mitsubishi integrally geared compressor

Mitsubishi Heavy Industries, Ltd.Technical Review Vol. 41 No. 3 (Jun. 2004)

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MHI Integrally Geared TypeC o m p r e s s o r f o r L a r g eCapacity Application andProcess Gas Application

This paper introduces an outline of the structures, features, development, examples of actual deliveries, andthe future outlook for integrally geared centrifugal compressors (hereafter called geared compressors) that havebeen delivered by MHI to various markets since 1967. In this paper, a detailed explanation is presented of largesize geared compressor trains used for PTA (purified terephthalic acid) plants and geared compressors used forfeeding fuel gas to gas turbines.

1. Introduction1. Introduction1. Introduction1. Introduction1. Introduction

Applications for the geared compressors manufacturedby Mitsubishi Heavy Industries, Ltd. (MHI) have beenexpanding gradually since the first machines were de-livered in 1967. The total number of machines deliveredsince then has reached more than 500 units.

Traditionally, geared compressors have been used asa source of factory air supply, integrated into a packagethat also includes intercoolers, a lubricating oil unit, and

drive unit. Recently, however, as these machines haveadvanced into various process fields, they have becomeincreasingly larger in size and are being applied in anever-expanding range of applications.

The scope of application of MHI's geared compressorsis shown in Fig.1Fig.1Fig.1Fig.1Fig.1. As can be seen in the figure, MHI'sgeared compressors are capable of producing pressuresas high as 6.0MPa (A) and intake flow rates as large as300 000Am3/H, thereby being readily able to fulfill re-cent market requirements.

This report introduces (1) large geared compressortrains used for PTA plants that are increasing each year,and (2) fuel gas geared compressors used to feed fuel gasto gas turbines used in combined cycle power plants. Anoverview of the structure and characteristics of a gearedcompressor is also given.

2. Structure and characteristics2. Structure and characteristics2. Structure and characteristics2. Structure and characteristics2. Structure and characteristics

Many centrifugal compressors operate in petrochemi-cal , other chemical , and air separation plants.Centrifugal compressors are roughly classified into twotypes. The first type is a single shaft multistage com-pressor consisting of a series of impellers installedtogether on a single shaft which are the main compo-nents for pressurizing the gas [Fig.2Fig.2Fig.2Fig.2Fig.2(a)]. The impellersare located on the shaft between two bearings that sup-port it. In the case of such a single shaft multistagecompressor, a gear unit is coupled between the com-pressor and drive unit, when the rotational speed ofthe drive unit is less than that required for the com-pressor.

The other type of centrifugal compressor has a built-in gear, namely a geared compressor. A gearedcompressor has an overhang type pinion shaft. Eachimpeller is attached to the end of its own high-speed pin-

1 000

100

10

1100 1 000 10 000 100 000 1 000 000

Fig. 1 Scope of application of geared compressors

Dis

char

ge p

ress

ure

(k

/cm

2 -ab

s)

Fuel gascompressor

Integrally geared typecentrifugal compressor

Inlet flow rate (Am3/h)

NAOTO YONEMURA*1

YUJI FUTAGAMI*1

SEIICHI IBARAKI*2

*1 Hiroshima Machinery Works*2 Nagasaki Research & Development Center, Technical Headquarters

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ion shaft within the gear unit. The pinion shafts are lo-cated around the circumference of a bull gear that rotatesat a low speed [Fig.2(b)].

The speed of each pinion shaft can be set by matchingthe number of teeth in each pinion to conform to the op-timum speed of each impeller.

When designing a geared compressor, it is very im-portant to conduct an overall technical evaluation of allthe basic mechanical components of the compressor suchas the impellers, gears, bearings, shaft seals, casing, etc.,since each impeller is directly installed onto pinion shaftsthat rotate at high speeds. MHI can produce and supplyall the geared compressors to high levels of mechanicalreliability, by designing each component based on designcriteria established through extensive tests carried outwithin the company.

