variable speed turbo couplings used as pump drive in...

Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 9–12, 1999.European Desalination Society and the International Water Association.

0011-9164/99/$– See front matter © 1999 Elsevier Science B.V. All rights reserved

Desalination 125 (1999) 181–189

Variable speed turbo couplings used as pump drive indesalination plants

Ralph HöfertVoith Turbo GmbH & Co. KG, Voithstrasse 1, 74564 Crailsheim, Germany

Tel. +49 (7951) 32432; Fax +49 (7951) 32650; email: [email protected]

Abstract

The requirement for reduction in energy consumption is met by the use of speed control in pumps of MSF and ROdesalination plants. This paper serves to show the benefits of using speed control for the a.m. application. Automaticstart-up can be performed. The energy savings will pay for the additional investment.

Keywords: Couplings; Variable speed couplings; Pump drive; Speed control; Hydrodynamic couplings

1. Introduction

In the following are descriptions of variousapplications where speed control is being used inseawater desalination plants for plants workingon the vaporization principle as well as also withreverse osmosis (RO). In these applications, brinerecirculation pumps, boiler feed pumps, coolingwater pumps and pipeline pumps, which areintegrated as important functional groups into thecontrol circuits of the whole plant, are beingused. The highest availability and reliability, highprofitability, low operating and initial costs,longest life as well as easy maintenance are themain criteria for use in desalination plants.

Therefore, special attention must be given tothe fact that mass flows are controlled by a speedcontrol of the corresponding centrifugal machineinstead of throttling. The useful effects are shownby means of examples in this abstract.

2. Speed control

2.1. Reasons for speed control of brinerecirculation pumps and circulation pumps in ROprocesses

The most critical phase in desalination units isdefinitely the start-up of the system. Althoughthis start-up should, in theory, only happen once

R. Höfert / Desalination 125 (1999) 181–189182

in 3 years for regular maintenance, malfunctionsor repair work will very likely require thisprocedure more frequently than expected. In astandard designed unit, this start-up requires theoperation of the main pumps in controlledcavitation, with all valves almost fully closed toallow for heat to be fed into a MSF system orpressure to be built up to osmotic pressure in ROsystems. Any improper operation may causesevere damage not only to the pump and thevalve but also to the membranes due to suddenpressure rises.

All pump manufacturers will agree that pumpspeed control would allow a fully automatic start-up with practically no risk in case of improperoperation. Consider the repair costs and theproduction loss of one accident.

The main reason for pump speed control is toachieve variable flow rates with definedpressures. As a benefit, this arrangement is themost economical approach if one considers theoperating costs of the plant. In MSF processesvariable flow rates are mandatory to compensatevariations of brine inlet temperatures anddifferent water entry temperatures. The desiredproduct flow rate will also determine the requiredbrine flow rate.

RO processes must compensate for thevarying feed water temperatures. Betweencleaning intervals variable pressures and flowrates will compensate the membrane fouling andcompaction by increasing feed water and brineflow.

To avoid excessive physical stresses to themembrane’s polymeric structure, the feedpressure should be reduced at highertemperatures, and the permissible flow is keptconstant. In contrast, pressure is increased atreduced temperatures.

2.2. Effects of speed control

It is not the intention of this presentation tofully describe all direct details, but as an

Fig. 1. Pump characteristic curve. Pt, power required withthrottle control; PP, power required at pump shaft withspeed control; PVTC, driving power into variable speedturbo coupling.

introduction, the following must be known. Apump for desalination plants should be designedto deliver 110% flow at the designed head(pressure) (Fig. 1).

If no speed control is installed, the onlypossibility to achieve the desired flow rate is toartificially increase the resistance of the system,e.g., closing a valve. This higher pressure withmore driving power than otherwise needed willresult in throttling losses. On the other hand, aspeed control will vary the characteristic of theinstalled pump (Fig. 2).

It should be mentioned that throttlinginfluences the efficiency of such a pump morethan speed control. Throttling definitely causescavitation beyond the incipient point of theNPSH characteristic, forcing the designed toselect pumps for good NPSH behavior rather thangood efficiency. Speed control always operatespumps above those limits.

Last but not least, speed control is the onlyefficient means of smoothly increasing flow and

R. Höfert / Desalination 125 (1999) 181–189 183

Fig. 2. Performance curve of a centrifugal pump.

pressure at the desired automatically controlledvariables. As a controller input, the brinerecirculation inlet temperature, the desiredosmotic pressure or the product flow rate can beused.

2.3. Operating principle of variable speedcoupling

In some tens of thousands of installations, ourhydrodynamic turbo couplings have proven theirreliability, availability and maintainability. Itssturdy design has yielded availability figureseven better than 99.8% determined in nuclearpower plants. Although quite a number ofdifferent designs have been built, the basicprinciple is always the same (Fig. 3).

A pump impeller accelerates the operatingfluid (in most cases oil) whose kinetic energy istransmitted without any metal contact to a turbine

Fig. 3. Operating principle of the turbo coupling.

wheel. Closely built into a rotating chamber,without any pipework, these units transmit theenergy with a minimum slip of 1.5–3.5% atdesign point.

