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8/13/2019 Pump Selection for the LNG Industry
1/5
eprinted from July 2012 HYDROCARBON
ENGINEERING
Thecraftofchoice
8/13/2019 Pump Selection for the LNG Industry
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With the evolving demands of the LNG market and the need for different
types of pumps comes a strong emphasis on proper selection for a given
application. From receiving terminals to peak shaving facilities and, more
recently, floating applications, the changing demands of the global market
have influenced the way pumps are selected (and therefore designed) to meet the specific
needs of the customer. Several factors including application type, liquid, installation location,
and the evolution of worldwide standards and codes have added complexity to the pump
selection process. It is important to be aware of the history of how the industry has influenced
LNG pump design and how those designs have been applied to meet ever more stringent
codes and regulations, as well as the increasing demands of the industry.
The present dayOver the past several years, LNG market growth has increased exponentially. With worldwide
consumption and demand increasing, new markets within the industry have emerged to meet
the needs of both the consumer and the producer. In the case of a custom pump
manufacturer, it is the process requirements, specific application and varying specifications of
the customer that provide the framework against which the selection of equipment is made.
Several factors must be considered when optimising a process or system: from increased
efficiencies and improved net positive suction head (NPSH) and pump down levels, to
KELSEY AZCARATE, EBARA INTERNATIONAL CORPORATION, USA, DETAILS
THE COMPLEXITIES OF PUMP SELECTION AND DESIGN, AGAINST THE
BACKDROP OF THE EVOLVING GLOBAL LNG MARKET.
PUMP&valve REVIEW2012Introduction
HYDROCARBON
ENGINEERING Reprinted from July 2012
8/13/2019 Pump Selection for the LNG Industry
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eprinted from July 2012 HYDROCARBON
ENGINEERING
equipment size, and codes and standards. For these reasons, the
process of selecting and/or designing a pump or expander is
complex and multi faceted.
The installation locations of cryogenic pumps and
expanders range from import and export terminals to newer
technology such as floating applications. Equipment needs vary
with regard to selection and design. Likewise, international
codes, standards and project specifications will vary depending
on factors such as region, installation and contractor. One
example would be a US based import facility with a send out
capacity of approximately 28 metric tpy and a storage capacity
of 800 000 m3. A send out facility is typically comprised of
several high pressure send out pumps operating in series as well
as in tank pumps to transfer the liquid from the tank. However,
selecting the quantity of pumps based on flow and head
requirements and space available at the site is just the first step
to properly selecting a pump for a customers specific
application.
Data presentation and pumpselection fundamentalsWhen an inquiry is received, datasheets and a specification
package including commercial terms and conditions are
typically provided, thus allowing the pump manufacturer to
produce an adequate proposal. The amount of detail varies
depending on the stage of the request for proposal (FEED, EPC
select or purchase phase). However, the technical parameters
included in the datasheets represent only the minimum
information needed to prepare an estimated pump
performance in the form of datasheets and curves. Further
requirements imperative to the design of the pump or
expander are listed in the remainder of the specifications.Pump and motor performance curves and datasheets are
generated to depict the overall performance parameters and to
summarise the requirements of the pump or expander being
selected for a customers application. The typical curve will
show a head versus flow graph together with efficiency, a rated
power curve and an NPSH curve (Figure 1). The datasheets will
summarise all of the important design parameters, such as
minimum and maximum continuous flow; shut off head; design
pressures; power requirements; synchronous speed and motor
rating(s) in terms of load; efficiency; and current. Providing a
picture of a pump or expanders future performance helps the
customer understand how the equipment will fit into their
process.
The creation of datasheets and curves begins with hydraulic
combinations consisting of an impeller, diffuser vane and
inducer. The specific hydraulic combination is derived from
coefficients established from the basic requirements for flow
and head. Several adjustments to existing hydraulics can be
made to meet specific duty requirements that do not fall
within an otherwise existing hydraulic. For example, a diffuser
vane may need to be modified to adjust the best efficiency
point (BEP). Likewise, an impeller may need to be modified forthe same hydraulic combination to meet a head (pressure)
requirement. Adjustments are made to the estimated
performance data based on the physical design and presented
accordingly. Should the physical dimensions of the hydraulics
change, the housing or casing design details and overall column
diameter may also be affected.
