid
characteristics, a flowrate and pressure
– the basic parameters for selecting a
pump.
Nowadays, computers tend to be used
for pump system calculations using the
suction and discharge vessel pressuresalong with the process parameters and
piping detail to calculate the system
losses for the given liquid flow. Some
software even suggests a suitable pump
type but I would rather leave that to an
experienced equipment engineer!
The material to be pumped is of
course determined by the process
and the reactions in the process. The
physical properties of the fluids are
known. That is, viscosity, density and
vapour pressure, which are all dependent
on temperature.
While the above description may
seem simple – find a pump to transport
a set quantity of a liquid to some other
place – this is deceptive. Chemical,
petrochemical and refining processes do
not run on constant conditions. Nothing
is that simple. Flowrate changes, as
does pressure. Temperature is likely to
vary at different times throughout the
process and this in turn affects viscosity,
density, vapour pressure and so on. This
is likely to result in pressure variations
in the system.
All of these variables very much affect
the type of pump that can be used and
indeed, how it is used, so selecting thecorrect pump is not always that simple.
If the mechanical equipment engineer
does not know the whole story, the
stage is set for problems. An equipment
engineer who suspects he does not have
the whole story of how his equipment
will be used and what might be
expected from it would be well advised
to carefully check and question the
given details, and perhaps ask leading
questions!
varying flowrates
Most pumps are required to pump atvarying flowrates; therefore both the
maximum and minimum requirements
need to be specified. All pumps have
limitations. For example, a centrifugal
pump operating with a flowrate that’s
either too low or too high could well
face problems with axial or radial
thrust, net positive suction head (NPSH)
requirements exceeding that available,
vibration and so on. Over-estimating
the flowrate can also result in problems
during operation. An equipment
engineer will tell you that over-
estimating the flowrate by 10% will only
land you with a piece of equipment that
is too big and not working at its most
efficient or ’healthy’ operating point.
It often pays to keep a pump operating
pumps
www.tcetoday.com october 2008 tce 41
at its optimum load, so in the case of
the above-mentioned centrifugal pump
struggling with variable flowrates, it
could well be viable to fit it with a
variable speed drive. This would allow
for automatically adjusting the pump to
match the requirements of the system.
Obviously this doesn’t come free but the
energy saving made by always running
the pump at optimum often makes up
for the capital cost.Particular attention must be paid
to pumps operating in parallel. If
one parallel pump fails or shuts
down, the others could ’run out’ on
their performance curve to a point of
cavitation or where
they demand more
power than the driver
can supply.
Similarly, over-
specifying differential
pressure at the design
stage can lead to
similar problemswhen the pump is run
in the true installed
conditions (ie the real
system curve).
temperaturevariations
Pumping temperature
is obviously
important. The
mechanical
properties of pump
material change
with temperature,
especially extremes
of temperature.
Liquefied gases that
’auto refrigerate’ on
The G4T pump is used to inject CO2
into the North Sea. Photo courtesy
of Lewa
Below: Very large vertical
seawater intake pump.
Photo courtesy of Ebara
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Process engineers should keep in close contact with equipment engineering
colleagues, especially when selecting pumps, Peter Stevens explains
The right tools for the job
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speed one and thus more expensive.
If possible, NPSH available should be
increased by elevating a suction vessel
or measures such as increasing the
suction pressure slightly, reviewing
suction pipe layout/sizing, or reducing
temperature and thus vapour pressure.
If any of these options is practical, the
resulting saving could be significant.
varying viscosity
Varying viscosity will result in varying
pump performance in most pump types,
most significantly with centrifugal
machines. A high-viscosity liquid needs
a much larger motor for a cold start-up
than for normal operating conditions. It
may be possible to warm up the system
at a low flowrate to reduce the viscosity
and thereby reduce power consumption.
When the viscosity is reduced the pump
can be operated at full design flow.
However, if high viscosity can occur at
any time then the pump needs to be
selected for this and motors must be
sized accordingly.
density dilemma
The absorbed power of a centrifugal
pump is directly proportional to the
density of the fluid. Pump drivers
must be sized for the worst operating
case, which may be highest density.
But again it may well be possible that
this higher density is only a temporarycase, often for start-up, and the pump
can temporarily operate at reduced
flow, cutting the power consumption.
A smaller motor could be installed
together with smaller cable, starter and
so on. Careful consideration of density
at various operating conditions could
mean the difference between installing
a low-voltage and a medium- or high-
voltage motor.
don’t supersize
A marginally-oversized process estimate
could make all the difference betweenan ordinary driver and a larger one
requiring force feed oil lubrication. I
have had cases where a quick check of
the process parameters has made all
the difference between an oversized
energy-hungry driver and standard
one with normal lubrication. This is
a very considerable cost and space
saving particularly if there is more
than one pump set involved. This also
brings to mind an old saying: “If you
do not require a piece of equipment,
you have saved all that money in
capital cost, installation, operation
and maintenance. But best of all if you
do not own something, it cannot go
wrong!”
In another instance, a site expansion
release of pressure
may require
pumps made out
of specialist or
unusual materials.
Such possibilities
need to be
highlighted.
Pumps are
sometimes
required to
withstand
’steam
out’
conditions,
perhaps to clean
out or decontaminate
the pump for inspection
or repair. This is often a higher
temperature than normal operation and
not only affects the pump itself butalso such items as gaskets and ’O’ rings
in the mechanical seals. ’Steam out’ is
not always noted on initial data sheets
but can have significant impact.
Some process reactions or ‘upset’
conditions can result in high
temperatures or higher than normal
suction pressures. The pump engineer
needs to be aware of this when
specifying a pump. Such conditions
may only occur when the pump is
static; in other applications the pump
may have to operate under these
abnormal conditions. This may well
require a different design, sealing
system, material selection or other
considerations.
