- Email: [email protected]
Design of two-phase turbines and geothermal electric power-plants in Hungary
0375 - 6505/85 $3.00 + 0.00 Pergamon Press Ltd. ~ 1985 CNR.
Geothermics, Vol. 14, No. 2/3, pp. 2 2 9 - 2 4 6 , 1985. Printed in Great Britain.
DESIGN
OF TWO-PHASE TURBINES ELECTRIC POWER-PLANTS
AND GEOTHERMAL IN HUNGARY
I. SEBESTYEN Rakoczi ut. 4, H-1072 Budapest, Hungary (Transmitted by the Government o[' Hungary) R.87 A b s t r a c t - - A description is given of a private project to produce electric energy from geothermal fluids in Hungary. Single-stage and multi-stage action and reaction turbines of the axial flow type are recommended for this purpose, together with the associated equipment and power plants.
INTRODUCTION In mid 1971 the author began work on the design of two-phase turbines for the expansion of saturated liquids. In June 1972 the author applied for a patent,* describing his design of axialflow type two-phase action turbines destined for multiple uses, but with priority given to geothermal electric power-plants. In May 1973 tests were made with two-phase nozzles in order to determine the deviation factor. His application for a patent was expanded in 1973 and 1979 and was issued in September 1983. A geothermal power-plant project, called " t o t a l flow concept", was described in April 1973 by Austin et al. Underlying their design is the same basic principle as that of the turbines and power-plants described by the present author in his application for a patent submitted in 1972. Work continued on this design in Hungary, in parallel with, and independent of research in the U.S.A. and elsewhere. C H O I C E OF H E A T E N G I N E Total extraction of the energy content of hot waters produced from a well can be achieved theoretically by expansion in some type of two-phase turbine. The other two methods, i.e. use of a steam turbine, accompanied by the flash process, and use of heat exchangers in a binary cycle, can at the moment be justified on practical grounds. The major problem is to design a two-phase turbine that is economically more attractive than these two methods. A two-phase turbine for the expansion of saturated liquid acts as a heat engine, because of the reduced internal energy of the medium. Considering that the flow of a saturated steam-free liquid, at the slightest drop in pressure, would be transformed into a so-called " m i s t " flow, it is obvious that the heat engine must be of aerodynamic design, since large volumes are flowing in an extremely fine distribution of drops. During expansion the drops would be further atomized partly as a result of boiling and partly of shear forces. TESTS W I T H N O Z Z L E S The fineness of drop distribution was studied in 1973 during tests with two-phase nozzles, conducted by the author with the aid of energy experts of the Pharmacological Plant EGYT, *Patent No. SE 1632/1972.
229
1. Sebestyen
230
Budapest. The two-phase nozzles were first designed on the basis of theoretical calculations, and imperfect expansion was found to be caused by delayed boiling. Thus the liquid entered the nozzle in such large quantities that, after full boiling had begun, there was not enough room for it to leave the nozzle at the outlet; as a consequence, expansion was delayed until the fluid had left the nozzle. The inlet diameter, i.e. the smallest cross-section of nozzle, had therefore to be narrowed. This was achieved by measuring the characteristics of the imperfect nozzles and by applying corrections during calculations. Figure 2 shows that the new nozzle (shown in Fig. I) was capable of producing a two-phase mixture of nearly parallel flow lines when expanding saturated hot water at a pressure of 11 bar abs to 1 bar, and also that expansion in the final cross-section was complete, as shown by measurements. Using the impulse scales shown in Fig. 3, in the form shown in Fig. 4, the impulses were measured on a buffer plate, and the deviation factor (loss factor) of the nozzle determined. f60 f45 90 ° back~,ards - ~ ~(~6 + ~ , ...... ~ ,
•.4,. x x
) /
/
.
.
Fig. 1. Two-phase nozzle designed specially to account for delayed boiling.
Fig. 2. Expansion test with two-phase nozzle, for 10
1 bar.
Geothermal Power-plants in Hungary
231
Fig. 3. Impulse measuring equipment.
Fig. 4. Impulse measurement.
