Pneu­matic sys­tems for in­dus­trial and man­u­fac­tur­ing

DEMM Engineering & Manufacturing - - HYDRAULICS & PNEUMATICS - BY AMIN AL­MASI

PNEU­MATIC SYS­TEMS are widely used in in­dus­trial and man­u­fac­tur­ing plants. How­ever, knowl­edge and skills of many en­gi­neers and op­er­a­tors on pneu­matic sys­tems and de­vices are of­ten lim­ited. A pneu­matic sys­tem is a com­bi­na­tion of pneu­matic cir­cuit and de­vices which use pres­sur­ized gas, usu­ally com­pressed air, to pro­duce me­chan­i­cal mo­tion; the mo­tion can be lin­ear or ro­tary, de­pend­ing on the type of ac­tu­a­tor. A ba­sic and widely used ac­tu­a­tor is a pneu­matic cylin­der, with max­i­mum force on the pis­ton rod be­ing de­ter­mined by air pres­sure and cross-sec­tional area of the pis­ton. Ex­am­ples of other pneu­matic ac­tu­a­tors in­clude: ro­tary ac­tu­a­tors, pneu­matic mo­tors, grip­pers, var­i­ous types of pneu­matic rod-less ac­tu­a­tors, etc.

Pneu­matic sys­tems are com­monly op­er­ated with com­pressed air or com­pressed in­ert gases. These sys­tems can be driven by a wide range of com­pressed gases such as car­bon diox­ide (CO2), dry ni­tro­gen (N2), etc; they also can be driven by com­pressed gases stored in cylin­ders al­low­ing for porta­bil­ity or some­times ap­pli­ca­tions in stand­alone emer­gency sys­tems. Pneu­matic sys­tems are used in many plants, ma­chiner­ies and fa­cil­i­ties be­cause a cen­trally-lo­cated and elec­tri­cally pow­ered air com­pres­sor sys­tem, that pow­ers pneu­matic de­vices, can usu­ally pro­vide mo­tive power in a bet­ter, safer, more flex­i­ble, and more re­li­able way than a large num­ber of other ac­tu­a­tors such as elec­tric ac­tu­a­tors, elec­tric mo­tors, etc.

Most in­dus­trial pneu­matic ap­pli­ca­tions use pres­sures of about 5.5 Bars to 9.5 Barg; al­though spe­cially de­signed pneu­matic sys­tems can be de­signed for higher pres­sures than the above­men­tioned such as 10 Barg, 11 Barg or more. Gas un­der high pres­sure can cause an ex­plo­sion if its stor­age tank/ves­sel is dam­aged; there­fore, op­er­at­ing pres­sures of pneu­matic sys­tems are usu­ally lim­ited to be­low 11 Barg or 12 Barg. Most pneu­matic circuits and de­vices run at low power rat­ings usu­ally around 2 to 30 kW; al­though, in spe­cific ap­pli­ca­tions, higher power rat­ings, say 90 kW or more, have been used. Two main ad­van­tages of pneu­matic circuits are their low ini­tial cost and sim­plic­ity. Be­cause pneu­matic sys­tems op­er­ate at rel­a­tively low pres­sure, the com­po­nents can be made of rel­a­tively in­ex­pen­sive ma­te­ri­als of­ten by mass production pro­cesses such as plas­tic in­jec­tion mould­ing, alu­minium die- cast­ing, or sim­i­lar. These pro­cesses cut sec­ondary ma­chin­ing op­er­a­tions and cost. Pneu­matic sys­tems are cost ef­fec­tive, ver­sa­tile and sim­ple sys­tems with stan­dard low cost com­po­nents which are widely used in dif­fer­ent ap­pli­ca­tions and ma­chines. This ar­ti­cle dis­cusses pneu­matic sys­tems for in­dus­trial and man­u­fac­tur­ing fa­cil­i­ties.

