CENTRIFUGAL COMPRESSORS

ABSTRACT:

BHEL Hyderabad is the only organization in the public sector in India actively involved in manufacturing high-pressure centrifugal compressors for the industrial applications. These are employed for the use in Fertilizer industry, Steel Industry, Refineries, Petrochemical plants, Pipe Line, Gas booster for Gas Turbines etc.

BHEL started manufacturing compressors with technical know-how from Nuovo pignone of italy, the world leaders in compressor technology, BHEL - Hyderabad has been at the forefront of the latest developments in compressor Technology. BHEL’s endeavour is to offer quality products, designed to international standards, through dedicated service. A new addition to the wide range of Centrifugal Compressors is the integrally geared and packed SRL Compressors popularly known as ‘ API 672 Compressors ‘.

TYPES AND FEATURES OF BHEL’S RANGE OF CENTRIFUGAL COMPRESSORS

The entire product range of BHEL compressors is broadly divided into fourteen models of MCL & 2MCL casings and eight models of BCL & 2BCL casings, making it possible to choose the optimum size of casing for any capacity. For higher operating pressures, the casings of BCL & 2BCL type have been further graded into five pressure levels, enabling selection of right casing thickness.

Features of different models are discussed below

A) Horizontally Split Compressor

MCL / 2MCL / 3 MCL / DMCL

These are multistage compressors for low and medium pressures with horizontally split casings. The horizontally split casing is either fabricated from plates or made of steel castings according to the duty of the compressor. The fabricated casing has several advantages and is preferred

over cast construction. The journal bearings and thrust bearings are of the tilted pad type.

MCL TYPE : These compressors can be utilized in fertilizer plants, ethylene plants, lube oil plants, refinery processes, city gas distribution etc.2 MCL TYPE : These compressors are provided with intermediate suction and discharge nozzles.

B) Vertically Split Compressor ( Process )

BCL / 2BCL / 3BCL / DBCL

C) Vertically split Compressor ( Pipeline )

PCL Compressures for high pressures.

D) Integrally geared

SRL 250 Compressors for low and medium pressures

Compressor Model Designation

2 BCL 40 7 / A

2 = No of phases

BCL = Constuctional feature

40 = Nominal Impeller dia ( cm ) 7 = No of Impellers

A = Pressure rating ( Up to 350 Atm )

Following are some of the developments in the recent past in the centrifugal compressors field.

- Multistage horizontal split casing design with design of 3D Impellers to maximize polytropic efficiency.

- Optimal design of impeller geometry adapted for low flow conditions (by slot welding technology for external welded impellers)

- Standard stage concept for optimum selection to suit any specific application.

- Fabrication of Casings- Compressors with high pressure ratios for process air service

( SRL type of compressors)- Compressor for highly corrosive service.- Some more features.

Multistage horizontal split casing design with 3D impellers:

Larger plant capacities call for higher sizes of compressor models and hence increase the project costs. To make the models more competitive a careful study has been made in evolving 3D impellers with aerodynamic flow channels. Especially where the tip mach number is more than 0.85 e.g. for gases having high molecular weight or when operated at low temperatures (say 300 C) it is necessary to design the flow channel more aerodynamically. Any mismatch between the vane orientation and the flow direction results in higher incidence losses and thus affecting the overall efficiency of the machine, resulting in narrow operating range. By analytical study the flow channel can be made smoother which however results in 3 dimensional shape of impeller vane profile.

A computer package program for design of 3D impeller has been acquired from M/s NORTHERN RESEARCH CORPORATION USA. The program gives an optimum set of 3 dimensional co- ordinates for the impeller vane geometry. Acceptable geometry can be arrived at by observing velocity distributions in the passage.

Following are the merits in the design of 3D –impellers.

1. Higher polytropic efficiency:

The following data indicates qualitatively the improvement in efficiency of 3D -impellers over 2D- impellers.

