Author: Daniel G. Teleoaca – Maritime Chief Engineer
Steam turbines have been a cornerstone in power generation for over a century, particularly in marine applications where they provide propulsion and electrical power. This article delves into the intricacies of a vessel steam turbine generator system, exploring its components, operational principles, maintenance requirements, common issues, and essential safety measures
Components and Working Principle of a Vessel Steam Turbine Generator
To accurately explain this, we will consider as an example the Mitsubishi ATD64CLM2 turbine generator which consists of the following components on a common bedplate:
- A main engine exhaust gas driven power turbine (P/T), with reduction gear and synchro-self-shifting (SSS) clutch.
- A steam turbine (S/T), driven by steam from the economiser whilst the vessel is under way at sea with reduction gear.
- An alternating current generator and auxiliaries. The bedplate incorporates the lubricating oil (LO) sump tank.
The steam passes through a trip valve, designed to shut off the steam instantly should a trip condition occur, and then through the nozzle control valves. The turbines normally exhaust to the condenser under vacuum conditions.
Labyrinth-type seals are used at the end of the turbine rotors to prevent the steam in these regions from leaking to atmosphere, and more importantly, to prevent air from entering the turbine where its internal pressure is less than atmospheric. The seals are formed by radially slotting sections of labyrinth into the packing rings, which themselves are likewise slotted radially into the turbine upper and lower casings. The peak and trough edges of these labyrinths are located adjacent to corresponding square radial grooves machined into the rotor shaft.
The clearances between the labyrinth edges and the rotor are minimized to reduce steam leakage between the inner (high gland steam pressure) areas and the outer (low gland steam pressure) areas. Adjacent axial clearance between the rotor and the labyrinths allow for the designed relative axial movement and expansion between the rotor and the casing.
Steam is supplied to the glands from a gland packing steam receiver mounted adjacent to the turbine. Where the turbine internal steam pressure is higher than the pressure in the gland housing, steam will enter the series of diaphragms from the turbine, as well as supplying the gland steam receiver, and is effectively throttled across each stage, causing its pressure to drop. The gland steam receiver is connected to the inner sections of the glands so that the steam supplied will pass outwards, and is led away from the outer glands to the gland condenser, which then drains to the vacuum condenser.
Where the pressure in the gland housing is greater than the internal turbine pressure at the shaft exit point, the steam available from the gland steam receiver will be drawn through the gland, effectively sealing it and preventing the ingress of air. The need for the steam to make-up or spill changes with the turbine load, ie, at high load the steam will generally be spilled out of the system, and at low loads the packing steam will need to be made up. Steam from the outer stages of the labyrinth is led to the main gland steam condenser. Due to the make-up and spill operation of the controller on the inner glands, the pressure of the steam at the leak-off point is always positive.
The 1st reduction gear is fitted between the steam turbine and the alternating current (AC) generator. It is of the single reduction, single helical type, the turbine pinion being solid, and used to drive the main LO pump and governor gear at one end. The gearing is supported by four oil lubricated bearings. The AC generator has four poles and is connected to the gear by a flexible coupling at its shaft end.
A main engine exhaust-driven power turbine is connected to the steam turbine shaft through the 2nd reduction gear and a synchro-self-shifting (SSS) clutch. The 2nd reduction gear is of the single reduction, double helical type. The pinion is solid.
The SSS clutch engages and disengages mechanically automatically. The slider of the SSS clutch moves to the engage side when the power turbine is rotating at precisely the same speed as the steam turbine, then the SSS clutch engages. The SSS clutch moves to the disengage side when the power turbine rotating speed becomes lower than the steam turbine rotating speed, then the SSS clutch disengages.
The LO system is supplied by an auxiliary motor-driven LO pump when the turbine is stopped or starting up. The auxiliary LO pump starts when the LO pressure falls to 0.6 bar and cuts-out when the turbine reaches 1.0 bar. A gear-driven integral LO pump supplies the system when the turbine is running normally. The pressure is regulated to the bearings and gears by a regulating valve, full delivery oil pressure is supplied to the trip valve.
The governing system consists of a Woodward Atlas SC digital speed control unit and an EG-3P proportional actuator, hydraulic servomotor and turbine nozzle valve. The governor has a very fast response to load change and is designed to prevent the turbine overspeeding, even in the event of the generator circuit-breaker tripping. The governor reacts to changes in speed by moving a linkage to the hydraulic servomotor which in turn opens or closes the steam inlet to the turbine nozzles.
