Rotors for Impulse Turbines

Rotors for Impulse Turbines
Rotors for Impulse Turbines

Although rotors for impulse turbines exhibit great variety in physical size, wheel diameter, number of wheels and other construction features, they may all be conveniently classified in one of three basic categories:

  1. Built-up rotors: Those rotors that are constructed by shrinking the wheels onto a shaft (Fig. 4.1)
  2. Solid rotors: Those rotors in which the wheels and shaft are machined from a single, integral forging (Fig. 4.2)
  3. Combination solid and built-up rotors: Those rotors in which some of the wheels are integral with the shaft and some are shrunk on (Fig. 4.3) There are several factors that determine the type of construction that is utilized for any particular turbine rotor application. The most significant of these factors are:
  4. Long-term operating experience
  5. Pitch diameter
  6. Maximum operating speed
  7. Steam temperature

Steam Temperature for Impulse Turbines

Because the need for solid rotor construction is so readily related to tip speed (r/min and pitch diameter), special caution is required to ensure that the last of the four factors listed above, temperature, is not ignored. The relationship between steam temperature and the effectiveness of shrunk-on wheels is fairly straightforward. The wheels are held in place by an interference fit, which is achieved when the heated wheel is placed in position and allowed to cool to the same temperature as the shaft.

As long as the wheel and shaft are at the same temperature, the desired interference fit or shrink is sustained. However, if during any transient operating condition a positive temperature differential develops between wheel and shaft, the desired shrink is reduced and may even be entirely lost. If this happens, the wheel is constrained from turning on the shaft by the key, but it may move axially with rather disastrous results.

It should be evident that the danger of incurring a large temperature differential between wheel and shaft becomes more real as the temperature of the steam to which the wheel is subjected increases. For this reason, an integral wheel is utilized whenever the maximum temperature for the stage exceeds 750°F (400°C) or stage inlet temperature exceeds 825°F (440°C).

Since stage temperatures diminish progressively from the first stage to the exhaust of a multistage turbine, a situation is frequently encountered in which stage temperatures dictate the need for integral wheels in some of the head end stages but become sufficiently low to permit shrunk-on wheels in the later stages. It is this situation that most commonly results in the combination solid and built-up type of rotor construction. This type of construction seems to be adaptable to conditions that are most commonly encountered on large condensing generator drive applications.

Built-Up Construction in Turbines

Now that the basic categories of steam turbine rotor construction and the uses of each have been discussed, perhaps some of the differences in the manufacturing of the various types can be best pointed out by attempting to trace very briefly a rough step-by-step manufacturing sequence for each of the rotor styles.

After being received, the machining of the rough forging for a builtup rotor shaft begins on an engine lathe where all facing and turning operations are accomplished. In the turning of any critical shaft diameters, such as journals, shaft ends, and underwheel diameters, approximately 0.015 to 0.020 in (0.35 to 0.5 mm) is left for grinding to final dimensions.

Provision is made for the location of the wheels by machining circumferential grooves in the shaft. These narrow shrink ring grooves are located axially on both sides of each intended wheel hub location. With turning and finish grinding operations completed, the next step in the machining sequence is normally the completion of all necessary milling operations.

These include the milling of a keyway for each of the wheels and for any other keyed rotor components such as couplings, thrust collars, and governor drive worm gears. Concurrently with the machining of the rotor shaft, the turbine wheels and blading are also in the manufacturing process. The rough wheel forging is machined to the desired profile, and the machining of the bore is completed.

After a final trimming cut on the wheel rim, the circumferential dovetail groove that is to receive the bucket roots is machined in the wheel rim. Special care is necessary to achieve the required fit of wheel dovetail to blade dovetail (Fig. 4.4). After completing the machining of the bucket airfoils and rivets, the roots on a set of buckets are custom machined expressly to fit the groove in the particular wheel that is to receive the blades.

This is necessary to achieve the close tolerances that are required to ensure a satisfactory degree of load sharing among the four bucket locks. The blades are assembled in the wheel by inserting each blade individually through a radial slot that is milled at one point in the wheel rim to provide access of bucket root to dovetail groove. Once in the groove each bucket may be driven around to its ultimate position in the wheel.

Buckets are, thereby, stacked in both directions from a point 180° opposite the access opening. The final bucket to be put in the wheel is a special locking bucket. This bucket has a specially formed root that is designed to fill the radial access slot in the wheel rim and lock all the other buckets in position.

This locking bucket (Fig. 4.5) itself is held in the wheel by a locking pin (or pins) fitted into a drilled and reamed hole that passes axially through the blade root and both sides of the wheel rim. After the blades are completely assembled in the wheel, the bucket shroud is attached (Fig. 4.6). Each shroud segment is placed over a group of blades (normally five, six, or seven blades per group) with the rivet on the tip of each blade extending through a drilled hole in the shroud segment.