T U R B I N E IN G E N E R A L An aerodynamic turbine which processes large volumes o f a two-phase mixture (containing drops that are being increasingly atomized during expansion) will clearly be similar to steam turbines and to the axial flow types in particular. Since two-phase turbines are similar to the latter, and are operated by the expansion of a saturated liquid, their operating conditions are, in some respects, more favourable than those of the so-called moist steam turbines. In the multistage turbines, steam from the dispersed liquid phase passing through the turbine will develop
232
I. Sebestyen
in all stages; droplets in the mist are in the boiling or explosion stages throughout their expansion. If using reactive stages, this phenomenon will also take place along the rotating blades. The two-phase nozzles of the first stage of two-phase axial-flow turbines are mounted at the inlet to a joint liquid chamber. The steam phase must be prevented from reaching the line connected to the turbine as, otherwise, some of the nozzles may receive steam and some may receive a mixed phase. The liquid chamber must therefore be sufficiently large to allow the separation of any steam bubbles that may flow in. These bubbles can be conveyed to some other utilization by the automatic steam separator located at the top of the chamber. The liquid entering the turbine may be saturated or at a pressure above that of saturation. In the latter case, liquid expansion will take place at the inlet of the nozzles of the first stage; when the liquid reaches the saturated state during expansion, boiling in the nozzles will begin. Because of flow losses, erosion (see below) and vibration effects, the turbines should preferably be designed for total inflow. In turbines with horizontal shafts and larger turbine wheels, gravity pressure along the nozzle inlets in the liquid chamber will vary greatly, but this will not affect expansion to any significant extent, because pressure changes along the inlet openings are not accompanied by differential temperatures. EROSION Erosive effects on the turbine are limited by the fact that the concentration of dispersed liquid during expansion of saturated liquid is considerably higher than during expansion of saturated steam in moist steam turbines. Therefore the liquid forms a flowing layer on the blades, and has a less erosive effect than drops. Erosion is also limited by the boiling of incident droplets, by boiling of the liquid film flowing to points of decreasing pressures, and also by the small inertia and large surface friction of the drops in the mist-like mixture, which hinders separation because of its short expansion time. Erosion can also be limited by reducing the pressure drop for each stage and by applying antierosive materials. R E G U L A T I N G SYSTEM The regulating system used is similar to that used in conventional turbines. Throttling can be achieved in the connecting line (in this case the separated steam must be diverted), or at the nozzles. Varying the flow while some nozzles are completely shut is not recommended because of pulsating flow in the wheel channels during rotation (breaking of the liquid film). A description is given below of two throttling devices for controlling turbine velocity, but with larger time constants. When the generator linked to the turbine is connected to a larger power grid of stable frequency, velocity is maintained at a constant value by the grid itself. T H E NEW T W O - P H A S E N O Z Z L E Because of its large specific volume the fluid leaving the nozzles must have a large crosssection, especially if expansion takes place in the vacuum, and despite the high flow velocity. At the inlet of the nozzle the size and number of the steam bubbles are low and this results--together with the increasing velocity--in a minimal sectional area. The multi-sectional nozzle, shown in Fig. 5, has reduced sections in both directions, which do not reduce flow to any great extent; the nozzle length is kept to a minimum, and, at its narrowest point, square or circular cross-sections can be designed. One important feature of the two-phase nozzle is the series of inlet holes along one spatial coordinate. Along another spatial co-ordinate, these inlet holes continue in separate channels that
Geothermal Power-plants in Hungary
233
Fig. 5. Three-dimensional and sectional drawings of multi-sectional nozzle.
narrow then gradually widen to an integral and continuous surface at the nozzle outlet. Nozzle diameter can also be designed in the third spatial co-ordinate but can also be given uniform dimensions, depending on operating conditions. These multi-sectional nozzles are recommended for two-phase turbine units of higher capacity, as simple two-phase nozzles normally require large turbines. A special technique has been developed for machine-tooling the nozzle disc to the above-described design. SINGLE-STAGE A C T I O N TURBINE FOR POWER-STATIONS This type of turbine is recommended when the inlet pressure is relatively low. Figure 6(A) is a scaled design of this turbine fitted with multi-sectional nozzles. The technical characteristics of this turbine are as follows: pressure range: 3 . 9 - 0 . 8 bar abs capacity: 1.78 MW velocity: 1500 rpm wheel diameter: 1.66 m (middle circle) blade length: 0.41 m fluid absorbed: 218.93 m3/h.
234
1. Sebestyen °
4
\ I
/
/I 15
J
3
6
Fig. 6. Two-phase axial action turbine with a single stage and multi-sectional nozzle. (A).(I) Nozzle disc, (2) liquid chamber, (3) inlet stub, (4) automatic steam separator, (5) turbine wheel, (6) outlet stub. (B). Tw'in-flow construction.
Figure 6(B) shows a two-sided twin turbine construction, built symmetrically to the inlet stub. Application of multi-sectional nozzles to this turbine would not increase turbine diameter unduly, despite the long turbine blades. STEAM S E P A R A T I O N For faultless operation of two-phase turbines the steam bubbles must be removed from the liquid. Separation of steam bubbles can be achieved by the equipment shown in Fig. 7. The inlet stub (1) is either horizontal or tilted slightly, and is connected tangentially to the cylindric equipment. At the bottom of the separator is a partition surface (5) surrounded by a liquid moment of m o m e n t u m reducing blade bank (6). Consequently the kinetic energy of circulating liquid (2) will be transformed to pressure in the lower liquid space. While the liquid phase is leaving through outlet stub (8), the steam phase flows through the moment of m o m e n t u m reducing blade bank (3) and through the radially located lamellar drop separators (4) to throttle valve (9). This valve is operated by level regulator (10), so that steam pressure will ensure that the liquid level in the equipment is kept constant and the liquid is forced through blade bank (6) into the lower space.
235
Geothermal Power-plants in Hungary 9
Fig. 7. Steam separating equipment for two-phase bubble mixture. (1) Inlet stub, (2) circulating layer, (3) moment of momentum reducing facility for steam, (4) drop separator, (5) partition surface, (6) moment of momentum reducing facility for liquid, (7) static liquid space, (8) outlet stub, (9) steam throttle valve, (10) level regulator, (11) precipitation collector, (12) precipitation outlet, (13) tapping stub, (14) steam outlet stub.