Mach No inlet flow 2D impeller 3D impeller % improvement coefficient efficiency efficiency in efficiency of 3D impeller over 2D impeller

0.85 0.08 75% 81.5 % ( 8.6 )

ii. Higher polytropic head coefficient:

Mach No inlet flow 2D impeller 3D impeller % improvement coefficient head co- head co- efficient efficient

0.85 0.08 0.4875 0.566 16%

From the above, it is clear that because of higher efficiency of 3D impellers the compressor power consumption will be low. The aspect of higher polytropic head combining with higher tip speeds results in less number of impellers for a given compressors ratio. Hence the design of the machine becomes more compact. With the twisted blade geometry at the inducer portion conventional methods are not suitable for manufacturing 3D impellers. These impellers are manufactured by using five axis NC milling machining facility. Another method to produce impellers is by precision casting. Though the manufacturing cost of the impellers are high, the difference in the cost between 2 dimensional impeller design and 3D impeller design gets compensated with in an tear because low operational cost and compactness of the machine in case of 3D impellers.

OPTIMAL DESIGN OF IMPELLER GEOMETRY ADOPTED FOR LOW FLOW CONDITIONS (EXTERNAL WELDED IMPELLERS):

Considering requirement of impellers handling low flows specially for high-pressure compressors a new chapter in manufacturing technology of impellers geometry is thus opened. This has incidentally helped in extending the range of 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 flow. Generally for vanes 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. The steps of the manufacturing technology essentially are the following.

- Milling on vertical NC machines.- Milling of vanes on disc.- Corresponding slot milling on second forged disc.- Welding.- Matching of vane centerline with corresponding slot centerline

within an accuracy of 0.1 mm and fusion of parent material on to the vane material by TIG pulses.

- After the root fusion the remaining groove is filled with weld deposit by manual or automatic means.

FEATURES OF NEW DESIGN:

- Design of impeller geometry can be aimed for low flows with low vane angles without any sacrifice in efficiency (By way of having higher b2/D2 ratio)

- The operating range with these impellers is more than impeller employed with a compromise due to manufacturing constraint.

- Impellers calling for the design with outlet vane angles in the range of 15 to 180, results in long channel passages, hence appreciably better guidance for the flow.

- Because of no weld deposit in the gas passages the flow channels are clean. The improved surface finish has its own contributions to the overall efficiency.

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STANDARD STAGE CONCEPT:

Centrifugal compressors belong to the ‘ Tailor made ‘ category product. However keeping in view of short delivery requirements cycles for

the product and availability of powerful computer systems led to possibility of standardization of no. of components. In this effort the main focus is on the basic element of the compressor i.e. the impeller with diaphragm known as a stage.

The variety of industrial gases to be handled with molecular weights ranging from 3 to 120 makes the design more complex. Molecular weight is a very important factor affecting the operational speeds, geometry of impeller etc.

Standard impeller are based on the following philosophy:

- Inlet capacity coefficient has been split into different ranges.- For each range of capacity coefficient a family of impeller geometry is developed with different outlet angles yielding different levels of head coefficient.- Series of impeller geometry in a family developed by further splitting the range of capacity coefficients. Acceptable geometry of the impeller is arrived at observing the basic necessities of relative velocity ratios and minimum disturbance of the flow angle with respect to the vane angle.

Impeller family Range of inlet capacity Outlet vane angleIdentification coefficient x 10-4 of the impeller Geometry

B 189 – 1275 580

Q 180 – 564 450 F 43 – 210 150 – 160 - 170 Effective selections can be made by processing through computer by varying the parameters such as impeller diameter, speed, family of impeller group etc. The selection is accepted on the criteria of power absorbed and the range of operations.

Following are the merits in this concept.

- Storing of mass data of impeller geometry on computer system makes it convenient to generate different kinds of information required for predicting the critical speed of the machine, stresses in the impellers, vibrational amplitude. Through a plotter program it is possible even to generate the cross-section of the compressor stages.

- Selection of the impeller geometry is made, for which the performance is established through actual tests or extrapolated from the test results of the similar family. Thus makes the performance predictions more reliable.- Engineering cycle time is reduced to a greatest extent because of the fact pre-planning can be done for the engineering documents, tooling schedules preparation.- Advance planning of raw materials like forgings and castings with minimum allowances for final finishing.

These impellers can be made by any one of the conventional methods viz. vane welding type, milling and welding type, electro erosion and the slot welding process.

COMPRESSORS WITH HIGH PRESSURE RATIO IMPELLERS( SRL COMPRESSORS ):

Single and multistage compressors for low and medium pressures, are mainly used as blowers or boosters in industries, refineries and petrochemicals plants, when large volumes of gases have to be handled at low pressures and a packaged, integrally geared design is required for compactness and economy.

