ECONOMIC SURGE TANK DESIGN BY SOPHISTICATEDHYDRAULIC THROTTLING

STEYRER PETER

Verbundplan GmbHP.O.B. 161, 5021 Salzburg, Austria

Tel. (++43-662) 8682-22353Fax (++43-662) 8682-165

e-mail: steyrerp@verbundplan.co.at

ABSTRACTThe author reports on an economic surge tank design for of the hydraulic system ofhigh-head, peak-load storage power plants. The operation of storage power plantsrequires a completely free operation without any restrictions on changes in loading orflow of neither the pumps nor the turbines. Examinations of traditional, simple shaft-or chamber-type surge tanks show their ineffectiveness due to the required chambervolume and the resulting costs. This demand led to the development of a moreeffective throttling device in connection with dual chamber surge tanks. Several surgetanks with this sophisticated system of the so-called reverse-flow throttle are alreadyunder operation in Austria.

Keywords: Free operation, differential surge tank, unsteady flow, reverse flow throttle,damping of oscillation

SELECTION OF SURGE TANK TYPEThe principle demand on a surge tank is to compensate the mass oscillation of thewater flow in the pressure tunnel of load changes of turbines and/or pumps, whereasthe construction type in connection with a suitable throttling device should effect in amost powerful damping of the amplitude already in the very first period of oscillation.

Partial or full-load rejection leads to on upsurge oscillation, whereby the maximumpressure is limited by the bearable stress of the concrete lining of the power tunnel.Load demand, however is followed by a downsurge oscillation and the dampingeffect of the throttling device should avoid reaction on the turbine or pump. In thiscase the minimum pressure must not come below the elevation of the power tunnel.

For the design of the Häusling pumped-storage power plant and later for rebuilding ofa new waterway of Gerlos high-head power plant an investigation for the mosteconomic type of surge tank fulfilling the operational requirements has been carriedout. Four types of surge tanks with different throttling devices were investigated witha specific computer software developed by Verbundplan and the results compared.

- (type 1) Shaft surge tank with orifice- (type 2) Chamber surge tank with symmetric orifice- (type 3) Differential surge tank with asymmetric orifice- (type 4) Differential surge tank with reverse flow throttle

The obvious different characteristics and damping effects of chamber surge tanksand differential types are compared as for example in figure 1 for a single load casefull load rejection. The graphs simply show the benefit of differential surge tanks dueto the much more effectiveness in damping of the oscillation.

Fig. 1: Different Characteristics and Damping Effect

SELECTION OF A SUITABLE THROTTLING DEVICEShaft surge tanks (type 1) and simple chamber surge tanks (type 2) usually areequipped with simple throttle blends or asymmetric orifices. For the latter the ratio ofupsurge to downsurge losses varies from 1:2 about to 1:3 depending on thegeometric construction.

New methods were required to get this ratio up higher for economical surge tankdesign. Such asymmetric reacting throttling devices can be used in principle only withdifferential surge tanks (type 3). These consist of two separate hydraulic systems:The lower chamber narrows at the end to a ventilation pipe with much smallerdiameter, leading upward into the upper chamber. The second system consists of theupper chamber and shaft. The throttling device is located at the bottom of the shaftand dramatically retards emptying of the shaft and the upper chamber. The pressureis controlled by the level in the ventilation pipe which drops very fast, because as itempties suddenly and unhindered into the lower chamber.The so-called reverse flow throttle was developed based on an idea of Thoma. Itconsists of a steel torus similar to a spiral casing of a Francis turbine (fig. 2). Thedownsurge oscillation produces a vortex flow which stabilizes within a few seconds.The water is forced to exit the torus through a small connection pipe rectangular tothe plane of the vortex flow and is discharged into the lower chamber. This change offlow direction results in very high pressure losses, these are 20 – 50 times higherthan in reverse direction (type 4).

Fig. 2: Surge Tank with Reverse Flow Throttle

ECONOMY OF SURGE TANKS IN COMPARISONFor the Häusling pumped storage plant the difference between maximum andminimum reservoir level is 110 m. For comparison of the 4 types of surge tanks theextreme pressure in the power tunnel was the common criteria for calculating the flowresistance of the throttling device.The dimension of the shaft and the elevation for upper and lower chamber wasexpected the same for all kinds of chamber type surge tanks. The shaft surge tank isnot directly comparable but it would have been needed a vertical shaft with diameter15,0 m, a height of 190 m, and a volume of 33.600 m³. The results for the other threetypes are shown in the following table:

Surge tank Type Load- Upper Chamber Lower Chambercase Volume % Volume %

2-chamber surge tankwith symmetric orifice

2 12

4623 m³7219 m³

109171

3226 m³5496 m³

128218

2-chamber differentialtank with asymmetric

orifice (ratio 1:3)

3 12

3452 m³5639 m³

82134

2841 m³4697 m³

113187

2-chamber differentialtank with reverse flow

throttle (ratio 1:30)

4 12

2316 m³4223 m³

55100

1990 m³2516 m³

79100

The investigation for combined loading cases (fig. 3) shows that by using a modernreverse flow throttle (type 4) the volume for the lower chamber can be decreased toat least less than half the size, the volume for the upper chamber to less than twothird in comparison to type 2.

