Compressors are mainly of two types: 1. Dynamic Compressors 2. Positive Displacement Compressors .

GAS COMPRESSORS
Introduction:
Compressors are widely used to raise the pressure of a gas,
especially in Oil and gas industries.
1. When gas is separated in bulk/test separator,
gas pressure is often too low to meet the pressure required
for further utilization.
2. For the export of gas.
3. For gas lift and gas injection purpose.
4. For fuel gas purpose.

Types of Compressors:
Compressors are mainly of two types:
1. Dynamic Compressors
2. Positive Displacement Compressors
Each of the above type are classified as follows:
1.(a) Axial Compressors
(b) Centrifugal Compressors
2.(a) Reciprocating Compressors
(b) Rotary Compressors
GAS COMPRESSORS

Types of compressors
Dynamic compressors Positive Displacement compressors
Axial Centrifugal Reciprocating Rotary
GAS COMPRESSORS

Principles:
• Positive Displacement compressor, operates by volumetric displacement
• Operates with reciprocating motion or with rotary motion
GAS PISTON CYLINDER
Greater the reduction in volume
Greater the increase in Pressure
High Head and Low Throughput Machines
Reciprocating Compressor:
A compressor that operates by volumetric displacement is called a
positive displacement compressor

Depends on its Function it is categorised:
Single Acting Reciprocating Compressor:
DISCHARGE
SUCTION
CYLINDER
PISTON
One revolution: One forward stroke & one back ward stroke
One suction stroke & one discharge stroke
One discharge per revolution
Compression taking place on one side of the piston

Double Acting Reciprocating Compressor:
One revolution:
• Two suction stroke &
Two discharge stroke
• Suction stroke and Discharge
stroke simultaneously
• Two discharge/Two suction
per revolution-
HEAD END CRANK END
SUCTION
DISCHARGE
DISCHARGE GAS JACKET
SUCTION GAS JACKET
Compression taking place on both side of the piston

Compressor’s Components:
External Components:
DRIVER
FLYWHEEL
FRAME/CRANKCASE
CROSSHEAD
COMPARTMENT
SEAL COMPARTMENT
CYLINDER
PULSATION
DAMPENER
SUCTION VALVE
DISCHARGE VALVE

External Components:
1. Driver:
• Electric motor, Gas Engine and Gas/Steam driven turbines
2. Fly Wheel:
• To avoid any surge of power due to reciprocating action
3. Frame/Crankcase:
• To provide high strength at a reasonable weight.
• The frame serves as a reservoir for lubricating oil
4. Cross Head Compartment:
Access through side door to carry out alignment and clearance checks.
5. Seal Compartment:
Houses the gas and oil scraper seal.
Access through side door to carry out leakage checks.

External Components:conti-
6. Cylinder:
•Covered with water jackets for cooling purpose.
• Piston travels to and Fro with a liner.
7. Pulsation Dampener:
• To absorb the intermittent pulsation due to reciprocating action.
•Each on suction and discharge
8. Suction / Discharge valves:
• Acts as Non return valves.
• Suction valves fitted on top and discharge valves are fitted on bottom
of the cylinder.

Compressor’s Components:
Internal Components:

Internal Components:
1. Crank Shaft:
• To convert rotary motion from driver to reciprocating motion through
connecting rods and cross-head.
• Oil holes are drilled for the lubricating purpose.
OIL
PASSAGE
COLLECTING
RING
COUNTER
WEIGHT
CRANK
CASE

2. Connecting Rod:
• Connect the crank shaft and the cross head.
• Both ends are equipped with heavy duty sleeve bearing.
CROSS HEAD END CRANK END
SLEEVE BEARING
ADJUSTING WEDGE
ADJUSTING BOLT

3. Cross Head:
• Connects the connecting rod and piston rod.
• Made of cast steel and have white metal surfaces.
• Lubricated through main oil system.
SHIM
CROSSHEAD SHOE
PISTON ROD
LOCK NUT
CONNECTING ROD
CROSHEAD PIN
CROSSHEAD GUIDE

4. Cross Head Guide:
• Housing is provided with inspection holes and covers.
• Easy access through this doors for maintenance/inspection.
5. Piston Rod:
• Made of high quality steel alloy.
• Are heat treated to provide a harder and stronger finish.
• Heavy chrome plated to protect against H2S.