PNEU­MATIC SYS­TEMS

Pneu­matic sys­tems are power sys­tems us­ing com­pressed air (or other com­pressed gases) as the work­ing medium for the power trans­mis­sion. An air com­pres­sor sys­tem con­verts the me­chan­i­cal energy of the prime mover (of­ten elec­tric mo­tor) into pres­sure energy of the com­pressed air. This trans­for­ma­tion fa­cil­i­tates the trans­mis­sion, stor­age, and con­trol of energy. This also can pro­vide a safer al­ter­na­tive for ar­eas where elec­tri­cal power is risky to be em­ployed. Af­ter com­pres­sion, the com­pressed air should be pre­pared and cleaned us­ing dif­fer­ent fil­tra­tion and dry­ing stages. There have been many fail­ures or dam­ages in pneu­matic sys­tems due to con­tam­i­na­tions or dirt in com­pressed air; there­fore these steps of fil­tra­tion, dry­ing, etc. need great care.

Pneu­matic sys­tems have nu­mer­ous de­sir­able fea­tures. These sys­tems are sim­ple, low cost, ef­fec­tive, high per­for­mance and re­li­able. There have been many ad­van­tages for such a sys­tem; for in­stance, air is avail­able every­where and it can be vented to the at­mos­phere; all these are re­duc­ing the com­plex­ity, cost, and weight of the over­all sys­tem. Pneu­matic sys­tems have been used for some con­sid­er­able time for car­ry­ing out sim­ple or com­plex me­chan­i­cal tasks in a wide range of ap­pli­ca­tions. In re­cent decades, such sys­tems have played an im­por­tant role in the de­vel­op­ment of au­to­ma­tion sys­tems through dif­fer­ent ar­range­ments and con­fig­u­ra­tions such as mecha­tronic sys­tems, etc. As a rule, many mod­ern pneu­matic com­po­nents are de­signed for a max­i­mum op­er­at­ing pres­sure of 9 – 10 Barg. How­ever, it is rec­om­mended to op­er­ate the same be­tween 5 and 8 Barg for safer and more re­li­able use.

AIR SUP­PLY SYS­TEM

One of notable ben­e­fits of pneu­matic sys­tems arises out of the sim­ple fact that air is free. The air sup­ply for a par­tic­u­lar pneu­matic ap­pli­ca­tion should be suf­fi­cient and of ad­e­quate pres­sure, tem­per­a­ture and qual­ity. Air is drawn from the at­mos­phere via an air fil­ter and raised to re­quired pres­sure by an air com­pres­sor set. The air is com­pressed to around 1/ 7th or 1/8th of its vol­ume with the help of a prop­erly se­lected air com­pres­sor set; the air com­pres­sor sys­tem has usu­ally “n+1” con­fig­u­ra­tion (one standby). The air tem­per­a­ture is raised con­sid­er­ably by com­pres­sion. Be­fore the air can be used it should be cooled. Air also con­tains a sig­nif­i­cant amount of wa­ter vapour; all these re­sult in the for­ma­tion of con­den­sa­tion. To en­sure proper qual­ity of the air, air treat­ment equip­ment is uti­lized to pre­pare the air be­fore it is ap­plied to a pneu­matic sys­tem and ac­tu­a­tors. In other words, the air com­pres­sor should be fol­lowed by a cooler set and air treat­ment unit.

An air treat­ment unit is usu­ally a com­bi­na­tion of com­pressed-air fil­ter, dryer, and reg­u­la­tor. The com­pressed-air fil­ter and dryer pack­age per­forms the job of fil­ter­ing all con­tam­i­na­tions from the com­pressed air flow­ing through it as well as con­den­sates, traces of wa­ter and wa­ter vapour. The pur­pose of the reg­u­la­tor is to keep the op­er­at­ing pres­sure vir­tu­ally con­stant re­gard­less of any fluc­tu­a­tion. Reg­u­la­tors are fit­ted with proper com­po­nents that re­act to changes in the down­stream air pres­sure. In other words, a reg­u­la­tor at­tempts to au­to­mat­i­cally main­tain a con­stant (pre-set) pres­sure within a pneu­matic cir­cuit as long as the sup­ply (reser­voir) pres­sure is greater than the re­quired cir­cuit pres­sure.

It is al­ways rec­om­mended to use at least two mea­sure­ment/con­trol sources, for in­stance, a reg­u­la­tor and a pres­sure gauge, to mon­i­tor and con­trol the per­for­mance of a pneu­matic sys­tem. Ev­ery pneu­matic sys­tem should have a pres­sure relief valve to pre­vent over pres­sure con­di­tions that can de­velop.

