SYED ASGHAR ABBAS NAQVI
INTERMITTENT FLOW CONTINOUS FLOW
SLIDING VANE LIQUID PISTON HELICAL LOBE SCREW
LOBED BLOWER MECHANICAL PISTON
In a reciprocating compressor, a volume of gas is drawn into a cylinder, it is trapped, and compressed y piston and t!en disc!arged into t!e disc!arge line" #!e cylinder valves control t!e flow of gas t!roug! t!e cylinder$ t!ese valves act as c!ec% valves" #!ere are two types of reciprocating compressor"
Reciprocating Compressor Types
&ingle ' (cting compressor
It is a compressor t!at !as one disc!arge per revolution of cran%s!aft"
Dou le ' (cting )ompressor
It is a compressor t!at completes two disc!arge stro%es per revolutions of cran%s!aft" Most !eavy*duty compressors are dou le acting""
Different parts of dou le acting compressor are listed elow"
&uction valve" &uction gas +ac%et" ,iston" )ylinder" Disc!arge valve" Disc!arge gas +ac%et
Construction of Reciprocating Compressors
-eciprocating compressors can two main groups"
e divided into
/as end" ,ower end"
Cylinder & Liner
,iston reciprocates inside a cylinder" #o provide for reduced reconditioning cost, t!e cylinder may e fitted wit! a liner or sleeve" ( cylinder or liner usually wears at t!e points w!ere t!e piston rings ru against it" Because of t!e weig!t of t!e piston, wear is usually greater at t!e ottom of a !ori1ontal cylinder" ( cylinder liner is usually counter ored near t!e ends of t!e outer ring travel i"e" counter ores are made +ust a!ead of t!e points w!ere t!e end piston rings stop and reverse direction" &!oulders may form in t!e liner w!ere t!e ring2s travel stops unless counter ores are provided"
For low speed compressors 3upto 445 rpm6 and medium speed compressors 3445*755 rpm6, pistons are usually made of cast iron" 8pto 9: diameter cast iron pistons are made of solids" #!ose of more t!an 9: diameters are usually !ollow 3to reduce cost6" )ar on pistons are sometimes used for compressing o;ygen and ot!er gases t!at must e %ept free of lu ricant"
(s t!e compressor reac!es operating temperature, t!e piston and rod e;pand more t!an lineralves #!ere are normally t!ree types of valves, t!ese are 1. Plate al e. !. C"a##el al e. $. P%&&et al e.
?eavy*duty compressors use a form of plate valve" #!e part t!at closes against a valve seat is a flat metal plate" #!e plates are !eld tig!tly against t!e seat y a set of springs"
#!ese valves use c!annel s!aped plates instead of flat plates" ( ove eac! c!annel is a owed, steel tension spring" &pring pus!es from t!e stop plate and c!annels cover t!e slots in t!e valve seat"
#!ese are s!aped li%e t!e valves in an automo ile engine" #!ese are separate, round poppets to seat against !oles in t!e valve seats" ,oppets are made of low friction material 3e"g" Ba%elite6, t!ey provide a low* pressure drop and are often used w!en ratios of compression are low and also for !ig! flow rates"
#!e valves are of automatic type" #!ey open due to t!e effect of t!e differential pressure etween cylinder and t!e suction or disc!arge c!am ers, and close due to t!e force of springs acting on t!e plates plus t!e differential pressure across t!e valves"
%a&or Components of "al#es
#!e main ody of valve t!at wit!stands t!e differential pressure w!en t!e valve is closed
,arts of reciprocating compressor t!at assist in transferring power and converting rotary motion into reciprocating motion are grouped in t!is category"
)ran% case supports t!e cran%s!aft" (ll earing supports are ored under setup condition to ensure perfect alignment" )ran%case is provided wit! easy remova le covers on t!e top for inspection and maintenance" #!e ottom of t!e cran%case serves as t!e oil reservoir" ( main pump for lu rication of t!e cran% mec!anism is placed on t!e s!ield mounted on t!e side opposite t!e coupling and is driven y compressor"
#!e cran%s!aft is uilt in a single piece" On t!e inside of t!e s!aft are !oles for passage and distri ution of lu e oil" Main Bearings #!e main earings are uilt in two !alves, made of steel, wit! inner coating of antifriction metal"
#!e connecting rod !as two earings" #!e ig end earing is uilt in two !alves" It is made of metal wit! inner coating of antifriction metal" #!e connecting rod small end earing is uild of steel, wit! inner coating of antifriction metal" ( !ole runs t!roug! t!e connecting rod for it2s entire lengt!, to allow passage of oil from t!e ig end to t!e small end us!"
