Desalination 276 (2011) 60–65

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The integration of water vane pump and hydraulic vane motor for a smalldesalination system

Yong Lu ⁎, Yuanyang Zhao, Gaoxuan Bu, Pengcheng ShuNational Engineering Research Center of Fluid Machinery and Compressors, Xi'an Jiao Tong University, China

⁎ Corresponding author at: National Engineering Reseand Compressors, Xi'an Jiaotong University, No. 28 Xian710049, China. Tel.: +86 29 82663812; fax: +86 21 82

E-mail address: luyong_xjtu@hotmail.com (Y. Lu).

0011-9164/$ – see front matter. Crown Copyright © 20doi:10.1016/j.desal.2011.03.023

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 August 2010Received in revised form 4 February 2011Accepted 5 March 2011Available online 7 April 2011

Keywords:Reverse osmosis (RO)Energy recoveryVane pumpHydraulic vane motor

An integration of water vane pump and hydraulic vane motor was introduced in small reverse osmosis (RO)system, in which the hydraulic vane motor was used as energy recovery device. The hydraulic performanceand energy consumption of one combination of pump and hydraulic motor were investigated under differentoperation conditions. This type of integration pump reduces energy cost and simplifies the system setup, thusit could be an alternative choice for small RO desalination system. Our results demonstrated that theimprovement of volumetric efficiencies of the pump and the hydraulic motor was the main factor to increasethe prototype pump performance.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

An energy recovery device (ERD) reduces the energy consumptionof pressure driven membrane systems by utilizing energy remainingin concentrate stream. Such devices have been well developed andused in recent years, especially in SeaWater Reverse Osmosis (SWRO)system. Energy recovery technologies have been widely reported bymany engineers [1,2]. According to MacHarg's classification, there arethree types of energy recovery approaches. Class I ERDs use theprinciple of positive displacement (or referred to as isobaric energyrecovery) technologies to directly transfer the pressure energy inconcentrate flow to new feed flow. Commercial examples are EnergyRecovery Inc.'s PX pressure exchange device [3,4], Calder's DWEERERDs [5,6], SIEMAG's pressure exchanger system (PES) [7,8], KSBgroup's SalTec's DT device [9,10], Aqualyng system [11], RO Kineticsystem [12] and Spectra Watermakers’ Clark Pump energy recoverypressure-amplifier [13,14]. The energy transfer efficiency (ETE) ofClass I devices can reach above 90%. The operation characteristicswere studied, for instance, by Wang et al [15,16]. Class II ERDs arebased on centrifugal principle, and they used as a single stand-aloneunit consisting of a pump section and a turbine section. The pumpsection converts the mechanical energy captured by the turbinesection from concentrate flow back to pressure energy in feed flow.Commercial examples are FEDCO's Hydraulic Pressure Booster (HPB)[17] and Pump Engineering Inc (PEI)'s Hydraulic TurboCharger (HTC)

arch Center of Fluid Machineryning West Road, Xi'an, Shaanxi663792.

11 Published by Elsevier B.V. All rig

[18]. The ETE of Class II devices can get about 70% with flexibleinstallation location in systems. The principle of Class III ERDs is similarto that of Class II devices. These ERDs are connected with high pressurepump and electric motor as a mechanical power assistant unit.Commercial examples include Francis Turbines and Pelton Wheels[19,20]. The ETE of Class III devices is close to 70%. In 2002, Danfosscompany introduced seawater hydraulic components in RO system[21,22], in which an axial piston pump (APP) and an axial piston motor(APM) are connected to a double shafted electric motor. The APM in thesystem is used as ERD. Theworking process is similar to that of the ClassIII ERDs. Compared with APP alone, the APP–APM can save about 50%power consumption during the pressure operation range of seawaterdesalination. The energy consumption is between 3 and 5 KWh/m3witha production flow under 25 m3/d and a recovery from approximately30% to 46%. Danfoss is also developing a combination with variablewater motor to improve system performance [23].

Clark Pump system can be used in a very small system, in contrastto most of other ERDs which are usually applied to large or middlescale SWRO plants. The Class I ERDs are generally rigged in parallelwith high pressure pump, and they require booster pump andoperation control valves which increase system complexity. Never-theless, the ERDs for small RO system have not been fully developeduntil now except the Danfoss APP–APM system.

