Engineering Design Guideline- Compressor Rev2

5/28/2018 Engineering Design Guideline- Compressor Rev2

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Practical EngineeringGuidelines for Processing

Plant Solutions www.klmtechgroup.com

March 2008

Aiut hor :

Ai L LingKLM Technology Group#03-12Block Aronia, RiverriaCondovilla,Taman Tampoi UtamaJalan Sri Perkasa 281200 Johor Bahru .

COMPRESSOR SELECTION ANDSIZING

(ENGINEERING DESIGN GUIDELINE)

Checked by:

Karl Kolmetz

TABLE OF CONTENT

INTRODUCTION

Scope

Compressor

(A) Dynamic Compressor

(I) Centrifugal Compressor

(II) Axial Flow Compressor

(B) Positive-Displacement Compressor 1

(I) Rotary Compressors 1

(II) Reciprocating Compressors 1

Design Consideration 1

DEFINITIONS 1

NOMENCLATURE 1

THEORY 1

(A) Selection of Compressor 1

(B) Compression Process Theory 2

(I) Ideal or Perfect Gas Theory 2


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Practical EngineeringGuidelines for Processing Plant

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COMPRESSOR SELECTION AND SIZING(ENGINEERING DESIGN GUIDELINE) March 2008

These design guideline are believed to be as accurate as possible, but are very general and not for specific design caseThey were designed for engineers to do preliminary designs and process specification sheets. The final design must alwabe guaranteed for the service selected by the manufacturing vendor, but these guidelines wi ll greatly reduce the amount of front engineering hours that are required to develop the final design. The guidelines are a training tool fo r young engineers a resource for engineers with experience.

This document is entrusted to the recipient personally, but the copyright remains with us. It must not be copied, reproducor in any way communicated or made accessible to third parties without our written consent.

(II) Compression Cycle 2

(III) Compression Stages 3

(IV) Reciprocating Compressor Sizing 3

(a) Sizing Procedure 3

(b) Theoretical Maximum Capacity 3

(c) Approximate Horsepower 3

(d) Compressor Displacement and Clearance 3

(e) Cylinder Bore Requirements 3

(f) Frame Load 4

(V) Dynamic Compressor- Centrifugal Compressor Calculation 4

APPLICATION

Example 1: Reciprocating Compressor Sizing- Pure gas Methane 4

Example 2: Reciprocating Compressor Sizing- Mixture of Hydrogen

REFEREENCES 5

SPECIFICATION DATA SHEET6

English Unit

Metric Unit 6

CALCULATION SPREADSHEET

Reciprocating Sizing Calculation Spreadsheet (Excel Format) 6


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Figure 9.5: The Piston at Full Expansion Stroke 3

Figure 10: Typical Clearance vs Cylinder Bore 4

Figure 11: Size of cyl inder vs. rated discharge pressure 4


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INTRODUCTION

Scope

This design guideline covered the selection and sizing methods of compressors used in thtypical refining and chemical industrials. It assists engineers to understand basic design ocompressors and increase their knowledge in selection and sizing.

The selection section includes the explanation for the suitability and type of compressorfor specific process, depending on their delivered head and capacity. The section alsincludes the comparison of differences for axial compressor, centrifugal compressor anreciprocating compressors.

All the important parameters used in the guideline are explained in the definition sectiowhich helps the reader understand the meaning of the parameters or the term used.

The theory section included sizing theory and formulations for the reciprocating compressoand centrifugal compressor designs.

In the application section, two cases examples are included to guide the reader by usinsimplifier step-by-step method to do reciprocating compressor sizing base on feed streamproperties.

In the end of this guideline, example specification data sheet is included which is createbase on industrial example. Calculation spreadsheet is included as well and to aid usemore understand to apply the theory for calculations.


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Compressor

A compressor is a piece of equipment that compresses gas, used to handle largvolumes of gas at pressure increase from 1.5 psi to several thousand pounds pesquare inch.

Compressor are manufactured depending on application and can be class into three maicategories; positive-displacement, dynamic and thermal type. In this guideline the majotwo categories compressors will be discussed which positive-displacement and dynamtype.

(A) Dynamic Compressor

Dynamic compressor or continuous-flow compressors are machines that the flow continuous. These include centrifugal compressors and axial flow compressors. Thestypes of compressor are widely used in chemical and petroleum refinery industry fospecifies services. They are also used in other industries such as the iron and steeindustry, pipeline booster, and on offshore platforms for reinjection compressorsCompared to positive displacement type compressor, dynamic compressor are mucsmaller in size and produce much less vibration.

(I) Centrifugal Compressor

In centrifugal compressor, the gases flow enters the impeller in an axial direction and exitin a radial direction. The gas fluid is forced through the impeller by rapidly rotating impelleblades. The kinetic velocity energy from rotating impeller is converted to pressure energypartially in the impeller and partially in the stationary diffusers. The diffuser consists of vaneless space, a vane that is tangential to the impeller, or a combination of both. Thesvane passages diverge to convert the velocity head into pressure energy.

