Introduction to Positive displacement compressors

Petroskill course

Hector Nguema Ondo Perez

Positive Displacement compressorsTable of contents

• Reciprocating compressors

Basic parts of reciprocating compressors P-V Diagrams Controlling a compressor

• Type of clearance Overhead Stabilizer Compressor #5

• Rotary compressors• Type of rotary compressors• Design of rotary compressors• Operation of rotary compressors

• Constructing Reciprocating compressors Valves

Pistons

Rod packing• Lubrication

• Cooling and safety controls

• Operating Compressor Procedures• Pre-start up• Start up • Shutdown• Normal Operations

Reciprocating compressorsReciprocating compressor basic parts

Frame: convert rotary motion into linear motion

• As the shaft turns motion is transferred to a throw on the crankshaft connected to the crosshead. Crosshead slides transferring motion to the piston rod.

Reciprocating compressorsReciprocating compressor basic parts

Distance Piece: provide isolation between the process side (cylinders) and the frame

• Crosshead guide end of the piston rod is isolated from the crankcase by a packing, which provides seal to prevent leakages.

Cylinder: creates internal passages for the gas flow and access to the suction and discharge valves

• Cylinder is cast with an outer wall and main bore at the center.

• Cylinder must match the stroke of the frame and the selected bore diameter required for the pressure and flow condition

Reciprocating compressorsReciprocating compressors

Single-acting Double-acting

• In the single-acting compressor, the forward stroke is the compression stroke. 

• The backstroke is the suction stroke.

• Forward stroke

• Crank end Suction Valve

• Head End discharge valve

• Backstroke

Reciprocating compressorsInterpreting P-V diagrams

P

V

Discharge

Suction

CompressionExpansion

A: Piston reaches head-end of the cylinder

B: Suction valve opens

D: Discharge valve opens

C: Piston reaches crack-end of the cylinder

Expansion of clearance gas

Fresh Gas enters cylinder

Gas inside cylinder is

compressed

Compressed gas leaves cylinder

Overhead Stabilizer Compressor #5

Reciprocating compressorsControlling a compressor

Throttling: Change capacity or rate of flow through the compressor

Compressor ratio (R)

• R is low (2.0) increase HP

• R is between 2 and 2.5 minimum impact

• R is above 2.5 decrease HP

Clearance Pockets: Varying clearance pocket increase or decrease amount of gas entering the compressor

Excessive clearance volume → Shutoff

• Cylinder capacity and Volumetric efficiency are 0

Reciprocating compressorsTypes of clearance pockets

It is always in use and cannot be adjusted by operators.

Fixed-volumeHOFV

The hand wheel opens a valve which allows for a fixed amount of clearance when needed. 

Once installed, these types of pockets have a fixed volume.  The amount of clearance may be changed by changing the size of the pocket

Screw or flanged Variable

The hand wheel makes it possible to adjust the amount of clearance as needed. 

Reciprocating compressorsControlling a compressor

Unloading: An unloaded compressor compresses less gas, uses less horsepower in the motor, and will be more efficient

Controlling speed: Reducing speed reduces fuel used and decrease operating costs.

• Unload one end of a single cylinder.

• To reduce the pressure and temperature of the packing, unload the crank end.

• Unload all of the crank-end suction valves at one end of a multi-cylinder.

• As gear trains and belt drives reduce speed from the driver, they increase the torque delivered to the compressor shaft

Rotary compressorsRotary compressors

A rotary-type lobed blower has two impellers that trap and transfer gas.  Because of the clearance space, some gas could leak backwards.

Lobed blowers Sliding-Vane Compressors

As the rotor turns, the pocket gets smaller while the vanes move into the rotor, compressing the gas.  At maximum compression, the gas is released out of the discharge port.

Screw compressors

Gas is displaced by helically-lobed rotors. The reduction in volume increases the pressure of the gas.

Liquid-piston compressors

As the rotor turns, gas enters from the two inlets in the stationary port chamber. The blades displace the gas toward the discharge ports in the stationary port chamber

Rotary compressorsOperation of rotary compressors

• Gas is drawn between the top side of rotors and the lobes

• As rotors rotate the inlet port is sealed

• Rotation reduce the gas volume increasing pressure

• When the gas is exposed to the outlet port the gas is discharged

Rotary compressors

Propane refrigerant compressor

• Compression is achieved by the meshing of the two helical rotors on parallel shafts

• Capacity control is achieved by means of a slide valve in the casing

Positive Displacement CompressorsCompressor valves

Plate Valves Poppet Valves

Channel Valves

• Capable of handling large flows by moving a sealing plate to open or close a flow path

• The springs push from the stop plate and the channels cover the slots in the valve seat

• These valves provide a low pressure drop and are often used when compression ratios are high. 

Positive Displacement CompressorsPistons and Piston Rings

Piston Rings

The rings seal by a close fit against the cylinder and against the flat sides of the grooves

Rider BandsRod Packing

Positive Displacement CompressorsBores, Liners, Crosshead and bearings

Cylinder Bore and Liner Crossheads

Bearings

A film of oil separates the crosshead pin and the crankshaft from the bearings. 

