Economic Surge Tank Design by Sophisticated

Hydraulic Throttling

 

STEYRER Peter

 

Verbundplan GmbH

P.O.B. 161, 5021 Salzburg, Austria

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

Fax (++43-662) 8682-165

e-mail: steyrerp@verbundplan.co.at

 

 

Abstract

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

 

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

 

Selection of Surge Tank Type

The principle demand on a surge tank is to compensate the mass oscillation of the water flow in the pressure tunnel of load changes of turbines and/or pumps, whereas the construction type in connection with a suitable throttling device should effect in a most 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 maximum pressure 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 damping effect of the throttling device should avoid reaction on the turbine or pump. In this case 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 of a new waterway of Gerlos high-head power plant an investigation for the most economic type of surge tank fulfilling the operational requirements has been carried out. Four types of surge tanks with different throttling devices were investigated with a 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 tanks and differential types are compared as for example in figure 1 for a single load case full load rejection. The graphs simply show the benefit of differential surge tanks due to the much more effectiveness in damping of the oscillation.

 

 

Fig. 1: Different Characteristics and Damping Effect

 

Selection of a Suitable Throttling Device

Shaft surge tanks (type 1) and simple chamber surge tanks (type 2) usually are equipped with simple throttle blends or asymmetric orifices. For the latter the ratio of upsurge to downsurge losses varies from 1:2 about to 1:3 depending on the geometric construction.

 

New methods were required to get this ratio up higher for economical surge tank design. Such asymmetric reacting throttling devices can be used in principle only with differential surge tanks (type 3). These consist of two separate hydraulic systems: The lower chamber narrows at the end to a ventilation pipe with much smaller diameter, leading upward into the upper chamber. The second system consists of the upper chamber and shaft. The throttling device is located at the bottom of the shaft and dramatically retards emptying of the shaft and the upper chamber. The pressure is controlled by the level in the ventilation pipe which drops very fast, because as it empties suddenly and unhindered into the lower chamber.

The so-called reverse flow throttle was developed based on an idea of Thoma. It consists of a steel torus similar to a spiral casing of a Francis turbine (fig. 2). The downsurge 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 to the plane of the vortex flow and is discharged into the lower chamber. This change of flow direction results in very high pressure losses, these are 20 - 50 times higher than in reverse direction (type 4).

 

 

Fig. 2: Surge Tank with Reverse Flow Throttle

 

Economy of Surge Tanks in Comparison

For the Häusling pumped storage plant the difference between maximum and minimum reservoir level is 110 m. For comparison of the 4 types of surge tanks the extreme pressure in the power tunnel was the common criteria for calculating the flow resistance of the throttling device.

The dimension of the shaft and the elevation for upper and lower chamber was expected the same for all kinds of chamber type surge tanks. The shaft surge tank is not directly comparable but it would have been needed a vertical shaft with diameter 15,0 m, a height of 190 m, and a volume of 33.600 m³. The results for the other three types are shown in the following table:

 

 

The investigation for combined loading cases (fig. 3) shows that by using a modern reverse flow throttle (type 4) the volume for the lower chamber can be decreased to at least less than half the size, the volume for the upper chamber to less than two third 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 a difference of only 20 m between maximum and minimum reservoir level. The results show the same or even a greater improvement by use of a surge tank with reverse flow throttle:

 

Surge tank	Type	Load-	Upper Chamber		Lower Chamber
		case	Volume	%	Volume	%
2-chamber surge tank with symmetric orifice	2	1 2	4191 m³ 7415 m³	71 126	3226 m³ 7635 m³	127 299
2-chamber differential tank with asymmetric orifice (ratio 1:3)	3	1 2	3545 m³ 6324 m³	60 107	2841 m³ 5011 m³	111 197
2-chamber differential tank with reverse flow throttle (ratio 1:30)	4	1 2	3159 m³ 5889 m³	54 100	1990 m³ 2549 m³	78 100

 

As a result of these investigations and economic reasons a reverse flow throttle was installed lately at Gerlos power station [6] where the reservoir level varies by only by 15 m (in operation since 1993).

 

Nature Testing

The efficiency of the reverse flow throttle has been tested several times at all six established plants by nature testing. The reverse flow throttles are equipped with five electric (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 for example show that the vortex flow stabilizes nearly immediately (fig. 5). The pressure in the axis of the torus (E3, E4) is lowered about 135 m(1,35 N/mm²) within 20 s. The pressure 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 the dramatic flow resistance caused by the vortex. In the following upsurge oscillation there is nearly no difference in pressure. This shows that in the reverse flow direction only form losses are produced. The computer model results compare well with the measured graphs.

 

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

 

Conclusion

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

 

Power Plant Owner	Kaunertal TIWAG	Malta ÖDK	Mayrhofen TKW	Rosshag TKW	Häusling TKW	Gerlos TKW
Installed T Capacity P	390 MW	730 MW 290 MW	345 MW	230 MW 240 MW	360 MW 360 MW	200 MW
Maximum T Discharge P	53 m³/s	80 m³/s 2 m³/s	92 m³/s	50 m³/s 36 m³/s	65 m³/s 50 m³/s	42 m³/s
Torus Diameter	6,4 m	8,2 m	7,8 m	6,3 m	7,4 m	6,0 m
Resistance 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 für Elektrizitätswirtschaft ÖZE, Jg. 25, Heft 10, 1972

(4)   gspan j.: "Untersuchungen an der hydraulischen Rückströmdrossel von Wasserschlössern"
Wasserwirtschaft 69, Heft 12, 1979

(5)   heigertH g., STEYRER P.: "Surge tanks for Peak-Load and Pumped-Storage Power Plants - Development and Realization" XXIV IAHR-Hydraulic Congress, D-011, Madrid, 1991

(6)   stäuble h., STEYRER P.: "The First Stage to Refurbishing Power Station Gerlos" Tunnel, Gütersloh, 1994

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