At Turbo Tech

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Table of Contents

Background....................................2

Design Features.............................3

Construction Features................... .4

Custom Design & Manufacturing....5

Turbo Performance.........................6

Piping & Instrumentation.................9

Pump Selection w/ Turbo...............10

Installation......................................11

ComparisonImpulse Turbine vs. Turbo .......12

Comparison

PX vs. Turbo............................14

Interstage Boosting........................16

Dual Turbine System......................17

The AT Turbo like its generational predecessors recovers hydraulic

energy from the high pressure concentrate (brine) stream in the

reverse osmosis (RO) process and transfers that energy to a feed

stream. That feed stream may be seawater going into a single stage

RO membrane block, or it may be first stage brine stream being

boosted in pressure for a second stage membrane block for further

recovery of permeate.

This unique approach offers many advantages to the RO designer

and users. This manual will explain in detail these advantages and

innovations that make the AT Turbo the most efficient and cost ef-

fective energy recovery unit available today. The manual shows howto estimate Turbo performance as well as how to apply the Turbo to

a variety of RO systems.

About the AT Turbocharger

The AT Turbocharger represents a tech-

nological breakthrough in Reverse Os-

mosis energy recovery performance.

Advance turbo machine design soft-

ware, 3D CAD and CAM software plus5 axis machining now enables PEI to

custom design and manufacture com-

plex geometry impellers and casing flow

passages that have resulted in Turbo’s

efficiency increases of 40% or more.

World class efficiency plus all of the

other features and advantages of the

Turbo are explained in detail in this

manual.

harnessing the power of...

L I Q U I D E N E R G Y



2

AT Turbo Background

Like the previous generation PEI Turbos, the AT Turbo transfer pressure energy from one liquid stream to a second liquid

stream. However, with the use of new technology the AT Turbo does this energy transference much more efficiently.

The AT Turbo consists of a pump section and a turbine section. Both pump and turbine sections contain a single stage

impeller or rotor. The turbine rotor extracts hydraulic energy from the brine stream and converts it to mechanical energy. The

pump impeller converts the mechanical energy produced by the turbine rotor back to pressure energy in the feed stream.

Thus the AT Turbo is entirely energized by the brine stream. It has no electrical, external lubrication, or pneumatic require-ments.

Figure 1 illustrates the operation of the AT Turbo in a single stage SWRO system. The feed stream from the high pressure

pump provides a flow of 1000gpm (227m3/hr) at a pressure of 588psi (40.5bar) to the pump section of the Turbo. The impelle

in the pump section increases the total feed stream pressure to 1000psi (68.9bar). The membrane block produces 400gpm

(90.9m3/hr) of permeate and rejects 600gpm (136m3/hr) of brine. The brine, which is now at a pressure of 980psi (67.5bar)

enters the turbine section of the AT Turbo. The turbine rotor depressurizes the brine while extracting the energy in the form of

high speed rotational torque. The brine, now depressurized to 5psi (brine exhaust can be any value, even hundreds of psi) is

exhausted to the discharge piping.

It is readily apparent that the reduced discharge pressure of the high pressure pump will have a large effect not only on

reducing operating cost, but also on reducing both initial capital and maintenance cost. More details on this important aspec

of the AT Turbo Total Life Cycle Cost will be explained later.

Also note, the Turbo eliminates the brine control valve, which is another major expense and maintenance item in SWRO

plants. Further note, that the Turbo is mechanically independent of the high pressure pump. Thus the Turbo can be used with

any type of feed pump and without any modifications to the pump, motor, or base.

In a later section of the manual the use of the Turbo in multi stage SWRO systems will be explained and again it will clear tha

the Turbo is more than just energy recovery. The Turbo, which incorporates a pump with a turbine into a single unit, opens

many new possibilities for the RO designer.

Figure 1



Design Features:The Turbo addresses the major issues facing the RO system

designer and user, including simplicity of design and opera-

tion, efficiency, reliability, ease of field servicing and versatil-

ity of use through the following design features.

• Type: The Turbo is an integral turbine driven cen-trifugal pump. The turbine is a single stage radial in-

flow type. The pump is a single stage centrifugal type

with its impeller mounted on the turbine shaft. The

unit is entirely energized by the high pressure brine

stream.

• Casing: The US Patent Pending AT Turbo casing

consists of an outer pressure casing designed for

1500 psi maximum pressure and inner radially split

volute insert casing. The volute insert is designed

by CFD software and completely machined on CNC

milling machines, thereby achieving the ultimate in

dimensional control, surface finish, and hydrualic ef-

ficiency.

• Impellers: The AT Turbo’s feature custom engineered

AND manufactured impellers. Using the most ad-

vance computational fluid dynamics pump and tur-

bine design software, 3D CAD and CAM systems

and 5 axis milling machines, each impeller design is

optimized and manufactured for maximum efficiency.

• Balancing: The entire rotating assembly is balanced

to ISO G3 standards (gyroscope tolerance) on PEI’s

computer controlled high speed balancer.