New series of SRL Compressors have been developed for process air requirement with high-pressure ratios. The new series of compressors use independent impellers, each of one being driven at its own optimum speed by means of an integral bull gear and pinion assembly. Overall pressure ratio with air can exceed 20:1.

These compressors can be designed and manufactured for supplying air-to-air separation unit.

COMPRESSORS FOR HIGHLY CORROSIVE SERVICE:

In addition to the introduction of the latest developments enumerated above a new compressor has been developed to handle highly corrosive gas. This is specifically significant, as it is a case of imported substitution.The cross section of the compressor is a single stage over hung design since the pressure head required to be developed is low. The major components are made of austenite stainless steel material. The compressor is provided

with single set of oil seals arrangement to seal the gas leaking from the shaft ends to atmosphere. An auxiliary seal oil system is provided to perform the duty. Bearing housing is rigidly clamped to the casing with suitable locations in the casing.

SOME MORE FEATURES:

- Provisions of two kinds of sealing at the shaft ends for the compressors handling cryogenic fluids are the recent necessities.Mechanical seal arrangement to minimize the shaft end leakages of gas to outside atmosphere to the bare minimum during normal run of the compressor.- For compressors handling toxic gasses tight shut off seals is one of the recent requirements. These seals arrest the gas leakage to outside atmosphere. When the unit is not in operation and where it is not afford to depressurize the system by venting.

COMPUTER AIDED DESIGN OF CENTRIFUGAL COMPRESSORS

It has often been said that the computer age will do for man’s mind what industrial revolution did for his muscle. High speed and high accuracy in problem solving made them an important design tool. Computer aided design ( CAD ) is a technique in which man and the computer are blended into a problem solving team, intimately coupling the best characteristics of each, so that this team works better than either alone. The main benefits that accrue out of CAD are- Reduction in design cycle time- Reliability of equipment- Design optimization

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APPLICATION OF CAD TO CENTRIFUGAL COMPRESSORS

Centrifugal compressors occupy an important position in process plants and they must be extremely reliable. Reliability of the equipment is built at the design stage itself. The design of compressor involves right from the evaluation of thermodynamic properties to rotor synchronous response complex mathematical equations. Also the calculation procedure is highly iterative. To carry out these calculations manually is not only time

consuming but error prone. Application of CAD overcomes these short comings. The important aspects of computer program used in the design of centrifugal compressors are described below.

SOFTWARE FOR EVALUATING THERMODYNAMIC PROPERTIES OF GASES

One of the most important aspects in the design of centrifugal compressors is the correct evaluation of thermodynamic properties of gases or gas mixtures being handled. Till recently Mollier Charts were used for the above purpose. This method is tedious and time consuming. More over, it is difficult to obtain Mollier charts for gas mixtures.

To over come this difficulty an equation of state is used. An equation of state is a functional relationship between pressure, temperature and volume. An equation of state, theoretical or empirical makes possible easy interpolation ( and possible extrapolation). It condenses the availability PVT data to a great extent. Using and equation of state, the errors involved in the evaluation of derivatives and integrals are reduced, as taking slopes by graphical differentiation involve large errors and are tedious. The accuracy depends on how well the equation of state represents the PVT data.

The Benedict-webb-Rubins (BWR) equation of state is the best available in the field of gas and liquid for light hydrocarbons. This equation of state has 8 numerical constants varying for different gases. To predict more exactly the properties of simple components Starlings modification of BWR equation is used. This equation has 11 numerical constants.

For non-hydro carbons and hydrocarbons for which the eight coefficients of BWR equation are not available the generalized BWR euation as given by Cooper and Gold Frank is used. For gas mixtures the coefficients are obtained by mixing rules given by BISHNOI & BOBINSON.

A computer program based on above equations of state is being used. The input to the program consists of gas composition and ranges of pressure, temperature over which the properties are required. The graphical output of this program is used for the purpose.

SOFTWARE FOR SELECTION OF COMPRESSOR STAGES

Centrifugal compressors of BHEL-NP design employ standard tested stages. A stage is a combination of impeller, vane less diffuser and return vane channel. Depending upon the impeller blade outlet angle the stages are divided into different families as B,Q, F,G and H. In each family of impeller there are different types of stages. The polytropic efficiency of an impeller is expressed as a polynomial of inlet flow coefficient with 7 numerical constants. Each of these constants as a function of family, type, diameter and tip speed Mach number of impeller.