6. Stuffing Box:
• Used for separation of the compressor cylinders to the atmosphere as well as
between the cross-head section and distance piece.
•Stuffing boxes can e lubricated or non-lubricated, water cooled or
non-water cooled.
7. Packing:
• Prevents compressed gas leak along
the piston rod.
• Packings are Metallic rings, fibre, plastic
or metal.
• Carbon or Teflon - used when packing
must be free of lubricant
OIL INLET
PISTON ROD
GAS VENT LINE

Reciprocating Compressor
8. Piston:
• Solid and Hollow Piston
UNDER
7 INCH
OVER
7 INCH Hollow
• Made of Cast iron - usually made solid
• Low-speed compressor(up to 330 rpm.
• High-speed compressor(330 - 600 rpm)
• More than 7” in dia. Usually made hollow..
• Manufacturers specify the amount of clearance required.
• To assemble cold piston, make the piston end clearance greater at the head-end
of the cylinder.( 1/3 on the crank case end and 2/3 at the head-end)

9. Piston ring:
• Provide a seal to minimise a leak.
• Also carry heat from the cylinder.
• Compressor’s cooling system carries most of the heat.
TAPERED PORTION
OF ROD
GROOVE
PISTON ROD
PISTON
RINGS
LOCK NUT
• Rings are made of material that wears more rapidly than cylinder or liner.
• Made of bronze, cast iron, bakelite, teflon etc.
• Material cause low friction.
• Ring grooves should be very accurate.
• Minimum allowable clearance 0.3 mm between piston and cylinder walls.
RING
EXAPANDER

10. Cylinder assembly:
• Cylinder bore and Liner - to reduce reconditioning cost.
• Worn liner needs to be replaced.
• Due to the weight of the piston, wear is usually at the bottom of the cylinder.
LUBRICATOR INLET
CYLINDER WALL
Misalign liner:
• Oil holes in the liner always aligned with the lubricator inlet.
• Misalign liner can cause plug or close the inlet.

11. Bearings:
• Bearing should be inspected for cracks, chips, scratches and wiping.
• Typical cause for high temperature-
- blockage in the oil supply.
- Inadequate oil flow.
- tight bearing clearance.
- Crankshaft misalignment
12. Oil wiper Rings:
• To prevent crankcase oil from passing into the cylinder.
• To prevent condensate from entering the crankcase.
• Normally two or three wiper rings are used in an oil seal.
• Two types
- Designed to turn back a large volume of oil along the rod.
- Oil wiped from the rod drains into an annular and back into the crankcase.

Reciprocating Compressor - Operations & Controls
Auxiliary System:
• Lube Oil System
• Cooling Water System.
• Safe Guarding System.
Lube Oil System:
Functions:
• Lubrication
• Cooling.
• Cleaning.
• Sealing.

Parts to be lubricated:
 Main bearings
 Connecting rod bearings
 Crosshead pin
 Crosshead
 Piston assembly
Oil primarily flows to the main bearings.
Normal Oil pressure is around 2 bar(g) at about 55 C.
Through drilled holes in the crankshaft the oil flow to the connecting rod
bearings.
The connecting rod itself has a drilled channel directing the oil to the crosshead pin
bearing.
After having lubricated and cooled this bearing,
the oil flows back to the tank.

Lube oil pressure Low Low-
Creates the compressor shutdown.

Cylinder Lubrication:
• To minimise the friction between piston rings and liner.
• Drop wise lubrication with the force feed lubricator.
• Lube oil between the rings help to cool and to seal - prevent gas leak from
cylinder to atmosphere.
DELIVERY
TUBE
GLYCERIN
SIGHT
GLASS
PLUNGER
CAM
OIL INLET
PUMP SHAFT HAND CRANK
LL Flow Alarm
LL Level Alarm

• Lubricator is directly driven by the crankshaft.
• Through the sight glass lube oil flow can be checked.
• Handwheel is provided to turn a couple of revolution before start-up.
• Lubricating flow to be maintained in such a way that a layer of oil is
formed on the cylinder liner surface - to make the friction coefficient of
piston rings and liner low.
• The number of lubricating point depend on cylinder size, rpm etc.

LUBRICATOR PUMP
1st STAGE
1st STAGE
1st STAGE
2nd STAGE
3rd STAGE
4th STAGE
CYLINDER LUBRICATION
GAS PACKING LUBRICATION
DRIVE END

Cooling Water System:
To evacuate the heat from
- Cylinders
- Cylinders covers(HE and CE)
- Lube oil
- Stuffing Box
• Water enters to the cavities of the cylinder from the bottom and leaves the
cylinder from the top.
• At the discharge of the pump, low pressure switch is installed.
• Normal pressure should be 3 bar(g), Low pressure alarm at 1 bar(g)
• Cooling water temp. high at cylinder outlet may trip the compressor.
• Cooling water flow low also may trip the compressor.

PROCESS:
• To compress the off-gas from the Separators.
• To increase the gas pressure for the injection purpose.
• Reciprocating compressors are high head and low through put machines.
• Multi stage compressors with intercoolers and scrubbers.
• Inter stage scrubbers - To separate liquid form the compression and
cooling.
• Inter-stage coolers - To cool the discharge gas after compression and bring
it to the suction temperature for the next stage.