In this paper, a combination of a water hydraulic vane pump and avane hydraulic motor is presented in a small desalination system. Suchpumps are positive displacement type, which have smaller flowpulsation, lower noise, compact configuration and simple. An integra-tion of the pump and the hydraulic motor can be easily applied in a ROsystem. In the following experiments, the hydraulic performance andenergy consumption of the prototype pump will be investigated.

hts reserved.

61Y. Lu et al. / Desalination 276 (2011) 60–65

2. Description of the prototype pump and system

2.1. The technical characteristics

The prototype pump is modeled on a high pressure balanceddouble vane pump which is one type of common pumps and widelyused in hydraulic systems. Fig. 1 shows a schematic diagram of thecross-section in a cartridge design. The pump mainly includes a shaft,a front casing, a body, a hinder casing, a pump element and a hydraulicmotor element. The pump element and the hydraulic vane motorelement are directly driven by a shaft and separated by a seal. Thefeatures of the two elements are fixed displacement, twelve vanes andintegral hydraulic balancing.

In the current work, the materials of prototype pump componentsare selected to satisfy the seawater corrosion requirement. Seawater hashigh electrical conductivity and strong electrochemical corrosionwhichdemand the use of corrosion-resistant materials and the metallicmaterialsmust complywith electrochemical series. The vital parts of theprototype pump are made of duplex (ferritic/austenitic) stainless steel(SAF 2205). Vanes are fabricated of polyimide–imide (PAI) materialfilled with graphite. The sealing is made of polytetrafluoroethylene(PTFE).

2.2. Membrane separation system with the prototype pump

Fig. 2 presents a simple schematic diagram of the membraneseparation process with the prototype pump. The pump elementsupplies high pressure (HP) feed flow and the hydraulic motorelement provides energy recovery from high pressure (HP) concen-trate flow. A relief valve adjusts the operation pressure of membranemodule.

Since the vane pump and the hydraulic motor are positivedisplacement machines and serially connected on a shaft, the highpressure feed flow and the high pressure concentrate flow aredependent on the displacements and volumetric efficiencies of thepump and the motor, when the rotation speed of the electric motorand the operation pressure of the membrane module are determined.Therefore, this membrane separation system is a self-controlledpressure system, which eliminates many auxiliary control equip-ments applied in other ERDs. The recovery variation of membranemodule can only go through the replacement of different displace-ment pump element or motor element. When the membraneseparation system is set up, the performance of membrane module,as well as the volumetric efficiencies of pump element and motorelement should be precisely calculated.

BPump Element

FrontCasing

Shaft

LP Inlet HP Outlet

Fig. 1. Cross-section view o

3. Description of the experimental setup

An experimental rig was set up to simulate the RO process, thehydraulic performance and energy consumption of the prototypepump.Fig. 3 shows the schematic diagram and Fig. 4 shows the picture of theexperimental rig in the laboratory. The setupmainly consists of a waterreservoir, pressure relief valves, pressure transducers, flow transducers,throttles, a piston pump, a cooler and an electric engine. The flow rateand pressure were measured by flow transducer and pressuretransducer, respectively, and then processed by the computer. Thecooler controlled the system temperature variation. The electric powerconsumed by the prototype pump (P) was measured by electric powermeter and a frequency converter adjusted the rotation speed.

The experimental setup includes two routes, route I and route II. Inroute I, the loadpressureof thepumpelement (pSH)was setby the throttlevalve. The outlet flow rate (QS) simulated the feed flow. In route II, thepistonpumpsimulated thehighpressure concentrateflow(QB1) from themembranemodule to drive the hydraulic motor element. The outlet flowrateof thehydraulicmotor (QB2)was comparedwithQB1 to check the sealsituation. The loadpressureof thehydraulicmotor (pBH)wascontrolledbythe relief valve, and the outlet pressure (pBL) was about 1 bar.

The volumetric efficiency is an important index to describe therelationship between internal leakage and load pressure. Volumetricefficiencies of the pumpelement (ηVP) and the hydraulicmotor element(ηVM) were calculated in different load pressures at a certain rotationspeed.