Pump selectionSeveral factors should be considered when selecting and
designing a pump for a specific application. As with any
application, the most crucial information required to begin the
selection process is flow, head, liquid type and its specificgravity, voltage, frequency and suction pressure. This basic but
very important information provides the framework from
which selections are made. Using a library of proven hydraulics
and the ability to modify proven hydraulics to meet specific
needs, a pump can be selected to meet the basic requirements.
In cases where the duty requirement falls within the
parameters of an existing hydraulic without modification, pump
and motor datasheets and curves are generated and the output
data necessary complete system design is produced. The
number of stages is modified to determine how to achieve the
required head rise or pressure drop considering all other factors
such as efficiency, best efficiency point (BEP), NPSH levels and
shut off head. This is just the beginning of the selection process
and since the overall goal is to optimise the proven hydraulic to
provide customers with the best pump or expander possible,
the technical selection is taken a few steps further.
Consider a pump and its input requirements whose duty
fits into a hydraulic with an 8 in. discharge and a 15 in. impeller
with 13 stages. Looking first at the BEP, the goal is to fall within
specific flow parameters. For example, API 610 requires that the
rated flow fall within 80 110% of the best efficiency flow rate.
Likewise, the BEP value should be within a specific range of the
impeller trim parameters. Should the BEP fall outside the range
of values for which the existing hydraulic combination is
designed, a shift in the curve would need to be made.Once the desired performance characteristics have been
achieved, further research must go into determining the physical
changes that must be made to the hydraulics. In several cases,Figure 1.Performance curves.
8/13/2019 Pump Selection for the LNG Industry
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HYDROCARBON
ENGINEERING Reprinted from July 2012
the diffuser vane machining alone can be revised to
meet the requirements. However, in cases where
diffuser vanes are new, the design work required to
submit a curve to a customer suddenly becomes
more complicated. At this stage, the entire pump
design must be considered, encompassing each
hydraulic component and its housing, and interface
dimensions throughout the entire assembly.
Specific considerations include how vanes and
their housings match up to the impeller and/or
inducer, pump casings, shaft length, and motor size
and its casing; all while taking into account the
additional factors such as efficiency and NPSH
levels. Figure 2 depicts a high pressure pump and all
of its components, with the cut away showing the
importance of each part and its interfaces.
In order to optimise the performance of each
pump and the customers process requirements,
factors such as high efficiency and low NPSH
levels are key. In an application where a pump isrequired to unload a tank, the NPSH levels should
be as low as possible. Again, the proper hydraulic
combination must be selected and designed to
meet this condition. Likewise, high efficiency is
desired and the proper combination together
with the number of stages is paramount to
achieving the best possible efficiencies. Motor
power is often limited by site parameters as well
as shut off head, since these affect downstream
piping and interface requirements. It is clear from
these basic first steps that there are several
important factors impressing on the pumpselection process.
Developing a cost basisOnce a selection has been made based on basic
information and an overview of customer
specifications, the remainder of the specifications
and standards for which the proposal is being built
should be noted. Review is focused on key factors
that affect the design of the pump: from shut off
head requirements and NPSH levels to motor ratings
and column sizing for retractable style pumps; all of
which affect performance and design. At this point in the process,
the technical requirements and ideal resultant design have been
identified but the financial impact still needs to be addressed. If
necessary, further changes may be made to reduce cost.
Larger columns and higher shut off head, higher NPSH levels,
and efficiency are essential to pump design, but these factors
also effectively increase costs or decrease potential income to
the purchaser or end user. Consider a case where all of the
specification requirements have been met, but better efficiency
can be achieved by increasing the amount of stages through a
higher specific speed. The customer may wish to trade the
increased length to gain efficiency. All designs must ultimately
be considered to optimise for performance and reliability.