Fig. 3Fig. 3Fig. 3Fig. 3Fig. 3 shows the results of an analysis for the vibra-tion stability of a pinion shaft, as an example ofcomponent design. There is an aerodynamic exciting forcethat is generated in the impeller, one of the various ex-citing forces that cause the shaft to vibrate. Fig. 3 showsa comparison of the analysis results, when the aerody-namic exciting force is considered, and when it is notconsidered. From the figure, it can be seen that stabilityis reduced when the aerodynamic exciting force gener-ated in the impeller is considered.

As indicated by the above, it is necessary to conduct aprior examination taking the exciting force into consid-eration in order to analyze the vibration behavior of ageared compressor. In addition, the behavior of the ro-tor varies significantly as the tangential force producedin the gear due to the operating condition changes.Hence, it is also important to analyze the vibration ofthe pinion shaft under partial loads.

Depending on the type of plant, the main compres-sor train may be composed of different rotatingmachines such as a geared compressor, single shaft

multistage compressor, expander, steam turbine, and adrive motor. Despite such a complex train, MHI can alsopropose the train that is best suited for a given plantbecause the company has extensive experience in thedesign and manufacture of every kind of rotating ma-chine. The train is selected based on careful mechanicalexaminations of the component machines, a comprehen-sive evaluation of the train composition satisfying allrequirements for the plant (gas balance and power bal-ance), and its operating method.

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Fig. 3 Comparison of change in shaft system vibration stability depending on existence of impeller exciting forceThe logarithmic decrement that indicates shaft stability has a tendency to decrease with the existence of an exciting force.

Casing

Impeller

Shaft

Gas flow

LoadLoad

Direction of eccentricity

: The eccentricity of the shaft generates an impeller exiting force.

Larger

Smaller

Stability

Loga

rithm

ic d

ecre

men

t

: Case in which the exciting force is not considered.: Case in which the exciting force is considered.

Forward rotation primary mode

Forward rotation secondary mode

Load

Diaphragm

Labyrinth seal

Roller bearing

Fig.2 Comparison of single shaft multistage compressor and geared compressor (Integrally geared type compressor)

Impellers

Casing

Shaft seal

(a) Construction of single shaft multistage type compressor

(b) Construction of geared compressor


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MHI also provides combined type geared compres-sors that ut i l ize the character ist ics o f gearedcompressors, in which the rotational speed of each pin-ion shaft can be freely selected. In this type of gearedcompressor, different application impellers or expanderimpellers for recovering power are installed in one gearunit casing.

Fig. 4Fig. 4Fig. 4Fig. 4Fig. 4 shows examples of combined type geared com-pressors. Adopting a combined type geared compressoroffers the benefit of remarkably reducing the amount ofspace necessary for installation, since the different com-pressor casings can be integrated into a single casingand saving can be made on auxiliary equipments. Inaddition to the above-mentioned merits, various lossesincluding mechanical losses of rotating machines, suchas bearing losses can be reduced, and the size of the lu-bricating unit can be significantly reduced. The overallresult is that significant savings in required power canbe achieved.

Fig. 5Fig. 5Fig. 5Fig. 5Fig. 5 shows a photograph of a combined type gearedcompressor that consists of a compressor and expanderfor a PTA plant.

3. Expansion of applications for geared compressors3. Expansion of applications for geared compressors3. Expansion of applications for geared compressors3. Expansion of applications for geared compressors3. Expansion of applications for geared compressors

3.1 Large size geared compressors3.1 Large size geared compressors3.1 Large size geared compressors3.1 Large size geared compressors3.1 Large size geared compressorsA typical example of an application for large geared com-

pressors is a compressor train used in a large PTA plant.

The capacity of PTA plants has been steadily increas-ing in recent years. Plants generally ranged in size fromaround 250 000 to 400 000 tons/year in the 1990s. In thisdecade, PTA plants recently installed have capacitiesranging from 500 000 to 700 000 tons/year, with futureplants expected to increase even further to capacities aslarge as 1 million tons/year.