The equipment shown in Fig. 8 is suitable for the separation o f a " m i s t " flow. Steam passes from the lower section o f the cyclone, with inlet at (1), t h r o u g h line (2) into the upper space (3) where it flows through lamellar d r o p separators (4) into the steam mist separator cyclone (12). If the equipment is used for phase separation in two-phase and steam turbines, then, as a result of steam throttling at valve (10), the pressure in the cyclone with inlet (1) will ensure that the liquid from the liquid collector (8) is uniformly forced out into the liquid chamber (11) o f the two-phase turbine. REGULATING EQUIPMENT Regulation o f two-phase turbines can be achieved by installing a hydrocyclone in the feedline; throttling takes place in the hydrocyclone, by narrowing or enlarging the hydrocyclone inlet. This type o f regulation is also useful in geothermal power-plants for separating deposits. The solution applied to the inlet section o f the hydrocyclone is shown in detail in Fig. 9(A).
236
1. Sebestyen
I0 3 I I
1/
11
Fig. 8. Phase separating cyclone system for two-phase mist-like mixture. (1) Inlet stub, (2) cyclone outlet, (3) space for turning, (4) drop separator, (7) precipitation outlet, (8) liquid collector, (10) steam throttle valve, (11) liquid chamber, (12) steam mist separator cyclone.
Any two-phase bubble flow or two-phase flow in the feed-pipe leads to a malfunctioning of the throttle valve and the dynamic effects will place considerable strain on the structure. This strain can be reduced by means of the equipment shown in Fig. 9, separating the steam bubbles existing upstream of the two-phase throttle valve (1) and using a separate steam throttle valve (4). Regulation can also be influenced by pressure prior to the turbine, or by turbine velocity. The liquid inlet to hydrocyclone (7) and the two-phase throttle valve (1) are used mainly to control the flow volume of pure liquid, while the volume of steam is controlled by the level regulator with the aid of steam throttle valve (4). Pure steam entering the hydrocyclone will force the turbulent liquid in a parabolic surface down to the lower end of lift pipe (9), while the lift pipe will receive a mixed phase of very low density that has undergone a slight gravity pressure drop during its upward flow. G E O T H E R M A L P O W E R - P L A N T S DESIGN A description is given of the designs for two geothermal power plants, together with their relative flow diagrams and main technical data.
Geothermal Power-plants in Hungary
237
Fig. 9. Regulating equipment. (1) Hydrocycloneinlet, (2) steam bubble separator, (3) drop separator, (4) steam throttle valve, (5) inlet, (7) hydrocyclone, (8) blowdown facility, (9) lift-pipe. (A). Hydrocyclone inlet.
Power-plant for geothermal liquid, equipped with two-phase turbines and steam turbine (Figs 10-/2t Characteristics of liquid inlet: Characteristics at low expansion limit: Total capacity of power plant: Capacity of two-phase turbine units: Liquid absorbed by power plant: Net efficiency of power plant:
5 bar; 152°C 0.08 bar; 41.5°C 8.653 MW 7.134 MW 15 m V m i n 8.1%.
Power-plant for geothermal liquid, equipped with two-phase and steam turbines (Figs 13 and 14) Characteristics of liquid inlet: Characteristics at low expansion limit: Total capacity of power plant: Capacity of two-phase turbines: Velocity of two-phase turbines:
5 bar; 152°C 0.08 bar; 41.5°C 9.266 MW 2.33 MW 450 rpm
238
I. Sebeslyen Velocity of steam turbines: Liquid a b s o r b e d by power plant: Net efficiency of power plant: Area of power p l a n t building:
3000 r p m 15 m 3 / m i n 8.65% 33.5 x 25 m.
The capacity of a steam t u r b i n e geothermal power plant, with the periodical throttling adopted nowadays, could clearly increase by more than 30% if the throttles were replaced by
r
t-ig. 10. Po~er-plant with t~o-phase turbines and steam turbine. (2) (4) Fittings; (6), (7) regulating equipment;(8), (15) phase separating equipment; (18) steam turbine; (21) direct-contact condenser with dov,npipe; (24) vapour condenser used for blow,down; (1) (IX,')t~o-phase turbines.
13
A iI
IV
,
v--
,m
Fig. I}. Overhead view of power-plant shown in Fig. 10. (I) Saturated liquid inlet; (5) flow meter; (1'), (11') precipitation channel.
Geothermal Power-plants in Hungary
239
[--]
~6
3
4
Fig. 12. Side view of power-plant shown in Fig. 10. two-phase turbines. If the power plant consists entirely of two-phase turbines, capacity could increase by more than 20%. The surplus obtained in the former will be the result of the higher efficiency of the steam turbines, but the second solution has the advantage of lower capital and maintenance costs. SPECIAL TWO-PHASE TURBINES AND GEOTHERMAL POWER-PLANTS
Two-phase reaction turbine In this type of turbine pressure and enthalpy will decrease in both the stators and rotors. Consequently, assuming a normal 50% reaction, the velocity of the fluid reaching the blades is
I. Sebestven
240
',4
"
: '
21
~]
~2
23
Z34 tig. 13. Power-plant with t,so-phase and steam turbines. (2) (4) Fittings; (6),17) regulating cquipment; (8), ( lO), l l2), (15) (17) phase separating equipment; (9), ( l l ) , ( 1 3 ) [ w o - p h a s e t u r b i n e s ; (18) (20) steam turbincs (14), (21) (23) direct-contact condenser ~ith dounpipc.
