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An introduction to GE-frame-9-E gas turbine engine compressors

This gas turbine is also known as “PG9171E” or “MS9001E”. Throughout this article the air inlet of gas turbine is considered to be the fwd end and the gases exit is to be the aft end. The fwd and the aft of each individual component are defined in the same manner. The direction of the air flow inside the compressor is defined as the downstream direction and opposite is to be the upstream direction. The right and left are defined by standing on the fwd end and looking downstream. The shaft of gas turbine rotates at 3000 rpm counterclockwise as viewed looking downstream.This gas turbine consists of the following main components:

  1. The compressor
  2. The combustion system
  3. The turbine
  4. The exhaust assembly
  5. The bearing

The compressor

The compressor is a 17-stage axial flow compressor with variable inlet guide vanes (IGV). Air is compressed to a pressure ratio of 12 to 1. Each stage consists of set of rotating blades and set of stator stationary blades. Compression is achieved in each stage .As the rotating blades increase the relative velocity of the air the stator stationary blades convert the gained kinetic energy into a pressure rise and guide air into the following stage at the proper angle.

Fig 1) An open view of the compressor

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The compressor consists of two major components:

f​ig 2) A typical stage of the compressor

The compressor rotor has 17 stages of rotating blades. It is assembled of 15 individual wheels and 2 stub shafts. All wheels are held together with 16 tie bolts and nuts.

The fwd stub shaft is machined to provide the following features:

The aft stub shaft is machined to provide following features:

Fig 3) The Rotor main parts

 

 

 

 

 

 

 

 

 

The compressor rotor blades are airfoil shaped and are attached to the wheels by means of dovetail arrangement. The first wheel blades are mounted on the wheel portion of the fwd stub shaft and they have only aft spacers while the next 15 wheel blades have fwd and aft spacers. The 17-stage blades are mounted on the wheel portion of the aft stub shaft and they have only fwd spacers. These spacers are placed to maintain relative position between the rotor and the stator blades. Rotor blades are held an axial position by staking at both ends of every dovetail slut. Compressor blades cannot be replaced with the rotor in position. Wheels must be disassembled.

The compressor stator consists of four main components:

​Fig 4) The Stator mail parts

 The inlet casing is located at the fwd end of the gas turbine. The blue painted section at the fwd of the casing is called “the bell mouth”. The inner bell mouth is positioned to the outer bell mouth by 8 airfoil shaped struts. The bell mouth function is to direct air normally and uniformly to the inlet guide vanes for efficient air flow control and better inlet flow co-efficient. The lower half of the bell mouth provides support for bearing number 1. A stationary labyrinth seal is installed at aft end of the inlet casing to prevent suction of contaminates into the compressor. Variable inlet guide vanes located at the aft end of the inlet casing. The function of these guide vanes is to control the amount of air across the compressor. The total number of the inlet guide vanes is 64 blades. Each blade stem is inserted into a hole machined on the inlet casing. Each four of them are supported from below by one inner segment. A pinion gear is installed to each blade stem and is keyed in the position. These pinions are rotated by a gear which is assembled to the control ring. The control ring is positioned by a hydraulic actuator to obtain desired guide vanes opening angle. The opening angle for this gas turbine ranges from 34 to 84 degrees.

Fig 5) “Inlet Guide Vanes”

The fwd casing contains the first 4 compressor stator vanes. The lower half is equipped with two trunnions used with other trunnions on the turbine shell to lift the gas turbine to or off the turbine base. It also features a mounting point for the turbine support plate.

The aft casing contains the 5th to the 10th compressor stator vanes. A groove is machined at the fwd face around the 5th wheel blades where air is extracted for cooling and sealing functions through two ports on the upper half and two on the lower half. Another groove is machined around the 11th Wheel blades where air is extracted for surge protection during transient operation.

The discharge casing is the final portion of the compressor casing. Being the largest single casting, it is the key stone of the gas turbine Structure. It connects the compressor to the turbine section and supports the combustion system. The discharge casing contains last 7 stages of compressor vanes and 2 rows of exit guide vanes (EGVs). The discharge casing consists of outer and inner cylinders. These inner and outer cylinders are connected by means of 12 struts. These struts flare out to meet the large diameter of the turbine shell where providing spacing for transition pieces in between. The inner cylinder is extended to the fwd side by the inner barrel. The inner and outer cylinders of the discharge casing form the compressor diffuser which converts some of the kinetic energy of compressed air into a pressure rise. At the fwd end of the inner barrel a honeycomb seal is installed to make with a labyrinth teeth on the aft stub shaft. A brush seal is running against the rotor’s smooth surface. This arrangement is also known as high pressure packing (HPP). There function of this arrangement is to control the amount of compressed air leakage inside the inner barrel. This air despite been used to cool first 4 wheel spaces, the amount should be minimized to improve unit efficiency and minimize air leakage inside bearing 2 seals. The lower half of inner cylinder supports the bearing number 2 and provides the opening for lube oil supply and drain piping. The upper half of the inner cylinder has an opening for the vent pipe. Seals are installed at these opening to prevent compressed air from escaping inside the inner cylinder. The discharge casing also supports the turbine 1st stage nozzles. The 1st stage nozzle support ring is mounted on the aft end of the inner cylinder. The stationary vanes of the compressor are also air foil shape. The blades of first 8 stages are mounted by dovetail arrangement to ring segment. These ring segments are inserted into circumferential grooves on the casing. The blades of the last 9 stages have square base dovetail and are inserted directly into the circumferential grooves on the casing. 2 rows of exit guide vanes are located at the end of compressor. These stationary vanes help in reducing the rotation of the air flow and increasing the pressure.