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20 Explanations of Professional Terms for Industrial Steam Turbine (II)
Created on 11.03
Understanding turbine components, auxiliary systems and operating behaviour
An industrial steam turbine’s performance depends on more than its steam conditions and power rating. Internal components, lubrication, sealing, installation quality and operating behaviour all contribute to reliable service.
This second article in the YTS terminology series introduces 20 additional terms to help plant owners, engineers and procurement teams better understand equipment specifications, technical drawings and maintenance discussions.
1. Rotor
The rotor is the turbine’s rotating assembly. Depending on the design, it includes the shaft, wheels or drum, moving blades and associated components.
Steam applies force to the moving blades, producing torque that is transmitted to the driven equipment. Rotor design must account for mechanical strength, thermal behaviour, balance and vibration.
2. Turbine Casing
The casing is the stationary enclosure surrounding the turbine’s steam path. It contains steam and supports or locates internal stationary components.
Many industrial turbines use a horizontally split casing to provide maintenance access. Casing construction depends on operating pressure, temperature, size and the turbine’s mechanical arrangement.
3. Nozzle
A nozzle is a shaped steam passage that accelerates and directs steam toward the moving blades.
As steam expands through a nozzle, part of its energy becomes velocity. Nozzle geometry and condition affect steam flow and the direction at which steam enters the blade row. Deposits, erosion or damage can impair performance.
4. Diaphragm
A diaphragm is a stationary partition used in many multistage impulse turbines. It carries the nozzles between adjacent rotating wheels and helps separate regions operating at different pressures.
Seals near its inner diameter restrict steam leakage along the shaft. Diaphragm condition therefore affects both steam distribution and internal leakage.
5. Turbine Stage
A turbine stage is an arrangement of stationary and moving blade passages that carries out part of the steam expansion and energy transfer.
A multistage turbine divides the overall duty among several stages. The appropriate number depends on steam conditions, speed, aerodynamic design and economic requirements; stage count alone does not establish equipment quality.
6. Impulse Stage
In an impulse stage, most of the pressure reduction occurs in the stationary nozzles. The resulting high-velocity steam changes direction through the moving blades, producing force on the rotor.
The pressure change through the moving blades is relatively small. The term describes how expansion is distributed within the stage.
7. Reaction Stage
In a reaction stage, steam expands through both stationary and moving blade passages. The moving passages also act as nozzles, contributing to the force that drives the rotor.
Reaction designs require suitable treatment of axial forces and clearances. Neither “impulse” nor “reaction” alone determines which turbine is best for a particular project.
8. Journal Bearing
A journal bearing supports the shaft radially and helps control its lateral position.
Many industrial turbines use hydrodynamic journal bearings, where shaft rotation develops a load-carrying oil film. Bearing geometry, oil condition, temperature and supply all influence operation. The bearing also contributes to the dynamic behaviour of the rotor.
9. Thrust Bearing
A thrust bearing carries axial load and locates the rotor along its axis.
Axial forces arise from steam pressure, blade loading and other mechanical effects. The thrust bearing helps maintain the intended axial relationship between rotating and stationary components. Its load direction and capacity must match the machine’s operating requirements.
10. Shaft Gland Sealing
Shaft gland seals restrict leakage where the rotating shaft passes through the casing.
In pressurised regions, they limit outward steam leakage. In regions below atmospheric pressure, an appropriate sealing arrangement helps limit inward air leakage. Labyrinth seals are commonly used, although the complete arrangement varies by turbine design and operating condition.
11. Lubricating Oil System
The lubricating oil system supplies suitable oil to bearings and, where applicable, other components such as a gearbox. It reduces friction and carries away heat.
A pressure-fed arrangement commonly includes a reservoir, pumps, filters, coolers and instrumentation. Required oil supply during start-up, normal operation and coast-down must be considered separately. Lubricating oil and hydraulic control oil may use shared or separate systems.
12. Turning Gear
Turning gear rotates the rotor slowly during specified periods before start-up or after shutdown.
Its purpose includes reducing uneven rotor heating or cooling that could cause thermal bowing. Turning arrangements may be manual or powered, depending on the machine. Their use and duration are established for the particular turbine.
13. Critical Speed
A critical speed is a rotational speed at which an excitation can produce resonance in the rotor-bearing system.
Vibration may rise as the rotor approaches such a speed. Critical speeds depend on the complete system, including rotor geometry, bearings and supports. They are considered when defining operating ranges and start-up or shutdown behaviour.
14. Differential Expansion
Differential expansion is the difference in thermal growth between rotating and stationary turbine parts.
During heating or cooling, the rotor and casing may change temperature at different rates. Their relative growth changes internal clearances. This measurement helps assess whether the machine remains within its designed clearance limits during thermal transients.
15. Shaft Alignment
Shaft alignment establishes the required relationship between the rotational centre lines of connected machines.
It involves both angular and offset alignment. Thermal growth must also be considered: the correct cold alignment may include deliberate offsets so that the shaft train reaches its intended alignment at operating temperature. Flexible couplings still require appropriate alignment.
16. Main Stop Valve — MSV
The main stop valve provides rapid interruption of the principal steam supply to the turbine when required.
Its main function is isolation and protective shut-off, rather than continuous load regulation. Some turbine arrangements combine stop and throttling functions, so the valve arrangement should be understood from the specific equipment drawings and functional description.
17. Steam Control Valve
A steam control valve regulates the steam admitted to the turbine.
Changing its opening adjusts the energy entering the machine. Depending on the application and control mode, this supports regulation of speed, electrical load or a steam system parameter. Turbines may use one valve or several valves with a defined opening sequence.
18. Steam Drainage
Steam drainage removes condensate that accumulates in steam piping, valve bodies and designated turbine locations.
Condensate commonly forms when steam contacts cooler surfaces during warming. Effective drainage helps limit water accumulation, water hammer and water entry into the turbine. Drain locations and discharge arrangements must suit the pressure conditions and equipment layout.
19. Generator Synchronisation
Generator synchronisation is the process of establishing suitable electrical conditions before connecting a generator to an energised busbar or grid.
Correct phase sequence must be established, while voltage, frequency and phase angle must be within the permitted closing limits. Automatic synchronising equipment coordinates speed, excitation and breaker closing to achieve the required connection conditions.
20. Load Rejection
Load rejection is a sudden reduction or loss of the load carried by the turbine.
For example, opening the generator circuit breaker can remove electrical load while steam continues supplying mechanical energy. The resulting imbalance can accelerate the rotor. Control response, valve behaviour and protective functions must therefore be coordinated for the specified load-rejection duty.
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