Power equipment instructor robert a mclaughlin zaili theo zhao
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POWER EQUIPMENT INSTRUCTOR: ROBERT A. MCLAUGHLIN ZAILI THEO ZHAO. AUXILIARY TURBINES & CONTROLS. Learning Objectives. The various steam turbine types Steam turbine classifications Pressure drop and steam velocity change in turbine components.

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Learning Objectives

  • The various steam turbine types

  • Steam turbine classifications

  • Pressure drop and steam velocity change in turbine components.

  • Applications and limitations of auxiliary steam turbines

  • The methods of speed control and over speed protection

  • Gland seal assembly

  • The three factors that determine the effectiveness of a labyrinth seal assembly

Steam Turbine Classification by staging

  • There are two main categories of steam turbines based on the method of energy transformation taking place inside the turbine or how the steam is expanded.

  • The two methods are

    • Impulse and

    • Reaction.

  • High pressure impulse turbines are more commonly used in the U.S., whereas reaction turbines tend to be used more in Europe and Asia.

  • Impulse turbines:

    • tend to be smaller than reaction turbine of comparable power and

    • are more durable and

    • have longer time between overhaul than reaction turbines.

  • Reaction turbines have a slightly higher operating efficiency but are usually used in low pressure steam environments.

Steam Turbine Classification by staging

  • Inflating a balloon and then releasing it to fly around the room uncontrolled, is an illustration of the reaction principle in action.

  • A water wheel is a good example of an impulse turbine.

    • However, it is also possible to create rotational force and power through the reaction principle.

1 Simple Impulse Turbine (Delaval)

  • It consists of a set of nozzles followed by a single set of rotating blades attached to a wheel or disk.

  • The nozzles

    • Convert thermal and pressure energy into velocity or kinetic energy

  • the moving blades

    • Capture some of this velocity

    • Turn it into rotational mechanical power.

Simple Impulse Turbine

  • There is only one pressure drop across the nozzle and essentially zero pressure drop across the moving blades.

  • The ideal ratio of the blade speed to the nozzle steam velocity to obtain maximum work from the stage is ½.

    • The blades would be moving approximately ½ the speed of the nozzle velocity.

Simple Impulse Turbine

  • It is only found in small auxiliary turbines and has limited use.

  • It essentially consists of a casing top and bottom with bearing on each end to support a rotor assembly.

  • Disks or wheels are attached to the rotor shaft or are an integral part of the rotor as a casting.

Simple Impulse Turbine

  • Attached to the disk are blades which convert the kinetic energy of the steam created in the nozzle into mechanical energy to drive any type of rotating equipment.

  • Applications of auxiliary turbines include:

    • Pump drives, forced draft fans, compressors,

    • feed pumps, generators and

    • just about anything that can be driven by an electric motor.

2 Pressure Compounded Impulse (Rateau)

  • It consists of two or more simple impulse stages in series on one rotor.

  • The same basic rules apply for pressure drops and velocity changes as depicted in the illustration.

  • The ideal blade velocity is equal to ½ nozzle velocity

Pressure Compounded Impulse

  • Rateau turbines are very common in large power generation turbines.

  • They are more efficient than Curtis or Delaval turbines and will extract thermal and pressure energy from the steam in small increments.

  • The use of this type of turbine for auxiliaries is usually limited to turbo-generator units, where more power is needed at greater efficiency.

3 Velocity Compounded Impulse (Curtis)

  • It consists of a set of nozzles followed by two or more sets of moving blades attached to one wheel or disk.

  • There is also a set of redirectional fixed blades in between the rotating blades to direct the steam fro the first set of blades to the next.

  • There is still only one pressure drop and this occurs across the nozzles.

Velocity Compounded Impulse

  • Theoretically there is no pressure drop across the moving or re-directional blades.

  • Velocity increases across the nozzle and decreases across the moving blades.

  • Pressure and velocity relationships for the velocity compounded impulse turbine are depicted in the illustration below.

Velocity Compounded Impulse

  • Curtis turbines are very common today in auxiliary steam turbines.

  • They are also used as astern elements in marine propulsion power plants as they will extract the maximum amount of energy from the steam in a single impulse stage.

  • They are also very rugged and durable but rather expensive to construct, therefore they are less common today in stationary power station turbines.

  • The ideal blade velocity is equal to 1/4 nozzle velocity

Reaction turbine stages (parsons)

  • It consists of a set of fixed blades (nozzles) and a set of moving blades each of which is shaped like nozzles.

  • Therefore, there are two pressure drops per stage in a reaction turbine.

  • Approximately 50% of the momentum exchange takes place in the fixed assembly and 50% takes place in the rotating assembly.

Reaction turbine stages (parsons)

  • They are currently not used for auxiliary equipment and are not common in marine propulsion applications mainly because of their size, weight and shorter life expectancy.