The oil is supplied under pressure through holes in the bearing surfaces. 

If breakage occurs between the piston and the crosshead:

• Faulty alignment• Improper rod Material• Operating above the max

load

If there is excessive wear in the cylinder bore, the entire body does not have to be replaced.  Only the worn liner needs to be replaced

Positive Displacement CompressorsLubrication systems

Splash system• Supply of oil is maintained in the crankcase

and splashed up by the rotation to lubricate the bearings and the crosshead

• Centrifugal force is used to lubricate the crank pin. The oil is forced up to the crank pin through an oil passage.

Force-Feed system• Oil is pumped

under pressure to the required parts. 

Positive Displacement CompressorsFilters & Relief Valves

Self-cleaning filters

Bypass Relief Valves

• Opening the bypass when the filter is blocked allows oil from the pump to return to the crankcase.  In this system, the compressor shuts down when the bypass valve is open to protect against the failure of lube oil pressure and supply

• The self-cleaning filter is designed to be cleaned by turning a wiper so that it sweeps over the holes in a cylindrical screen.

Positive Displacement CompressorsOil Flow

• Oil enters passages drilled in the crankshaft.  The passages lead to the crank pin.  The oil lubricates the crank pin and enters the passages drilled in the connecting rods. 

Positive Displacement CompressorsCooling system

•  A compressor's cooling system is designed to remove some of the heat of compression.

• An intercooler cools the gas between the stages, and an aftercooler cools the gas after it leaves the compressor.

Positive Displacement Compressors

Positive Displacement Compressors

2nd Suction Scrubber

Suction bottle

Reciprocating Gas Compressor Frame

Compressor Engine

Intake

exhaust

Engine Exhaust silencer

Gas cooler

Lube oil tank

Overhead Stabilizer Compressor #5

Positive Displacement CompressorsSafety Controls

Spring Loaded Safety Valve

• The pressure needed to open the valve depends on the compression of the spring. 

Rupture Disc

• Safety valves or rupture discs prevent excessive pressure from building up in the compressor

• Safety valves discharge the gas to a vent or flare line.  Safety valves must be installed between the cylinder outlet nozzle and the first valve in the discharge line.

Positive Displacement CompressorsSafety Controls

Governor Overspeed Trip

Mechanical governor

Hydraulic governor

• Keep the engine running at a constant speed

• Control the amount of fuel or fuel-air mixture by opening and closing the engine's throttle or mixing valve

• Connected to the engine crankshaft through the gears

• Hydraulic fluid is used to control the operating mechanism of the governor

• The trip pin is held in its recess by a compression spring.  When the engine overspeeds, centrifugal force pushes the trip pin out of its recess in the driveshaft and trips the shut-down switch.

Positive Displacement CompressorsSafety control

Oil Pressure Control Water Temperature Control

• The low oil pressure shutdown device shuts off the compressor when the oil pressure is low.

• Some devices cut off the fuel supply and also shut down the ignition

• Activate if the cooling water temperature rises dangerously high.

• Monitor the engine jacket water pressure

Positive Displacement CompressorsPLC Cause & Effect Chart

Water Level

Oil & Lubrication

Total Trips

Vibration & Speed

1st Stage

2nd Stage

Positive Displacement Compressors

• All compressor parts are in good operating condition

• Vent Valves are open and compressor block valves are closed

• The incoming gas is clean and free of liquid

• The lubrication and cooling system are operating or are ready to operate

• Safety Valves and other automatic control devices are properly set

Pre-start up procedure

Positive Displacement Compressors

• Open the suction equalization valve for 30 seconds

• Open the suction block valve and close suction equalization valve

• Open discharge block valve

• Engine Fuel system block Valve is opened

• ENGINE START Sequence is initiated

• Engine starts and compressor is an unloaded condition during warm-up

• After Lube Oil temperature is above SP, manual LOAD becomes enabled. It is recommended to load slowly in incremental steps.

• Sidestream supply is made available to the compressor.

Start-up procedure

Positive Displacement Compressors

• LEVEL ONE S/D: Close process side stream block valve, open the process recycle/bypass valves to unload the compressor, stop the drive engine, close engine fuel system block valve, close discharge block valve, close process suction block valve and close process suction equalization valve.

• LEVEL TWO S/D: Similar to LEVEL ONE S/D, but without delays

• LEVEL THREE S/D: Similar to LEVEL TWO S/D but opens the Blowdown valve until pressure is less than 50Psig

Shutdown procedure

Positive Displacement Compressors

• Check Oil Levels Daily (Operator Rounds)• Inspect lube oil for contaminants (MEGPL LAB)• Check Lube oil Pumps ( Operator Rounds)• Change Oil Filter cartridges (Maintenance)• Calibrate instruments daily ( I&E PMR)• Replace pneumatic filter regulator filters every 6 months

(Maintenance)• Open drains on all pneumatic filter regulator for 10 sec every week

Maintenance Activities

ANY QUESTIONS??

Thanks for coming!

Hector Nguema Ondo Perez