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GRC Transactions, Vol. 29, 2005

767

KeywordsPerformance and reliabilities of geothermal turbine, axial ex-haust turbine, reduction of turbine exhaust loss, Hellisheidi, axial exhaust, pressure recovery efficiency, integral shroud blade, 30 inch last blade

ABSTRACT

Geothermal power production has excellent features in view of low carbon dioxide emission and energy security for the countries that do not produce natural resource such as oil, natural gas and so on.

Geothermal steam contains impurities such as silica or chloride and non-condensable gas in comparison with thermal power generation. Impurities or non-condensable gas cause scaling or corrosion related problem.

In addition, geothermal steam is usually produced at satu-rated temperature conditions. And the countermeasures should be taken against scaling, corrosion and erosion problem for higher reliability and performance.

In this paper, recent technologies that were applied to Hellisheidi and improved reliabilities and performance are introduced, focusing on several latest technologies for the performance.

Introduction of the ProjectHellisheidi is located about 20 km east of Reykjavik in

Iceland. Hellisheidi Geothermal Power Plant is a new plant, which has two steam turbine generators with the capacity of 40MW each.

The purpose of the power station is to utilize the high temperature geothermal field for co-generation of thermal and electrical power energy.

Hellisheidi Geothermal Power Plant is presently under construction. The commercial operation will be expected to start on September 1st 2006 for Unit-1 and October 1st 2006 for Unit-2.

The development of the Geothermal Power Station is car-ried out by following formation.• Owner : ORKUVEITA REYKJAVÍKUR• Consultant : VGK Consulting Engineering Ltd.• Contractors

- EPC for power stationTurbine Generator : MitsubishiCold End : SPX Cooling Technologies GmbH Balcke-Durr GmbH

(Not including civil construction)

Technologies Applied for HellisheidiFrom the viewpoint of economy, reliability and perfor-

mance, the optimum design was carried out in Hellisheidi. The design specifications of this plant are shown in Table 1.

Two steam turbine generators are arranged in a turbine house. Steam turbines are of single-cylinder, single-flow which is more economical compared with double flow type, and of axial-exhaust type which is higher performance compared with down and top-exhaust type.

Technologies Applied to Hellisheidi Geothermal Power Plant

Hisanori Matsuda

Mitsubishi Heavy Industries, Ltd.1-1 Akunoura-machi, Nagasaki 850-8610, Japan

Table 1. Specification of Plant for Hellisheidi.

Plant Cycle Single Flash, Condensing

Output 40MW×2 units

Steam Condition

• Pressure 0.75 MPa

• Temperature 167.8 degree C

Exhaust Pressure 0.01 MPa

Turbine6 stages (Impulse + Reaction), Axial Exhaust, Condensing

Condenser Shell and Tube Type

Cooling Tower Mechanical Draft Counter Flow Type

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In addition, to maintain both economy and high perfor-mance, 30-inch last stage blades were applied. The design specifications of these units are shown in Table 2.

The condenser is of shell and tube type and installed at the same level as the steam turbine to minimize the turbine house. Two condensate pumps and two deaerator condensate pumps with 100% capacity and gas removal system are inside the turbine house.

The control system of the plant and equipment were designed, taking minimum manpower for operation and su-pervision and maintaining high reliability.

Axial Exhaust Turbine

The axial exhaust turbine is applied to improve both economy and performance.

The condenser is installed at the same level as the steam turbine to minimize the turbine house as shown in Figure 1. Axial exhaust turbine enables to reduces the crane height from floor level +0.0m to lower than 35 % of top exhaust turbine and to lower than 64% of down exhaust turbine (Figure 2).

In case of top and down exhaust type, the streamline for exhaust steam is forced to curve due to the structure (Figure 3). Axial exhaust turbine remarkably improves the pressure recov-ery efficiency about 4 times higher than top and down exhaust turbine (Figure 4).

Integral Shroud Blade (ISB)

The turbine sectional assembly is shown in Figure 5. ISB design was used in the rotating blades of all stages (Figure 6). As there is no tenon riveting or welding at stubs, the durability against SCC (Stress Corrosion Cracking) or corrosion fatigue inherent to geothermal turbines is remarkable increased, thus improving the turbine reliability.

30 inch Integral Shroud Last Blade

To develop a large-capacity and high performance geother-mal turbine in a single- cylinder arrangement, longer last-stage blade is indispensable. Consideration from the viewpoint of both strength and performance is to be taken of developing longer last-stage blade because of increased tensile stresses on the rotor disc and blade fitting area. The 30-inch ISB blade, which is the longest blade used in geothermal turbines, was adopted for the Hellisheidi project.

Table 2. Specification of Turbine.

Type of Turbine -

SC1F-30”(Single-Cylinder, Single Flow, Impulse-reaction

condensing turbine)Exhaust type - Axial exhaustRated Speed rpm 3,000rpmNumber of Stage - 6

Last Blade Height mm 762 (30inch)

Figure 1. Sectional Arrangement.

Figure 2. Crane Height (from Floor Level +0) Ratio.