• Bearing: The Turbo uses three bearings all of which

is lubricated by the feed or brine flow. The pump and

turbine center bearing are the hydrodynamic journal

type. The thrust bearing is of the hydrostatic type which

utilizes high pressure water in an annular groove to

balance net rotor axial thrust. Standard material for

all bearings is resin impregnated carbon graphite.

Optional material is solid ceramic aluminum oxideUnits equipped with ceramic bearings also use a

plasma sprayed ceramic coating on the shaft bearing

surfaces.

• Shaft Seals: Shaft seals or more precisely the lack

of shaft seals is one of the outstanding design fea-

tures that contribute to the Turbo’s high reliability. Me-

chanical seals or shaft packings are the most main-

tenance intensive parts on nearly all pumps and are

responsible for the most downtime. And because the

Turbo’s rotor is fully enclosed by the casing, there

are no shaft penetrations to the atmosphere, hence

no seals.

• Multiple Turbine Nozzles: The Turbo is equipped

with two nozzles and a control valve that allows brine

flow and pressure to be regulated without energy

wasting throttling or bypassing. See page 8 for addi-

tional information on this important feature.

• Pipe Connections: All feed (pump) and brine (tur-

bine) pipe connections are Victaulic type. For larger

units or very high pressure applications ANSI 600#

class flanges are available.

• Unit Base: Turbo bases are available in Delrin plas-

tic, stainless steel, or painted carbon steel. All bases

are bolted with SS bolting and are drilled for sole plate

bolting.

Figure 2



4

Operation and

Construction FeaturesBesides its outstanding efficiency, the AT Turbocharger

brings many benefits, some obvious, and others not

so obvious, however, they all add up to increase total

value to the RO designer and user. Consider the fol-

lowing Turbo characteristics:

1. Fexibility of installation, can be placed next toRO block, greatly reduces brine piping cost

– no double shaft motors, brine sumps, ex-

tended bases and foundations.

2. Significantly reduces high pressure pump size

(number of stages), motor size, motor starter,

switch gear and transformers

3. Able to discharge brine against backpressure

– no brine sump or pump

4. Brine is not exposed to atmosphere, thereby

minimizing odor and corrosion

5. Reduces load on high pressure pump –

resulting in greater pump reliability – shorter

and stiffer shafts for centrifugal pumps,

reduced frame and crankshaft loading and

cooler oil temperatures for positive

displacement plunger pumps.

6. Can be used with any type of high pressure

feed pump.

7. Turbos are very compact, small space and

foundation requirements

8. Turbo do not generate flow or pressure

pulsation

9. Low noise and vibration operation – Turbo also

reduces high pressure pump noise

10. No shaft seals – very high reliability11. No oil or grease lubrication – bearings are

water lubricated

12. Brine pressure and flow regulated with Turbo

auxiliary nozzle and valve, no energy wasting

throttling or bypassing

Materials of Construction

The standard and optional material of construction for AT Tur-

bocharger are:

Part Standard Optional

Casing Duplex Stainless Super Austenitic

Steel Stainless Steel

Alloy 2205 AL6XN

Impellers AL6XN

Bearings Resin/ Ceramic:

Carbon Graphite Aluminum Oxide

Retaining Rings SS316 Passivated

External Bolting SS304

Duplex alloy 2205 is a superior material for crevice and pitting

corrosion resistance in high chloride environments. Alloy 2205

has twice the tensile strength of SS316L . The welding char

acteristics of 2205 are very good and post weld heat treat-

ment to maintain corrosion resistance is not required. The

nominal composition of Alloy 2205 is:

Cr 22%

Ni 5%

Mo 3%

N 0.15%

Fe balance

Custom engineering impellers utilizing turbomachine design

software.

TESTING All Turbochargers are individually tested for performance

mechanical integrity, and hydrostatic pressure. All data acqui

sition is by electronic instrumentation and computer interface

Test data is documented to identify the unit test and test con

ditions. The complete test report becomes a part of the unit’s

job history file.



Customer Specific Hydraulic Design

And ManufacturingIn the past, every Turbo was designed and manufactured to

achieve its best efficiency point (BEP) at the customer’s spe-

cific duty point. Casing hydraulic passages (volute, diffuser,

and nozzles) were machined to the correct dimensions for

duty point operation. The cast impellers were trimmed for

maximum efficiency at the design reject ratio. The AT Turbo,however, goes much beyond that already high level of cus-

tom engineered equipment produced by PEI.

Now, thanks to a highly disciplined design process which in-

cludes computational turbo machine software, 3D CAD and

CAM software and 5 axis CNC milling machines, PEI can cre-

ate a completely unique turbochargers with turbine rotor and

pump impellers designed with complex 3D blade geometry.

The results of this process, plus the inherent efficiency ad-

vantage of high speed operations, are simply the most effi-

cient pumps and turbines available today.

Machined wax turbine impeller for

investment casting.

3D machining accomplished through use of Master Cam

software and 5 axis machining center.

HAAS 5 axis machining center.

*Custom Machined Volute Insert technology

patented by Pump Engineering, Inc.