Base on application requirement, centrifugal compressors can be design for single-stagmachines, either direct-driven or geared (Figure 1), and or are design for executed i

multistage configuration (Figure 2). Both single- and multistage machines are generallmade up of standardized components. There are two principal casing types: (1) horizontallsplit casing and (2) vertically split casing (barrel-type compressors). The nozzlconfigurations can be selected over a wide range.


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Typical centrifugal for the single-stage design can intake gas volumes between 100 t

150,000 inlet acfm. A multi-stage centrifugal compressor is normal considered for inlevolume between 500 to 200,000 inlet acfm. It designs to discharge pressures up to 235psi, which the operation speeds of impeller from 3,000 rpm to higher. There have limitatiofor velocity of impeller due to impeller stress considerations, the limitation velocity of 0.8 t0.85 Mach number at the impeller tip and eye. Centrifugal compressors are designed drivby electrical motor, steam turbine, or gas turbines.

Figure 1: Single-stage centrifugal compressor.

Impeller

Seals

Casing Inlet Guide

Vane

Bearings

Diffuser Vanes

Gears


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Figure 2: Multi stage centri fugal compressor.

(II) Axial Flow Compressor

Axial flow compressors are used mainly as compressors for gas turbines. They are used ithe steel industry as blast furnace blowers and in the chemical industry for large nitric aciplants.

Compare to other type of compressor, axial flow compressors are mainly used foapplications where the head required is low and with the high intake volume of flow. Theare available in sizes producing pressures in excess of 100 psi at intake volumes betwee23,500 to 588,500 acfm.


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The component of axial flow compressor consist of the rotating element that construct from

a single drum to which are attached several rows of decreasing-height blades having airfocross sections. Between each rotating blade row is a stationary blade row. All blade angleand areas are designed precisely for a given performance and high efficiency. The use omultiple stages permits overall pressure increases up to 30:1. The efficiency in an axial flowcompressor is higher than the centrifugal compressor.

The operation of the axial flow compressor is a function of the rotational speed of thblades and the turning of the flow in the rotor. The stationary blades (stator) are used tdiffuse the flow and convert the velocity increased in the rotor to a pressure increase. Onrotor and one stator make up a stage in a compressor. One additional row of fixed blade(inlet guide vanes) is frequently used at the compressor inlet to ensure that air enters th

first stage rotors at the desired angle. In addition to the stators, another diffuser at the exof the compressor further diffuses the gas and, in the case of gas turbines, controls itvelocity entering the combustor.

Driver of axial flow compressor can be steam turbines or electric motors. In the case odirect electric motor drive, low speeds are unavoidable unless sophisticated variablefrequency motors are employed.


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(B) Positive-Displacement Compressor

Positive displacement compressors can be divided into two types rotary compressors anreciprocating compressors. In all positive displacement machines, a certain inlet volume ogas is confined in a given space and subsequently compressed by reducing this confinespace or volume. At this now elevated pressure, the gas is next expelled into the dischargpiping or vessel system.

(I) Rotary Compressors

Rotary compressors are machines of the positive-displacement type. Such units aressentially constant volume machines with variable discharge pressure. The volume can bvaried only by changing the speed or by bypassing or wasting some of the capacity of thmachine. The discharge pressure will vary with the resistance on the discharge side of th

Figure 3: Axial-flow compressor. (Allis-Chalmers Corporation.)


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system. Rotary compressors are generally classified as of the straight-lobe type, screw

type, sliding-vane type, and liquid-piston type.

(II) Reciprocating Compressors

Reciprocating compressors are used mainly when high-pressure head is required at a lowflow. Reciprocating compressors are furnished in either single-stage or multistage typesGenerally, the maximum allowable discharge-gas temperature determines the maximumcompression ratio.

On multistage machines, intercoolers are provided between stages. These hea

exchangers remove the heat of compression from the gas and reduce its temperature tapproximately the temperature existing at the compressor intake. Such cooling reduces thvolume of gas going to the high-pressure cylinders, reduces the power required focompression, and keeps the temperature within safe operating limits.

Typical compression ratios are about 3 per stage to limit discharge temperatures tperhaps 300

oF to 350F. Some reciprocating compressors have as many as six stages, t

provide a total compression ratio over 300.


