4.1 GENERAL

The steam turbine and turbine auxiliary will consist of:

Steam turbine

Condenser

Condensers for air and NCG extraction

Blade washing water injection system

Turbine room crane

4.2 STEAM TURBINE

The function of the steam turbine is to accomplish conversion of the thermal energy of steam produced by the geothermal energy required to drive the generator. The generator converts the mechanical energy to electrical energy, which is transmitted through, the generator breaker to the generator step-up transformer.

Two steam turbine generators are arranged in a turbine house. The two steam turbines will be single-cylinder, single-flow, down-exhaust, impulse-reaction condensing type, arranged longitudinally and installed indoors.

The condenser is of direct contact type and installed under the steam turbine.

Two hot well pumps with 50% capacity and gas removal system are inside turbine house.

The control system of the plant and equipment were as such designed to require minimum operator intervention and supervision and maintain high reliability.

Main steam pipe will ensure a safe and reliable operation of the steam turbines. Take the emergency into account, one bypass that vent to sky passed steam silencer will be installed in the main steam piping .

4.2.1 Main data of steam turbine

Type of steam turbine	single-cylinder, single-flow, impulse-reaction condensing type
Model of steam turbine	N6.6-0.8
Rated power	6.86 MW
Pressure before main steam valve	0.8+0.2-0.3 MPa (a)
Temperature before main steam valve	171.9+10-15 degree C
Rated steam inlet flow	56.7 t/h
Design cooling water temperature	33 Degree C
Rated extraction pressure	0.06 ata
Rotation direction	Clockwise, looking to the generator from the end of steam turbine
Height of turbine center line (to platform on operation floor)	630 mm
Weight of the largest piece of turbine	13.5 t
Weight of turbine rotor	6.9 t
Max. dimension of steam turbine	5500x3150x3265
Stages of steam turbine	1 regulating stage + 9 pressure stage

4.2.2 Major features of steam turbine

The turbine shaft and generator couplings are amply strong to withstand shocks during operation, including generator short circuit conditions and synchronizing out of phase. The couplings will be designed so as to permit easy and rapid disconnection, re-assembly and alignment.

The steam turbine can operate under idling condition after trip-out not less than 15 minutes.

The rear part of the cylinder is equipped with the vent valve for low vacuum protection.

The shaft amplitude, shaft displacement and revolution rate at the main bearing are with 4-20mA standard output for connection to the DCS monitoring interface.

The turbine will be designed with sufficient drainports for moisture removal particularly during start up. The design will include passage ways and interstage drainage where appropriate to avoid erosion damage of low pressure blading in the wet region. The steam turbine will be equipped with one drain flash tank for maximum removal of water and the drain is let into the condenser through the drain flash tank.

The automatic main steam valve and foundation will be fixed by hangers. The steam turbine will be equipped with electric driven turning gears in power of 3kW. The turning gears can be automatically retreated and it will be equipped with manual operating device.

The steam turbine will be equipped with steam lock pressure automatic regulating device and steam lock air extraction device. The steam supply system for steam lock can automatically regulate its pressure under any working condition of the turbine. The steam lock system will be equipped with valves.

The steam turbine will be supplied with complete lubricating and control oil system. The oil system is for supply of oil to the regulation and security system, and for bearing lubrication and turning gears. The oil system will consist of reliable main oil supply equipment and auxiliary oil supply equipment. The oil tank in volume of 5 cubic meters will be equipped with exhaust fan and explosion-proof motor. The oil tank will be installed with emergency drain valve. Oil piping will be installed with exposed stem cast steel valves. Two oil coolers in volume of 2x100% will be supplied and the outlet piping of oil coolers will be installed with strainers.

Steam from the geothermal field will be sent to the steam turbine via the main steam piping. The main steam will pass through pneumatic stop valves before entering the steam turbine.