22
21 24
/ 11
"
19
/16
j
i
18
4:
/15
/
li 8
A
k~
13
11
--4
Fig. 14. Top ~,iew of power plant shown in Fig. 13.
241
Geothermal Power-plants in Hungary
only half the velocity occurring in action turbines, thus reducing blade erosion. If more stages are applied, the velocity of the droplets entering the blade banks will further decrease. This type of turbine is shown in Fig. 15.
3
2
4
6
5 7
,.y-
VIi
Fig. 15. Two-phase multi-stage axial reaction turbine with vertical shaft. (I Inlet stub, (2) liquid chamber, (3) automatic steam separator, (4) closing-steam inlet, (5) equalizing disc, (6) nozzle disc, 17) turbine ,~'heel.
242
I. Sebestyen
Four-stage reaction turbine with vertical axis: pressure range: 3 . 9 - 0 . 0 8 bar abs capacity: 2.2 MW velocity: 1500 rpm fluid absorbed: 220 m3/h maximum wheel diameter: 1.72 m (middle circle) maximum blade length: 0.573 m. The mean velocity reaching the turbine blades is about 60 m/s. Through inlet stub (4) of the turbine casing steam can be led below equalizing disc (5). This turbine can also be constructed with a horizontal shaft.
Two-phase turbine with steam inlet In the turbines described above the steam phase reaching the turbines had to be avoided. The two-phase turbine with steam inlet has a liquid vaporizing nozzle bank, followed by a mixingchamber where steam arriving through a steam inlet stub can be led into the mist-like two-phase mixture. The amount of steam which economically can be admixed has lower and upper limits and the m a x i m u m is about 10°70. This type of turbine, the five-stage reaction turbine with steam inlet and vertical axis (Fig. 16), has the following characteristics: pressure range: 3.9 - 0.08 bar abs 3 bar abs steam inlet into turbine: 2.8 MW capacity: 1500 rpm velocity: 50 kg/s liquid absorbed: 2 kg/s steam absorbed: 1.729 m (middle circle) maximum wheel diameter: 0.576 m. maximum blade length: Figure 17 shows a flow diagram of a power plant unit equipped with such a turbine, which can be used to expand saturated liquid with a steam content of less than 10%.
Small turbines for power-stations Precipitation of dissolved salts inside the turbine creates maintenance problems. The turbine shown in Fig. 18 was designed with this problem in mind. It has a horizontal shaft, is fitted with a "fly-wheel" with bearings only on one side, and is the single-stage action type with axial flow. The turbine casing is not split horizontally. Liquid chamber (1) is closed by circular cover (2). Being suspended on lifting-ring (3), it can be removed from this position and, still strung on the shaft, can be propped so that the maintenance personnel can clean the nozzles in the open liquid chamber from the inlet side. On the outlet side of the turbine the discharge pipe extension (4) can be removed from the discharge pipe by using lifting-ring (5) providing easy access to the turbine rotor and blades and also to the outlet section of the nozzles. At this point the turbine wheel can be removed from the shaft. Thermal water power-plant The power-plant design shown in Fig. 19 exploits thermal wells with a temperature of less than 100°C for electric power production. A hydrocyclone (3) is connected through lift pipe (4) to two-phase turbine (5), which is linked directly with the barometric downpipe condenser (7). Feed pipe (1) of the hydrocyclone is connected to the hydrocyclone throttle inlet (2) described earlier. Technical data are as follo~ s:
Geothermal Power-plants in Hungary
243
J
j2
~ 5
Fig. 16. Two-phase multi-stage vertical-shaft reaction turbine with steam inlet. (l) Inlet stub, (2) liquid chamber, (3) steam line from automatic separator, (4) spraying nozzle disc, (5) steam inlet stub, (6) equalizing disc, (7) turbine wheel.
244
1. Sebestyen ! 4
/
//
) /
/
2
3
Fig. 17. Power-plant with two-phase turbine with steam inlet. (ll Regulating equipment, (2) steam separating equipment, (3) two-phase turbine with steam inlet, (4) condenser, (5) vapour condenser used for blowdown.
2
.
.
,
3
t
.
5 /d
.
.
Fig. 18. Two-phase axial action turbine with a single-stage flying-wheel. (1) Liquid chamber; (2) liquid chamber cover plate; (3), (5) lifting-ring; (4) discharge pipe extension; (6) inlet stub.
inlet water temperature: 95°C 3 m3/min liquid absorbed by power plant: lower pressure for expansion: 0.06 bar turbine capacity: 565.7 kW turbine speed: 1000 rpm turbine wheel diameter: 1.52 m (in the middle circle) turbine blade length: 0.5 m thermal efficiency of power plant: 8.5% net efficiency of power plant: 5.5% net electric power in case of cooling towers: 420 kW rate of return on investment can be expected within 5 years. The easy maintenance of this type of power plant should be emphasized. The electricity produced can be connected via a synchronous generator to a larger power network, thus eliminating regulation problems.
245
Geothermal Power-plants in Hungary
I
z~
7
2~
I
i
/
.
-.
