Intro to Centrifugal Compressor Components
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CENTRIFUGAL COMPRESSOR COMPONENTS 1. Casing For high pressure compressor, it need outer and inner casing to support its pressure. Commonly, the nozzles are at the outer casing and inner parts such as diaphragm, impeller and shaft are at the inner casing. Inner casing can be opened as top inner casing and bottom inner casing in order to install the parts. Outer Casing Most compressor manufacturers have standard castings or forgings of their various housings and impellers from which they make up a compressor for a particular set of operating conditions. Inner Casing The inner casing have the parts been installed. During final fabrication, the inner casing will be inserted into the outer casing (housing) which connect all the inlet/outlet nozzles. The casing size limits the flow, which can be passed through it while the impeller castings limit the maximum and minimum head. The construction features of the two models in use are: i. Horizontally split casing design The horizontal plane in the middle and consists of an upper and lower part. All necessary connections, such as suction and discharge nozzles, intermediate suction and discharge nozzles, wherever required, and lube oil inlet and drain connections are integral with the lower half. Internal parts can be accessed just by lifting the upper part which needs no major dismantling of piping. For inspection of bearings, there is no need to remove the upper half. Only bearing cover removal is adequate.
Figure 1: Horizontally Split Casing
Vertically split casing design These are used when the working pressure and type of gas demand such an arrangement. All internal parts are similar to the horizontally split type casing, but the diaphragm seals and the rotor bundle are inserted axially in a forged steel barrel casing. Ends are closed with end covers, the lower half of the bearing housing is integral with the end cover. By removing the end cover, it is possible to withdraw the complete internal assembly and have access to the internals like seals, diaphragms and rotor,
without disturbing the outer casing. There is no need to remove end covers for bearing inspection.
Figure 2: Vertically Split Casing
2. Diaphragms The function of the diaphragm is: y y y i. ii. iii. To form the dynamic flow path of the gas inside the compressor. To form the separation wall between one Compressor stage and the subsequent one. To convert the kinetic energy of the gas leaving the impeller into pressure energy.
They are of three types: Suction diaphragm intermediate diaphragm Discharge diaphragm
The diaphragm is generally made of cast steel. However, based on operating conditions, alloyed cast iron, forged steel or stainless steel materials are also used. In small and medium size casings, the diaphragms are fabricated from plates.
Figure 3: Interstage Seals Diagram
3. Rotor Assembly The basic function of the centrifugal compressor rotor is to impart the required compression energy to the gas. The rotor forms the heart of the centrifugal compressor, consisting of the shaft, Impellers, spacers, bushes, Balancing drum, thrust collar, Coupling hub and thrust bearing. The impellers are hot shrunk and keyed. The shrinking of impeller and balancing piston is necessary to ensure that the impeller does not get slackened due to the centrifugal forces during start up and normal running of the compressor. This would otherwise result in vibrations on the rotor system. Rotor must perform its function with a deflection less than the minimum clearance between rotating and stationary parts. The loads involved are the torques, the weight of the parts, and axial gas forces. The rotor, during assembly is balanced stage wise. The rotor assembles with some equipment and parts as shown below.
Figure 4: Rotor Assembly
a. Impellers An impeller in centrifugal compressor imparts energy to a fluid. The impeller consists of two basic components: i. An inducer such as an axial-flow rotor ii. The blades in the radial direction where energy is imparted by centrifugal force There are three types of impellers. The differences are as shown below: Type of impeller Radial blades Advantages 1. Reasonable compromise between low energy transfer and high absolute outlet velocity 2. No complex bending stress 3. Ease in manufacturing 1. Low outlet kinetic energy 2. Low diffuser inlet Mach number 3. Surge margin is widest of the three High energy transfer Disadvantages Surge margin is narrow
Backward curved blades Forward curved blades
1. 2. 3. 1. 2.
Low energy transfer Complex bending stress Difficulty in manufacturing High outlet kinetic energy High diffuser inlet Mach number 3. Complex bending stress 4. Difficulty in manufacturing
Figure 5: Semi-open Impellers Diagram
b. Shafts The shaft is made out of forged alloy steel and the impellers, spacers and the balancing drum are shrunk fitted on it. Spacers of stainless steel material are used to protect the shaft against gas erosion and corrosion. The shaft is made by turning and grinding operations. Journal bearing zones of the shaft is ground and burnished with the diamond burnishing technique to improve the surface finish and to keep the total run outs within the permissible limits.
Figure 6: Rotor Shaft
c. Bearings The bearings in turbomachinery provide support and positioning for the rotating components. Radial support is generally provided by journal or roller bearings, and axial positioning is provided by thrust bearings. i. Radial bearings The heavy frame type gas turbines use journal bearings. Journal bearing may be either full round or split; the lining may be heavy, as in large-size bearings for heavy machinery, or thin, as used in precision insert-type bearings in internal combustion engines.
Figure 7: Radial Bearings
Thrust bearings The most important function of a thrust bearing is to resist the unbalanced force in a machine s working fluid and to maintain the rotor in its position (within prescribed limits).
Occasionally this type of bearing is used for light loads (less than 50lb/in) and in these circumstances, the operation is probably hydrodynamic due to small distortions present in the normally flat bearing surface.
Figure 8: Thrust Bearings
Figure 9: Bearing Diagram
4. Seals Seals are very important and often critical components in turbomachinery, because they spin at very high speed. In order to prevent frictions and wears, seals play the most important roles for these matters. i. Labyrinth seals The labyrinth is one of the simplest of sealing devices. It consists of a series of circumferential strips of metal extending from the shaft or from the bore of the shaft housing to form a cascade or annular orifices. The major advantages of labyrinth seals are their simplicity, reliability, tolerance to dirt, system adaptability, very low shaft power consumption, material selection flexibility, minimal effect on rotor dynamic, back diffusion reduction, integration of pressure, lack of pressure limitations, simple to manufacture and tolerance to gross thermal variations.
Figure 10: Labyrinth Seals
Mechanical seals A typical mechanical seal has two major elements. They are the oil-pressure-gas seal and breakdown bushing. This seal normally have buffering via a single ported labyrinth located inboard of the seal and a positive shutdown device which will attempt to maintain gas pressure in the casting when the compressor pressure is at rest and the seal oil is not being applied.
Figure 11: Mechanical Seal
Dry gas seals The use of dry gas seals in process gas centrifugal compressors has increased over the last thirty years, replacing traditional oil film(mechanical) seals in most applications. Dry gas seals are basically mechanical seals consisting of a mating ring, which rotates, and a primary ring, which is stationary. As gas enters, it is sheared towards the gap to act as seals. During normal operation, the running gap is approximately 3 microns.
Figure 12: Dry Gas Seals