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SEMINAR REPORT ON ABOUT THE ORGANISATION
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INTRODUCTION
A simple definition of compressor is a device used to pressurize a fluid including liquids and gases. There are many different kinds of compressors, but the main purpose of using a compressor is to raise the pressure of a gas.
A compressor converts shaft power to raise the enthalpy of the gas. The gas enters the compressor at a low pressure and exits at a high pressure. The rotating shaft is attached to a blade assembly; the rotating blades push on the gas and increase the pressure and thereby increasing the enthalpy. Compressors can be either Positive displacement type or Dynamic type
CLASSIFICATION OF COMPRESSORS
Not too many years ago, it was common practice to use reciprocating compressors when high pressures were required. Dynamic-type machines were used only where larger volumes and lower pressures were involved. Dynamic compressors were usually called blowers when air or gas was compressed to about 40 psig. The term compressor was applied to any such machine where air or gas was compressed to a final pressure over 50 psig. In recent years, this differentiation has become meaningless. Industry now uses the term compressor for all types of machines compressing air or gas
CENTRIFUGAL COMPRESSORS
Centrifugal Compressor is one of the most critical equipment in process industry. As these machines are highly power intensive, efforts towards improving their efficiencies and reliabilities has been the goal of the designer. Considerable improvement in efficiencies of these machines has been possible by incorporating new design concepts and state-of-art technologies in the product and the associated systems. Following are the developments in the centrifugal compressors field
1. Optimal design of impeller geometry adapted for low flow conditions
2. Standard Stage Concept
3. Design of three dimensional impellers
4. Use of dry flexible couplings
5. Use of dry gas seals
6. Use of Low solidity diffuser vanes
Optimal design of impeller geometry adapted for low flow conditions: Considering the requirement of impellers handling low flows especially for high pressure compressors a new chapter in manufacturing is thus opened. This has incidentally helped in extending the range operation of impellers. The basic fact in the design of impellers is that higher the outlet vane angle higher the head coefficient and higher flow handling capacity. This means we have to employ low outlet angles for impeller vanes handling low flows. Generally for vane angles less than 37.5 degrees, we require long channel passages. It is difficult to manufacture these impellers by any traditional methods. The efficiency and head realized in this case are definitely low. External welded impeller manufacturing technology has over come the above difficulties. In this the design of impeller geometry can be aimed for low flows with low vane angles without any sacrifice in efficiency. The operating range with these impellers is more than the impeller employed with a compromise due to manufacturing constraint. Impellers calling for the design with outlet vane angles in the range of 15-18 degree, result in long channel passages, hence appreciably better guidance for the flow.
3MCL Compressors
These are multistage compressors which generally in corporate more than these compression stages in a single casing. As a rule they are used in series where different gas flows have to be compressed the various pressure levels i.e. by injecting and/or extracting gas during compression.
DMCL Compressors
2 compressor stages are arranged in parallel in a single casing. The fact both stages are identical and the delivery nozzle is positioned in the centre of the casing makes this solution the more balanced possible
2.2.8. PCL compressors
For compression of very high volumes, casings having double flow arrangement are used. This reduces the casing size and permits the compressors rotating speed to be maintained within the speed range of steam turbines. So that turbines can be directs coupled to the compressor, eliminating gear boxes. This compressor having two suction and one discharge nozzle
2.3.3. Diaphragms
Diaphragms are in two halves split along the horizontal centerline. Depending upon the characteristics of the machine and depending on the pressure and gases to be handled the raw material its quality is selected. In a machine, diaphragms are
Suction diaphragm
Intermediate diaphragm
Discharge diaphragm
Depending upon the type of construction, diaphragms are:
Cast diaphragms for low-pressure difference.
Milled vanes diaphragms for high pressure.
In a BCL type of compressor, suction and discharge diaphragms are generally cast ones and the intermediate diaphragms are fabricated ones, whereas in low pressure i.e. in MCL type of compressors, all the diaphragms are cast types.
2.4. ASSEMBLY OF THE COMPRESSOR
Assembly of rotor
Preparation of top and bottom pack of diaphragms, each concentrically assembled.
Pinning the upper pack of the diaphragm to the lower pack, to ensure the correct relative position of each to one another.
Dismantle the upper half diaphragm from lower and position them on a concentrically inspection fixture.
Assemble the seals at individual diaphragm with suitable adjustments.
Mount the rotor on concentricity inspection fixture.
Determine the correct position of rotor axially, at which the passage of all impellers and diaphragms will be ideally located with respect to one another.
Mount the one-ring seals and oil seals at the respective position on the rotor.
Assemble the upper half of the diaphragm over the lower, with rotor inside.
Gradually guide the entire diaphragm pack with rotor into the casing using assembly fixture
Assemble the journal bearing
Assemble the thrust bearing.
IMPELLERS
Impeller is the vital rotary part in the functioning of the compressor. The fluid (gas or air) enters the impeller axially at the eye of the impeller and then flows radially out of the impeller. The gas goes through the diffuser to the return channel and further goes into the next impeller. It is one of the most stressed components of the compressor, demanding highly precise manufacturing methods. Each impeller is dynamically balanced and subjected to over speed test. They are mounted, shrink fitted and keyed on the shaft, which is coupled to an external source (generally electric motor). This source imparts the required energy and makes it to rotate.
