2. Compressors
Compressors:
Move air or gas in higher differential pressure ranges from 35 psi to
as high as 65,000 psi
Blowers:
Move large volumes of air or gas at pressures up to 50 psi
Fans:
Move air or gas at a sufficient pressure to overcome static forces.
Discharge pressures range from a few inches of water to about 1
psi
Bloch H.P. & Hoefner J. J. (1996). Reciprocating Compressors, Operation and Maintenance. Houston TX: Gulf Publishing Company
5. PD vs Dynamic Compressors
Positive Displacement (PD) Compressors:
Increase the pressure of fluid by reducing its volume
Dynamic Compressors:
Increase the pressure of fluid by converting momentum into
pressure rise, the angular momentum is transferred to the
fluid from a continuously rotating member
6. Reciprocating Compressors
Reciprocating compressors consist of a piston
working within a cylinder to physically compress
the gas contained in that cylinder.
a volume of gas is drawn into a cylinder,
it is trapped,
and compressed by piston
and then discharged into the discharge line
7. Single Stage Vs Multi Stage
Single Stage Compression:
The Pressure is increased from inlet to discharge pressure in a
single step compression.
Multi Stage Compression:
The pressure is increased from inlet to discharge pressure in
several steps of compression.
E.g. The two-stage compressor adds a second piston to the
process that fires after the first to further compress the air in
the tank
10. Single Acting Vs Double
Acting
Single Acting Reciprocating Compressors:
It is a compressor that has one discharge stroke per revolution
of crankshaft.
Double Acting Reciprocating Compressors:
It is a compressor that completes two discharge strokes per
revolution of crankshaft. Most heavy duty compressors are
double acting.
11. Working of Reciprocating
Compressors
First, the intake gas enters the suction manifold, then
flows into the compression cylinder
where it gets compressed by a piston driven in a
reciprocating motion via a crankshaft, and is then
discharged.
Depending upon the construction of the compressor, a
single stroke of a piston may comprise of both the
intake and discharge of the gas.
A sequence of events describing how compression
occurs in a reciprocating compressor can also be
represented on a PV-Diagram.
20. Parts of Reciprocating
Compressors
Crankshaft and Bearings
Connecting Rod and Cross Guide
Pistons and Rods
Rod Packing
Cylinders
Valves
Distance Piece
Variable Volume Clearance Pocket (VVCP)
21. CrankShaft
May be casted or forged (for Power > 150 hp)
Counter weights to minimize rotary unbalance and
reciprocating unbalance.
24. Crosshead Guide
Supports and guides the crosshead
Installed, adjusted and doweled to the frame at the
factory
25. Crosshead bushing
Provide protection to the crosshead
Bronze or steel backed aluminum
Interference fit
Receives lubrication through a bore in the connecting
rod
26. Shoes and Shims
Actual riding surface
Jack bolts provided for shim adjustment
Aluminum or cast iron
Lubricated by oil thrown off crankpins
27. Piston
Piston transmit energy from the crankshaft to the gas
in the cylinder
Piston rings are used for sealing but the wear rate
should be minimum
Ring material may be metallic or non-metallic
Piston rod is a threaded to the piston and transmits
the reciprocating motion from cross head to piston.
Weight of Piston contributes to compressor shaking
forces therefore, it is kept minimum.
29. Piston Rod Packing
Provide seal around piston rod and across
the flat surface of adjacent cup
Prevents the leakage of gas into the
distance piece
Tie studs are for alignment and not for
tightening
End cup is fitted with gasket to prevent gas
leakage around the case.
cup
Piston
rod
Tie stud
30. Packing Types
Fully lubricated:
Oil is supplied to the packing from the mechanical lubricator. The cases
are drilled so that oil will be carried to the oil cups and then to the piston
rod through a drilled hole in the oil cups
Mini-lubricated:
Same as fully lubricated, except that oil to the packing is decreased
approximately 30 percent of normal flow.