Temperature has of course an effect
on vapour pressure and thus the NPSH
available. Suction and NPSH conditions
need to be calculated and considered
for all operating cases. Make an error
here and you’ll create a significant
problem or even catastrophic pump
damage. Perhaps the one parameter
that affects the pump selection more
than others is NPSH. If at all possible,the system should be designed to give
the best possible suction conditions
for the pump and not simply to prevent
cavitation. A higher NPSH available
could well mean the difference between
being able to select a simple single-
stage horizontal end suction pump and
having to use a vertical multistage
canister pump installed in a concrete
pit in the ground – two very different
pump types with two very different
price tags (not to mention installation
and maintenance) to achieve the same
basic duty. Another alternative may
be a slower running double suction
(impeller) pump for a lower NPSH
requirement. However, slower pumps
are bigger than a comparative higher
project called for an additional
centrifugal pump. A simple review of the
existing equipment revealed that the
existing pump had more than enough
spare capacity for the new requirements
– without any modification whatsoever.
All that was needed was a slight change
to the control valve. No new pump, no
foundations, no excavation, no added
power cables or motor controls, no
pipework, valves and so on. And the
pump would be operating at a more
efficient duty point. One less pump to
service too!
it’s good to talk
What is absolutely essential is that
the process engineer and equipment
engineer appreciate each another’s
roles and constantly communicate
with each other. This is especially
important if there are changes in process
requirements in the physical properties
of the pumpage.
Another source of potentially very
useful information is of course the
operators on the plant itself. It is quite
surprising what one can find out from
such sources. It is unusual, but does
happen that pumps are completely
wrecked once in operation. In one such
case, we eventually discovered that the
operators had a habit of ‘throttling’ the
flow by means of the suction valve! Not
something that the pump supplier ordesign engineer would have thought
about without investigating.
whose fault is it?
When a pump fails it is usually the
pump’s fault. More to the point, it’s
the pump that gets the blame. In fact,
most of the time it is the selection,
installation or operation that determines
how well a pump will work, or indeed
fail. After the duty requirements are
known, the equipment engineer uses the
process data to produce the mechanical
data sheet, which in turn is sent with
some form of requisition to a number
of suppliers. The suppliers select and
propose a suitable pump. Then, the
evaluation begins.
commercial evaluation
Commercial evaluation does not
generally impact mechanical engineers.
However, there are of course some
occasions when it does become
interesting. The capital cost of a piece
of machinery is one consideration, but
then there is also the running cost.
A few percentage points difference
in efficiency of a high energy pump
can mean a great deal of difference
in operating cost. As a very simple
example, 4% difference in efficiency of a
Above: High-speed
pump. Photo
courtesy of Sundyne
pumps
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5 MW driver at a cost of €40/MWh could
be worth €320,000 over five years.
An old generalisation was that a one
percentage point saved in efficiency
would pay for the cost of a (bare-shaft)
pump during its operating lifetime.
technical evaluationTechnical evaluation on the other
hand is of much greater interest to
the equipment engineer. It very much
seems to be their mindset to take
things to pieces and see how they
look and work in detail. Essentially
that is what is done in a technical
evaluation but on paper, or rather
a computer spread-sheet nowadays.
The actual requirements are compared
with the pumps proposed by the pump
suppliers in their bids; basic design,
orientation, construction, impeller type,efficiency, absorbed power, suction
capability, metallurgy, compliance
with specifications, scope of supply,
quantities, testing and so on. Typically,
we also review reference installations
of similar machines on similar service.
With larger pumps, higher pressures
and more exotic requirements, it would
be naïve to purchase a machine from a
manufacturer without a successful track
record. For example, this is particularly
relevant with respect to thick section
castings in exotic materials.
Another area that has over the
years become more and more important
is pump noise. In many cases there
are maximum permissible noise
levels; sometimes this is enshrined in
legislation, and plants may be asked
to reduce noise levels as part of their
environmental consideration. Another
aspect is the reduction or complete
exclusion of emissions of hazardous
or toxic materials from equipment.
Mechanical seals and sealing systems
can only do so much though this
technology continues to develop,
particularly in the area of dry gas seals
(now extensively used for process
gas compressors). However, glandless
technology is making inroads, in thepump market in particular. The size
and capabilities of such machines
continues to develop in leaps and
bounds, particularly in canned motor
and magnetic drive pumps.
A thorough evaluation of vendor
bids should include a review by other
engineering disciplines including
electrical, process, piping, metallurgy
and insulation. Thorough evaluation is
the watchword here: it is far better to
spend the time to check and double
check all parameters at this stage,
before the machine arrives on site
with, for example, an undersized motor
or connection flanges pointing in the
wrong directions for the piping.
summary
Process conditions and the increasing
complexity and scale of plants
continue to challenge suppliers and
the capability of the equipment they
have to offer. Pressures are rising,
temperature ranges getting more
extreme, and machines are getting
larger requiring more powerful drivers,
bigger castings and forgings in exotic
materials. All of these requirements
have to be satisfied, along with higher
plant production rates, more stringent
environmental requirements, higher
availability (mean time between
failure), lower running and maintenance
costs, and if possible lower capital cost.
I am sure that for the imaginable future
there will be a great deal of work for
the pump, the people that apply them,
and the people that engineer them,
manufacture them and use them.
After all it’s only a pump! tce
Peter Stevens
(peter_stevens@
fwuk.fwc.com) is
principal rotating
equipment
engineer (Foster
Wheeler Energy)
pumps
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