Hy­draulics and pneu­mat­ics

DEMM Engineering & Manufacturing - - CONTENTS - By Amin Al­masi

Re­cip­ro­cat­ing com­pres­sors are the most common com­pres­sor world­wide. The ad­vance­ments of re­cip­ro­cat­ing com­pres­sor tech­nol­ogy have made it pos­si­ble to achieve high pres­sures and high ca­pac­i­ties in com­pact and ef­fi­cient com­pres­sor pack­ages us­ing high-speed, vari­able-speed re­cip­ro­cat­ing com­pres­sors.

Mod­ern high-speed re­cip­ro­cat­ing com­pres­sors are man­u­fac­tured in the 800 to 1600 rpm speed range; they can also of­fer a su­pe­rior op­er­a­tional flex­i­bil­i­ties us­ing vari­able-speed op­er­a­tion usu­ally in a wide-speed range. How­ever, more so­phis­ti­cated de­signs and tech­niques are re­quired for th­ese mod­ern ma­chines.

For ex­am­ple, some of high-speed re­cip­ro­cat­ing com­pres­sors op­er­ate less ef­fi­ciently than tra­di­tional low-speed com­pres­sors due to higher dy­namic ef­fects and higher f low-rates that lead to higher as­so­ci­ated f low re­lated losses through cylin­der valves and pul­sa­tion con­trol de­vices.

Also, with higher power re­cip­ro­cat­ing com­pres­sors op­er­at­ing over a rel­a­tively large high speed range, cylin­der valve op­er­a­tion could be more com­pli­cated, pul­sa­tion am­pli­tudes tend to be sig­nif­i­cantly higher, all th­ese lead­ing to higher dy­namic losses and high risk vi­bra­tions on the cylin­ders, cylin­der valves and sur­round­ing pip­ing and fa­cil­i­ties.

Ad­vanced tech­niques such as mod­ern pul­sa­tion con­trol tech­niques are needed to ac­com­mo­date the in­crease in com­pres­sor power and the range of run­ning speeds as well as vari­a­tions in op­er­a­tions.

The f low re­sis­tances play a crit­i­cal role in any re­cip­ro­cat­ing com­pres­sor pack­age. The di­rect ef­fect of the f low re­sis­tance in the suc­tion and dis­charge gas pas­sages is the in­crease of dis­charge and suc­tion pres­sure ra­tio inside the com­pres­sor cylin­der.

The flow re­sis­tance has some ef­fects on the suc­tion and dis­charge gas tem­per­a­ture vari­a­tions; such ef­fects should prop­erly be mod­elled and eval­u­ated care­fully.

A mod­ern de­vel­op­ment in the re­cip­ro­cat­ing com­pres­sor in­dus­try is the step-less ca­pac­ity con­trol sys­tems. They are ad­vanced and ex­pen­sive sys­tems that are su­pe­rior op­tions for some ap­pli­ca­tions – where they can of­fer con­sid­er­able op­er­at­ing cost re­duc­tion and a bet­ter en­ergy man­age­ment.

This sec­tion fo­cuses on a step-less ca­pac­ity reg­u­la­tion sys­tem for re­cip­ro­cat­ing com­pres­sors in the ca­pac­ity range be­tween 20 per­cent and 100 per­cent.

This sys­tem is based on the so called “re­verse flow reg­u­la­tion prin­ci­ple”. The prin­ci­ple works by rev­ers­ing part of the to­tal gas that has been taken into the cylin­der by con­vey­ing it back to the suc­tion cham­ber by hold­ing the suc­tion valve open for a con­trolled and vari­able pro­por­tion of the com­pres­sion stroke.

There­fore, the dis­charge ca­pac­ity and power re­duc­tion are vir­tu­ally pro­por­tional. Such a step-less ca­pac­ity con­trol sys­tem uses a very so­phis­ti­cated hy­draulic sys­tem to un­load the suc­tion valve for a con­trolled pro­por­tion of the com­pres­sion stroke. There is a re­la­tion­ship be­tween the vari­a­tion of the ca­pac­ity and the open­ing time of the suc­tion valve in a com­pres­sion stroke.

This op­er­a­tion needs com­pli­cated and high per­for­mance con­trol and hy­draulic sys­tem. A new ca­pac­ity reg­u­la­tion sys­tem usu­ally con­sists of a hy­draulic unit, a distrib­u­tor, an un­loader sys­tem, and a com­plex con­trol sys­tem.

Such a step-less ca­pac­ity con­trol sys­tem is rec­om­mended for a rel­a­tively re­cip­ro­cat­ing com­pres­sor (say above 1.5MW) that should be op­er­ated ex­tended pe­ri­ods of part-load op­er­a­tion.

On the other hand, a step-less ca­pac­ity con­trol sys­tem is a com­plex and ex­pan­sive sys­tem which re­quire spe­cial op­er­a­tion and main­te­nance. The com­mis­sion­ing, re­pair, main­te­nance and over­haul of such a sys­tem are costly and spe­cial which can only be done by di­rect in­volve­ment of step-less ca­pac­ity con­trol sub-ven­dor spe­cial­ist(s) which as­so­ci­ated with sig­nif­i­cant costs and com­plex­ity. A step-less ca­pac­ity con­trol sys­tem is only rec­om­mended for large com­pres­sors with op­er­a­tion pro­files that re­quired such a wide range and flex­i­ble ca­pac­ity con­trol. This is not rec­om­mended for small and medium size re­cip­ro­cat­ing com­pres­sors in or­di­nary ser­vices that most of the time will work at full-load or de­fined sim­ple part-load steps. A step-less ca­pac­ity con­trol sys­tem is a mod­ern, com­pli­cated and ex­pen­sive ca­pac­ity con­trol de­vice that should only be used in right ap­pli­ca­tions.