Micro-flow:
There is no oil supplied directly to the packing. The only oil that reaches
the packing is carried over on the piston rod from the crankcase.
Fully non-lubricated:
No oil is supplied to the packing and no oil carryover from the crankcase
is allowed to reach the packing. The crankcase oil is prevented from
reaching the packing by means of an oil deflector collar attached to the
piston rod
31. Packing Rings
It seal along the piston rod, between
themselves and the packing case
Tangential Cut Ring:
The tangent ring is cut into three segments so that
each cut lies on the side of an equilateral triangle
As wear occurs, the ring segments will close radially to
compensate while still maintaining sealing contact at
the tangential joints
33. Packing Ring Sets
Radial and Tangent Set:
An overlapping seal joint that prevents gas passage
along the rod from the outside periphery toward the
rod bore. It has no through escape
The inner radial cuts of this tangent ring are arranged
so that they occur approximately between the radial
cuts of the first ring in the pair
Rings are doweled to maintain this staggered cut
arrangement
Match marks are always on the pressure side
Seals on the compression stroke but allows backflow
into the cylinder on suction stroke
34. Packing Ring Sets
Radial, Tangent and Back-up Set:
Used in high pressure compressors
The radial packing ring is installed in the groove on the
pressure side toward the cylinder. The tangential-cut ring
is installed between the radial and the back-up ring
Single acting- Seal in only one direction i.e. Seals on the
compression stroke but allows backflow into the cylinder
on suction stroke
Back-up ring is anti extrusion ring and prevents extrusion
of radial and tangent rings, It also help in dissipation of
heat from the rod
Generally radial and tangent rings are non metallic while
the backup ring is metallic
Used between 700 to 1500 psi
35. Packing Ring Sets
Tangent and Back-up Ring Set:
Single acting- Seal in only one direction i.e. Seals on the
compression stroke but allows backflow into the cylinder on
suction stroke
They are not dowelled together
No pressure limitation and has been used to handle
thousands psi
Double Tangent Ring Set:
Double acting therefore, installed with either side facing the
pressure
Dowelled to prevent the misalignment
Used for vacuum application, usually behind the vent to
prevent leakage into the distance piece
Pressure Breaker Ring:
Not intended to seal but to break or slow down gas passage
36. Valves
Spring Loaded, Gas actuated
Permits only one way flow of gas either into or out of
the cylinder
Components of a Valve:
Seat
Sealing Element
Device to limit travelling of sealing element
39. Valve Configurations
Rectangular Element valve:
Lift is approximately 0.100” to 0.200”
The rectangular shape of the elements eliminates the use of a
cross ribbing system to provide structural integrity of the valve
seat and guard
This enables the use of long uninterrupted slots in the seat
and guard and provides good flow area.
These valves generally make excellent use of available valve
port areas to provide flow area at moderate valve lifts
Feather Valve
Channel Valve
Reed Valve
40. Valve Configurations
Concentric Ring Valve:
These valves use one or more relatively narrow metallic
or nonmetallic rings arranged concentrically about the
center line of the valve
Lift is controlled by a flat stop that limits the ring
motion
wide application range and are used in low- to high-
speed application
Lifts are as high as 0.16”
the top end of application speed is in the area of 1200
rpm
The flow efficiency of these valves is not as good as
straight element valves, but it is good for the pressure
ratios generally used.
41. Valve Configurations
Ported Plate Valve:
Ported plate valves are similar to the concentric ring
valves, except the rings are joined together into a single
element
The number of plate edges available for impact is reduced
Spring is generally provided by coil springs that directly
contact the plates, eliminating the buttons used in
concentric ring valves.
Flow efficiency of these valves is roughly equivalent to the
concentric ring valves
The geometry of the sealing element is not as flexible as
the concentric ring valves
They are high-speed valve although it can be applied
equally well to low-speed machines
42. Valve Configurations
Disc / Poppet Valve:
They can be found in applications ranging from a few
hundred psi to over 50,000 psi
Maximum lift is approximately 0.300”
Springing is provided by individual coil springs for each
poppet.
