steam turbine

Proprietary & Confidential

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Steam Turbines Classifications, Applications & Operation

GGSR, Bathinda (HMEL)

Navneet Singh


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Description

Steam turbine is a prime mover that converts thermal energy in pressurized steam into useful mechanical work - for its rotation. Turbine, in-turn, is used to power various rotating equipment like compressors, pumps, generators etc.

Operating Principle:

Steam at high temperature and pressure contains the potential energy.

Potential energy of the steam is converted into a mechanical work through expansion in a nozzle and impact or reaction with a blade.

Mechanical Work of many sets of blades attached to a shaft produces rotational power.

An ideal steam turbine is considered to be an isentropic process, or constant entropy process. No steam turbine is truly isentropic, typical isentropic efficiencies range from 2090% based on the application of the turbine.


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Description

Applicable Standards:

API-611: General Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services API-612: Petroleum, Petrochemical and Natural Gas industries-Steam Turbines -Special Purpose Applications NEMA standards SM 23 / SM24


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Classification

Based on their size and usage (API):

General-purpose steam turbines (API-611)

Spared/non-essential service Generally backpressure design Limited steam pressure/temp Limited speed

Special-purpose steam turbines (API-612)

Not-Spared/critical service Backpressure/condensing/extraction design

Not limited by steam pressure/temp Not limited by speed

Based on working principles:

Impulse turbines (action turbines) - Steam expansion occurs only in the nozzles and / or in the

stationary blades, not in the rotating blades.

Reaction turbines - Steam expansion occurs in both the stationary and the rotating blades.


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Reaction Impulse Comparison


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Cross Section


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Classification


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Configuration

Straight HP

Tandem HP

Tandem LP


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Selection Criteria

Inlet steam pressure and temperature, exhaust steam pressure

Driven Equipment power/speed (rated max.-min.) and max allowable over-speed for trip

Speed control (manual or type of process signal)

Site conditions: indoor/outdoor and ambient conditions

Cooling water data (pressure, temp and cleanliness)

Any off-normal steam or driven equipment operating conditions

Type of driven equipment and service:

Centrifugal or positive displacement

Normal or quick start

Continuous or standby duty

Site electrical rating (if electrical accessories are involved)

Specifications:

Steam cost evaluation

Sound level requirements


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Steam supply/exhaust conditions

I. Non condensing (Back-pressure) turbine - Most widely used for process steam

applications at refineries, pulp and paper plants, and desalination facilities where large

amounts of low pressure process steam are needed. The exhaust pressure may be

controlled by a regulating valve to suit the needs of the process steam pressure.

II. Condensing turbine Most commonly found in electrical power plants. Exhaust steam is

in a partially condensed state, at a pressure well below atmospheric.

III. Extraction turbine Steam is released from various stages of the turbine, and used for

industrial process needs to improve overall cycle efficiency. Extraction flows may be

controlled with a valve, or left uncontrolled (bleed)

IV. Reheat - Reheat turbines are also used electrical power plants. Using reheat in a cycle

increases the work output from the turbine

V. Induction - Introduce low pressure steam at an intermediate stage to produce additional

power


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Operation: Start-up (Small turbines)

Small turbine with oil sump lubrication Check points:

1. Check driven machine instruction for starting and operating procedures and readiness.

2. Check and top up correct oil to correct levels in the bearing and governor housings.

3. Drain all condensate from low points in the inlet steam line, from the casing, exhaust line.

Drain valves must be left open while the turbine is started.

4. Warm up by gradually opening the exhaust line shut off valve or the bypass steam valve. Open

the exhaust line shut off valve fully, when the casing equals temp of exhaust steam.

5. Admit sufficient steam through the Steam inlet-line valve to turbine so that the rotor starts

slowly spinning. Observe vital parameters and open valve till rated speed.