3~ 1\
. ""
~
~f8
0o-o-
--I>o . i,o~>~ ~ L°oo.~ Fig. 19. Thermal water power-plant with two-phase turbine. (I) Liquid inlet, (2) hydrocyclone inlet (3) hydrocyclone, (4) lift-pipe, (5) two-phase turbine, (6) electric generator, (7) direct-contact condenser with downpipe, (8) precipitation channel, (9) cooling tower.
I. Sebestyen
246
CONCLUSIONS The energy content of the fluid recovered in the liquid phase from geothermal systems can be extracted and used for electric power production by means of 2 - 3 MW two-phase axial-flow turbines and 10 MW power generating units and moderately sized machines. The major problem is guaranteeing continuous operation of machines and equipment in spite of scaling, which is even more troublesome than erosion. Acknowledgement--Photographs by Cs. Kardos.
Geothermics, Vol. 14, No. 2/3, pp. 2 2 9 - 2 4 6 , 1985. Printed in Great Britain.
DESIGN
OF TWO-PHASE TURBINES ELECTRIC POWER-PLANTS
AND GEOTHERMAL IN HUNGARY
I. SEBESTYEN Rakoczi ut. 4, H-1072 Budapest, Hungary (Transmitted by the Government o[' Hungary) R.87 A b s t r a c t - - A description is given of a private project to produce electric energy from geothermal fluids in Hungary. Single-stage and multi-stage action and reaction turbines of the axial flow type are recommended for this purpose, together with the associated equipment and power plants.
INTRODUCTION In mid 1971 the author began work on the design of two-phase turbines for the expansion of saturated liquids. In June 1972 the author applied for a patent,* describing his design of axialflow type two-phase action turbines destined for multiple uses, but with priority given to geothermal electric power-plants. In May 1973 tests were made with two-phase nozzles in order to determine the deviation factor. His application for a patent was expanded in 1973 and 1979 and was issued in September 1983. A geothermal power-plant project, called " t o t a l flow concept", was described in April 1973 by Austin et al. Underlying their design is the same basic principle as that of the turbines and power-plants described by the present author in his application for a patent submitted in 1972. Work continued on this design in Hungary, in parallel with, and independent of research in the U.S.A. and elsewhere. C H O I C E OF H E A T E N G I N E Total extraction of the energy content of hot waters produced from a well can be achieved theoretically by expansion in some type of two-phase turbine. The other two methods, i.e. use of a steam turbine, accompanied by the flash process, and use of heat exchangers in a binary cycle, can at the moment be justified on practical grounds. The major problem is to design a two-phase turbine that is economically more attractive than these two methods. A two-phase turbine for the expansion of saturated liquid acts as a heat engine, because of the reduced internal energy of the medium. Considering that the flow of a saturated steam-free liquid, at the slightest drop in pressure, would be transformed into a so-called " m i s t " flow, it is obvious that the heat engine must be of aerodynamic design, since large volumes are flowing in an extremely fine distribution of drops. During expansion the drops would be further atomized partly as a result of boiling and partly of shear forces. TESTS W I T H N O Z Z L E S The fineness of drop distribution was studied in 1973 during tests with two-phase nozzles, conducted by the author with the aid of energy experts of the Pharmacological Plant EGYT, *Patent No. SE 1632/1972.
229
1. Sebestyen
230
Budapest. The two-phase nozzles were first designed on the basis of theoretical calculations, and imperfect expansion was found to be caused by delayed boiling. Thus the liquid entered the nozzle in such large quantities that, after full boiling had begun, there was not enough room for it to leave the nozzle at the outlet; as a consequence, expansion was delayed until the fluid had left the nozzle. The inlet diameter, i.e. the smallest cross-section of nozzle, had therefore to be narrowed. This was achieved by measuring the characteristics of the imperfect nozzles and by applying corrections during calculations. Figure 2 shows that the new nozzle (shown in Fig. I) was capable of producing a two-phase mixture of nearly parallel flow lines when expanding saturated hot water at a pressure of 11 bar abs to 1 bar, and also that expansion in the final cross-section was complete, as shown by measurements. Using the impulse scales shown in Fig. 3, in the form shown in Fig. 4, the impulses were measured on a buffer plate, and the deviation factor (loss factor) of the nozzle determined. f60 f45 90 ° back~,ards - ~ ~(~6 + ~ , ...... ~ ,
•.4,. x x
) /
/
.
.
Fig. 1. Two-phase nozzle designed specially to account for delayed boiling.
Fig. 2. Expansion test with two-phase nozzle, for 10
1 bar.
Geothermal Power-plants in Hungary
231
Fig. 3. Impulse measuring equipment.
Fig. 4. Impulse measurement.
