WHAT IS COMPRESSORCOMPRESSOR ARE THE MACHINES REQUIRED FOR TRANSPORTATION/MOVEMENT OF GAS IN A SYSTEM.THE PURPOSE OF COMPRESSON IS TO INCREASE THE PRESSURE OF GAS FROM ONE LEVEL TO ANOTHER.

CLASSIFICATION OF COMPRESSORSTWO PRINCIPLE METHOD ARE USED TO COMPRESS GASSES.FIRST IS TO TRAP A VOL.OF GAS AND DISPLACE IT BY POSITIVE ACTION OF PISTON OR ROTATING MEMBER(POSITIVE DISPLACEMENT) .THE SECOND METHOD IS ACCOMPLISHED BY THE MECHENICAL ACTION OF CONTURED BLADES WHICH IMPART VELOCITY AND HENCE PRESSURE TO THE GAS.

COMPRESSORS

POSITIVE DISPLACEMENT DYNAMIC ( TURBO )(COMPRESSION OF FLUID BY REDUCINGVOL.OF COMPRESSION CHAMBER)

RECIPROCATING ROTARY

HELICAL LIQUID STRAIGHT SLIDE RING LOBE VANE CENTRIFUGALMIXED FLOW AXIAL

DYNAMIC TYPE COMPRESSORTHE MACHINE IN WHICH GAS IS COMPRESSED BY DYNAMIC ACTION OF IMPELLERS IMPARTING VELOCITY AND PRESSURE TO FLOWING GAS. THE VELOCITY HEAD IS CONVERTED INTO PRESSURE PARTIALLY IN THE ROTATING ELEMENT AND PARTIALLY IN STATIONARY DIFFUSER OR BLADE.

CENTRIFUGAL COMPRESSORTHE MACHINE IN WHICH VELOCITY AND PRESSURE ARE IMPARTED TO THE GAS IN RADIAL DIRECTION BY ONE OR MORE IMPELLER DIFFUSER COMBINATION.

AXIAL COMPRESSORTHE MACHINE IN WHICH VELOCITY AND PRESSURE ARE IMPARTED TO THE GAS IN AXIAL DIRECTION BY ONE OR MORE MOVING AND STATIONARY ROW OF BLADES.

MIXED FLOW COMPRESSORTHE DYNAMIC MACHINES WITH IMPELLER,WHICH COMBINE SOME CHARACTERSTICS OF BOTH CENTRIFUGAL AND AXIAL TYPE.

COMPRESSOR SURGE CONTROL PARAMETERS

CONSEQUENCES OF SURGING1. Rapid flow and pressure oscillations cause process instabilities .

2. Rising temperatures inside the compressor 3. Tripping of the compressor 4. Mechanical damage

WHAT IS SURGINGSurging is defined as a self oscillation of the discharge pressure and flow rate, including a flow reversal. Every centrifugal or axial compressor has a characteristic combination of maximum head and minimum flow. Beyond this point, surging will occur. During surging, a flow reversal is often accompanied by a pressure drop.

RECIPROCATING COMPRESSORReciprocating compressors are the best known and most widely used compressorsof the positive displacement type. They operate on the principal of reducing thevolume (increase in pressure), of specified quantity of trapped gas in an enclosureand then compressed gas is pushed out of the enclosure.

FUNCTIONING OF RECIP COMPRESSORA simple reciprocating compressor consists of a Crankcase, crank, piston rod,cylinder, piston and valves. Earlier reciprocating compressors were single acting i.e.gas or air was handled on one side of piston/cylinder. Now double acting is commonly used due to inherent advantage and higher efficiency.

P-V DIAGRAM OF RECIP COMPRESSOR

SCHEMATIC OF RECIP COMPRESSOR

MULTISTAGING WITH INTERCOOLINGReduced work of compression, as due to intercooling, compression is closer to isothermal (gives rise to minimum work of compression). This results in to savingof power and smaller sizes of subsequent stages.Lower discharge temperature and hence selection of material of construction for cylinder and its components and results in smaller size of subsequent stages.Limits pressure differential. This reduces excess strains in the frame.Good volumetric efficiency as compression is done in more than one stage and hence compression ratio is controlled.

CYLINDER LUBRICATIONBig reciprocating compressor cylinders are lubricated to avoid wearand tear of liner, piston rings, rider rings and stuffing box. Lubricants are injected in drops and are lost with the process gas. It is necessary for the lubricant to be compatible with the process gas Generally recip compressors above 100 KW are lubricated type.