The steam turbine will be equipped with automatic main steam valve, which is kept in open position by the oil pressure of turbine. Once the pressure is lost, the valve is tripped and closed,

The condenser neck and the turbine low pressure cylinder will be connected by expansion joints, and be rigidly supported at the bottom. The expansion joints will compensate the downward expansion of the condenser and the rear cylinder.

The thrust force of steam piping beyond the allowed range by the manufacturers will not be applied onto the turbine casing.

4.2.3 Major auxiliary equipment of steam turbine

1	Automatic main steam valve
1	Double-stage steam jet ejector
1	Drain flash tank
1	Oil tank	V=3 m3
2	Oil cooler	F=12.5 m2
1	Centrifugal auxiliary oil pump	Q=40 m3/h, H=1.25 MPa
1	AC gear oil pump	Q=20.5 m3/h, H=0.353 MPa
1	DC gear oil pump	Q=20.5 m3/h, H=0.353 MPa
1	Gland cooler	F=20 m2
1	Turning motor	N=3 kW
1	Jet condenser
1	Oil filter

All the equipment covered in this section are in conformity to ASME and IEC standards or equivalents.

4.2.4 Steam turbine control, regulation and security system　

Steam turbine monitoring: One set of monitoring instruments in 8000 series, to be supplied with steam turbine.

Steam turbine control: EHC electrical/hydraulic regulation system. The controller will be model 505 from WOODWORD USA. The turbine vendor will supply one WOODWORD 505 and one CPC, and perform configuration and one site commissioning.

Steam turbine security: An emergency security system for over speed prevention will be supplied. It will immediately close the main steam valve and control valves to cut off the steam source in case of failure. Additional over speed protection devices and standby assistant emergency stop devices will be supplied.

Automatic protection devices to close the main steam valve and governor valve under the following emergency conditions:

Over speed

High bearing return oil temperature

Low lubrication oil pressure

Big thermal expansion

High bearing metal temperature

High bearing vibration

Trip from electric remote signals

Low condenser vacuum

Big axial displacement of rotor

Electric control devices stop

From the viewpoint of economy and performance, compact and high efficient heat exchange is requested for the condenser. For that purpose, direct contact type of condenser will be applied. The condenser is composed of main cooling part, where approximately 90% of steam is condensed, and cooling part. Spray jet type was applied to main cooling part and tray type was applied to main cooling part. The condenser neck and the low pressure cylinder of the steam turbine will be connected by expansion joint, and rigidly supported at the bottom.

The vertical spray water pipes are installed in the steam condensing part and swirl type spray nozzles are mounted on vertical spray water pipes in horizontal direction. The arrangement of the nozzles was so designed as to taking into consideration of no interference with water film of adjacent nozzles.

The condenser is designed for maximum continuous operation conditions of the steam turbine. The steam turbine exhaust pressure will be 0.06 ata, the circulation cooling water temperature will be 30 degree C at the operation condition of maximum exhaust.

The condenser will be designed to receive the following exhaust steam, drain and return water, and with good performance of deaerating.

Exhaust steam from steam turbine

Condensate from steam turbine

Drain from steam lock cooler

Warming pipe drain of the main steam piping at start and stop of turbine

Drain of trunk drain flash tank

Drain of steam turbine and etc.

The condenser air extraction system provides for the venting of the condenser steam space for removal of non-condensable gases during turbine generator operation. It also provides the capacity to rapidly reduce the condenser pressure from atmospheric before unit startup, allowing admission of steam to the condenser. The vacuum breaker valve allows air to be vented into the condenser to reduce the rolldown time of the turbine generator.

The condenser will be installed with two liquid-ring vacuum pumps of 100% volume.

The condenser air extraction system includes:

Main condenser vent line

liquid-ring vacuum pump

One condenser vacuum breaker

Piping and valves

The vacuum breaker installed is to break condenser vacuum and reduce the turbine roll down time.

Geothermal steam contains harmful impurities such as silica or chloride and much non-condensable gas in comparison with thermal power plant. Impurities or non-condensable gas cause scaling or corrosion related problem. In addition, geothermal steam is usually saturated. And the measures shall be taken against scaling, corrosion and erosion problem for higher reliability and performance.