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Figure 4: Low pressure cylinder with double acting piston. (Cooper Industries)


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Design Consideration

Operating Condition

(I) Fluid properties:-

a. Gas Composition- In design of compressor, full gas compositions data are verimportant. It should be analysis and listed in compressor specification sheet witeach component name, molecular weight, boiling point. This data are importandetermined the volume flow rate of the compressed gas, average moleculaweight, compressible factor, and specific heat ratio.

b. Corrosiveness-Corrosive gas stream constituents must be identified for aoperating conditions including transients. This important because corrosion gas awet H2S in compression service can cause stress corrosion cracking of higstrength materials.

c. Fouling tendency- The compressor design specification sheet should include thfouling tendency of the gas and specify compressor flushing facilities if requiredThis is an important parameter for the selection of the type of compressor design

Axial and high speed, single stage centrifugal is not suitable for fouling serviceFlushing allows helical screw and conventional centrifugal compressors to bused in a fouling service.

d. Liquid in gas stream-Liquid in the gas stream should be avoided because iharmful to compressors. For feed stream into compressor that content liquid igas stream, liquid separators and heat tracing and insulation of compressor inlelines should be provided when ambient temperature is below the dew point of thgas at the compressor inlet or when handling hydrocarbon gas componentheavier than ethane.

e. Inlet pressure Gas stream inlet pressure should be specified in compressospecification sheet for the lowest value; this is to meet the guarantee performancof compressor. The allowance pressure drop of 0.3 psi in through compresso

inlet hood, screen, filter and piping should be expected.

f. Discharge pressure - Centrifugal and axial compressors, the discharge pressurspecified is at the discharge flange. Meanwhile reciprocating and rotarcompressors, the specification should note that the discharge pressure specifie


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is downstream of the pulsation suppression device for reciprocating compressor

and downstream of the discharge silencing device on rotary compressors.

g. Inlet temperature- For gas stream temperature is affects the volume flow ratecompression service head requirements, and required power. Because of thinlet temperatures for compression process should be specified full range.

h. Discharge temperature- Discharge temperature of Compressor is affected by inletemperature, pressure ratio, gas specific heat values, and the efficiency ocompression. This temperature is important in determining compressomechanical design, gas fouling tendency, process compression stage selection

and cooler and piping design.

(II) Mechanical design of compressor:-

a. Casing & cylinder- Maximum allowable working pressure is much parameter listein compressor specification sheet. This data together with maximum allowabltemperature are use for compressor manufacturer to fabricate the casing cylinder that can withstand for MAWP & maximum allowance temperature ohigher. For centrifugal and axial compressors, the casing MAWP is computed b

adding the maximum inlet pressure to the maximum differential pressure that thcompressor can develop at the most severe combination of conditions. Meanwhilfor reciprocating compressor cylinders and screw compressor casings, the MAWshould exceed the rated discharge pressure by 10 percent or 25 psi, whichever igreater.

b. Casing & cylinder- Maximum allowable temperature for the centrifugal or axiacompressor should be the maximum discharge temperature anticipated over thcompressor speed and capacity range and include a margin of approximate

50F. For reciprocating compressor cylinders and screw compressor casings, thspecified maximum allowable temperature should exceed the rated discharg

temperature by 50F.

c. Piping flanges and rating Connection of piping line sizes, flange rating anfacing for all compressor inlet and outlet should be indicated in desigspecification sheet.


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d. Shaft and piston rod sealing The type of shaft or piston rod seal and supportin

auxiliary should be specified

e. Lube oil and seal oil system -The function of these systems is primarily to providan uninterrupted supply of clean and cooled fluid to the bearings and seals of thcompressor, gear and driver. These are important systems for compressors, thusystem design of these systems should be clearly stated in design specificatiosheet.

(III) Process compression stages

Compression ratio is the relation of discharge pressure (P2) over the suctiopressure (P1) for a compressor, P2/P1. For the high-pressure compression servicethe compressor is design for multiple process compression stages and sometime thcoolers are included between the stages to remove the heat of compressionReasons for providing process compression staging are:

a. To limit the discharge temperature of each stage to acceptable levels from thstandpoints of both compressor design restraints and the fouling tendency of thcompressed gas.

b. To make side streams available in the compression sequence at intermediat

pressure levels such as in process refrigeration systems.

c. To reduce compressor stage" inlet temperatures thereby reducing the amount owork (head) required to achieve a given pressure ratio.

d. To satisfy differential pressure and pressure ratio limits of various compressotypes, e.g., axial thrust load limitations for centrifugals and axial, piston rod streslimits in reciprocating compressors, and rotor lateral deflection and axial thrust iscrew compressors.

e. Provide the condition for include intercooler between stages, that will help reduc

horsepower require for compression, and keeps the temperature within safoperating limits.


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DEFINITION

Adiabatic / Isentropic This model assumes that no energy (heat) is transferred to ofrom the gas during the compression, and all supplied work is added to the internal energof the gas, resulting in increases of temperature and pressure.