Fig. 3: Loading cases for comparison of different surge tanks

For a similar figuration of hydraulic system the same comparison was done with adifference of only 20 m between maximum and minimum reservoir level. The resultsshow the same or even a greater improvement by use of a surge tank with reverseflow throttle:

Surge tank Type Load- Upper Chamber Lower Chambercase Volume % Volume %

2-chamber surge tankwith symmetric orifice

2 12

4191 m³7415 m³

71126

3226 m³7635 m³

127299

2-chamber differentialtank with asymmetric

orifice (ratio 1:3)

3 12

3545 m³6324 m³

60107

2841 m³5011 m³

111197

2-chamber differentialtank with reverse flow

throttle (ratio 1:30)

4 12

3159 m³5889 m³

54100

1990 m³2549 m³

78100

As a result of these investigations and economic reasons a reverse flow throttle wasinstalled lately at Gerlos power station [6] where the reservoir level varies by only by15 m (in operation since 1993).

NATURE TESTINGThe efficiency of the reverse flow throttle has been tested several times at all sixestablished plants by nature testing. The reverse flow throttles are equipped with fiveelectric (E1 – E5) and two hydraulic (H1, H2) pressure measurement devices.

Fig. 4: Measurement Devices for Monitoring of Reverse Flow Throttle

The results obtained during a resonance load case at Häusling power plant forexample show that the vortex flow stabilizes nearly immediately (fig. 5). The pressurein the axis of the torus (E3, E4) is lowered about 135 m(1,35 N/mm²) within 20 s. Thepressure along the circumference of the spiral casing (E1, E2) reacts much slower.The measured graph of H2 corresponds exactly to E3, E4 and the graph of H1 to E1,E2. The rapid increase in pressure differential between the graphs shows thedramatic flow resistance caused by the vortex. In the following upsurge oscillationthere is nearly no difference in pressure. This shows that in the reverse flow directiononly form losses are produced. The computer model results compare well with themeasured graphs.

Fig. 5: Nature Test – Resonance loading case in turbine mode

CONCLUSIONAt present six differential surge tanks with reverse flow throttle are under operation athigh-head power plants in Austria, with wide spread of varying differences inreservoir level and turbine/pump discharge. All of them work satisfactorily and it is torecommend to make already in the design stage an economic comparison weathersuch a sophisticated design could mean an improvement to a new project.

Power PlantOwner

KaunertalTIWAG

MaltaÖDK

MayrhofenTKW

RosshagTKW

HäuslingTKW

GerlosTKW

Installed TCapacity P

390 MW 730 MW290 MW

345 MW 230 MW240 MW

360 MW360 MW

200 MW

Maximum TDischarge P

53 m³/s 80 m³/s2 m³/s

92 m³/s 50 m³/s36 m³/s

65 m³/s50 m³/s

42 m³/s

Torus Diameter 6,4 m 8,2 m 7,8 m 6,3 m 7,4 m 6,0 mResistance Ratio 1:50 1:28 1:17 1:17 1:29 1:31

TIWAG Tiroler Wasserkraft AG ÖDK Österr. Draukraftwerke TKW Tauernkraftwerke

REFERENCES:(1) SEEBER G.: "Das Wasserschloß des Kaunertal-Kraftwerkes" Schweizerische

Bauzeitung, Zürich, 1970/1(2) HEIGERTH G.: "Drossel- und Differential-Wasserschlösser von Regelkraftwerken

mit freier Betriebsführung" Thesis, Vienna University of Technology, 1970(3) GSCHAIDER F., EWY G., HEIGERTH G.: "Triebwasserführung, Wasserschlösser

und Bachbeileitungen der Zemmkraftwerke" Österreichische Zeitschrift fürElektrizitätswirtschaft ÖZE, Jg. 25, Heft 10, 1972

(4) GSPAN J.: "Untersuchungen an der hydraulischen Rückströmdrossel vonWasserschlössern"Wasserwirtschaft 69, Heft 12, 1979

(5) HEIGERTH G., STEYRER P.: "Surge tanks for Peak-Load and Pumped-StoragePower Plants – Development and Realization" XXIV IAHR-Hydraulic Congress,D-011, Madrid, 1991

(6) STÄUBLE H., STEYRER P.: "The First Stage to Refurbishing Power StationGerlos" Tunnel, Gütersloh, 1994

(7) STEYRER P., SAMETZ L.: "Surge Tanks with Reverse Flow Throttle"International Symposium on Pumped Storage Development, Nanjing, 1994