)ross!ead fastens piston rod to t!e connecting rod" #!e sliding surfaces of cross!ead are coated wit! antifriction metal i"e" a ited s!oes" #!ese are of interc!angea le type" #!at permits it to slide ac% and fort! wit!in t!e cross!ead guides" #!e s!oes !ave c!annels for t!e distri ution of lu e oil" #!e lu rication is o tained under pressure$ it comes out from t!e two guides of t!e cross!ead slide ody" )onnection etween connecting rod and cross!ead is reali1ed y means of a gudgeon pin" #!e piston rod is connected to t!e cross!ead y nut arrangement, tig!tened y means of !ydraulic device"
Lu ricants reduce friction and t!erefore wear etween moving compressor parts" Lu ricant also serves as a coolant"
/enerally, two types of systems are uses to lu ricate t!e positive displacement compressors" &,L(&? &@EM It is used in older mac!ines" ( supply is maintained in t!e cran%case" Oil is splas!ed up y t!e rotation of t!e cran% and t!e counter weig!t into t!e collecting ring" )entrifugal force t!rows t!e oil outward t!roug! an oil passage to t!e cran% pin"
Forced Feed System
Oil is pumped under pressure to t!e re=uired parts" Following are t!e main parts of systemA -(IBEOil from t!e cran%case first passes t!roug! a coarse strainer" #!e strainer is remova le so t!at c!ips or ro%en off pieces can e cleaned out of t!e strainer"
*ypass Relief "al#e
#!e compressor will e completely damaged if t!e oil flow is apprecia ly reduced or stopped y a plugged filter" In order to prevent t!is damage, forced feed system is fitted wit! a ypass relief valve, a low*pressure alarm and ypass s!utdown" If t!is filter ecomes plugged, inlet pressure to t!e filter increases" If t!e inlet pressure e;ceeds spring pressure, t!e ypass valve opens allowing oil to return to t!e cran%case" Oil pressure in t!e system falls, t!e low*pressure alarm causes t!e compressor to s!utdown"
Oil Circulation System
#o %eep earings lu ricated, oil is supplied under pressure to t!e earings" #!e lu rication system %eeps a supply of cool and clean oil flowing to t!e system" ( sufficient =uantity of oil is stored in a reservoir"
Oil flows from t!e pump to cooler" #!e oil in t!e cooler flows t!roug! t!e tu es" #!e cooling water on t!e s!ell side a sor s !eat from t!e oil" /enerally, oil temperature is %ept etween C5 ' D5°)" Oil cooler t!an C5°) can lead to condensation of water inside t!e cran% case" Moisture can promote sludging of oil" #emperature of oil !ig!er t!an D5 ) can decrease strengt! of earing materials 3Ba itt6, t!at can lead to premature failure" In order to maintain t!e desired temperature range, t!e oil system !as a ypass valve upstream t!e cooler t!at is controlled y a t!ermostat"
Lu)ricated Piston+Pac ing
Clearance Poc et
Pac ing Lu)rication
Pe()%*)+ Ma)#te#a#+e ,E e(- ./// "%0(1 %2 %&e(at)%#3
Lu rication system /udgeon pins of cross!ead ' )!ange t!e oil" )lean filters and oil sump" ' )!ec% and tig!ten t!e fastening screws, if necessary"
Loc% nuts of cross!ead ' )!ec% and tig!ten if necessary" ,iston rings E rider rings -od pac%ing ' If compressor operation s!ows deterioration, proceed for replacement" ' If t!e e!avior of t!e compressor indicates deterioration proceed for replacement" ' )!ec% t!e seats, springs, rings and lift" -eplace if necessary"
)ompressor valves N%te'
(fter F555 !ours of operation it is advisa le to replace all parts in movement"
E e(- 14/// "%0(1 %2 O&e(at)%# Main earings E connecting rod earings )ross!ead )ran%s!aft )!ec% wear and clearances"
)!ec% clearances etween s!oes and guides" Inspect for any scoring caused y dirt in oil" )!ec% tig!tening of counterweig!t" )!ec% for wear and ovali1ing" )!ec% for wear, and tig!tening of nut"
)ylinder liner ,iston rod
%aintenance Procedures & Safety ,contd-.
(fter maintenance, mac!ine s!all e rotated slowly for at leas on revolution to ensure no interference" W!enever a component is disassem led" Ma%e sure of t!e a sence of defects efore re*using" #!e person responsi le for maintenance s!ould ensure t!at all parts installed are perfectly clean" (fter, maintenance on cylinders and sealing rings ensure t!at all t!e lu e oil pipes are filed wit! oil" #!is could e c!ec%ed y loosening t!e pipefitting" #ig!ten t!e pipe fitting again"
%aintenance Procedures & Safety ,contd-.
Do not use a c!isel to remove gas%ets attac!ed to t!e surface" )arefully slide a lade etween gas%et and surface" Damaging to t!e compressor*sealing surface can e avoided in t!is way" If t!e earings are found damaged, perform t!oroug! cleaning to avoid furt!er damage due to contamination of oil y metallic particles"
Problems & Troubleshooting
T(%05le E;cessive noise or a normal noise" P%11)5le Ca01e Misalignment and looseness" Re6e*-estore correct tig!tening of flyw!eel )!ec% support earings" )orrect alignment" ,erform correct tig!tening of foundation" -estore compressor design operating conditions" -estore t!e minimum condensate level in %noc% out drum" Increase fre=uency of drawing clean separators" (dd drain valves at points w!ere li=uid accumulates" -epeat assem ly, following t!e correct order" -estore t!eir fastening" )orrect position of t!e valves" -estore t!e surfaces on w!ic! t!e valve rests"
E;cessive press pulsation" E;cessive entrainment of condensate
Incorrect assem ly Loosening of cylinder valves
Pro)lems & Trou)les$ooting
contdT(%05le ( normal temperature gas disc!arge P%11)5le Ca01e1 Insufficient cooling Re6e*Increase cooling fluid rate" )lean t!e cooling system" 8tili1e a fluid t!at gives lower deposits" (pply ade=uate tig!tening tor=ue" -eplace damaged gas%ets" -estore sealing surface" -estore clearances etween piston rings and t!eir grooves" -eplace worn out rings" )orrectly install pac%ing seal" -estore proper clearance" -estore < replace piston rod"
Lea%age of gas from gas%ets
/as flow rate not at re=uired value"
Inade=uate piston ring seal"
E;cessive lea%age of gas from pac%ing"