An air re­ceiver is fit­ted to act as a buf­fer and re­duce pres­sure fluc­tu­a­tions. Com­press­ibil­ity of air/gas makes it nec­es­sary to store a large vol­ume of pres­surised air in an air re­ceiver (air reser­voir/ ves­sel), to be drawn on by the load. The air reser­voir stores the pres­surised air used to op­er­ate the pneu­matic cir­cuit com­po­nents; it acts like a pneu­matic bat­tery. Sig­nif­i­cant amounts of energy can be stored in a pres­surised air re­ceiver. For this rea­son, such reser­voir should be strong and ro­bust; safety pro­vi­sions and pre­cau­tions should be fully con­sid­ered for such a ves­sel. This sys­tem is de­liv­ered the air to an air dis­tri­bu­tion sys­tem. Many plants, fac­to­ries and man­u­fac­tur­ing units pro­duce com­pressed air at one cen­tral sta­tion and dis­trib­ute an air ring main to all places on the site in a sim­i­lar way to other util­ity ser­vices such as elec­tric­ity, wa­ter, gas, etc.

PNEU­MATIC VALVES

Valves con­trol the flow (“on/off”) and the speed of air flow. Pneu­matic valves can be op­er­ated by hand, me­chan­i­cally, elec­tri­cally (so­le­noid) or air ( pi­loted). So­le­noid valves are elec­tri­cally op­er­ated valves that con­trol the di­rec­tion and flow of pres­sur­ized air to and from pneu­matic ac­tu­a­tors or circuits. They are widely used in plants and units. These valves can be ei­ther “mono-sta­ble” or “bi-sta­ble”. Mono-sta­ble valves are spring re­turn to a de­fault con­di­tion. Bi-sta­ble valves have no pre­ferred or de­fault con­di­tion thus re­main­ing where it was last po­si­tioned. There have been many dif­fer­ent types of pneu­matic valves. For ex­am­ple, a widely used valve is three- port, two- po­si­tion so­le­noid valve. This means the valve has three ports and two pos­si­ble con­di­tions (pass­ing or not pass­ing) and it is elec­tri­cally op­er­ated (so­le­noid).

PNEU­MATIC AC­TU­A­TORS

A pneu­matic ac­tu­a­tor or tool is a type of power de­vice driven by com­pressed air. These de­vices and ac­tu­a­tors are usu­ally more com­pact, cost ef­fec­tive and safer to run and main­tain than their elec­tric power de­vices equiv­a­lents, and have a higher power-to-weight ra­tio, al­low­ing a smaller, lighter, more cost ef­fec­tive de­vice to ac­com­plish the same task. In gen­eral, pneu­matic de­vices, ac­tu­a­tors and tools are cheaper than the equiv­a­lent elec­tric- pow­ered de­vices or tools. Gen­eral grade (stan­dard) pneu­matic ac­tu­a­tors are com­monly cheaper than any other de­vices. Pneu­matic ac­tu­a­tors are be­com­ing in­creas­ingly pop­u­lar, and have al­ways been ubiq­ui­tous in in­dus­trial and man­u­fac­tur­ing fa­cil­i­ties.

A pneu­matic ac­tu­a­tor is of­ten a cylin­der type lin­ear ac­tu­a­tor; it mainly con­sists of a pis­ton, a cylin­der, and some valves or ports. The pis­ton is equipped with a di­aphragm or seal set, which keeps the air in the cylin­der, al­low­ing air pres­sure to force and move the pis­ton. The larger the size of the pis­ton, the larger the out­put force can be. Hav­ing a larger pis­ton can also be good if air sup­ply is rel­a­tively low or might be vari­able and dropped to a low value, al­low­ing the same forces with less pres­sure in­put. Lin­ear pneu­matic ac­tu­a­tors are avail­able in many dif­fer­ent con­fig­u­ra­tions. These cylin­ders are fit­ted with pis­tons of var­i­ous types, di­am­e­ters, strokes or lengths. They are spec­i­fied as sin­gle act­ing (pow­ered in one di­rec­tion) or dou­ble act­ing (pow­ered in both di­rec­tions). Sin­gle act­ing spring re­turn cylin­ders are more eco­nom­i­cal with re­spect to air con­sump­tion. How­ever, their ap­pli­ca­tions are lim­ited to some ser­vices where such a spring re­turn sys­tem is ac­cept­able; dou­ble act­ing cylin­ders are com­monly used in many ser­vices.