Non-Lubricated TypeThere are many services in which oil in any form in the compressed gas, is not acceptable such as instrument air compressor and some process gas compressors.

CYLINDER COOLINGin big compressors where heat generation is very high. Water jackets are cast as part of the cylinder. Water is circulated in the cylinder jacketfrom external source.In some case the cylinder jacket temperature is to be maintained few degrees higher than the process gas temperature to avoid condensation during standstill, this is possible with water-cooled cylinders by maintaining the water temperature with use of heaters.

COMPARISIONRECIP- USED FOR DISCH.PR 828 BAR HYPERCOMPRESSOR ARE USED FOR LDPE PRODN ARE UP TO 3500 BAR.CENTRIFUGAL- DISCH.PR.UP TO 1035 BAR.CAPICITY- RECIP.COMPR.CAPACITY DEPENDS UPON CYL.SIZE,NO.OF THROW AVAILABLE AND DRIVER SPEED.CENTRIFUGAL COMPR.CAN DELIVER UP TO 680000 M3/HR BUT MIN.FLOW IS LIMITED BY SURGE POINT.MOLECULAR WT- RECIP.COMPR.PERFORMANCE DOES NOR AFFECTED BY MOL.WT.BUT IN CENTRIFUGAL COMPR,COMPRESSION RATIO IS HIGHLY DEPENDENT ON MOL.WT.HEAD IS DEVELOPED BY INCREASING K.E. WHICH LATER ON CONVERTED TO PRESSURE IN DIFFUSER.AMOUNT OF K.E.IS A FUNCTION OF GAS VELOCITY AND MOL.WT.COMPRESSION RATIO-THE MAX.COMPRN.RATIO A RECIP.COMPRESSOR CAN HANDLE IS LIMITED BYGAS DISCH.TEMP.TYPICAL COMPRESSION RATIO IS BETWEEN 1.2 TO 4.IN CENTR.COMPR.IT IS DEPENDENT UPON GAS M.WT,NO.OF STAGES,COMPR.SPEED.

COMPARISION CONTD.RELIBILITY-RECIP.COMPR.IS PROBALY NOT AS RELIABLE AS CENTRIFUGAL AS IT HAS MORE MOVING PART (PISTON RING,PRESSURE PACKING,RIDER RING )WHICH LEADS TO MORE WEAR AND TEAR THAN ANY PART OF CENTRIFUGAL COMPRESSOR.IN ADDITION RECIP.COMPR.HAS VALVES WHICH IS A MECH.DEVICE(SPRING LOADED CHECK VALVE) REQUIRE CONSIDERABLE MAINTENANCE.ANOTHER FACTOR IS CLEANINESS OF PROCESS GAS.WEAR LIFE OFMOVING PART AND VALVES WILL BE LONGER IF PROCESS GAS IS FREE FROM LIQUID AND SOLID DEBRIS.RELIBILITY OF CENTRIFUGAL COMPRESSOR IS 98-99 PERCENT.TYPICALLY MAINTENANCE INTERVAL IS HIGH IN CENTRIGUGAL COMPR.

COMPARISION CONTD.CAPITAL AND OPERATING COST-RECIP.COMPR HAVE LOWER CAPITAL BUT HIGH OPERATING COST THAN CENTRIGUGAL COMPR.

DOs & DONTs

Dos-Always ensure all start-up permissives like lube oil pressure /temperature, process gas suction pressure / temperature, suction /discharge valve opening etc. are fulfilled before start up. Ensure all operating parameters are within operating window as specified in the data sheet. Ensure functioning of all the protective devices for safe operation of machine.Ensure proper Standard Operating Procedures are followed during operations. Monitor regularly lube oil pressure and temperature..

DOs & DONTsCONTD. Monitor regularly lube oil level in the sump and top up if required. Always ensure proper grade of oil is being topped up as recommended by OEM. Ensure proper guards are mounted on the machine that no rotating parts are directly exposed. Ensure draining and purging of the machine before taking it into maintenance. Ensure machine is positively isolated mechanically and electrically before taking it into maintenance. Use proper certified tools & tackles while working on machine.

DOs & DONTsCONTD.DONts Do not start the machine without understanding the control, protective and monitoring system. Do not allow cooling water to cool bearing oil sump temperature to below recommended oil temperature. Do not start dismantling the machine without understanding dismantling procedure. Do not disable or bypass any protection for the machine without understanding the consequences and concerning the experts.

Do not change any alarm or trip value setting without concerning the experts.

Do not start maintenance work if the machine is not properly isolated.