The ηVP is described by:

ηVP =QS

QS0⁎100% ð1Þ

where QS0 is the outlet flow rate of pump element without loadpressure.

The ηVM is obtained from:

ηVM =QB10

QB1⁎100% ð2Þ

where QB10 is the inlet flow rate of hydraulic motor element withoutload pressure.

In the experiments, QB1 was the same as QB2 because of well seal,so the permeate flow (Qp) is defined as:

Qp = QS−QB1 ð3Þ

HP Inlet

Hydraulic Motor Elementody

HinderCasing

LP Outlet

f the prototype pump.

Electric motor Prototype pump

Relief Valve

PermeateMembrane module

LP concentrate flow HP concentrate flowFeed flow

HP feed flow

Fig. 2. Schematic diagram of the membrane separation process with the prototype pump.

62 Y. Lu et al. / Desalination 276 (2011) 60–65

The system recovery is given by:

η =Qp

QS⁎100% ð4Þ

From Eqs. (1) and (2), the system recovery can also be describedas:

η = 1−QB10

QS0

1ηVMηVP

ð5Þ

The QS0 and QB10 are determined by displacement of the certainelement and rotation speed, so the η is decided by ηVP and ηVM. Qp andηwere calculated based on experimental results to describe the effectof the volumetric efficiencies of two elements in different conditions,and did not describe the change trend of a certain RO membranesystem with the load pressure variation.

QB1

Safety Valve

Accumulator

Piston Pump

ElectricPower Mete

Computer

Manometer

FlowTransducer

T

Throttle Valv

FilterCooler

Relief Valve

Wa

Fig. 3. The schematic diagram

During the test, the pressure medium was tap water. The absolutefiltering precision was 5 μm. The main test parameters were asfollows:

• The maximum pSH was 42 bar and pBH was about 1.5 bar less thanpSH to simulate the pressure drop in membrane module.

• The rotation speed range was 1100–1400 rpm.• The fluid temperature was 21 °C and the temperature variation wasless than 2 °C.

4. Results and discussion

4.1. Flow rates of the prototype pump

The flow rates of pump and hydraulic motor elements were studiedunder four rotation speeds conditions, from 1100 rpm to 1400 rpm.Fig. 5 presents the experimental results about the discharge of the pump

P

QB2

QS

PBL

PBH

PSL

PSH

M

PrototypePump

FrequencyConverter

r

PressureTransducer

hermometer

e

ter Reservoir

of the experimental setup.

Fig. 4. The experimental setup.

Fig. 6. Volumetric efficiency versus load pressure.

63Y. Lu et al. / Desalination 276 (2011) 60–65

element (QS), the inlet flow rate of the hydraulic motor element (QB1)and the calculated results of the permeate flow (QP).

When the load pressures of pump element and hydraulic motorare increased from 1 bar to 42 bar and 40.5 bar respectively, QS isdecreased and QB1 is raised. Correspondingly QP is reduced. QS isdecreased from 3.75 m3/h to 2.96 m3/h in 1400 rpm and from2.96 m3/h to 2.22 m3/h in 1100 rpm; QB1 is increased from 1.69 m3/h to 2.29 m3/h in 1400 rpm and from 1.35 m3/h to 1.86 m3/h in1100 rpm. Accordingly QP is reduced from 2.06 m3/h to 0.67 m3/h in1400 rpmwhich is decreased by 67.5% and from 1.62 m3/h to 0.36 m3/h in 1300 rpm with decreased by 77.8%.

4.2. volumetric efficiencies and system recovery

From Fig. 5, QP declines as the load pressures increase. This isbecause the leakage of two elements increase and the effect of thevolumetric efficiencies on the calculated permeate flows. Fig. 6 showsthe experimental results about the volumetric efficiencies of pumpelement (ηVP), the hydraulic motor element (ηVM). AppropriatelyFig. 7 presents the calculated results of the system recovery (η).

As the load pressures increased, ηVP, ηVM and η are decreasedalmost linearly. When the load pressures are at the maximum, ηVP isreduced to about 79% in 1400 rpm and 74.8% in 1100 rpm. ηVM isreduced to 73.8% in 1400 rpm and 72.3% in 1100 rpm. Correspond-ingly η is decreased from 54.4% to 22.6% in 1400 rpm with decreased

Fig. 5. Flow rate versus load pressure.

by 58.5% and is from 54.6% to 16.2% in 1100 rpm with decreased by70.3%.