Flow efficiency is excellent because of the high lift and
streamlined poppet head
This makes an ideal valve for low-pressure ratio, high-gas
density applications where valve losses are very important
and sufficient pressure drop can be generated to drive the
valve open
The top end of application speed is in the area of 1200
rpm
Poppet valves are ideally suited to the low-speed,
medium-pressure, low-ratio efficiency service typical of
gas transmission applications.
43. Variable Volume Clearance
Pocet (VVCP)
Control capacity of fix speed compressor
Reduce load on driver
VVCP open:
Greater clearance volume
More trapped gases
The gas expands on intake stroke and take space in cylinder
Effective intake stroke length decreases
Consequently capacity of compressor decreases
Notas del editor
The Thermodynamic Cycle
Compression occurs within the cylinder as a four-part cycle that occurs with each advance and retreat of the piston (two strokes per cycle). The four parts of the cycle are compression, discharge, expansion and intake.
At the conclusion of a prior cycle, the piston is fully retreated within the cylinder at V1, the volume of which is filled with process gas at suction conditions (pressure, P1 and temperature, T1), and the suction and discharge valves are all closed. This is represented by point 1 (zero) in the P-V diagram.
As the piston advances, the volume within the cylinder is reduced. This causes the pressure and temperature of the gas to rise until the pressure within the cylinder reaches the pressure of the discharge header. At this time, the discharge valves begin to open, noted on the diagram by point 2.
With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3.
Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4.
When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1.
Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3.
Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4.
When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1.
Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3.
Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4.
When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1.
Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3.
Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4.
When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1.
Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3.
Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4.
When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1.
Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
Metallic Piston Rings:
CAST IRON*Dirty or corrosive gases
BRONZE*High pressure*High temperature
Non Metallic Piston Rings:
PTFE –TEFLON
*Carbon filled -Non-lube and lubricated
*Glass andMolyfilled -Lubricated
*Bronze filled -Non-lube and lubricated (mostly air service) High temperature, non-corrosive
*Ryton
•THERMOPLASTICS-(Lubricated)
The concentric ring valve uses one or more relatively narrow rings arranged concentrically about the centerline of the valve
These valves have the advantage of a low stress level due to the lack of stress concentration points.
The disadvantage is that it is difficult to maintain uniform flow control with the independent rings.
The ported plate valves, are similar to the concentric ring valve except that the rings are joined into a single element.
The advantage is that the valve has a single element making flow control somewhat easier. Because of the single element, the number of edges
available for impact is reduced. The valve may be mechanically damped, as this design permits the use of damping plates.
It has the disadvantage that because of the geometry used, the stress is higher due to the potential of high stress concentration.
The poppet valve consists of multiple, same-size ports and sealing elements.
The advantage of the valve is that has a high flow efficiency due to the high lift used and the streamlined shape of the
sealing element.
The disadvantage is that the valve is not tolerant of uneven flow distribution. The valve is most commonly used in gas transmission
service and in low speed, low-to-medium compression ratio compressors. There appears to be an increase in the use of poppet valves
in hydrocarbon process service because of the ease of maintenance.
Reciprocating compressor always have a certain amount of clearance or un-swept volume at the end of the cylinder. Some of this comes from the valve pockets and the rest from the small distance between the piston head and the cylinder head. This clearance space effects the capacity of the compressor because the vapor tapped in this volume doesn't leave the cylinder at the end of the compression stroke. It just re-expands on the suction stroke and takes up space that would other wise be used by low pressure vapors entering the cylinder. Designers take advantage of this condition to help control the capacity of fixed speed compressors. It can be done by adding a fixed volume to the compressor cylinder's head or by adding a variable volume device with a handwheel or pneumatic operator.