The attachment is made by peening over the head of each blade rivet. After completion of the bucketing and shrouding procedures, each wheel is statically balanced with any necessary corrections being made by grinding material from the wheel rim.

There are special cases where integrally shrouded blades are used in built-up rotor construction. These are essentially lightly loaded blades (as used in geothermal applications), and the feature is to eliminate stress risers (riveted junction) in the anticipated corrosive environment (Fig. 4.7). In addition, the integral shroud will aid in thermodynamic performance. Integrally shrouded blades are not standard practice.

In preparation for the rotor assembly the wheels are placed in a gasfired furnace and heated as required to achieve the necessary bore expansion. The actual shrinking-on procedure is normally accomplished with the rotor supported in vertical position with the exhaust end down.

Starting with the last stage each wheel is removed from the furnace in turn and lowered down over the governor end of the shaft to its proper position where it shrinks tightly on the shaft as the wheel cools. Each wheel must be turned to align the keyway in the bore with the key that is prepositioned in the shaft keyway. Keyways for adjacent wheels are oriented 180° apart on the shaft and this, in turn, establishes an oppositely oriented 180° alternate spacing of locking buckets.

Each wheel is preceded and followed onto the shaft by a heated ring that shrinks into a previously machined shrink ring groove in the shaft to provide positive axial positioning of the wheel. The shaft material for built-up rotor construction for engineered turbines is usually per ASTM A293, Cl. 3, a chrome-molybdenum-nickel alloy steel. The forging is purchased with a proper heat stability test per the requirements of ASTM A293. The commercial specification for these materials is given in Table 4.1

Rotors for Impulse Turbines

In an impulse turbine, the rotor plays a critical role in converting the kinetic energy of steam into mechanical energy. It is a rotating assembly that consists of a shaft, a series of blades, and a coupling to connect to the driven machinery. The blades are carefully designed to interact with the high-velocity steam jets emitted from the nozzles, transforming the steam’s momentum into rotational force.

Key Components of the Rotor for Impulse Turbines

  1. Shaft: The shaft is the central core of the rotor, providing structural support and transmitting torque from the turbine blades to the driven machinery. It is typically made from high-strength materials, such as forged steel or stainless steel, to withstand the high rotational stresses and operating temperatures.
  2. Blades: The blades are the primary components responsible for converting the kinetic energy of steam into rotational force. They are strategically positioned around the rotor circumference, forming a series of buckets that capture and redirect the steam jets, imparting angular momentum to the rotor.
  3. Coupling: The coupling connects the rotor to the driven machinery, allowing the transmission of mechanical power. Different types of couplings, such as flexible couplings or rigid couplings, are used depending on the specific requirements of the application and the driven machinery.

Design Considerations for Impulse Turbine Rotors

  1. Blade Design: The design of the blades is crucial for optimizing the energy transfer from the steam jets to the rotor. The blade profile, material selection, and blade angle are carefully considered to maximize efficiency and minimize energy losses.
  2. Rotor Balancing: Proper balancing of the rotor is essential to ensure smooth operation and prevent vibration-related problems. This involves precisely distributing the mass of the rotor components and blades to minimize imbalances that could cause excessive vibrations during operation.
  3. Material Selection: The materials used for the shaft, blades, and other rotor components must withstand the high operating temperatures, pressures, and stresses encountered in impulse turbines. Common materials include high-strength steels, stainless steel, and special alloys designed for high-temperature applications.

Manufacturing Techniques for Impulse Turbine Rotors

  1. Forging: Forging is a common method for producing rotor shafts, particularly for larger turbines. This process involves heating and shaping the steel billet into the desired shape under heavy pressure, resulting in a strong and durable component.
  2. Casting: Casting techniques are often used to manufacture turbine blades. The blades are typically cast in molds using high-quality materials that can withstand the operating conditions.
  3. Machining: Precision machining techniques are employed to finish the rotor components and ensure accurate dimensions and surface finishes. This includes turning, milling, and grinding operations to achieve the desired tolerances and specifications.

Maintenance and Inspection of Impulse Turbine Rotors

  1. Regular Inspections: Regular inspections of the rotor are essential to detect any signs of wear, erosion, or damage to the blades, shaft, or other components. Visual inspections, ultrasonic testing, and other non-destructive testing methods are used to identify potential problems.
  2. Balancing Checks: Periodic balancing checks are conducted to ensure the rotor remains in balance and prevent vibration-related issues. This involves measuring and correcting any imbalances that may have developed during operation.
  3. Repair or Replacement: If the rotor is damaged beyond repair, it needs to be replaced with a new one. Proper assembly, balancing, and alignment of the new rotor are crucial for ensuring optimal turbine performance and longevity.

In summary, rotors play a critical role in impulse turbines, converting the kinetic energy of steam into mechanical power. Their design, manufacturing, and maintenance are crucial aspects of ensuring the efficient and reliable operation of impulse turbines in various applications.

EMS Power Machines

EMS Power Machines
EMS Power Machines

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