Adjusting the oil level within these couplingsin a continuous way by means of a so-calledscoop tube, a stepless variable speed can beachieved within the required flow rates for start-up and operation of the pumps, providing allbenefits mentioned before for such a speed-controlled design. The additional costs of such acoupling pay off with energy savings within avery short time. In fact, the initial costs are onlyslightly higher than the costs of a goodcontrollable valve no longer needed in variablespeed-driven desalination units.

3. Examples of speed control

The points mentioned above apply likewise toother large pumps in the plant which arediscussed below.

3.1. Brine recirculation and brine blow-downpumps

By using variable speed turbo couplings, therequirements as to maximum rigidity in case ofunfavorable environmental conditions with


Fig. 4. Drive of brine recirculation pumps by 866 SVNL 22 vert. in Rewais desalination plant. Output power, Pa = 660 kW;output speed, na = 968 rpm.

Fig. 5. Sectional view of the brine recirculation pumpdrive line in the Ruwais desalination plant.

Fig. 6. 750 SVNK vert. in the drive of brine blow-downpumps at the Al-Taweelah desalination plant. Outputpower, Pa = 200 kW; output speed, na, 950 rpm.


Fig. 7. Variable speed geared coupling driving boiler feedwater pump at the power station in Moorburg, Germany. Outputpower, Pa = 9860 kW; output speed, na = 4860 rpm.

minimum capital expenditure are fulfilled(Figs. 4 and 5).

Since 1982, four variable speed turbocouplings, type 866 SVNL 22 vert., are drivingbrine recirculation pumps in Ruwais to theoperators’ full satisfaction. Since 1993, sixvertical variable speed turbo couplings aredriving brine blow-down pumps in Al-Taweelah(Fig. 6).

3.2. Boiler feed pump

The main reason for this typical application isto provide speed regulation at the pump by usinga simple constant speed electric motor. The fluidcoupling allows a nearly unloaded quick start-upof this electric motor by reducing the time of highinrush current to only a few seconds. Ifnecessary, the fluid coupling’s scoop tube controlbrings the pump up to speed in 5 s. Theincorporated gears of the coupling (Fig. 7) allowthe pump manufacturer to design the pump forthe speed with the best efficiencies. The use of

Fig. 8. Power splitting principle in a multi-stage variablespeed drive.

speed regulation not only permits keeping thepump with better efficiency values for changingflows, but also it reduces wear and noise andincreases the lifetime of the pump. It alsoprotects the pump under minimum flowconditions.


Fig. 9. VORECON multi-stage variable speed drive, typeRW 12-11 F5, for the drive of a boiler feed pump inMoabit (Berlin, Germany). Output power, Pa = 4210 kW;Input speed, ne = 1490 rpm; output speed, na = 6000 rpm.

Fig. 11. VORECON multi-stage variable speed drive,type RWE 10 F3, for the drive of boiler feed pumps inGran Canaria, Spain. Output power, Pa = 1931 kW; inputspeed, ne = 1490 rpm; output speed, na = 3600 rpm.

Special attention should be paid to thisapplication, which is arranged in the powerstation part of the seawater desalination unit;because of the relatively high power (several

Fig. 10. VORECON multi-stage variable speed drive,type RW 14-12 F7, for the drive of boiler fee pumps atthe Superpower Station, (Mannheim, Germany). Outputpower, Pa = 8500 kW; input speed, ne = 1490 rpm; outputspeed, na = 5000 rpm.

Fig. 12. Variable speed multi-disc coupling MDC.

efficiency over the whole regulation range whilemaintaining the known advantages of hydro-dynamics; for example, the greatest reliabilityand availability with minimum serviceexpenditure are met with the multi-stage variablespeed drive VORECON.

The VORECON drive combines hydro-MW), the consideration of efficiency is of great

importance. The reqirements as to very high

dynamic components (torque converter and/or


turbo coupling) in connection with a super-imposed planetary gear in a housing. The highefficiency can be achieved due to the powersplitting principle (Fig. 8): about 80% of thepower is mechanically transmitted and thussubject to a very low portion of mechanicallosses (only about 20% of the power istransmitted hydrodynamically). Figs. 9–11 showapplications of the VORECON multi-stagevariable speed drive as a boiler feed pump drive.

3.3. Cooling water pumps

Changing water temperatures and varyingwater level are the factors for the necessary of acooling water pump control in order to providean optimum adaptation to the operatingconditions, guaranteeing economic operation ofthe plant. Especially for this kind of application,i.e., with high power consumption at lower pumpspeeds, the Voith variable speed multi-disccoupling MDC (Figs. 12 and 13) is extremelysuitable because of its compact design, low spacerequirements and low capital expense required.