6. Close all drain lines when no condensate is observed.

7. Check for abnormality, vibration, bearing housing temperature, noise.


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Operation: Start-up (Large turbines)

Large turbines with full/partial condensing operation

Pre-start-up checks and start-up checks

1. Check driven machine instruction for starting and operating procedures and readiness.

2. Check instrument trip system.

3. Commission bearing oil and governing oil system. Observe oil pressures

4. Drain all condensate from low points leave drain valve open.

5. Commission the condensing system.

6. Commission the cooling water to the gland steam condenser.

7. Warm up the main steam inlet line up to the turbine, steam venting through silencer.

8. Start turbine barring


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Warm up and Start-up curve:


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Start-up:

1. Open the Emergency Stop Valve.

2. Check barring mechanism is disengaged

3. In case of electronic governor give start-up command from the governor panel.

4. In case of manual start up follow start-up sequence as described by the manufacturer.

5. Admit sufficient steam through the Steam inlet line valve to turbine so that the rotor starts slowly

spinning. Continue to open the slowly until the speed governor assumes control of turbine speed.

6. Line up extraction (if extraction turbine). Turbine controller may not allow speed increase after a

limit without enabling extraction.

7. Close all drain lines when no condensate observed.

8. Check for leakage, vibration, bearing temperatures, noise.

9. Monitor the turbine operation until stable operation is reached.


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Operation: Performance monitoring

The below given parameters are generally monitored, alarmed at near critical vales and connected to trip the turbine at unacceptable values: 1. Oil and Bearing Temperatures 2. Bearing housing and Shaft vibrations 3. Casing expansion 4. Steam Parameters


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1. Oil and Bearing Temperatures Oil supply/return temperature Oil inlet to bearings pressure Oil cooler outlet temperature Oil filter delta P Bearing metal temperature Thrust bearing temperature

2. Bearing housing and Shaft vibrations

Shaft relative vibrations Bearing housing vibrations Shaft axial position monitoring


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3. Casing/rotor expansion Shaft and casing relative expansion Shaft eccentricity measurement

4. Steam Parameters

Inlet Steam temperature and pressure Inlet Steam flow Wheel chamber pressure and temperature


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Speed control / governing

Steam turbine governing is the procedure of controlling the flow rate of steam into a steam turbine so as to maintain its speed of rotation as constant or at a desired set point.

1. Throttle governing: The flow rate is controlled using a partially opened steam control valve. The reduction in pressure leads to a throttling process in which the enthalpy of steam remains constant

2. Nozzle governing: Flow rate of steam is regulated by opening and shutting of sets of nozzles rather than regulating its pressure

3. By pass governing: Occasionally the turbine is overloaded for short durations. During such operation, bypass valves are opened and fresh steam is introduced into the later stages of the turbine. This generates more energy to satisfy the increased load

4. Combination governing: Combination governing employs usage of any two of the above mentioned methods of governing. Generally bypass and nozzle governing are used

5. Emergency governing: When the speed of shaft increases beyond 110%. Balancing of the turbine is disturbed - Vibrations. Failure of the lubrication system. Vacuum in the condenser is quite less or supply of coolant to the condenser is inadequate


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Safety devices

Manual shutdown Push button (Local as well as remote Control room)

Rotor over-speed monitors turbine rotor speed and will shutdown turbine when maximum

allowable speed (trip speed) is attained (Electronic over-speed protection)

Over-speed trip bolt (Mechanical)

Excessive process variable signal monitors all train process variables and will shutdown

turbine when maximum value is exceeded

Two independent governors are needed for safe turbine operation

Trip valve exerciser allows trip valve stem movement to be confirmed during operation

without shutdown

Lubrication safety:

Auxiliary oil pump

Emergency oil pump

Lube oil rundown tank


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Steam turbines as HMEKL

Power plant:

STG 1 & 2 - Double Extraction cum condensing turbine 35 MW each

STG 3 Full back pressure turbine - 32 MW

Process plants:

RGC DHDT Fully condensing turbine

RGC VGO - Fully condensing turbine

RGC CCR - Fully condensing turbine

WGC DCU - Fully condensing turbine

WGC FCCU - Fully condensing turbine

MAB FCCU - Fully condensing turbine


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Thank you

steam turbine

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