Isothermal- This model assumes that the compressed gas remains at a constantemperature throughout the compression or expansion process. In this cycle, internaenergy is removed from the system as heat at the same rate that it is added by thmechanical work of compression. Isothermal compression or expansion more closelmodels real life when the compressor has a large heat exchanging surface, a small gavolume, or a long time scale (i.e., a small power level). Compressors that utilize inter-stag

cooling between compression stages come closest to achieving perfect isothermacompression. However, with practical devices perfect isothermal compression is noattainable. For example, unless you have an infinite number of compression stages witcorresponding intercoolers, you will never achieve perfect isothermal compression.

Axial ly spli t-A joint that is parallel to the shaft centreline.

Bearing Is a device to permit constrained relative motion between two parts, typicalrotation or linear movement. Compressors employ at least half a dozen types of journabearings. Essentially all of these designs consist of partial arc pads having a circulageometry.

Blades- Rotating airfoils for both compressors and turbines unless modified by aadjective.

Impeller -Is a rotor inside a shaped housing forced the gas to rim of the impeller tincrease velocity of a gas and the pressure in compressor.

Inlet volume flow- The flow rate expressed in volume flow units at the conditions opressure, temperature, compressibility, and gas composition, including moisture content athe compressor inlet flange.Maximum allowable working pressure (MAWP) - This is the maximum continuoupressure for which the manufacturer has designed the compressor when it is operating aits maximum allowable temperature.

Maximum allowable temperature The maximum continuous temperature for thmanufacturer has designed the equipment.


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Maximum inlet suction pressure The highest inlet pressure the equipment will bsubject to in service.

Normal operating condition The condition at which usual operation is expected andoptimum efficiency is desired. This condition is usually the point at which the vendorcertifies that performance is within the tolerances stated in this standard.

Polytropic -This model takes into account both a rise in temperature in the gas as well asome loss of energy (heat) to the compressor's components. This assumes that heat maenter or leave the system, and that input shaft work can appear as both increased pressur(usually useful work) and increased temperature above adiabatic (usually losses due t

cycle efficiency). Compression efficiency is then the ratio of temperature rise at theoretica100 percent (adiabatic) vs. actual (polytropic).

Process compression stage -Is defined as the compression step between two adjacenpressure levels in a process system. It may consist of one or more compressor stages.

Radially split -A joint which is perpendicular to the shaft centerline.

Rated discharge pressure -Is the highest pressure required to meet the conditionspecified by the purchaser for the intended service.

Rated discharge temperature -Is the highest predicted operating temperature resultinfrom any specified operating condition.

Rod reversal-Is a change in direction of force in the piston-rod loading (from tension tcompression or vice versa) that result in a load reversal at the crosshead pin during eacrevolution.

Rotor - The rotors are usually of forged solid design. Welded hollow rotors may be applieto limit the moment of inertia in larger capacity compressors. Balancing pistons to achievequalization of rotor axial thrust loads are generally integral with the rotor. Rotating bladeare located in peripheral grooves in the rotor.

Surge- The volume flow capacity below which a centrifugal compressor becomeaerodynamically unstable.


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NOMENCLATURE

Ap Piston area, in2

Ar Piston rod area, in2

bhp Horsepower, hpCL Cylinder clearance volume as decimal fraction of displaced volumeCp Specific heat at constant pressure, Btu /(Ib

oF)Cv Specific heat at constant volume, Btu/(Ib

oF)

D Impeller diameter, inDISP Cylinder displacement, ft3/ming Gravitational constant, 32.2 ft/s2

ghp Gas horsepower, hpH Head, ftk Isentropic exponent, dimensionlessLt Tensile load, IbLc Compressive load, lbM Molecular weightN Rotative speed, r/minNc Compression efficiencyNm Mechanical efficiencyn Polytropic exponentn1 Number of moles

P Absolute pressure, psiaPa Atmospheric pressure, psiaP2or Pd Discharge pressure, psiaP1or Ps Suction pressure, psia (flange)Pc Absolute critical pressure, psiaPr Reduced pressure, dimensionlessQ Capacity, million standard ft3/day (ref. 14.7 psia, 520R)V Volume, ft3

V1 Suction specific volume, ft3/Ib

VE Theoretical volumetric efficiency, dimensionless

R Universal gas constant, 10.73 RIb.mole.

psia.fto

3

or 1545/MW

Rp Pressure ratio across cylinder (flange to flange)rpm Rotative speedrs Compression ratio per stagert Overall compression ratio ( pressure discharge/ pressure intake)


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S Stroke, in.

s Number of stagesT Absolute temperature, oRT2orTd Discharge temperature,

oRT1 or Ts Suction temperature,

oRTc Absolute critical temperature,

oRTr Reduced temperature, dimensionlessu Peripheral velocity, ft/sW Gas flow, Ib/minZ Average compressibilityZs Compressibility factor at suction conditionsZstd Compressibility factor at standard conditions

Greek letters

Hydraulic efficiency Pressure coefficient

Engineering Design Guideline- Compressor Rev2

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