DATA: Customer

Requirements

Generate hydraulic design

Generate CNC program

Machine volute inserts*

Hydro & Performance Test

Unit ready for shipping

OK

TEST

YES

NO

Machine AT 3D impellers



6

AT TURBO PERFORMANCEGenerally, an energy recovery turbine (ERT) is rated as hav-

ing a certain efficiency based on the conversion of hydraulic

energy into mechanical shaft energy. However, in reverse os-

mosis where the process is driven by pressure energy, the

shaft energy generated by the ERT is normally transferred to

the feed pump which then converts that energy back into pres-

sure energy in the feed stream.

The most accurate measure of ERT efficiency for an RO sys-

tem is the ratio of hydraulic energy returned to the feed stream

to the amount available in the brine stream. This ratio is called

the Hydraulic Energy Transfer Efficiency (HETE), or nte

and is defined as:

Nte = Hout

/ Hin

[1]

Where Hout

= Hydraulic energy transferred to the feed stream

Hin

= Hydraulic energy available in the brine stream

The Hydraulic Energy Transfer Efficiency provides the tru-

est method to evaluate and compare the energy recovery ef-

fectiveness of all energy recovery devices, including impulseturbines, flow work exchangers, as well as the Turbo.

Unlike conventional ERTs, the energy transfer efficiency of

the Turbo is independent of pump efficiency. The reason for

this is that the Turbo contains its own pump, so the complete

energy transference occurs within the Turbo. So unlike an

impulse turbine or reverse running multistage pump, the

Turbo’s rotor speed is completely independent of the motor/

high pressure pump. This means the Turbo can be designed

for high speed operation which is the most efficient and cost

effective design. Pump efficiencies of 90%+ are possible for

larger AT Turbos.

The useful work of the AT Turbo is expressed as the “Boost

Pressure”. This is the pressure rise that occurs between the

Turbo’s pump inlet and pump discharge. To apply the Turbo

to an RO system, the boost pressure needs to be calculated.

Use Figure 3 to find the approximate Hydraulic Energy Trans-

ference Efficiency for the Turbo. For example, at a feed flow

rate of 1000gpm the AT Turbo displays an nte of about 73%.

FIG 3 AT Turbo Hydraulic Energy Transfer Efficiency (nte)

Knowing nte makes calculation of the Turbo pressure boost,

∆∆∆∆∆P, very simple:

∆∆∆∆∆P = (Nte) (Rr) (Pbr – Pe) [ 2 ]

Where Rr = ratio of brine flow to feed flow

Pbr = brine pressure at turbine inlet

Pe = exhaust pressure of Turbo

(brine pressure leaving Turbo)

The brine pressure drop, ∆∆∆∆∆Pbr is defined as:

∆∆∆∆∆Pbr = Pbr – Pe [ 3 ]

Now substituting numerical data into our formula for the

1,000gpm (227m3/hr) example as follows:

Qf = 1,000gpm (feed flow)

Qb = 600gpm ( brine flow)

Qp = 225gpm(product flow)

y = 40% (recovery ratio)

Rr = 60% (reject ratio)

Pm = 1,000psi (membrane pressure)

Pbr = 980psi (brine pressure)

Pe = 5psi (brine exhaust pressure)

A design objective is to make ∆P as large as possible to ob-

tain maximum energy recovery. Equations [ 2 ] and [ 3 ] indi-

cate that increasing ∆∆∆∆∆Pbr will accomplish that. Since Pbr is

usually a given value based on membrane design, try to keep

turbine exhaust pressure to a minimum safe level. Although

the AT Turbo can discharge brine at a high pressure, this ex-haust pressure does reduce the available recoverable energy

In this example the use of the AT Turbo reduced the high

pressure feed pump discharge pressure from 1000psi to 588

psi. Thus not only will the AT Turbo system be the most en-

ergy efficient, but it will contribute considerable capital cost

savings as well.

A Note on Energy Efficiency Comparisons

When comparing the stated efficiency of the Impulse Turbine

and Reverse Running Pump Turbines with the Turbo’s Hy

draulic Energy Transfer Efficiency, be aware that their effi

ciencies are given as the ratio of mechanical shaft output tobrine energy input. To get a true comparison of Hydraulic

Energy Transfer Efficiency to mechanical efficiency, multi-

ply the Impulse Turbine efficiency by the feed pump efficiency

and any other component such as a V belt speed reducer

Variable Frequency Drive or part load reduction of motor effi

ciency. As an example, a 86% efficient Impulse Turbine

coupled to a 77% efficient centrifugal pump would have com

bined efficiency Hydraulic Energy Transfer Efficiency o

66.2%. Further if a full sized motor were used, there would be

a reduction in motor efficiency due to part load operation tha

would now bring nte to 64.8%.

Thus the 64.8% nte of the Impulse Turbine/Pump and not the86% mechanical efficiency, is the true measure of Energy

Recovery Turbine efficiency.