A pneu­matic mo­tor or com­pressed air en­gine is a type of mo­tor which does me­chan­i­cal work in form of ro­tary mo­tion by ex­pand­ing com­pressed air. Ro­tary mo­tion is sup­plied by dif­fer­ent types and mod­els such as a vane type air mo­tor, pis­ton air mo­tor, etc. In other words, some types rely on pis­tons and cylin­ders; oth­ers use tur­bines or sim­i­lar. Pneu­matic mo­tors have been of­fered in many forms and shapes over the past 150 years, rang­ing in size from hand-held tur­bines to air en­gines of up 500 kW (or even more).

SPEED OF RE­SPONSE

Pneu­matic sys­tems use com­press­ible air/gas; for this rea­son, a pneu­matic sys­tem is slower in re­spond­ing to loads, es­pe­cially sud­den out­put loads, than other sys­tems such as a hy­draulic sys­tem. Sim­i­larly, torque or force re­quires time and out­put mo­tion to build-up. Re­sponses to sud­den out­put loads of­ten show ini­tial over­shoot. Much more com­plex net­works or other damp­ing means are usu­ally re­quired to de­velop sta­ble re­sponse in closed-loop sys­tems. On the other hand, there are no harm­ful shock waves anal­o­gous to the tran­sients that can oc­cur in hy­draulic sys­tems, and pneu­matic sys­tem com­po­nents last com­par­a­tively longer.

The low stiff­ness of pneu­matic sys­tems is an­other in­di­ca­tor of rel­a­tively long re­sponse time. Res­o­nances can oc­cur be­tween the com­press­ible gas and equiv­a­lent sys­tem in­er­tias at lower fre­quen­cies. Even the rel­a­tively low speed of sound in con­nect­ing lines con­trib­utes to re­sponse de­lay, adding to the dif­fi­culty of closed-loop sta­bil­i­sa­tion. For­tu­nately, it is pos­si­ble to con­struct so­phis­ti­cated pneu­matic sys­tems to achieve sta­bil­i­sa­tion at a point. Such pneu­matic sta­bil­is­ing means are com­mer­cially avail­able and are im­por­tant el­e­ments of closed-loop pneu­matic con­trol sys­tems. The con­trol of pneu­matic sys­tem is al­most ex­clu­sively by valves, which con­trol the flow from a pneu­matic pres­sure source. Low- pres­sure outlet ports should be large enough to ac­com­mo­date the high vol­ume of the ex­panded air/gas.

One of re­ported prob­lems in some pneu­matic sys­tems is caused by slug­gish pneu­matic ac­tu­a­tors, which presents some dif­fi­cul­ties such as it can limit the con­trol band­width of ac­tu­ated sys­tems. Too of­ten, a rel­a­tively low speed of re­sponse in pneu­matic sys­tems is a prob­lem and should be con­sid­ered and ver­i­fied par­tic­u­larly for circuits and ap­pli­ca­tions which re­quire fast re­sponses. Gas/air com­press­ibil­ity makes pneu­matic sys­tems 1 or 2 or­ders of mag­ni­tude slower than hy­draulic sys­tems. There have been some ap­pli­ca­tions where hy­draulic sys­tems were used in­stead of pneu­matic sys­tems only be­cause of the speed of re­sponse.

PNEU­MAT­ICS VS HY­DRAULICS

Both pneu­mat­ics and hy­draulics are ap­pli­ca­tions of fluid power. Pneu­mat­ics uses an eas­ily com­press­ible gas (air, etc.), while hy­draulics uses rel­a­tively in­com­press­ible liq­uid me­dia such as hy­draulic oil. As an in­di­ca­tion, hy­draulics ap­pli­ca­tions com­monly use pres­sures from 50 to 350 Barg and spe­cial­ized hy­draulics sys­tems may ex­ceed 500 Barg. Pneu­matic sys­tems use op­er­at­ing pres­sures far lower than ones of hy­draulic sys­tems, as noted, some­where be­tween 6 to 10 Barg. Con­sid­er­ing op­er­at­ing pres­sure range, hy­draulic sys­tems are more com­pact with lower power-to-weight ra­tio; pneu­matic sys­tems there­fore re­quire larger ac­tu­a­tors than hy­draulic sys­tems for the same load; al­though pneu­matic ac­tu­a­tors are still smaller than elec­tri­cal ones.