The following are possible mechanisms of precipitation for the silica that is carried over into the steam:

There is a relationship between the solubility curve of non-crystal silica and the silica concentration in the hot water after the cyclone separator process. These relationship shows that the hot water after having passed through the separator is over-saturated with respect to silica. This implies that the mist contained in the steam after separation is over-saturated, or nearly over-saturated, with silica. Therefore, if the temperature decreases, and flashing, boiling etc. occur in the mist, even slightly, over-saturated will lead to silica precipitation. Some of the silica scale is thought to originate in over-saturation, but it is difficult to qualitatively estimate how much silica scale is produced by this mechanism.

Small pressure drops occur at the main stop valve, the sub-stop valve and the strainer, and this causes flashing of the hot water mist. This flash probably produces much of the silica scale adhering to these areas. Repeated precipitation of silica in the mist leads to progressive silica scale forming on the surfaces.

Numerical simulation and field experiments using real geothermal steam confirmed that the temperature of the first-stage turbine nozzle at the steam outlet side is higher than the steam temperature. This causes the mist covering the nozzle surface to boil; its water content vaporizes, and the dissolved components in the mist, such as silica, precipitate and solidify on the nozzle outlet.

Over-saturation, flash and boiling of the mist are possible mechanisms for scale adherence. In these three cases, injecting water into the steam (preventing flash and boiling) and in lowering the concentration of the silica in the mist (preventing over-saturation). Washing off of the accumulated scale is also expected.

The turbine water injection equipment directly atomizes the circulating water (steam condensed water) through the injection nozzle into the main steam piping. The injection nozzle is positioned some distance before the main stop valve, and the distance allows the injected water and steam to thoroughly mix before they reach the steam turbine inlet. To prevent the steam turbine from corroding, wetness measuring equipment was installed before the turbine inlet.

The results of the turbine water injection tests indicated that water injection for two hours every two weeks was the most preferable, and the station will be operating stably with this routine.

Blade washing system utilizes water from the hot well pump outlet, pressurizes it by a pump and injects it into the main steam line. The injected water dissolves and removes scale deposited on the first stage nozzles.

It is difficult to efficiently separate minute mist particulates in the steam without changing their state. However, if the injected water is directly atomized into the steam line and the relatively large water droplets thus produced catch the mist particulates, it is easy to separate them downstream. This permits reducing the amount of mist in the steam and leads to a considerable decrease in the scale-adherence rate to the turbine nozzle, etc.

The equipment has ten nozzles, which directly atomize circulating water into the main steam piping, and two mist eliminators, which efficiently remove the injected water.

Water injection is accomplished with a venturi tube configuration. The mist separating equipment will be abbreviated as MSP (Mist Separator).

To reduce scale formation at the Geothermal Power Station, turbine water injection equipment was installed. Water injection tests indicated that water injection of 1-2% of the steam weight is effective in preventing scale accumulation and in removing it. The steam turbine can be operated satisfactorily without causing shaft vibration, corrosion and so on.

By installing and operating the MSP upstream, the purity of the steam was improved, and significant improvements in the preventing of scale accumulation were achieved.

The noticeable effects of running the mist separating equipment and the water injection equipment are: reduced reconditioning costs for scale removal and avoiding output reductions

4.6 Turbine room crane

The turbine room crane will be provided for maintenance and overhaul of the steam turbine, generator and auxiliary equipment.

The crane will be an overhead, double girder bridge type with motor operated main and auxiliary hoists, trolley and bridge. Control of the crane will be performed from a crane supported operating cab.

Technical data of crane:

Span:		13.5 m
Lifting weight:	Main hoist	20 t
	Auxiliary hoist	5 t
Lifting height:	Main hoist	18 m
	Auxiliary hoist	18 m
Lifting speed:	Main hoist	3.5 m/min
	Auxiliary hoist	8 m/min
	Crane bridge	20 m/min
	Trolley	20 m/min