∆∆∆∆∆P = ( .73 ) ( 600gpm/1000gpm ) * ( 980psi – 5psi ) = 427



E f f i c i e

n c y ( % )

Feed Flow (gpm)

HTC-AT Efficiency Curve

HTC-AT Efficiency Chart

600 gpm

980 psi

Extra high pressure brine piping

Pelton Impulse

Turbine

86% efficiency

294 hp Output

1000 hp Motor

motor net power

467 hp

High Pressure

7 stage pump

77 % efficiency

1000 gpm

1000 psi

Pump Power =

761 hp

1000 gpm

30 psi

PIT Transfer efficiency = .86 (PIT n) x .98 (*) x .77 (pump n) = 64.8% nte

1000 gpm

585 psi00 hp motor

AT 900 Turbo

(placed next to membrane

rack for reduced piping)

Pump Power = 420 hp

1000 gpm

1000 psi

71% nte

1000 gpm

30 psi

High Pressure

4 stage pump

77% efficiencyTurbo Transfer efficiency = 71% nte

* percentage reduction of motor efficiency due to part load operation

600 gpm

980 psi

600 gpm

5 psi

Figure 3

Figure 4

Figure 5



8

Volute

Auxiliary No

TurbImp

Valve

Auxiliary Nozzle

Primary Nozzle

Brine

AT Turbocharger Auxiliary Nozzle and Valve

All AT Turbocharers (see Fig 6) are equipped with a Main Nozzle and a secondary Auxiliary Nozzle (AN) and Auxiliary Nozzle

Control Valve (ANCV). The Main Nozzle is sized to provide a concentrate system resistance (concentrate pressure) equal to

the maximum design pressure at the design concentrate brine flow rate.

The auxiliary nozzle is sized to about 20-25% of the area of the Main Nozzle.

The ANCV controls flow to AN in the turbine casing. The ANCV will provide

a 20-30% pressure range at a constant brine flow.

Note that the ANV does not bypass flow around the

Turbo. It is a unique way to achieve variable area

nozzle flow without the energy wasting throttling and

bypassing valve arrangements needed by constant

speed reverse running pump turbines.

The ANCV can be supplied either as a manual or

as a power actuated unit.

AT Turbocharger Installation and PID

For single stage seawater RO systems, the Turbo is locatedhydraulically between the discharge of the high pressure feed

pump and the membrane block. Its actual physical location can be optimized to reduce the length of both brine and feed

piping. A brine control valve is not needed when using the Turbo. Brine pressure and flow can be trimmed through use of the

Auxiliary Nozzle Control Valve (ANCV).

Fig. 7 is a preferred PI&D arrangement of a turbo installation for non VFD driven centrifugal feed pump motors. Note that a

flow control valve (FCV) is used to throttle pressure for membrane recovery control. The FCV is used in conjunction with the

ANCV to achieve the proper flow capacity and pressure. The FCV is opened or closed to maintain the required flow rate, while

the ANCV is open to operate the membrane at a lower pressure and closed to operate the membrane at a higher pressure.

Figure 6

Figure 7



Piping and Instrumentation Diagrams

Figure 8 Two Stage - Brine staged utilizing AT

Turbo as interstage booster pump.

Figure 9Second Pass - utilizing AT Turbo as

interstage booster pump.

Figure 10 Single stage SWRO with PD pump.



10

Pump selection with the TurboTM is easy, although a few guide-

lines should be followed. Always use the highest expected

pressure conditions in calculating TurboTM pressure boost. This

would typically be the pressure at the end of the membrane

life with the highest expected TDS feed at the lowest expected

temperature.

If the recovery or feed flow may vary significantly, it is a good

idea to check with PEI to ensure the TurboTM can efficiently

accommodate the conditions.

Positive Displacement Pumps

With positive displacement (PD) pumps such as reciprocat-

ing plunger types, flow rate is established by pump displace-

ment and RPM. Membrane pressure is controlled by adjust-

ment of the ANCV on the TurboTM. PD pumps do not need

any type of feed pressure control device such as a feed throttle

valve.

Due to the very high reliability of the TurboTM, the PD pump

and motor can be rated to deliver the required pump discharge

pressure allowing for the Turbo

TM

boost. Such downsizing of the pump and motor will save considerable capital and main-

tenance costs and energy use.

CAUTION! Always use good pulsation dampening on the

feed pump discharge. Severe flow and pressure pulsations

can be damaging to pressure gauges, flow meters, piping and

possibly the TurboTM . See PEI’s installation manuals for addi-

tional information

Centrifugal Pumps

These pumps must be sized for maximum pressure condi-

tions. When the required membrane pressure is lower than

the rated pump discharge pressure excess pump deliverypressure must be eliminated.

A feed throttle valve is the most common way to regulate this

pump pressure. This valve is partially closed during initial

Feed Pump Selection with the AT Turbooperation of the plant. As the membrane pressure require

ment increases with plant age, the valve is opened to obtain

increased pressure to the RO train.

Another way is to use a variable speed pump drive such as a

variable frequency drive (VFD). The pump speed is adjusted

as required to obtain the desired pump pressure. The advan

tage of this technique is that the pump power consumption is

minimized and the expensive control valve can be eliminated

Centrifugal Pump Example

The goal of this analysis is to find the performance require

ments of the feed pump. Operating conditions are indicated

in Figure 11 including the projected lowest and highest mem

brane pressure.