Fig. 6Fig. 6Fig. 6Fig. 6Fig. 6 shows an example of a compressor train usedfor a 500 000 tons/year PTA plant. It is essential to opti-mize the equipment arrangement and operation methodused in the compressor train of a PTA plant. The reasonfor this is that, in addition to electric motors, admissionlow pressure steam turbines and off-gas expanders arealso used as drive units, in order to recover the power.The low pressure steam and off-gas generated in theplant are used for these drivers.

Extremely high mechanical reliability is required forthis type of compressor train, since it plays a major roleas the so-called heart of the plant feeding air to thereactor.

E1E2

C1C 2

C 3

C 3

C 4

C 4

C 2

C 2

C1

C1

Fig. 4 Example of combined geared compressor

Inter-heater

First stage intercooler

Intercooler

Third step intercooler

Motor

Motor

Third step intercooler

Pre-heater

Steam turbine

Second stage intercooler

Second stage intercooler

First stage intercooler

(a) Example in which a compressor and expander are combined

(b) Example in which two compressors are combined

Fig. 5 Combined geared compressor for PTA plant (Type in which a compressor and expander are combined)

170

0

Steam turbine

Fig. 6 Compressor train for a 500 000 tons/year PTA plantThe compressor train consists of an expander, motor, compressor, and steam turbine.

Expander Gas heater Motor Compressor

Air cooler Condenser


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Fig. 7Fig. 7Fig. 7Fig. 7Fig. 7 shows the results of torsional vibration analy-ses for various shafts, which were conducted to verifythe mechanical reliability of the PTA plant compressortrain. In these analyses, the overall torsional vibrationanalysis is carried out through the train as a whole, in-cluding the pinion shaft, wheel shaft, and the shafts ofall the drive units.

Fig.8Fig.8Fig.8Fig.8Fig.8(a) shows a large capacity impeller developed tocope with the needs of larger sized plants such as PTAplants, in place of conventional package type geared com-pressors. The large-scale impeller is made up of mainvanes and intermediate vanes, and the number of vanes

at the inlet is less than that of a conventional impeller.The height of each vane is about 50% greater than theheight of vanes on a conventional impeller, in order toincrease the capacity of the impeller. In addition, thenumber of vanes at the outlet of the impeller is increased,to reduce the load on each vane by about 30%, so thatthe efficiency of the impeller under high-pressure ratiosis improved. Fig.8(b) shows the flow analysis for a large-scale impeller. This is the result of a vortex analysis forcomplex three-dimensional flow inside the impeller,which is obtained using critical point theory. This is justone example of how MHI improves impeller performance

PTA PLANT TOTAL TRAIN

INFORMATION

Compressor Wheel Shaft +Motor Generator +Expander Wheel Shaft

3

4

5

26

SHAFT 1 2 3 4 5 6

LENGTH (mm)

WEIGHT (kg)

IP (kgfmms 2)

10 395.40 2 218.00 1 612.00 1 256.00 6 268.70 1 180.00

14 152.67 1 335.04 262.10 116.80 4 589.59 105.58

379 095.31 5 466.64 228.23 47.40 47 442.47 52.50

1st Mode9.9Hz

(0.49 N)63.5Hz(0.49 N)

174.4Hz(0.82N)

302.9Hz(1.14N)

50.6Hz(0.69N)

166.6Hz(0.51N)

Wheel Shaft

1 185 rpm

Expander Shaft

1 185 rpm

S/T Shaft

4 410 rpm

Compressor LP Shaft7 830 rpm

Compressor MP Shaft

12 790 rpm

Compressor HP Shaft

15 990 rpm

Compressor LP Pinion Shaft Compressor MP Pinion Shaft Compressor HP Pinion Shaft

Steam Turbine Shaft + S/T drive Pinion Shaft Expander Pinion Shaft

+

++ +

+

1

Fig. 7 Analysis of torsional vibration of compressor train for a PTA plantTorsional vibration analysis is carried out on the shaft systems of the train as a whole.