T U R B I N E IN G E N E R A L An aerodynamic turbine which processes large volumes o f a two-phase mixture (containing drops that are being increasingly atomized during expansion) will clearly be similar to steam turbines and to the axial flow types in particular. Since two-phase turbines are similar to the latter, and are operated by the expansion of a saturated liquid, their operating conditions are, in some respects, more favourable than those of the so-called moist steam turbines. In the multistage turbines, steam from the dispersed liquid phase passing through the turbine will develop
232
I. Sebestyen
in all stages; droplets in the mist are in the boiling or explosion stages throughout their expansion. If using reactive stages, this phenomenon will also take place along the rotating blades. The two-phase nozzles of the first stage of two-phase axial-flow turbines are mounted at the inlet to a joint liquid chamber. The steam phase must be prevented from reaching the line connected to the turbine as, otherwise, some of the nozzles may receive steam and some may receive a mixed phase. The liquid chamber must therefore be sufficiently large to allow the separation of any steam bubbles that may flow in. These bubbles can be conveyed to some other utilization by the automatic steam separator located at the top of the chamber. The liquid entering the turbine may be saturated or at a pressure above that of saturation. In the latter case, liquid expansion will take place at the inlet of the nozzles of the first stage; when the liquid reaches the saturated state during expansion, boiling in the nozzles will begin. Because of flow losses, erosion (see below) and vibration effects, the turbines should preferably be designed for total inflow. In turbines with horizontal shafts and larger turbine wheels, gravity pressure along the nozzle inlets in the liquid chamber will vary greatly, but this will not affect expansion to any significant extent, because pressure changes along the inlet openings are not accompanied by differential temperatures. EROSION Erosive effects on the turbine are limited by the fact that the concentration of dispersed liquid during expansion of saturated liquid is considerably higher than during expansion of saturated steam in moist steam turbines. Therefore the liquid forms a flowing layer on the blades, and has a less erosive effect than drops. Erosion is also limited by the boiling of incident droplets, by boiling of the liquid film flowing to points of decreasing pressures, and also by the small inertia and large surface friction of the drops in the mist-like mixture, which hinders separation because of its short expansion time. Erosion can also be limited by reducing the pressure drop for each stage and by applying antierosive materials. R E G U L A T I N G SYSTEM The regulating system used is similar to that used in conventional turbines. Throttling can be achieved in the connecting line (in this case the separated steam must be diverted), or at the nozzles. Varying the flow while some nozzles are completely shut is not recommended because of pulsating flow in the wheel channels during rotation (breaking of the liquid film). A description is given below of two throttling devices for controlling turbine velocity, but with larger time constants. When the generator linked to the turbine is connected to a larger power grid of stable frequency, velocity is maintained at a constant value by the grid itself. T H E NEW T W O - P H A S E N O Z Z L E Because of its large specific volume the fluid leaving the nozzles must have a large crosssection, especially if expansion takes place in the vacuum, and despite the high flow velocity. At the inlet of the nozzle the size and number of the steam bubbles are low and this results--together with the increasing velocity--in a minimal sectional area. The multi-sectional nozzle, shown in Fig. 5, has reduced sections in both directions, which do not reduce flow to any great extent; the nozzle length is kept to a minimum, and, at its narrowest point, square or circular cross-sections can be designed. One important feature of the two-phase nozzle is the series of inlet holes along one spatial coordinate. Along another spatial co-ordinate, these inlet holes continue in separate channels that
Geothermal Power-plants in Hungary
233
Fig. 5. Three-dimensional and sectional drawings of multi-sectional nozzle.
narrow then gradually widen to an integral and continuous surface at the nozzle outlet. Nozzle diameter can also be designed in the third spatial co-ordinate but can also be given uniform dimensions, depending on operating conditions. These multi-sectional nozzles are recommended for two-phase turbine units of higher capacity, as simple two-phase nozzles normally require large turbines. A special technique has been developed for machine-tooling the nozzle disc to the above-described design. SINGLE-STAGE A C T I O N TURBINE FOR POWER-STATIONS This type of turbine is recommended when the inlet pressure is relatively low. Figure 6(A) is a scaled design of this turbine fitted with multi-sectional nozzles. The technical characteristics of this turbine are as follows: pressure range: 3 . 9 - 0 . 8 bar abs capacity: 1.78 MW velocity: 1500 rpm wheel diameter: 1.66 m (middle circle) blade length: 0.41 m fluid absorbed: 218.93 m3/h.
234
1. Sebestyen °
4
\ I
/
/I 15
J
3
6
Fig. 6. Two-phase axial action turbine with a single stage and multi-sectional nozzle. (A).(I) Nozzle disc, (2) liquid chamber, (3) inlet stub, (4) automatic steam separator, (5) turbine wheel, (6) outlet stub. (B). Tw'in-flow construction.
Figure 6(B) shows a two-sided twin turbine construction, built symmetrically to the inlet stub. Application of multi-sectional nozzles to this turbine would not increase turbine diameter unduly, despite the long turbine blades. STEAM S E P A R A T I O N For faultless operation of two-phase turbines the steam bubbles must be removed from the liquid. Separation of steam bubbles can be achieved by the equipment shown in Fig. 7. The inlet stub (1) is either horizontal or tilted slightly, and is connected tangentially to the cylindric equipment. At the bottom of the separator is a partition surface (5) surrounded by a liquid moment of m o m e n t u m reducing blade bank (6). Consequently the kinetic energy of circulating liquid (2) will be transformed to pressure in the lower liquid space. While the liquid phase is leaving through outlet stub (8), the steam phase flows through the moment of m o m e n t u m reducing blade bank (3) and through the radially located lamellar drop separators (4) to throttle valve (9). This valve is operated by level regulator (10), so that steam pressure will ensure that the liquid level in the equipment is kept constant and the liquid is forced through blade bank (6) into the lower space.