  • However, some new reaction turbines are being built and used in the U.S. as demand for more efficient electric power generation increases.

  • Ideal blade velocity is equal to nozzle velocity

Condensing & non-condensing

  • Condensing turbines are ones that exhaust at below 14.7 psia or sub atmospheric pressures.

    • Condensing turbines exhaust into a vacuum, which is mainly caused by the reduction in volume of steam as it turns back into water.

    • An air ejector or vacuum pump will begin the process and assist in maintaining the negative pressure by removing air and non-condensable gasses from inside the condenser.


Condensing & Non-condensing

  • Non-condensing turbines exhaust at or above 14.7psia.

  • Most auxiliary turbines exhaust at around 25-35 psig and are non-condensing units.

Classification by direction of steam flow

  • Radial, axial or tangential flow refers to the direction of steam flow in relationship to the axis of the rotor or shaft.

  • Radial flow turbines have steam flowing perpendicular to the shaft axis.

Classification by direction of steam flow

  • Axial flow turbines have steam flowing parallel to the rotor axis.

Classification by direction of steam flow

  • Tangential flow (Pelton) turbines have steam flowing at a tangent to the rotor.

  • The turbine blades act like buckets as picture, the water-jet gradually being diverted from its original direction.

  • Some kinetic energy is lost, but pressure is exerted in the peripheral direction of the wheel before being exhausted – at lower velocity – to the side.

Classification by direction of steam flow

  • Turbines may also utilize dual or divided flow of the steam to help counteract axial thrust on the rotor.

Classification by number of casings

  • Auxiliary turbines utilize one casing to extract the energy from the steam.

  • Larger more powerful turbines often utilize more than one casing to extract work from the steam.

  • When two or more casing are used to extract work, the configuration is call “compounding of casings”

Classification by steam admission control

  • Auxiliary turbines use throttling control valves and hand operated nozzle control valves to vary the speed and power of the turbine.

    • Nozzle control valves can be opened and closed by either bar lift devices, cam devices or manually.

  • Another form of control which is not common today is called by-pass control.

    • By-pass control, cam lift is used on the cruising turbine out side the classroom.

Auxiliary turbineusage

  • Auxiliary turbine usage is found on any rotating equipment.

  • Steam turbines have

    • Very high starting torque,

    • Good high speed control and also

    • Good variable speed capability.

  • Generally when steam is available, motors in excess of 50 hp can be replaced by steam turbines at a weight savings.

  • Steam turbines are very reliable,easy to maintain and easy to operate.

  • Reasons for using steam turbines instead of electric motors are as follows:

    • Power density or weight/hp advantages over 50 hp

    • High rotational speed requirements

    • Good control of variable speed devices

Turbine Control & Safety Devices

  • Auxiliary turbines driving pumps will use constant pressure governors

  • Turbines driving generators or compressors will use constant speed governors.


Speed limiting governor

  • Speed limiting governors are used to prevent the turbine from going into an over-speed condition and are usually direct acting fly-ball type units.

  • The speed limiting governor takes over control of the steam admission valve when the rotor speed reaches about 107% of the rated speed of the turbine.

Over speed trip

  • In the event that the primary governor and the speed limiting governor fails to control the speed of the rotor, the over-speed trip is designed to stop steam flow into the turbine and thus protect the unit from over-speed damage.

  • The over- speed trip is actuated by centrifugal force and consists of a spring loaded pin attached to the end of the rotor.

Over speed trip

  • At about 110-115% of the rated speed of the rotor the pin pops out and trips a mechanical device that completely shuts off the steam supply to the turbine steam chest.

  • The rotor speed will automatically slow down but in order to restart the turbine, the trip mechanism must be manually reset.

Turbine Control & Safety Devices

  • Depending upon the size and cost of the turbine, other protective devices are used as follows:

    • Low Lube oil pressure trip

    • Relief valves and Sentinel valves

    • High back pressure trip

    • High vibration protection

    • High Exhaust Temperature protection

    • Hand trip

Shaft Sealing, Labyrinth Seals vs Carbon Seals

  •  All turbines must have some form of shaft seals to prevent steam from exiting the casing when the turbine is operating.

  • Mechanical labyrinth seals are common on most large steam turbines and carbon type seals are more common on small auxiliary turbines.

Shaft Sealing, Labyrinth Seals vs Carbon Seals

  • Shaft seals

    • Keep steam inside the casing, air outside the casing,

    • Help maintain vacuum in condensing units and

    • Reduce the amount of oxygen entering the system.

  • Three factors that determine the effectiveness of a labyrinth seal as follows:

    • The number of teeth in the seal

    • The teeth length

    • The condition of the teeth or sharpness of the teeth

Shaft Sealing, Labyrinth Seals vs Carbon Seals

  • Carbon packing

    • is not as effective as a labyrinth seal assembly but

    • it is cheaper and does have an application in small auxiliary turbines.


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