When the load pressures of pump element and hydraulic motorelement are low, the system recoveries are almost the same in tworotation speed conditions because ηVP and ηVM are close to 1. So fromEq. (5), η is determined by the displacements of two elements. Whenthe load pressures rise, the system recoveries are declined and hard tomaintain a fixed value as the volumetric efficiencies of two elementsdecreased rapidly. The η in 1400 rpm is higher than that of otherrotation speed conditions because the rotation speed also has a slighteffect on the volumetric efficiencies of two elements.

4.3. Energy consumption and energy saving

Firstly the power consumption of pump element (PP) wasinvestigated alone in the same range of load pressure under fourrotation speed conditions. Secondly the power consumption of theintegration of pump and hydraulic motor element (PI) was testedunder pressure conditions of the membrane module. Figs. 8 and 9present the power consumptions and the energy reduction percen-tages (E).

From Figs. 8 and 9, the power consumptions and energy savingincrease with the growth of load pressures. E is 20.2% in 1400 rpm andis 15.5% in 1100 rpm at the maximum operation pressure. When theload pressures are low, E is minor because of low power energy influid and small mechanical efficiencies of the pump and hydraulicmotor elements [24].

Fig. 7. System recovery versus load pressure.

Fig. 8. Power consumption versus load pressure.Fig. 10. Specific power consumption versus load pressure.

64 Y. Lu et al. / Desalination 276 (2011) 60–65

From PI and the corresponding QP, the specific power consumptionof the integration of two elements (WI) was calculated, and results arepresented in Fig. 10. WI increases quickly as the load pressures rise,which reach about 6.2 kWh/m3 in 1400 rpm and 10.4 kWh/m3 in1100 rpm at 42 bar. This is because QP and the volumetric efficienciesof two elements drop rapidly as the operation pressures increase. WI

in 1400 rpm is lower than that of other rotation speed conditions for ηhas a bit larger value in higher rotation speed.

5. Conclusion

This paper shows that the combination of a water hydraulic vanepump and a vane hydraulic motor is a feasible approach for smalldriven membrane system. The prototype pump was investigatedunder pressure conditions of membrane module. The energyreduction percentage was about 20% at the maximum operationpressure. From the experiment results, volumetric efficiencies of thepump and motor elements mainly affect hydraulic performance andenergy consumption of the prototype pump. Compared with otherERDs, the prototype pump is easier to operate and maintain. Thecartridge design of the prototype pump is simply to replace the pumpor hydraulic motor element to change displacement, which adapt therecovery ratio of membrane module. Like Danfoss APP–APM system,two elements in the prototype pump are fixed displacement andconnected by a shaft with the same rotation speed, so the membranesystem has constant recovery ratio and less flexibility. When the

Fig. 9. Energy reduction percentage versus load pressure.

performance of the membrane module and the pump/motor elementare accurately designed, the membrane separation system can reachoptimized operation conditions.

Nomenclature

Q S feed flow rate, m3/hQB1 flow rate of high pressure concentrate, m3/hQB2 flow rate of low pressure concentrate (drain), m3/hQp permeate flow, m3/hQ S0 the outlet flow rate of pump element without load pressure,

m3/hQB10 the inlet flow rate of hydraulic motor element without load

pressure, m3/h.ηVP volumetric efficiency of vane pump element,%ηVM volumetric efficiency of hydraulic vane motor element, %η system recovery, %pSL inlet pressure of vane pump, barpSH outlet pressure of vane pump, barpBH inlet pressure of hydraulic vane motor element, barpBL outlet pressure of hydraulic vane motor element, barP power consumption, kWPP power consumption of pump element, kWPI power consumption of the integration of two elements, kW

E energy reduction percentage, E =PP−PIPP

, %

WI specific power consumption of the integration of twoelements, kWh/m3

Acknowledgements

This research was funded by the Program for Changjiang Scholarsand Innovative Research Team in the University of China (Grant No.IRT0746).

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