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Geothermal Power-plants in Hungary 9
Fig. 7. Steam separating equipment for two-phase bubble mixture. (1) Inlet stub, (2) circulating layer, (3) moment of momentum reducing facility for steam, (4) drop separator, (5) partition surface, (6) moment of momentum reducing facility for liquid, (7) static liquid space, (8) outlet stub, (9) steam throttle valve, (10) level regulator, (11) precipitation collector, (12) precipitation outlet, (13) tapping stub, (14) steam outlet stub.
The equipment shown in Fig. 8 is suitable for the separation o f a " m i s t " flow. Steam passes from the lower section o f the cyclone, with inlet at (1), t h r o u g h line (2) into the upper space (3) where it flows through lamellar d r o p separators (4) into the steam mist separator cyclone (12). If the equipment is used for phase separation in two-phase and steam turbines, then, as a result of steam throttling at valve (10), the pressure in the cyclone with inlet (1) will ensure that the liquid from the liquid collector (8) is uniformly forced out into the liquid chamber (11) o f the two-phase turbine. REGULATING EQUIPMENT Regulation o f two-phase turbines can be achieved by installing a hydrocyclone in the feedline; throttling takes place in the hydrocyclone, by narrowing or enlarging the hydrocyclone inlet. This type o f regulation is also useful in geothermal power-plants for separating deposits. The solution applied to the inlet section o f the hydrocyclone is shown in detail in Fig. 9(A).
236
1. Sebestyen
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1/
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Fig. 8. Phase separating cyclone system for two-phase mist-like mixture. (1) Inlet stub, (2) cyclone outlet, (3) space for turning, (4) drop separator, (7) precipitation outlet, (8) liquid collector, (10) steam throttle valve, (11) liquid chamber, (12) steam mist separator cyclone.
Any two-phase bubble flow or two-phase flow in the feed-pipe leads to a malfunctioning of the throttle valve and the dynamic effects will place considerable strain on the structure. This strain can be reduced by means of the equipment shown in Fig. 9, separating the steam bubbles existing upstream of the two-phase throttle valve (1) and using a separate steam throttle valve (4). Regulation can also be influenced by pressure prior to the turbine, or by turbine velocity. The liquid inlet to hydrocyclone (7) and the two-phase throttle valve (1) are used mainly to control the flow volume of pure liquid, while the volume of steam is controlled by the level regulator with the aid of steam throttle valve (4). Pure steam entering the hydrocyclone will force the turbulent liquid in a parabolic surface down to the lower end of lift pipe (9), while the lift pipe will receive a mixed phase of very low density that has undergone a slight gravity pressure drop during its upward flow. G E O T H E R M A L P O W E R - P L A N T S DESIGN A description is given of the designs for two geothermal power plants, together with their relative flow diagrams and main technical data.
Geothermal Power-plants in Hungary
237
Fig. 9. Regulating equipment. (1) Hydrocycloneinlet, (2) steam bubble separator, (3) drop separator, (4) steam throttle valve, (5) inlet, (7) hydrocyclone, (8) blowdown facility, (9) lift-pipe. (A). Hydrocyclone inlet.
Power-plant for geothermal liquid, equipped with two-phase turbines and steam turbine (Figs 10-/2t Characteristics of liquid inlet: Characteristics at low expansion limit: Total capacity of power plant: Capacity of two-phase turbine units: Liquid absorbed by power plant: Net efficiency of power plant:
5 bar; 152°C 0.08 bar; 41.5°C 8.653 MW 7.134 MW 15 m V m i n 8.1%.
Power-plant for geothermal liquid, equipped with two-phase and steam turbines (Figs 13 and 14) Characteristics of liquid inlet: Characteristics at low expansion limit: Total capacity of power plant: Capacity of two-phase turbines: Velocity of two-phase turbines:
5 bar; 152°C 0.08 bar; 41.5°C 9.266 MW 2.33 MW 450 rpm
238
I. Sebeslyen Velocity of steam turbines: Liquid a b s o r b e d by power plant: Net efficiency of power plant: Area of power p l a n t building:
3000 r p m 15 m 3 / m i n 8.65% 33.5 x 25 m.
The capacity of a steam t u r b i n e geothermal power plant, with the periodical throttling adopted nowadays, could clearly increase by more than 30% if the throttles were replaced by
r
t-ig. 10. Po~er-plant with t~o-phase turbines and steam turbine. (2) (4) Fittings; (6), (7) regulating equipment;(8), (15) phase separating equipment; (18) steam turbine; (21) direct-contact condenser with dov,npipe; (24) vapour condenser used for blow,down; (1) (IX,')t~o-phase turbines.
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Fig. I}. Overhead view of power-plant shown in Fig. 10. (I) Saturated liquid inlet; (5) flow meter; (1'), (11') precipitation channel.
Geothermal Power-plants in Hungary
239
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Fig. 12. Side view of power-plant shown in Fig. 10. two-phase turbines. If the power plant consists entirely of two-phase turbines, capacity could increase by more than 20%. The surplus obtained in the former will be the result of the higher efficiency of the steam turbines, but the second solution has the advantage of lower capital and maintenance costs. SPECIAL TWO-PHASE TURBINES AND GEOTHERMAL POWER-PLANTS
Two-phase reaction turbine In this type of turbine pressure and enthalpy will decrease in both the stators and rotors. Consequently, assuming a normal 50% reaction, the velocity of the fluid reaching the blades is
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241
Geothermal Power-plants in Hungary
only half the velocity occurring in action turbines, thus reducing blade erosion. If more stages are applied, the velocity of the droplets entering the blade banks will further decrease. This type of turbine is shown in Fig. 15.