In an impeller, the energy transferred is in the form of kinetic energy, which is then converted into pressure energy in the diffuser. The pressure ratio of any compressor depends mainly on the impeller diameter, rotational speed and volume flow
2.5.2 Material of Impellers
They are made of low alloy steel which give high mechanical characteristics but with low carbon content to ensure satisfactory welding. This material has (also good for low temperature) having 2% chrome 1% molybdenum and 0.13 to 0.17% carbon.
Given that the weld and surround zone are the same, a phenomenon of intergranular corrosion would appear to occur with higher carbon content. It is for this reason that there is a need to limit the carbon content. The intergranular corrosion weakens the metallurgical bond between grains, which leads to the mechanical degeneration of the material. The presence of carbon in the steel greater than that soluble in the austenitic matrix determines the potential sensitivity of the material to intergranular corrosion.
This is prominent because carbon is principally responsible for the precipitation of the carbides and impoverishment in chromium renders the material sensitive to corrosion.
When the impellers are to be used in corrosive fluids, steel with higher chromium content is used such as X12Cr13 (13% of Cr), in particular corrosive conditions requiring very high strength, steels with still high percentage of chromium as high as 15 - 19% may be required. Where high degree of corrosion and higher stresses are present, recourse can be made to steel of 17-4 PH grade, with 17% of Chromium, and 4% of Nickel (precipitation hardened at lower temperature). The steel with 9% Ni (N9 COGNE) is used for impellers required to operate at very low temperatures up t
. STAGE DESIGN CRITERIA
The purpose of this calculation is:
-Verify that the machine is able to give the required head at the envisaged speed (to define the impeller diameters, vane outlet angle and rotating speed).
-Calculate the outlet impeller width as a function of specific volumes
-Verify the absorbed power.
-Calculate the adiabatic and polytrophic efficiencies.
Evaluation of velocities at nozzle C0 and at 1st impeller eye C1
Calculation of the enthalpy reduction corresponding to the kinetic energy increment:
Δh = (C12- C02)/(2g*427) (cal/kg)
CENTRIFUGAL COMPRESSOR TESTING
Every centrifugal compressor after assembly is subjected to several stringent tests to assess the mechanical performance. Thermodynamic performance test is also conducted whenever specified.
BHEL is manufacturing centrifugal compressor with technique from Nuovo pignone, Italy. So far more than 260 compressors had been successfully manufactured, tested and supplied to cater for various applications for several petrochemical, fertilizer and refinery plants.
As compressors are critical equipment a high quality of testing is required. This is ensured by quality testing methods and practices and modern state of are test facilities. BHEL is equipped with test facilities for carrying out the following tests for compressors and turbines as per international standards API 617, API 612, PTC – 10 and custom specification.
1. No load Mechanical run test.
2. Rotor Insensitivity test.
3. String Mechanical run test.
4. Completed unit test.
5. Thermodynamics performance test.
6. Static Seal/Gas leak test.
7. Full load, Full Speed, Full pressure test.
MRT testing procedure
The compressor can be directly driven by a variable speed steam turbine, or alternatively by a driven arrangement consisting of an electric motor, hydraulic coupling and gear box. By varying the oil let into the hydraulic coupling, it is possible to vary the speed of the secondary shaft from about 500 to 1000 to 3000 rpm. Further speed increase is obtained form the step-up gearbox, which in turn is connected to the compressor through a flexible coupling.
The compressor is tested under vacuum with the help of high capacity vacuum pumps. This will reduce the friction and windage losses and hence the power consumption and temperature rise during the test run. The compressor discharge hood temperature will shoot up, if discharge pipe of the compressor is connected to atmosphere instead of the vacuum line.
During the test rum, the bearings are supplied with oil form a centralized lube oil console. Which will pump oil at the required pressure and temperature. The system consists of twin coolers, high-grade duplex filters and control valves
Full speed, Full pressure and Full load test
This test is conducted is used to known the capability, mechanical performance of the compressor at full load, full speed and full pressure condition. Many times full load tests are prescribed for certain critical applications like hydrogen recycle compressors for Hydro-cracker where the compressors are required to be run by creating operating conditions close to the site. This is achieved by using a mixture of helium and nitrogen
CONCLUSION
Hence the study of centrifugal compressor is performed in B.H.E.L. with the help of the guide allocated to us. In this process we had studied the way the design is selected according to the need of the customers. After the selection of the design all the parts of centrifugal compressor such as impellers, diaphragm, rotor, oil seals, casing and many other parts are prepared. Each parts is further classified in many types each having its specification. The next step is the assembly of all the parts according to the layout. During assembly some minor tests are carried out such as hydraulic test and balancing test and after assembly of the centrifugal compressors all the major tests such as no load test, full load test, seal test and many more are being carried out .these test are carried out so that there is no malfunctioning of the components during the delivery of the compressors. During the production of the centrifugal compressor the main aim of the industry is the better quality and service with a better efficiency.