Axial Top Down Exhaust type

Cra

ne H

eigh

t (fr

om F

loor

leve

l +0)

R

atio

Axial Exhaust Down Exhaust Top Exhaust

Figure 3. Stream line compared for exhaust steam flow.

To Condenser

To Condenser

To Condenser

Figure 4. Pressure Recovery Efficiency Ratio.

Matsuda

Pres

sure

Rec

over

y Ef

ficie

ncy

Rat

io

Axial Top Down Exhaust type

1

769

The feature of 30-inch blade is integral shroud blade (ISB) design; that is, shroud is integrated to the blade profile (Fig-ure 7). During operation, adjacent blades come into contact

with each other at the shrouds due to a twist-back movement generated by the cen-trifugal force on the blades, as illustrated in Figure 8. The me-chanical damping induced by this physi-cal contact reduces the vibration stress to less than 20 % of con-ventional grouped blades (Figure 9). At the rated rotor speed, all blades are con-nected together at the integrated shrouds to form an infinity

Figure 5. Turbine Sectional Assembly.

Figure 6. ISB for All Stages.

Figure 7. Integral Shroud Blade (ISB).

blade group. This design reduces the number of vibration modes which require the tuning of blade frequencies in order to avoid the resonance. And therefore, reliability of the blade has improved.

With ISB construction, design loading of the blade, defined as maximum steam flow rate per unit annulus area (pounds/ft2/hr), is increased by 1.5 times relative to the conventional grouped blade, which makes a great contribution to higher capacity and reliability.

Rotor Design

Super low sulfur CrMoV rotor material was developed specifically for the corrosive environment in geothermal field and has been applied to many geothermal turbines. This rotor material was also applied to Hellisheidi. The feature of super low sulfur CrMoV is low susceptibility against stress corro-sion cracking in geothermal steam maintaining high strength and toughness.

In addition, we applied the large blade root and groove design to reduce the static (centrifugal) stress at the blade root and groove. Stress concentration occurs at the area where sec-tion area changes. In view of stress concentration, we need to specially care about the disc root and blade groove.

For disc root, large corner “R(Radius)” and “Taper shape” are applied to minimize stress concentration or SCC.

Figure 8. Shroud Contact during Operation.

Figure 9. Comparison of Vibratory Stress.

Centrifugal Force

Twist Back by Centrifugal Force

Leading Edge

Matsuda

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Figure 10 shows the improvement in centrifugal stress level of large groove compared with conventional groove.

Bow Blade

The bow configuration (Figure 11) designed by fully three-dimensional fluid dynamic method reduces the secondary flow loss which is one of the major blade losses in blade path

Figure 10. Comparison of Blade Groove.

caused by the generation of a vortex in the boundary layer. The bow blade design was used on stationary and rotating blades, thereby increasing turbine efficiency.

Erosion

Steam condition at turbine inlet is almost saturated and in-duces wetness in turbine, especially at last stage blade, is higher than thermal turbine case that may be superheated steam.

So, measures against erosion at last stage blade leading edge should be applied.

Following measures were developed and have been ap-plied to our geothermal turbines. And the effectiveness of following measures have been verified in long period of actual operation.

- Drain catcher at outer ring of diaphragm for removing condensed steam from blade path

- Drain groove dug at concave surface of last stage stationary blade for removing condensed steam from blade path

- Hollow nozzle at last stage for removing condensed steam from blade path

- Stellite strip on the leading edge of last stage blade to protect blade material from erosion

- Ample axial space between stationary blade and moving blade to decrease drain energy that cause erosion

Drain Catcher

The drain catchers provided at each stage are significantly important devices to avoid drain erosion and to decrease moisture loss. Water droplet is collected by the drain catcher. In this regard, the space between stages and the shape of the drain catcher, which affects the efficiency of drain catching, has been determined through an analysis of water droplet behavior in model testing. Drain erosion of the long blade is prevented by further decreasing the wetness in the steam flowing into the moving blade by grooves dug on the concave surface of the last stationary blade. Figure 12 shows the typical drain catcher.

Figure 11. Bow Nozzle. Figure 12. Typical Drain Catcher.

Matsuda

771

Hollow Nozzle

The drain on the stationary blade is evacuated through the slit on the stationary blade surface and the drain hole on the inside of the blade profile (Figure 13). A hollow nozzle

is more effective at removing the moisture, thereby protecting the rotating blade from erosion, than the typical groove on the concave surface of the stationary blade. For example, the Darajat project turbine utilize hollow nozzles at last two stages (both L-0 and L-1 stage), considering the high moisture content due to high main steam pressure and low exhaust pressure.

Reference

S. Saito, T. Suzuki, J. Ishiguro and T. Suzuki, 1998, “Development of Large Capacity Single-Cylinder Geothermal Turbine.” GRC Trans-actions, Vol.22.

Y. Nakagawa and S. Saito, “Geothermal Power Plants in Japan Adopting Recent Technologies.” World Geothermal Congress 2000.

Y. Uryu, 2004, “Technology for Reliable Geothermal Turbine.” GRC Transactions, Vol.28.

Figure 13. Hollow Nozzle.

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