Specification packagesAssuming the pump selection meets the requirements of the
specification package, deviations and exceptions must be taken
accordingly. All requisition package
specifications must be reviewed whose contents
range from pump and motor standards such as
API, NEMA, or DEP to customer specifications
such as electrical, instrumentation, welding,
piping, and quality inspection. There will be
cases where a specification may not apply to
the pump design at all. For example, API 610 in
general does not apply to cryogenic, submerged
motor pumps; nor do NEMA standards. The key
point is that specifications come in varying
degrees of magnitude, but considering each and
every one of them is important to both the
design and overall cost to the customer and
pump manufacturer.
As the industry has evolved, there has been
a dramatic increase in the amount and
complexity of specifications. This is all due to
the expansion of the market into regions where
standards may be different, safety codes havebeen enforced and environmental factors come
into play. Alternatively, this may simply be
because the cryogenic pump market has
steadfastly grown to the point that site design is
better understood. Furthermore, new markets
such as floating storage and regasification units
(FSRUs) have complicated the overall design of
the supplied equipment. This is due to new
physical design requirements, environmental
footprint and location. Moreover, factors such
as limited site space have created the need for
modularised construction, which must be takeninto account when designing each piece of
equipment: pumps and expanders included.
Behind the budgetAt this point, a pump or expander selection has
been made based on all of the specifications
and standards relevant to the inquiry and
deviations; exceptions or clarifications have
been compiled accordingly. Through these
exercises, the entire scope for the main
equipment has been determined, as well as
optional items such as spare parts, instrumentation, service and
quality requirements in order to produce a budget and price
summary for a proposal. In order to build a budget cost, a
baseline structure is built or an existing structure is utilised to
include the full pump or expander layout and its components.
For accuracy, a budget is built considering all major
components individually, as well as how they interface with
one another (not excluding weldments, motors, and electrical
components such as junction boxes and feedthrust). Once all
components have been determined and fit into the structure,
the specific design and their manufacturability must be
considered.
Manufacturability refers to whether or not a part can or
will be machined or forged from billet, or whether it can becast, or the type of machining. Cost and precision are key
factors in these types of decisions, as are lead times. Moreover,
when it comes to longer lead items whose design requires
Figure 2.High pressurepump cut away.
8/13/2019 Pump Selection for the LNG Industry
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larger amounts of materials and whose standards are strict, the
overall design must be determined in advance (for example,
suction vessels, headplates and suction valves). In essence, the
entire pump layout must be considered in order to create an
accurate structure.Due to the customisable nature of bespoke pump design
and the volatility of the materials for which the pump is
comprised, a budget can only loosely be based on past costs
(Figure 3). In effect, it is almost necessary to base a unit cost and
price on real time data, which can mean receiving individual
quotes for each component. These trends and index prices are
utilised to estimate current and future costs based on time of
manufacture. For instance, typical marine application orders
have deliveries more than one or two years past the purchase
order date. In essence, in order to develop an accurate estimate,
the quotation date, purchase order date, and delivery dates
must all be referenced. Several hours and resources must be
spent to produce an accurate budget whereby cost and price are
favourable to the manufacturer and the customer. Together with
increasingly complex specification packages and the
expectation to create an accurate and complete commercial
and technical proposal, the time required to prepare it can be
quite extensive.
ConclusionIn summary, the evolution of market demands has driven the
way equipment is supplied, and therefore quoted, to the
customer. A technical proposal can no longer be supplied
based on a standard set of assumptions or specifications.
Likewise, a commercial proposal can no longer be supplied
based on these same factors. In effect, a scope of supply is no
longer standard. The increasing requirements from customers
in worldwide regions within the LNG industry have added a
great deal of complexity to the process. This evolution willcontinue as the market demands for LNG increase and more
players are involved. As for the process of making a proper
selection, this too will adapt in time.
References1. http://cruonline.crugroup.com/steelferroalloys/priceindex/tabid/143/
default.aspx.2. http://www.kitcometals.com/charts/copper_historical_large.
html#1year.
Figure 3.Aluminum and stainless steel indexpricing.
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