Pneu­matic sys­tems of­fer spe­cific ben­e­fits which make them bet­ter op­tions for cer­tain ap­pli­ca­tions. There have been some ad­van­tages for pneu­matic sys­tems over hy­draulic ones. Sim­plic­ity of de­sign and con­trol is one of ad­van­tages of pneu­matic sys­tems. Ma­chines and mech­a­nisms are eas­ily de­signed us­ing stan­dard pneu­matic cylin­ders, pneu­matic ac­tu­a­tors and other com­po­nents, and op­er­ate via sim­ple con­trol (of­ten very sim­ple on- off logic). Al­though there have been some stan­dard­iza­tion on hy­draulic sys­tems, hy­draulic units are usu­ally de­signed and in­stalled for each ap­pli­ca­tion. Pneu­matic sys­tems usu­ally have long op­er­at­ing lives and re­quire lit­tle main­te­nance. Be­cause air/gas is com­press­ible, equip­ment is less sub­ject to shock damage. Gas ab­sorbs ex­ces­sive force, whereas oil in hy­draulics di­rectly trans­fers force. Com­pressed gas can be stored, so ma­chines and fa­cil­i­ties still run for a while if elec­tri­cal power is lost. Al­though ac­cu­mu­la­tors and other types of stor­age ves­sels have been used in hy­draulic sys­tems; the stored energy is very lim­ited. There is a very low chance of fire com­pared to hy­draulic oil. Fluid vis­cos­ity and its tem­per­a­ture vari­a­tions are vir­tu­ally neg­li­gi­ble with pneu­matic sys­tems.

First cost of an air cir­cuit is less than a hy­draulic cir­cuit but op­er­at­ing cost can be higher. Com­press­ing at­mo­spheric air to a nom­i­nal work­ing pres­sure re­quires a lot of power. Pneu­matic ac­tu­a­tors are usu­ally ex­pen­sive to op­er­ate; specif­i­cally air mo­tors are one of the most costly com­po­nents in plants to op­er­ate. On the other hand, air- driven ma­chines are usu­ally qui­eter and safer than their hy­draulic coun­ter­parts. Lower noise is mainly be­cause the power source (air com­pres­sor sys­tem) is in­stalled re­motely from the ma­chine in an en­clo­sure that helps con­tain its noise.

Be­cause air is com­press­ible, a pneu­matic ac­tu­a­tor can­not hold a load rigidly in place like a hy­draulic ac­tu­a­tor does. An air- driven de­vice can use a com­bi­na­tion of air for power and oil as the driv­ing medium to over­come this prob­lem, but the com­bi­na­tion adds cost to the cir­cuit. This is only used in very special ap­pli­ca­tions. Pneu­matic sys­tems are usu­ally cleaner than hy­draulic sys­tems. Leaks in an air cir­cuit do not cause house­keep­ing prob­lems, but they are ex­pen­sive.

BIO: AMIN Al­masi is a lead me­chan­i­cal en­gi­neer in Aus­tralia. He is char­tered pro­fes­sional en­gi­neer of En­gi­neers Aus­tralia ( MIEAust CPEng – Me­chan­i­cal) and IMechE ( CEng MIMechE) in ad­di­tion to a M. Sc. and B. Sc. in me­chan­i­cal en­gi­neer­ing and RPEQ ( Reg­is­tered Pro­fes­sional En­gi­neer in Queens­land). He spe­cialises in me­chan­i­cal equip­ment and ma­chiner­ies in­clud­ing cen­trifu­gal, screw and re­cip­ro­cat­ing com­pres­sors, gas tur­bines, steam tur­bines, en­gines, pumps, con­di­tion mon­i­tor­ing, re­li­a­bil­ity, as well as fire pro­tec­tion, power gen­er­a­tion, wa­ter treat­ment, ma­te­rial han­dling and oth­ers. Al­masi is an ac­tive mem­ber of En­gi­neers Aus­tralia, IMechE, ASME, and SPE. He has au­thored more than 150 pa­pers and ar­ti­cles deal­ing with ro­tat­ing equip­ment, con­di­tion mon­i­tor­ing, fire pro­tec­tion, power gen­er­a­tion, wa­ter treat­ment, ma­te­rial han­dling and re­li­a­bil­ity.

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