Pump Selection for the LNG Industry

8/13/2019 Pump Selection for the LNG Industry

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eprinted from July 2012 HYDROCARBON

ENGINEERING

Thecraftofchoice


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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


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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.


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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.


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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.

Pump Selection for the LNG Industry

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