Sunday, January 21, 2018

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2.3.1. GENERAL
Description :
The axial-flow compressor section consists of the compressor rotor and the inclosing casing. Included within the compressor casing are the inlet guide vanes, the 17 stages of rotor and stator blading, and the exit guide vanes.

In the compressor, air is confined to the space between the rotor and stator blading where it is compressed in stages by a series of alternate rotating (rotor) and stationary (stator) airfoilshaped blades.

The rotor blades supply the force needed to compress the air in each stage and the stator blades guide the air so that it enters in the following rotor stage at the proper angle. The compressed air exits through the compressor discharge casing to the combustion chambers. Air is extracted from the compressor for turbine cooling, for bearing sealing, and during startup for pulsation control.

Since minimum clearance between rotor and stator provides best performance in a compressor, parts have to be made and assembled very accurately.

Compressor SectionCompressor Section

Description :
The compressor rotor is an assembly of 15 individual wheels, two stub-shafts, each with an integral wheel, a speed ring, tie bolts, and the compressor rotor blades.

Each wheel and the wheel portion of each stub-shaft has slots broached around its periphery. The rotor blades and spacers are inserted into these slots and are held in axial position by staking at each end of the slot. The wheels and stub-shafts are assembled to each other with
mating rabbets for concentricity control and are held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction. After assembly, the rotor is dynamically balanced to a fine limit.

The forward stub-shaft is machined to provide the forward and aft thrust faces and the journal for the n° 1 bearing, as well as the sealing surfaces for the nø 1 bearing oil seals and the compressor low pressure air seals.

Compressor Rotor LocationCompressor Rotor Location

General :
The stator (casing) area of the compressor section is composed of four major sections :

  • inlet casing
  • forward compressor casing
  • aft compressor casing
  • compressor discharge casing

These sections, in conjunction with the turbine shell and exhaust frame form the primary structure of the gas turbine. They support the rotor at the bearing points and constitute the outer wall of the gas path annulus.

The casing bore is maintained to close tolerances with respect to the rotor blade tips for maximum efficiency.

Compressor Stator

Inlet casing :
The inlet casing is located at the forward end of the gas turbine.

Its prime function is to uniformly direct air into the compressor. The inlet casing also supports the n° 1 bearing housing, a separate casting that contains the n° 1 bearing. The n° 1 bearing housing is supported in the inlet casing on machined surfaces on either side of the inner bellmouth of the lower half casing. To maintain axial and radial alignment with the compressor rotor shaft, the bearing housing is shimmed, doweled and bolted in place at assembly.

The inner bellmouth is positioned to the outer bellmouth by eight airfoil-shaped radial struts that provide structural integrity for the inlet casing. The struts are cast into the bellmouth walls.

Inlet Casing

Variable inlet guide vanes :
Variable inlet guide vanes are located at the aft end of the inlet casing.

The position of these vanes has an effect on the quantity of compressor air flow. Movement of these guide vanes is accomplished by the inlet guide vane control ring that turns individual pinion gears attached to the end of each vane. The control ring is positioned by a hydraulic
actuator and linkage arm assembly.

Hydraulic Cylinder for IGV System


Cover for Ring Gear

Forward casing :
The forward compressor casing contains the first four compressor stator stages. It also transfers the structural loads from the adjoining casing to the forward support which is bolted and doweled to this compressor casing's forward flange.

The forward compressor casing is equipped with two large integrally cast trunnions which are used to lift the gas turbine when it is separated from its base.

Forward Casing

Aft casing :
The aft compressor casing contains the fifth through tenth compressor stages. Extraction ports in the casing permit removal of 5th and 11th stage compressor air. This air is used for cooling and sealing functions and is also used for starting and shutdown pulsation control.

Aft Casing

Discharge casing :
The compressor discharge casing is the final portion of the compressor section. It is the longest single casting. It is situated at the midpoint between the forward and aft supports and is, in effect, the keystone of the gas turbine structure.

The functions of the compressor discharge casings are to contain the final seven compressor stages, to form both the inner and outer walls of the compressor diffuser and to join the compressor and turbine stators. They also provide support for n° 2 bearing, the forward end of the combustion wrapper, and the inner support of the first-stage turbine nozzle. The compressor discharge casing consists of two cylinders, one being a continuation of the compressor casings and the other being an inner cylinder that surrounds the compressor rotor. The two cylinders are concentrically positioned by twelve radial struts. These struts flair out to meet the larger diameter of the turbine shell, and are the primary load bearing members in this portion of the gas turbine stator.

The supporting structure for the nø 2 bearing is contained within the inner cylinder. A diffuser is formed by the tapered annulus between the outer cylinder and inner cylinder of the discharge casing. The diffuser converts some of the compressor exit velocity into added pressure.

Discharge Casing

Blading :
The compressor rotor and stator blades are airfoil shaped and designed to compress air efficiently at high blade tip velocities. The blades are attached to their wheels by dovetails arrangements.

The dovetail is very precise in size and position so as to maintain each blade in the desired position and location on the wheel.

The compressor stator blades are airfoil shaped and are mounted by similar dovetails into ring segments. The ring segments are inserted into circumferential grooves in the casing and are held in place with locking keys. The stator blades of the last nine stages and two exit guide vanes have a square base dovetail that are inserted directly into circumferential grooves in the casing. Locking keys also hold them in place.

Compressor Rotor Blade

Compressor Stator Blades


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