When the electrical load on the turbine generator decreases, the surplus steam to the steam turbine is dumped to the vacuum condenser. When the generator is operating in parallel with the diesel generators, the governor operates to give correct load sharing.
The vacuum condenser system cannot be started until the water level in the condenser well is at working level. Top-up the condenser from the distilled water tank as necessary by opening the filling line valve.
Operating Procedure
Pre-Starting Procedure
Lube Oil System Check:
- Check the turbo generator lube oil sump level and drain any water.
- Replenish if the level is low.
- Start the lube oil priming pump from the local station and check the lube oil pressure.
- Set the pump to auto mode.
Vacuum Pump and Condenser:
- Ensure the vacuum pump operating water tank is filled to the normal level.
- Check the vacuum condenser condensate level and ensure the condensate pump is set to auto mode to maintain the level.
Steam Line Preparation:
- Open the steam drain valve to remove any condensed water from the steam line, preventing excessive hammering and vibration during startup.
Main Steam Inlet Valve:
- Open the main steam inlet valve for the turbo generator.
Gland Steam Pressure:
- Adjust the gland steam pressure to a normal level.
Cooling Water System:
- Ensure seawater valves for the vacuum pump cooler, T/G lube oil cooler, and vacuum condenser are open.
Starting the Turbine
Initial Checks:
Verify that the condensate vacuum, gland steam pressure, steam inlet pressure, and lube oil pressure are normal.
Turbine Startup:
- Start the turbo generator from the local station and close the drain in the steam line.
- Check first and second stage steam pressure, condenser vacuum, water level, lube oil pressure, and vibration levels.
Monitoring:
- Monitor the turbo generator speed, voltage, frequency, vacuum, condenser level, and other parameters.
Remote Control:
- Give control to the remote station from the local control and take the TG on load.
Post-Starting Procedures
Active Monitoring:
- Continuously monitor steam temperature, pressure, flow, bearing temperatures, and vibrations.
- Check the condenser vacuum system, turbine ejector system, and condenser level control system.
- Examine the leakage from the stuffing box.
Shutting Down Procedure
Gradual Reduction:
- Slowly reduce steam flow to avoid thermal shock.
Cooling Down:
- Allow the turbine to cool down gradually, maintaining lubrication and cooling systems.
Maintenance Requirement
Routine Maintenance:
- Inspections: Regular checks for wear, corrosion, and alignment issues.
- Lubrication: Ensure bearings and other moving parts are well-lubricated.
- Cleaning: Remove debris from turbine components to maintain efficiency.
- Balancing: Dynamic balancing of the rotor to prevent vibrations.
Preventive Maintenance:
- Oil System: Regularly check oil quality, levels, and filter conditions.
- Steam Quality: Monitor for moisture content to prevent erosion and corrosion.
- Control Systems: Calibrate sensors, valves, and actuators.
Overhauls:
- Annual Overhauls: Comprehensive checks and replacements of worn parts.
- Component Checks: Inspect and possibly replace bearings, seals, and blades.
Common Problems
- Wear and Tear: Continuous operation leads to degradation of components.
- Corrosion and Erosion: Due to moisture in steam or corrosive environments.
- Vibration: From imbalances or misalignments, leading to potential failures.
- Steam Leakage: Can reduce efficiency and cause damage if not addressed.
Safety Precautions
- Regular Maintenance: Ensures all components are in safe operating condition.
- Operational Training: Crew must be trained in emergency procedures and normal operations.
- Pressure Relief Systems: Essential to prevent overpressure scenarios.
- Temperature Monitoring: Detects overheating or cooling issues early.
- Vibration Analysis: Identifies potential mechanical issues before they escalate.
- Emergency Shutdown Systems: For quick response to critical situations.
- Fire Safety: Equip areas with fire detection and suppression systems.
- Safety Barriers and Signage: Prevent unauthorized access and ensure awareness of hazards.
In conclusion, the vessel steam turbine generator system is a complex yet highly efficient means of converting thermal energy into mechanical and electrical power. Its operation requires meticulous attention to detail, from the initial steam generation to the final electricity distribution. Regular maintenance, adherence to safety protocols, and prompt resolution of common issues are crucial for ensuring the longevity and reliability of these systems. By understanding and implementing these practices, maritime operations can harness the power of steam turbines safely and effectively, contributing to the vessel’s propulsion and electrical needs.
For more insights into how these systems work in real-world applications, check out this case study on PENG1005 – Simulation – MC90V Steam Turbine Startup and Generator Synchronization or watch this video on How Steam Turbine Works. These resources provide practical examples of how steam turbine generator works.
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