The above-mentioned advantages for lowerspeeds result from the different functionalprinciple compared to the variable speed turbocoupling. The torque to be transmitted isregulated by varying the disc package contactpressure. This varies together with the radialheight of an oil ring (control oil) formed in anannular chamber rotating with input speed. Theheight of the oil ring is changed by means of atilting scoop tube. The centrifugal force producesa rotational pressure resulting in an axial forceacting on the disc package. The stronger the axialforce, the higher the torque transmitted. At 95%speed, the discs are synchronized, resulting in apure mechanical power transmission, thuseliminating the slip losses.

Fig. 13 shows one of the two variable speedmulti-disc couplings that drives cooling waterpumps at the Dock Sud power station inArgentina in the test field.

Fig. 13. Variable speed multi-disc coupling, type MDC745 vert., for the drive of cooling water pumps. Outputpower, Pa = 1350 kW; output speed, na = 425 rpm.

3.4. Transport pumps

Here variable speed turbo couplings (seeFigs. 14 and 15) are used to adjust pump flowand pressure to variable flows to increase thepressure in pump stations in the event of changesin altitude and to adapt the head to variableconditions in shared pipeline systems. As a newdrive solution for drinking water pumps inseawater desalination plants, we would like tomention, in addition, the variable speed turbocoupling, type SVTW, using water as theoperating medium (originally developed to drivepumps in irrigating plants) where the circulatingwater is used as the power-transmitting medium.The requirements as to maximum possibleenvironment compatibility can be metsufficiently with this new drive variant.


Fig. 14. 1150 SVNL 21 G in the drive of town water pumps at the Al-Taweelah desalination plant. Output power, Pa =2675 kW; output speed, na = 961 rpm.

Fig. 15. 1000 SVNL 21 in the drive of drinking waterpumps at the Al-Taweelah desalination plant. Outputpower, Pa = 1026 kW; output speed, na = 970 rpm.

4. Reference list of Voith variable speed turboand geared variable speed couplings inseawater desalination plants

Since 1997, 82 variable speed turbo andgeared variable speed couplings have beenoperating at the operators’ full satisfaction(Table 1).

5. Conclusions

Variable speed turbo couplings and gearedvariable speed couplings — proven means ofspeed variation and with years of perfect casehistories in thousands of applications withcentrifugal machines — have been successfullyused for flow control. The reasons for thissuccess are simple functioning and robust design,low maintenance requirements, high reliabilityand availability at very competitive system costs.\

References

[1] G. Fechner and R. Pillkahn, Use of variable speedpumps for MSF and RO plants, Elsevier Science,Amsterdam, 1985.

[2] G.H. Peikert, Variable speed fluid couplings drivingcentrifugal compressors and other centrifugalmachinery, 13th Turbomachinery Symp., Houston,Texas, 1992.

[3] Variable speed multi-disc coupling MDC, VoithPrint, Cr 270, 1997.


Table 1Voith variable speed turbo and geared variable speed couplings

Year Company/code Qty. Type Power (kW) Speed (rpm) Driven machine

1977 KSB, BremanKSB Bremen S13

1 650 SVNL2 vert.

404 1490 Centrifugal pump

1981 Balcke Dürr, FrankenthalBalcke Umm Al Nar S1–S3

7 R 15 K 3100 5387 Boiler feed pump

1982 Torishima, JapanTorishima S4Incon, HomburgIncon S 1, Ruwais

4

4

866 SVNLvert.866 SVNL22 vert

660

660

963

968

Brine recyclingpumpCooling waterpump

1988 Thyssen Maschinenbau, WittenThyssen Al Taweelah S1

4 866 SVNL22 vert.

769 958 Centrifugal pump

1989 Thyssen Ruhrpumpen, WittenThyssen Umm Al Nar S1TMI La SpeciaTMI Sitra S1–S3

4

4

866 SVN22 vert.750 SVTL

700

912

962

1447

Centrifugal pump

Centrifugal pump

1991 KSB, FrankenthalKSB Abu Dhabi S1

3 1000 SVNL 21 1026 970 Drinking waterpump

1993 Thyssen Ruhrpumpen, WittenThyssen Al-Taweelah S2KSB, FrankenthalKSB Al-Taweelah S1–S4KSB, FrankenthalKSB Al-Taweelah S5–S6KSB, FrankenthalKSB Al-Taweelah S7–S9TLT OberhausenTLT Al-Taweelah S1–S3

1

18

5

6

12

866 SVN22 vert.R 14 K 375.1

1150 SVNL 21G

750 SVNK vert.

1150 SVNL21–18,5.3

769

2177

2675

200

1267

958

5442

961

950

962

Centrifugal pump

Boiler feed pump

Drinking (town)water pumpBrine blow-downpumpCentrifugal fan

1994 IDP, Worthington, ItalyWorthington Fontas S1

4 866 SVNL 22 685 964 Product (drinking)water pump

1995 KSB, FrankenthalKSB Al-Taweelah S10

1 R 14 K 375 2177 5442 Boiler feed pump

1999 Torishima, OsakaTorishima Al-Taweelah A2 S1

4 1150 SVNL21–18,5.3

1423 714 Portable waterpump

variable speed turbo couplings used as pump drive in...

Documents