Figure 11 indicates a TurboTM transfer efficiency of 64%. Us

ing “end of life” conditions, equation [3] calculates the TurboTM

boost (i.e. the pressure rise between points 2 and 3) as 352

psi. The required feed pump discharge pressure, Pp, is:

Pp

= Pm

– Ptc= 950 – 386 = 564 psi

Thus, the feed pump must be sized to deliver 550 psi outle

pressure.

For start-up conditions, the TurboTM boost is 348 psi which

yields a TurboTM feed inlet pressure of 536 psi. Note that the

feed throttle valve must be adjusted to “destroy” about 61 ps

of feed pressure.

If a VFD were used, the control valve would be eliminated

and the feed pump power consumption would be reduced

For this example, the pump for start-up condition without theVFD would require 184 hp. With a VFD, the input power drops

to 176 hp.

Please contact PEI with questions about feed pump selec

tion.

Feed Throttle

Valve

ηηηηηp

= 72%

ηηηηηm

= 93%

Feed

Pump

AT TurboTMηηηηηte = 64%

Start-up End of LifeFlow Pressure Flow Pressure

(gpm) (psi) (gpm) (psi)

1 420 30 420 30

2 420 564 420 564

2’ 420 502 420 564

3 420 850 420 950

4 280 830 280 920

5 280 5 280 5

’

Figure 11



InstallationCharacteristics of the AT Turbo that effect its installation in typical SWRO plant are as follows:

Compact Size and Weight

Because of its high speed operation, an AT Turbo is much smaller than an equivalent capacity motor driven pump, as

much as five to tens times smaller. For instance a 100gpm Turbo ways 40lbs vs 400lbs or more for a high pressure pump.

This factor makes it ideal for skid mounted or containerized system, where space restrictions are always an important

consideration.

Flexible Installation Location

The Turbo is mountable in any orientation. Units up to the AT 100 can be supported by its piping. For larger system thathave separate pump rooms, the Turbo ability to downsize the pump, motor, and associated electrical equipment will result

in a smaller building. Additionally, the Turbo can be located anywhere between the high pressure pump discharge and the

feed header to the membrane block. By placing the Turbo next to the membranes, considerable savings of high pressure

super alloy piping can be made.

Piping and Foundations

Victaulic pipe connections are standard on the AT Turbo from model AT 25 through model AT 2400. ANSI 600lb class

flanges however are available on all AT Turbo models and are standard equipment on models AT3600 and larger. Because

of their relatively small size and vibration free operation foundation requirements are very modest and are primarily de-

signed to support piping loads that the Turbo may be carrying.

Low Noise and Pulsation Free Flow

Highly efficient hydraulic design of the AT Turbo significantly minimizes noise generation to such an extant that it is not

audible over the background noise of a typical SWRO plant. In addition, because the Turbo downsizes the high pressure

pump and motor size and pressure requirements, there is a noise reduction associated with this equipment. The high

speed centrifugal principle of Turbo operation assures pulsation free smooth flow to the membranes.

Pressurized Brine Discharge

The AT Turbo can discharge brine (concentrate) against practically any level of backpressure. So there is never any need

for brine disposal pumps or gravity flow piping or trenches.

AT Turbo Recommended Requirements

· Pressure gauges or transducers should be installed near each Turbo pipe connection to permit monitoring of

Turbo performance.

· Suction Stabilizers and Discharge Pulsation Dampeners should always be used with reciprocating positive dis-placement pumps

· Perform all pipe cleaning and flushing before final installation and start up of the Turbo

Maintenance and OverhaulThe AT Turbo has no scheduled maintenance requirements. There are no shaft couplings to align, no shaft seals (lead-

ing cause of pump failure), no lubrication system or lubricants (second leading cause of pump failure), and no external

auxiliary services such as cooling water or pneumatic requirements.

See the Installation and Operation Manual for complete information.

Overhaul and Repair Thanks to its relative small size and single stage design, even the largest Turbo can be completely inspected and/or

overhauled in a few hours. All bearings are slip fit and O ring mounted, making their removal and installation a quick,

simple, and straightforward job. PEI stocks all parts necessary for any repairs that may occur.



12

Comparison of the AT Turbo and Pelton Impulse Turbine

As explained in the AT Turbo Performance section of this manual, Hydraulic Energy Transfer Efficiency is the only accurate

method of comparing Energy Recovery Devices. In this section,AT Turbo performance will be compared to a Pelton Impulse

Turbine performance for a relatively large SWRO train size of 2,641,000gpd (10,000m3/d). The comparison will cover the

entire hydraulic operating envelope from minimum to maximum membrane pressure requirements.