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242
I. Sebestyen
Four-stage reaction turbine with vertical axis: pressure range: 3 . 9 - 0 . 0 8 bar abs capacity: 2.2 MW velocity: 1500 rpm fluid absorbed: 220 m3/h maximum wheel diameter: 1.72 m (middle circle) maximum blade length: 0.573 m. The mean velocity reaching the turbine blades is about 60 m/s. Through inlet stub (4) of the turbine casing steam can be led below equalizing disc (5). This turbine can also be constructed with a horizontal shaft.
Two-phase turbine with steam inlet In the turbines described above the steam phase reaching the turbines had to be avoided. The two-phase turbine with steam inlet has a liquid vaporizing nozzle bank, followed by a mixingchamber where steam arriving through a steam inlet stub can be led into the mist-like two-phase mixture. The amount of steam which economically can be admixed has lower and upper limits and the m a x i m u m is about 10°70. This type of turbine, the five-stage reaction turbine with steam inlet and vertical axis (Fig. 16), has the following characteristics: pressure range: 3.9 - 0.08 bar abs 3 bar abs steam inlet into turbine: 2.8 MW capacity: 1500 rpm velocity: 50 kg/s liquid absorbed: 2 kg/s steam absorbed: 1.729 m (middle circle) maximum wheel diameter: 0.576 m. maximum blade length: Figure 17 shows a flow diagram of a power plant unit equipped with such a turbine, which can be used to expand saturated liquid with a steam content of less than 10%.
Small turbines for power-stations Precipitation of dissolved salts inside the turbine creates maintenance problems. The turbine shown in Fig. 18 was designed with this problem in mind. It has a horizontal shaft, is fitted with a "fly-wheel" with bearings only on one side, and is the single-stage action type with axial flow. The turbine casing is not split horizontally. Liquid chamber (1) is closed by circular cover (2). Being suspended on lifting-ring (3), it can be removed from this position and, still strung on the shaft, can be propped so that the maintenance personnel can clean the nozzles in the open liquid chamber from the inlet side. On the outlet side of the turbine the discharge pipe extension (4) can be removed from the discharge pipe by using lifting-ring (5) providing easy access to the turbine rotor and blades and also to the outlet section of the nozzles. At this point the turbine wheel can be removed from the shaft. Thermal water power-plant The power-plant design shown in Fig. 19 exploits thermal wells with a temperature of less than 100°C for electric power production. A hydrocyclone (3) is connected through lift pipe (4) to two-phase turbine (5), which is linked directly with the barometric downpipe condenser (7). Feed pipe (1) of the hydrocyclone is connected to the hydrocyclone throttle inlet (2) described earlier. Technical data are as follo~ s:
Geothermal Power-plants in Hungary
243
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~ 5
Fig. 16. Two-phase multi-stage vertical-shaft reaction turbine with steam inlet. (l) Inlet stub, (2) liquid chamber, (3) steam line from automatic separator, (4) spraying nozzle disc, (5) steam inlet stub, (6) equalizing disc, (7) turbine wheel.
244
1. Sebestyen ! 4
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Fig. 17. Power-plant with two-phase turbine with steam inlet. (ll Regulating equipment, (2) steam separating equipment, (3) two-phase turbine with steam inlet, (4) condenser, (5) vapour condenser used for blowdown.
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Fig. 18. Two-phase axial action turbine with a single-stage flying-wheel. (1) Liquid chamber; (2) liquid chamber cover plate; (3), (5) lifting-ring; (4) discharge pipe extension; (6) inlet stub.
inlet water temperature: 95°C 3 m3/min liquid absorbed by power plant: lower pressure for expansion: 0.06 bar turbine capacity: 565.7 kW turbine speed: 1000 rpm turbine wheel diameter: 1.52 m (in the middle circle) turbine blade length: 0.5 m thermal efficiency of power plant: 8.5% net efficiency of power plant: 5.5% net electric power in case of cooling towers: 420 kW rate of return on investment can be expected within 5 years. The easy maintenance of this type of power plant should be emphasized. The electricity produced can be connected via a synchronous generator to a larger power network, thus eliminating regulation problems.
245
Geothermal Power-plants in Hungary
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--I>o . i,o~>~ ~ L°oo.~ Fig. 19. Thermal water power-plant with two-phase turbine. (I) Liquid inlet, (2) hydrocyclone inlet (3) hydrocyclone, (4) lift-pipe, (5) two-phase turbine, (6) electric generator, (7) direct-contact condenser with downpipe, (8) precipitation channel, (9) cooling tower.
I. Sebestyen
246
CONCLUSIONS The energy content of the fluid recovered in the liquid phase from geothermal systems can be extracted and used for electric power production by means of 2 - 3 MW two-phase axial-flow turbines and 10 MW power generating units and moderately sized machines. The major problem is guaranteeing continuous operation of machines and equipment in spite of scaling, which is even more troublesome than erosion. Acknowledgement--Photographs by Cs. Kardos.