The head coefficient is expressed as a polynomial of product of inlet flow coefficient and specific volume ratio at inlet and outlet. The polynomial consists of 4 coefficients each of them is a function of family, type, diameter and tip speed Mach number of impeller.

From the tested data of a set of impellers in each family, the values of the above said coefficients are evaluated. This data is available as a direct access file on the computer. Part of the program consists of subroutines to evaluate gas properties.

The input program consists of the following

i) Inlet conditions of gasii) Discharge pressureiii) Gas compositioniv) Approximate speed and no. of stages in the trainv) Pressure drop across intercoolersvi) Impeller diametervii) Injection and extraction conditions

The program then selects the type of stages evaluates speed, phase polytropic efficiency and total power.

SOFRWARE FOR DESIGN OF 3-DIMENSIONAL IMPELLERS

For impellers operating at high tip Mach numbers ( above 0.9) it is necessary to match the blade angle with flow angle to minimize losses. This results in a twisted impeller blade. To carry out the design of 3-dimensional impellers, a computer program developed by Northern research Engineering Corporation (NREC) is being used. The program calculates the relative velocity distribution along eight streamlines from hub to shroud based on

prescribed loading method. The major input parameters are hub & shroud contours, blade thickness distribution, swirl distribution, blade blockage factor and outlet angle.

Output is in both tabular and graphical form giving the relative velocity distribution along 8 streamlines and blade shape in the form of line elements.

SOFTWARE FOR CALCULATION OF STRESSES IN CENTRIFUGAL IMPELLERS

The state of art in the present day centrifugal compressor design is to go in for high tip speeds. Calculation of stresses in the impeller is the basis for selecting the impeller material, type of heat treatment and interference for shrink fitting of impeller on to the shaft. A program based on the procedure given by B.P.C.H.O is being used.

The input to the program is the description of the impeller, tip speed and blade thickness distribution, blade root stresses, extension and deflection at various radii.

SOFTWARE FOR ROTOR DYNAMICS

An important part of the standard design procedure for a rotor is the calculation of its critical speeds. As a first approximation undamped critical speeds are calculated using PROHL’method. In this method isotropic supports are considered. A rather more satisfying approach to the flexural analysis of the rotor consists in introducing damped in the vibration system and considering anisotropic linear supports. Such a model enables one to face the following problems.

a) Study of stationary vibrations due to unbalances that are present in the rotor. These vibrations are synchronous with the rotor rotation.

b) Study of stationary vibrations due to the presence of one or more forces, with constant intensity and all of them rotating uniformly at the same speed, generally different from rotor speed.

c) Study of natural damped mode of vibration of the rotor and calculation of stability parameter. Stability measures the attitude of

the very mode towards damping. A small or even negative value of this parameter indicates that the system is unstable.

With only one computer program, that was developed for this purpose, one can face three types of problems previously described. Most of the formulae used in the program were drawn from the theory that is illustrated by J.W.LUND. The input to the program consists of physical description of the rotor, geometric characteristics of bearing, speed range over which synchronous response is required.

Program output consists of bearing characteristics (stiffness and damping coefficients) vs sommerfield number, amplitude of vibration at various speeds.

Torsional critical speeds for the entire train is carried out by means of a program based on Holzers method.

SOFTWARE FOR PLOTTING OF DIAPHRAGM RETURN VANE CHANNEL AND FAMILY OF IMPELLERS:

The flow entering the intermediate impeller of a compressor is made radial by deswirl vanes located upstream of the impeller.

Inlet angle of the vanes is obtained based upon the conditions of flow at the impeller outlet. The flow area varies linearly from inlet to outlet of return vane channel. A program based on above considerations calculates the vane profile.

The complete data pertaining to different types of impellers of a family is available as a direct access file. A computer program facilities plotting of complete series of impellers in a given family for a given diameter.

Accurate evaluation of thermodynamic properties, critical speeds, stresses contribute to the reliability of equipment. The next step is to generate component drawings and necessary information for machining on CNC machines.

COMPRESSOR COMPONENTS:

- Casing- Diaphragm- Rotor- End covers for barrel construction design - Sealing system

CASING:

The construction features of the two models in use are- Horizontally split casing design- Vertically split casing design

Horizontally split type 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. The MCL, 2MCL, 3MCL and DMCL compressors are of horizontal split design.