GAS
FROM
SEP.
1st STAGE 2nd STAGE 3rd STAGE
TO G/L
HEADER
CONDENSATE TO PRODUCTION SEPARATOR
TO FLARE KO DRUM
COOLER COOLER
COOLER
INTERNAL RECYCLE START UP
4th STAGE
COOLER
PIC PRCA
LOW SELECTOR
LOW SELECTOR
PRCA
PIC
PIC
PRCA
B / D
TO
FLA
RE
STATION RECYCLE LINE

• Off-gas from the Separator flows to the suction scrubber through ESD valve.
• Gas is compressed up to 1100 kpa(g) in the 1st stage of the compressor.
• Then gas enters into 2nd stage suction scrubber,via the fin fan type cooler, gas
cooled down to the required suction temperature for the 2nd stage.
• In the scrubber free liquid is removed and gas flows to the 2nd stage of the
compressor, where it is further compressed to the higher pressure upto 3000 kpa(g).
• Before gas flows to the 3rd stage and 4th stage, it flows to the respective fin-fan
coolers and scrubbers.
• Final stage discharge gas with higher pressure flows through the discharge
scrubber before it goes the gas-Lift or Injection header.
• Liquid from the 1st stage suction scrubber routed to flare knock-out drum
through level control valve and condensate from all discharge scrubbers routed
through respective LCV to the production separator or condensate recovery system

Scrubber:
• Cyclone Scrubber-Used to remove liquid and solid particles from a gas stream.
• Gas enters at a tangential direction.
• Some case swirl tube is provided to increase the centrifugal action.
• Centrifugal force throws the heavier and large particles away from the centre.
• Gas flows upwards, liquid falls downwards.
• Higher centrifugal force separates liquid and dust from the gas very easily.
Inter-stage scrubber:
• Multi-stage scrubbers - for maximum efficiency.
• Designed to remove the condensate- avoid entering liquid to the next stage.
• Final stage-remove before gas exported to header or gas grid.

Inter-stage Cooler:
• Normally Fin-fan type.
• Bundle of tubes with fins to increase the surface area for effective cooling.
• Air is being forced through the bundle of tubes.
• Air moves through the fins surrounded on the tubes, makes the effective heat
exchange from the gas which is flowing inside the tubes.
• Hot gas from each stage of the compressor passes to the cooler.
• Gas is being cooled to the required suction temp for the next stage.
Temperature Controls:
By two ways
• Variable speed motor:
- Temperature increase/decrease  Speed of motor varies
•On/Off control :
- Manual control
- Two speed control. i.e. high speed and low speed
- In winter: day time on high speed and night time on low speed
or keep minimum number of fans running.

Start-up and Shut-down (procedure in general):
Pre-start Check:
• Follow PTW system after maintenance work.
• Lining up of Compressor (Different steps for different compressors).
• Check auxiliary system. i.e. crankcase oil,lubricator lube oil, seal oil level, cooling
water system etc.
• Hand crank the lubricator.
• Cooler fans in operating condition.
Start -up:
• Start the prime mover/Motor (sequence starts)
• Suction ESD by-pass valve opens, equalise the pressure across main ESD.
• Vent valve closes.

• Main ESD opens.
• Compressor starts and will be on total recycle.
• Discharge start opening.
• By-pass start closing, observe discharge pressure.
• Open the consumer supply.
•Observe the temperature at discharge.
• Observe water jacket, lube oil, packings etc.
Shut-Down:
• Gradually unload the compressor.
• Allow it to run for some time to avoid unequal cooling of metal parts.
• Switch off main driver.
• After sometime switch off auxiliaries.

 Start/Stop Operation:
• Although there is local/remote switch, start-up is from main panel.
• The local function only acts as an interlock to the auto start, to enable maint.
• In local mode - not possible to open/close valves.
• Can be started only by remote(from control room)
 Auto Start - Start-up sequence:
• Once start initiate - no further operator intervention.
• Logic takes it through start sequence step by step.
• Monitor the process condition for correct response with in set time and go to
next step.
• Failure of process tends to “ Start sequence failure”
• Each hold - for process condition to be satisfied.

Start Sequence:Initiate the start( Sahi Nihyada Atn-K 1411)
• Step 1 Close vent valve
Start pre-lube pump start lubricator pump
• Hold 1 Vent valve closed
Lube oil flow not low low, lube oil header pressure not low low
• Step 2 Open purge valve
• Hold 2 Purge valve opened.
• Step 3 Start pressurising time
• Hold 3 Suction pressure not low low
• Step 4 Close purge valve
Start main drive motor, start cooling water pump, open suction
valve.