Example RO System Conditions:

Fluid: Seawater TDS 38,000 Temperature: 14 – 290 C Capacity: 2,641,000 gpd (10,000m3/d)

290C Summer Conditions - 0 years 140C Winter Conditions - 3 years

Feed 3986 gpm (906 m3/hr) 3986 gpm (906 m3/hr)

Flow

Pump 920 psi (63.4 bar) 1085 psi (74.8 bar)

Pressure

Membrane 885 psi (61 bar) 1050 psi (72.4 bar)

Pressure

Concentrate 2232 gpm (507 m3/hr) 2232 gpm(507 m3/hr)

Flow

Concentrate 865 psi (59.7 bar) 1030 psi (71bar)

Pressure

From the above membrane manufacture recommended design data, the extremes of throttle pressure is 200psi (maximum

pump discharge pressure 1085psi – minimum membrane pressure 885psi). This total pressure range defines the basic

Hydraulic Operating Envelope. At those times of reduced pressure operation, the excess pressure will be throttled by flow

control valve.

Presented below are the performance factors for an AT 3600 Turbocharger and a Pelton Impulse Turbine and centrifugal

high pressure pumps

AT 3600 + pump Pelton Impulse Turbine+ pump

Feed Pump Flow 3986 gpm (906 m3/hr) 3986gpm (906 m3/hr)

Turbo Boost Pressure (max) 433psi (29.9 bar)

Feed Pump Discharge Pressure 652psi (45 bar) 1085 psi (bar)

Feed Pump Efficiency .85 .85

Motor Efficiency .95 .95

Pelton Impulse Turbine Efficiency n/a .89

Hydraulic Transfer Efficiency .78 .748

The graph below depicts the performance of the AT 3600

and the Pelton Impulse Turbine over the Hydraulic Operating

Envelope defined above. Note that the pressure throttling

significantly diminishes the amount of recoverable concen-

trate energy available to the Impulse Turbine. In fact, the

pump discharge pressure throttling energy loss can be 20%

or more of total pumping energy and represents the greatestsource of loss and inefficiency in most SWRO high pressure

circuits equipped with Impulse Turbines.



750 800 850 900 950 1000 1050

2936

2176

2109

2039

1969

1899

18301763

1650

1500

1750

2000

2250

2500

2750

3000

3250

Membrane Pressure (psi)

Pressure Throttling with a Turbocharger Another advantage of feed stream pressure boosting with the Turbo is a reduction in throttling pressure differential. Taking the

above example, where there is 200psi difference between minimum and maximum membrane operating pressure, the AT

Turbo reduces the amount of throttling from 200psi to 130psi. Because Turbo Boost Pressure (TBP) is a function of Brine

Differential Pressure (BDP), the TBP in the example decreases from 433psi at a BDP of 1030psi to 364psi at a BDP at 865psi

The reduction in throttling pressure differential results in lower noise, less wear, and cavitation in the flow control valve.

HTC AT Benefits

• Feed pump and motor layout not affected.

• HTC AT can be located anywhere, including next to

the RO trains, reducing the amount of high pressure

piping.

• The HTC AT is able to discharge brine against any

backpressure and a brine disposal pump or sump is

not needed.

• The turbo boost pressure provided by the HTC AT

reduces the discharge pressure the main feed pumpmust produce thereby allowing for a pump with fewer

stages.

• Motor size reduced by 30% - 50%.

• Smaller transformer and motor starter or VFD

• The HTC AT is maintenance free! All three bearings

are process lubricated.

• No increase in power over complete membrane

pressure envelope.

• Brine flow and pressure can be regulated with reduced

energy-wasting throttling or bypassing.

• Small footprint and Victaulic pipe connections ensure

easy installation.

Pelton Impulse Turbine (PIT)

• PIT is connected to pump – a modified base plate, second

shaft and coupling are required.

• PIT must be located next to the pump. Pump room mus

accommodate longer base plate and heavier pumping

equipment.

• Brine sump with pump or gravity flow piping required

Level switches, alarms and valves may be needed too.

• Must develop full membrane pressure with main feed

pump requiring maximum number of pump stages.

• Full size motor with a double extended shaft may be

required. No size reduction in transformer or starter.

• Oil/grease lubricated bearings need periodic inspection

and maintenance. Brine disposal pump requires

additional maintenance. Large number of pump stages

increases the cost of spare parts. Double extended shaf

motor may be difficult to replace on an emergency basis

HTC AT vs. Pelton Impulse Turbine – a side by side comparison

Pump hp

with PIT

Net Motor

with PIT

Pump hp

with HTC

Figure 12

F l o w



14

Comparisons of the AT Turbo and the Pressure Exchanger

The Hydraulic Energy Transfer Efficiency of the Pressure Exchanger in RO service can be determined by analysis of the

efficiencies of all the components that are part of the high pressure circuit.

Fig 14 shows the PX system. Note that its basic design is comprised of the PX device, a high pressure pump, a booster pump

Additional and necessary components such as high pressure flow meters and valves have a minor effect on overall HETE, s

for sake of simplicity will not be considered in the energy analysis.

The PX Exchanger system uses two pumps. One pump’s capacity equals the permeate flow plus an additional flow equal to

the internal leakage losses of the PX, typically 3 – 4% of brine flow. This pump has to achieve the full membrane pressure

required by the feed water salinity plus an additional pressure head required by the increase in feed salinity due to brine mixin

in the PX device. The second pump is a booster pump whose capacity equals the brine flow and whose head rise equals th

membrane pressure loss (20 – 50psi) plus the inlet and outlet pressure losses of 20 – 25psi per each end of the PX device

giving a total differential pressure of 60 – 100psi.