Fig. 8 Large capacity impeller

(a) Conventional impeller and large capacity impeller

(c) Pinion shaft equipped with a large capacity impeller

Flow rate distribution (color contours) and total pressure loss (line contours)

Pressure surface

Directionof rotation

Center of vortex

(b) Flow analysis of large capacity impeller

Flow lines passing throughthe vortex center

Critical flow lines on Suction surface

Suctionsurface


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through the use of such flow analysis techniques(1). Apinion shaft with a large capacity impeller is shown inFig.8(c).

Since the gears also need to be larger in size as gearedcompressors increase in size and capacity, MHI also ac-tively carries out R&D concerning large sized gears.

3.2 Application for process gas3.2 Application for process gas3.2 Application for process gas3.2 Application for process gas3.2 Application for process gasIn this section, a geared compressor used to feed fuel

gas to the gas turbines in a combined cycle power plantis described, as an example of an application for suchcompressors in the process gas field. Fig. 9Fig. 9Fig. 9Fig. 9Fig. 9 illustratesthe structure of the compressor.

The fuel gas geared compressor is used to pressurizefuel gas and feed it to a gas turbine for power genera-tion. Hence, if the geared compressor fails, the gasturbine stops with the result that all power generationalso stops. Consequently, geared compressors need to beextremely reliable.

Providing a highly reliable control system, capable ofcoping with severe operating conditions such as load re-jection operation and DSS operation of the gas turbine,is essential for this type of geared compressor. Accord-ingly, MHI has established simulation technology for thefuel gas geared compressor. Simulations of the systemare to be conducted beforehand in order to assess con-trol response to load changes such as load rejection.

Tandem type dry gas seals are provided for the sealedparts, since the compressor handles high pressure fuelgas with a discharge pressure as high as 5.0 MPa (A).

MHI developed and delivered its first fuel gas feed-ing geared compressor in 2001. During in-company testscarried out before delivery, performance checks were doneon the compressor to confirm that specified performancelevels were achievable. Mechanical checks using full loadtests were also conducted. Furthermore, in order to dem-onstrate the reliability at the DSS operation, the trainwas started and stopped about 150 times. After this se-ries of starts and stops was completed, an overhaulinspection was carried out to check the soundness of theparts. The compressor has been operating stably sincedelivery without any problems. Based on these actualresults, the reliability of MHI’s geared compressorsachieved a high evaluation, resulting in the acceptanceof orders for a total of twelve machines since then.

Since the expansion of combined cycle power gen-eration using gas turbines is anticipated in the future,MHI intends to make further efforts to promote the saleof fuel gas geared compressors, based on these actualexperiences.

4. Future development4. Future development4. Future development4. Future development4. Future development

MHI intends to further improve large size geared com-pressors so that they can be applied to compressor trainson large PTA plants with capacities as high as 1 milliontons/year. Furthermore, it is MHI’s intention to applythese compressors to large scale air separation units usedfor GTL (gas to liquid) plants that convert natural gasinto liquid fuel in keeping with environmental regula-tory requirements.

Moreover, in the field of process gas, MHI also intendsto apply geared compressors to the pressurization of vari-ous process gases in petrochemical, oil refinery, and otherchemical plants, based on experiences gained with fuelgas geared compressors and the application technologyfostered through single shaft multistage centrifugal com-pressors.

5. Conclusion5. Conclusion5. Conclusion5. Conclusion5. Conclusion

The future market for geared compressors will requirefurther increases in capacity and high pressure, and thefields of their applications, including process gases, willcontinue to expand. MHI will continue its efforts to meetcustomer demands for ever-more efficient and powerfulcompressors and other technologies, by carefully assess-ing such needs from various plant processes and providingthe most suitable compressors to meet these needs.

ReferenceReferenceReferenceReferenceReference(1) Uchida et al., The Advanced CFD Technology for The Devel-

opment Turbo-Machinery, Mitsubishi Juko Giho Vol. 40 No.6(2003) p. 336

Fig. 9 Fuel gas geared compressorThis image shows a cut-away view of the structure of a two-stage type fuel gas geared compressor.

Naoto Yonemura Yuji Futagami Seiichi Ibaraki

mitsubishi integrally geared compressor

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