• Monitor 4 Suction valve open
• Step 5 Start cooler fans
• Hold 5 Cooling water pressure not low low
• Step 6 Open discharge valve
• Hold 6 Discharge valve opened
• Step 7 Start completed enable compressor loading

 Compressor Stop:
• Can be stopped by Operator
• Logic takes the compressor through a sequence to a safe halt and vent.
• Trips - The compressor is locked out. Needs to Reset to restart.
Stop Sequence:
• Step 1 Disable compressor loading
• Hold I Start-up bypass valve open
• Step 2 Stop main driver motor, close suction valve, close discharge valv
•Hold 2 Suction valve closed, discharge valve closed,
Main driver motor stopped.
•Step 3 Stop cooling water pump,
Stop cooler fans, stop lube oil pump, stop lubricator pump.

• Hold 3 Purge valve closed
Recycle header valve closed
• Step 4 Open vent valve
TRIP Functions:
Compressor Shut-down:
• Logic incorporates a shutdown function - shutdown the compressor and
auxiliaries, and vent to the atmospheric pressure.
• Logic lockout the system, reset is necessary for a re-start.
Initiation of a trip:
• On process condition or
• ESD by push button or Station trip.
• If one compressor trip - No effect on others, unless high suction pressure exists.

Trip Action:
• In the event of a trip, all electric drives will be stopped.
• Suction, Discharge, recycle and purge valves close and start-up bypass valve will
open.
• The vent valve will open
Over-rides on trip functions:
• Maintenance over-ride : For maintenance of that individual trip signal.
• An Auto start up over-ride:During start up,i.e. cooling water flow low low,
suction pressure low, lube oil header pressure low,
lubricator oil flow low etc.

Controls:
• Local skid mounted panel consist of pneumatic logic for control, alarm and
shut-down features.
• Unit control panel(UCP) located in the control room - consists of the logic
which controls the process operation and initiates the ESD action(i.e. station
process abnormal)
• Suction pressure - 175 kpa(g) - maintained by split range control system,
regulates the gas flow to the flare.
• Discharge pressure - 8800 kpa(g) - control by PC which is a part of compressor
load share and capacity control logics.
• No unloading valves to reduce the throughput.
• To avoid excessive recycling the compressor uses clearance pocket capacity
control.

Fixed-Volume Clearance Pocket
The clearance pocket is always in service

Hand Operated Fixed-Volume (HOFV) Clearance Pocket
The HOFV clearance pocket may be used

Different Clearance Pockets may be screwed into
or
flanged onto the cylinder
Once installed, such pockets have a fixed volume.

Variable-Clearance pocket
The hand-wheel makes it possible to adjust/vary the amount of clearance
As needed.

GAS FROM SEPARATORS
CLEARANCE POCKET CONTROL
K- 1411
GAS TO GASLIFT
MANIFOLD OR
EXPORT HEADER
RECYCLE (PRESSURE CONTROL)
START-UP RECYCLE
14-UZ-562
14-UZ-561
14-PRCA-309.1/2
14-UZ-478
14-UZ-481
14-UZ-484
LOW SELECT
COMPRESSOR PACKAGE BOUNDARY
OVER ALL K 1411 PROCESS CONTROL

•If recycle valve opens beyond a set position for more than pre-set time limit,
clearance pockets in the compressor will open - result in a decrease in a discharge
pressure, cause the recycle valve to close.
• If recycle valve closes beyond a set position for more than pre-set time limit,
clearance pockets in the compressor will close - result in a increase in a discharge
pressure, cause the recycle valve to open.
• Thus system allows the compressor to operate most efficiently.

Normal Operation Check:
• Makes routine inspections.
• Logging of routine process parameter - checklist.
• Check suction valves cover for any passing/faulty valve.
• Check all auxiliary systems for Pressure, temperature, delta pressure
across filter, strainer etc.
• Worn bearing produce drop in lub oil pressure.
• Lubricator level through sight glass and injection rate.
• Check for any abnormal sound.
• Preventive maintenance schedules involve monthly, quarterly, half yearly or
yearly inspection and checks.
• Routine test of lube oil sample.
• Check of liquid levels in all inter-stage scrubbers and its respective LCV’s
function.
• RV’s routine inspection and calibration.
• All safety guards for flywheel, belts, other moving parts.
• Comparison of checklist with previous reading for any deviation.
• Take and follow up all corrective action to run the unit safe.

Compressors are mainly of two types: 1. Dynamic Compressors 2. Positive Displacement Compressors .

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Compressors are mainly of two types: 1. Dynamic Compressors 2. Positive Displacement Compressors .