Example 150,000gpd SWRO plant (49.5gpm permeate, 111gpm feed)

operating at 45% recovery at a membrane pressure of 1,000psi

Thus the HETE of the PX Exchanger device is .658. Compare this number to the advertisedclaims of 94% efficiency for the PX. The difference between the claimed efficiency and true

HETE is in the losses due to internal leakage and inefficiencies of the booster pump. Addi-

tional losses will be produced by valve(s) necessary to control the PX.

AT Turbocharger System

For the Turbo example, first calculate AT Turbo boost

pressure utilizing the following forumula:

Reject Ratio x AT Turbo HTE x Brine Diff. Pres. = Turbo Boost

.55 x .60 x 995 psi = 322 psi Boost

The high pressure pump differential pressure then is:

1000psi (membrane pressure) – 20psi(suction pressure) –

322psi (turbo boost pressure) = 658psi

Reciprocating Plunger Pump Efficiency =. 85 (including V

belt losses).

60hp electric motor efficiency = .92

Total Pump Motor Power = 111 x 658 x .0005831/.85/.92 x

.746 = 40.62kW

Permeate Energy Rate is 13.67kW/1,000gal or 3.64kW/m3

PX Pressure Exchanger System

PX High Pressure Pump:

Q = 49.5 x 1.04 (internal leakage factor) = 51.5gpm

P = 995psi (1,000psi – 20psi suction pressure +

15psi Salinity Mixing Factor)

Reciprocating Plunger

Pump Efficiency = .84 (including V belt drive losses)

40hp motor efficiency = .90

Total Pump Motor Power *= 51.5 x 995 x .0005831/.84/.90 x .746 = 29.48 kW

Booster Pump: Q = 61.5gpm

P = 75psi

Pump used for this application will be a multistage centrifu-

gal with BEP efficiency of .52. 7.5 hp motor efficiency = .88

Total Pump Motor Power = 61.5 x 75 x .0005831/.52/.88 x

.746 = 4.38kW

Total energy usag: 33.86kW

Permeate Energy Rate: 3.04kW/m3 or 11.40kW/1,000gals

* Total Pump Motor Power hp = Q x P x .0005831/ pump efficiency/ motor efficiency/

vfd efficiency (if used)

Where Q = Pump Capacity in gpm

P = Differential Pressure Rise in psi

.0005831 = Water Horsepower Constant



PX High Pressure Pump:Q = 347gpm x (430 x .04) (internal leak factor) = 364gpm

P = 1045psi (1050psi – 20psi (suction pressure + 15psi

Salinity Mixing Factor)

The most reasonable choice for a high pressure feed pump

is a multistage centrifugal. A well designed segmental ring

type such as, Flowserve 4x11 A, will display an efficiency of

64.2%.

Total Pump Motor Power =

364 x 1045 x .0005831/.70 /.93 x .746 = 277 kW

Booster Pump selection would be an API 610 class single

stage centrifugal rated for 1,000psi suction pressure and

with an efficiency of .72Booster Pump: Q = 771gpm

P = 75psi

Booster Pump Motor Power =

424 x 75 x .0005831/.72 / .92 x .7457 = 20.87kW

Total energy usage: 298 kW

Permeate Energy Rate: 14.3 kW/1,000gals or 3.85kW/m3

For the AT Turbo example, first calculate the Turbo Boost

Pressure by:

Reject Ratio x AT Turbo HTE x Brine Differential Pressure

.55 x .721 x 1025psi = 406.5 psi

The high pressure pump differential pressure then is:

1050psi (membrane pressure) – 20psi (suction pres-

sure) – 406.5 psi (turbo boost pressure) = 623.5 psi

High Pressure Pump: Because the feed flow is higher (771gp

vs 347gpm) with the AT Turbo system a more efficient fee

pump is available. A typical axial split case multistage volu

pump displays an efficiency of .79 at this flow rate. 400hp Eletric Motor efficiency = .94

Total Pump Motor Power:

771 x 623.5 x .0005831/.79 /.94 x .7457 = 281.48kW

Permeate Energy Rate:13.52kW/1,000gals or 3.57kW/m3

Example 2500,000gpd SWRO plant (347gpm permeate, 771gpm feed) operating at 45% recovery with a membrane pressure of 1,050p

PX Pressure Exchanger System AT Turbocharger System

In the above example the HTE of the PX Pressure Exchanger is .647. It is clear in the second

example the AT Turbo is more efficient. However, efficiency is just one of many factors to

consider in energy recovery equipment selection. Capital cost, reliability, maintainability, ease

and simplicity of operation and control are other important factors, and in some cases more

important than efficiency alone. In all these areas, the AT Turbo is superior to the PX device.