Vertically split type 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. The BCL, 2BCL, DBCL, 3BCL and PCL type casings are of the vertical split design.

DIAPHRAGMS :

The function of the diaphragm isi) To form the dynamic flow path of the gas inside the compressor. ii) To form the separation wall between one Compressor stage and the

subsequent one.iii) To convert the kinetic energy of the gas leaving the impeller into

pressure energy.

They are of three types.

1. Suction diaphragm2. intermediate diaphragm3. Discharge diaphragm

The diaphragm are 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.

ROTOR:

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.

SHAFT:

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.

IMPELLER:When the impeller is rotating at high speed, air is drawn through the

eye of the impeller. The absolute velocity of the inflow air is axial. The magnitude and the direction of the entering relative velocity depend upon the linear velocity of the impeller at the radial position of the eye considered, as well as the magnitude and the direction of the entering absolute velocity. The impeller vanes at the eye are bent to provide shock less entrance for the entering flow at its relative entrance angle. The air then flows radially through the impeller passages due to centrifugal force. All the mechanical energy driving the compressor is converted into kinetic energy, pressure and heat due to friction. The purpose of the diffuser/diaphragm is to convert the kinetic energy that leaves the impeller into pressure. The air leaving the diffusers is collected in a spiral passage from which it is discharged from the compressor.

Impellers are most stressed components of the compressors demanding highly precise manufacturing methods. Impellers are identified depending on the methodology used during manufacturing. The different types of impellers, which are being manufactured, are:

1) Welded type of impeller

Welding technology is adopted for the impellers having gas passage width more than 30 mm. In this type of impeller, disc and counter disc are machined out of two separate forgings and vanes are bent to the required shape out of plate. Vanes are welded to the disc and counter disc from inside, followed by stress relieving, testing of welded parts, heat treatment and machining to correct profiles.

2) Milled & welded type of impeller (Internal welded type)

This technology is adopted for the impellers having gas passage width varying from 7 to 30 mm. In this type of impellers, vanes are milled on to

the disc (or counter disc) and then counter disc (or disc) is welded to the disc followed by heat treatment, testing, finish turning and balancing. The critical operation involved is the milling of vanes on disc (or counter disc) by special 3 dimensional milling machine.

3) Milled & welded type of impeller (External welded type)

This is the latest technology, which has been adopted for impeller manufacturing. The impellers having low outlet angles, small radius of curvature of vanes and narrow gas passages, could not be manufactured as mentioned above because of the limitations involved in the impeller manufacturing techniques. Subsequently, new technology is developed established i.e. External welded impellers.In this type the impellers are manufactured out of two separate forgings for disc and counter disc. The critical operations involved in manufacturing of external welded impellers are

i) Machining of vanes on counter disc and corresponding blind grooves on disc.

Milling is carried out on CNC machine center. This operation is critical in the sense that there should be perfect matching of the axis of the blind groove on disc and axis of vanes on the counter disc. This is being ensured by drilling peepholes on every blind groove.

ii) Welding of disc on to the counter disc

Disc is diametrically located on to the counter disc, coaxiality of groove and vanes axis is ensured through peepholes. The job is preheated and gas passages are closed by seal welding circumferentially to avoid the leakage of inert gas. Job is mounted on the special welding fixture, vanes curvature is centered w.r.t manipulator center and clamped. Root welding is done by TIG by fusing disc material on to the vanes and subsequently metal is filled up externally in the groove of disc by TIG pulse. This operation will be carried out special automatic welding machine having pulse generator and a special fixture.

4) Electro- eroded impeller:

This method is used where welding technology is not suitable for the impellers of small width gas passages. The impellers are manufactured out of a single forging. Gas passages are eroded in the forging & vanes are created by adopting EDM (Electrical discharge Machining) technique. The critical operation in manufacturing this type of impeller is the Electro-erosion of gas passages. Electrodes are machined to the shape similar to that of gas passages, and fed vertically downwards through a pilot hole. The pilot hole guides the electrode. Material is eroded by spark discharges between the electrode & the job across a gap usually filled with die electric medium and gas passages are created.

END COVERS :

BCL type of casing are closed at both ends with end covers which has got integral bearing and seal housing. End covers are always manufactured out of forgings.