Gas tur­bines

There is a wide range of gas tur­bines avail­able on the mar­ket for me­chan­i­cal drive and power gen­er­a­tion ap­pli­ca­tions. The speed vari­a­tion ca­pa­bil­i­ties are crit­i­cal re­quire­ments for the op­er­a­tional flex­i­bil­ity and the en­ergy man­age­ment of th­ese ma­chines. Some fa­mous sin­gle-gas tur­bine driv­ers such as “Frame 6”, “Frame 7”, and “Frame 9” can only of­fer very limited speed vari­a­tions. Th­ese gas tur­bines need the help of large vari­able-speed elec­tric mo­tor for the start-up. Split-shaft gas tur­bines might be heavy-duty ma­chines (such as “Frame 5” gas tur­bines) or aero-de­riv­a­tive ma­chines (such as LM2500 se­ries). Some gas tur­bines are multi-spool ma­chines, but with no free-power tur­bine, that still ex­hibit a large speed range, such as the LM6000.

The gen­er­ated power of a typ­i­cal gas tur­bine de­creases ap­prox­i­mately 0.7 per­cent for ev­ery one de­gree centi­grade in­crease in the am­bi­ent tem­per­a­ture. The am­bi­ent tem­per­a­ture change could im­pact both the gas tur­bine and the driven equip­ment such as a com­pres­sor or a gen­er­a­tor. A com­pres­sor train or a gen­er­a­tor train, which uses a gas tur­bine driver, should be able to man­age the power in dif­fer­ent am­bi­ent tem­per­a­tures.

The move in mod­ern plant to­wards aeroderiva­tives gas tur­bine driv­ers is mainly about the ef­fi­ciency, op­er­a­tional flex­i­bil­ity and a bet­ter en­ergy man­age­ment. As an in­di­ca­tion, aero-de­riv­a­tives gas tur­bines can of­fer ef­fi­cien­cies around 41–46 per­cent com­pared to 30–39 per­cent ef­fi­cien­cies for heavy-duty ma­chines.

Vari­able fre­quency drives (VFD) can of­fer high start­ing torque which makes them the ideal so­lu­tion to start large com­pres­sor trains in case of sin­gle-shaft gas tur­bine driv­ers. The VFD driven elec­tric mo­tor can func­tion not only as a starter, but also as a helper to in­te­grate the


En­hanced safety

gas tur­bine out­put in hot am­bi­ent tem­per­a­tures or as a gen­er­a­tor pro­duc­ing elec­tric power when the gas tur­bine out­put is higher than that re­quired by the com­pres­sor. Ma­jor plant op­er­a­tors are in­ter­ested in elec­tric mo­tors driven com­pres­sors and pumps. Many clients pre­fer this so­lu­tion as op­posed to gas tur­bine driven trains. The main rea­sons are:

Higher avail­abil­ity. Re­duced main­te­nance costs and downtimes.

Higher op­er­a­tional flex­i­bil­ity par­tic­u­larly by us­ing vari­able fre­quency drives (VFD). This so­lu­tion pro­vides ex­cel­lent start-up ca­pa­bil­i­ties as well as the com­pres­sor/pump speed vari­a­tion.

De­cou­pling of the plant pro­duc­tion and the am­bi­ent tem­per­a­ture. Re­duced plant emis­sions. The elec­tric mo­tor cost and de­liv­ery pe­riod are com­pet­i­tive com­pared to ones for a gas tur­bine.

A cen­tral­ized power gen­er­a­tion sys­tem can re­duce green­house gas emis­sions and sig­nif­i­cantly im­prove ther­mal ef­fi­ciency when com­pared to in­di­vid­ual gas tur­bine driven trains. Elec­tric mo­tor driven com­pres­sor (or pump) avail­abil­ity can be very high (more than 99 per­cent), def­i­nitely higher than the 95-96 per­cent typ­i­cally cited for large (con­ven­tional) gas tur­bine driven trains. Elec­tric mo­tors can be kept op­er­a­tional for six years or more with­out any shut­down.

How­ever, the elec­tri­cal power sup­ply has a great com­mer­cial im­pact (par­tic­u­larly on the ini­tial plant cost). All dif­fer­ent sce­nar­ios and con­di­tions should be con­sid­ered. For ex­am­ple, an elec­tri­cal mo­tor driven train can be the best op­tion from both cap­i­tal cost and op­er­a­tion cost points of view, when cheap elec­tri­cal power is avail­able and can be im­ported to the plant. Another ex­am­ple, when there are elec­tri­cal power de­mands ex­isted nearby and the elec­tri­cal en­ergy from a big­ger sized mod­ern, com­bined-cy­cle power plant can be sold.

Mod­ern, large com­bined cy­cle power plants can of­fer ef­fi­ciency more than 61 per­cent. In this way, us­ing elec­tri­cal mo­tor driven plants can re­sult in a higher ther­mal ef­fi­ciency. The losses in elec­tri­cal trans­mis­sions, and elec­tric mo­tor drives are against this higher ther­mal ef­fi­ciency. Typ­i­cally, the in­crease of ther­mal ef­fi­ciency is es­ti­mated by about 15-20 per­cent more than gas tur­bine driven trains and loses are cal­cu­lated about 3-4 per­cent. The over­all ef­fi­ciency in­crease of around 11-17 per­cent can be achieved us­ing a mod­ern power plant and large elec­tric mo­tor driven com­pres­sors and pumps in a plant. Adding to this, the higher re­li­a­bil­ity, avail­abil­ity and op­er­a­tional flex­i­bil­ity, the op­er­a­tional cost re­duc­tions can usu­ally jus­tify the ad­di­tional cap­i­tal costs.

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