Figure 13

Figure 14



16

The Dual Turbine System:

A Cost Effective and Reliable Method of Eliminating Pressure Throttling in RO Systems

In the comparison of AT Turbo and Pelton Impulse Turbine performance (page 12-13), the effects of feed stream throttling were clearly

noted. This type of energy loss is especially prominent with SWRO plants that see wide annual salinity and/or temperature changes.

There are a number of ways to minimize these losses. Permeate throttling can reduces these losses somewhat. Using variable

frequency drives (VFD) with centrifugal pumps to change motor driver speed, hence pump discharge pressure is a more effective

method to minimize pressure throttling. However VFD equipment for large plants is very expensive and VFD generate their own losses

of 2-3%. Additionally, as speed is changed to accommodate pressure changes, pump capacity also changes, which may result in

unacceptable capacity to the membrane

To address the problem of pressure throttling in SWRO plants, PEI has developed the Dual Turbine System (DTS) for both single stage

and two stage plants.

Figure 15 (left) and illustrates the DTS for

maximum energy recovery in SWRO

plants.

M

P T

3986

885

39866673986 gpm

30 psi

653 gpm

865 psi

1499 gpm

865 psi

PIT

Net Motor hp 1442

High Pressure

Pump 84% eff.

84% eff.

1834 gpm

0 psi

M

P TInterstage HTC

Control Valve

Reverse

Running

Pump

Turbine

Per

Figure 16 (right) and illustrates another DTS de-

sign utilizing the BCS two stage design with a

reverse running pump as the secondary turbine.Other design options include using an additional

TurboCharger to lower first stage pressure re-

quirements.



All pressure throttling is eliminated without resort to expensive VFD. The Impulse Turbine is of modest size (specific size depends on

flow conditions) and inexpensive compared to pressure throttling losses or a VFD. There are many variations to the basic DTS design.For instance reverse running pumps or additional Turbos can be in the place of the Impulse Turbine. However in most cases the

Impulse Turbine is a good choice because of its excellent part load efficiency.

For a complete Dual Turbine energy analysis, please contact Pump Engineering.

The Dual Turbine System (continued)

Energy Rate

750 psi - 1050 psi

DTS PIT

2.26 - 2.95 kW/M3 3.90 - 3.16 kW/M3

750 800 850 900 950 1000 1050

2936

21762109

2039

19691899

1830

1763

1650

1261 1317 1377 1440 1507 1577 1650

1200

1450

1700

1950

2200

2450

2700

2950

3200

Membrane Pressure (psi)

Pump hp

with PIT

Net Motor hp

with PIT

Pump hp

with HTC

Net Motor hp

with DTS

Fig. 15 shows a parallel flow arrangement of a AT Turbo as a feed pressure booster and a smaller Pelton Impulse Turbine as a

secondary ERT. Fig 16 is a similar arrangement, but with the AT Turbo being used as a interstage booster pump. In both cases the

method of operation is the same. At those times when Turbo boost pressure plus high pressure pump discharge pressure is in excess

of membrane requirements, a valve will open to divert flow from the turbine of the AT Turbo to the Impulse Turbine. Thus the AT Turbo

becomes a controllable variable speed pump, providing only sufficient pressure for the membrane process. The high pressure pump

runs at constant speed and output at its Best Efficiency Point. The Impulse Turbine recovers the concentrate energy not required by

the Turbo.

Figure 17

F l o w



18

Today, with the use of high pressure membranes (100bar), it is now possible to design two stage seawater reverse osmosis system with

recovery ratios of 60% and higher. This type of plant has become known as the BCS or Brine Conversion System. Higher recoveries in

SWRO plants can save capital and operating cost on intake and outfall structures and piping, pretreatment system size and chemica

usage. In additional this type of plant can be highly energy efficient when an AT Turbocharger is used as the interstage booster pump.

Fig 18 shows a flow schematic for a two stage 60% recovery system. As indicated in the schematic, an interstage booster pump is

required to increase the pressure from the 1st stage to the 2nd stage. The reason for the increased interstage pressure is to compensate fo

the increased salinity of the 2nd stage feed (which is the 1st stage concentrate).

AT Turbocharger for SWRO Interstage Booster Pump Application

Example:

Two stage SWRO plant of 60% recovery and capacity of 150,000gpd (174gpm feed flow)

The feed salinity will be 35,000ppm and temperature will be 280C. The above schematic indicates the flows and pressures of this design

The AT 100 model turbocharger provides a boost pressure of 497psi (34.2bar) with a HETE of .60. The feed pump is a PD pump at 87%

efficiency. In this example the permeate energy rate is 11.15kW/1,000 gal or 2.97kW/m3

.

Advantages of AT Turbo as an Intersatge Booster Pump · Combines booster pump and ERT in one unit

· AT Turbo is more efficient than a motor driven pump · The Turbo easily handles very high suction pressure

· No high pressure mechanical seals · No electric motor or controls

With larger BCS systems, the AT turbo efficiency is such that more boost pressure can be produce than what the 2nd

stage membranerequires. A new and patented Dual Turbine System is described in the previous section that can eliminate all pressure throttling in both two

stage BCS and single stage SWRO plants

Figure 18



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