Reciprocating Compressors

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Objective of the Training

• Acquisition of basic knowledge of reciprocating piston compressors,

as far as required for understanding valve-, control systems-, piston
ring- and pressure packing performance.

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2
Contents

• Single-Acting Reciprocating Compressor

Principle, compressor cycle, pV -diagram (indicator diagram)

• Double-Acting Reciprocating Compressor

compressor cycle, opening and closing of the compressor valves

• Multistage Reciprocating Compressor

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Crank Mechanism
Components, kinematics,
masses, rod forces

3
Basic principles - working principle

• Gas is compressed in a reciprocating cycle

Animation

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quick slow

- Basic Principles
- Double Acting Piston Compressor
- Multistage Compressor

4
Basic principles - indicator diagram

Pressure - time diagram Pressure - volume diagram

p,t p,V
shows the pressure in the cylinder shows the pressure in the cylinder
at a given time or crank angle at a given volume or piston position
indicator pressure

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time t volume V
TDC BDC TDC TDC BDC
360°
0° 180° 360° 0° 180°
crank angle crank angle
5
Basic principles - compression cycle

next cycle
begins

discharge

pressure p
expansion p,V-

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Diagram
compression

..... of the gas volume V

in the cylinder
suction

6
Basic principles - piston speed

piston speed c [m/s]

mean piston speed cm s.n

cmax cm =
30
piston stroke s [m]
n ... RPM

suction p1

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delivery p2

delivery pressure p2 3 2

pV- diagram

suction pressure p1 1
4
v
7
Basic principles - efficiency

pV - diagram = indicator diagram

pressure (shown in ideal form)

volume
clearance volume

suction volume

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Volumetric efficiency:
suction volume
swept volume η v=
swept volume
= piston area x piston stroke

Top Dead Centre Bottom Dead Centre

TDC BDC

piston position
8
Basic principles - single & double acting

single-acting
suction
side

delivery
side

drive cylinder

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double-acting
suction side

end

head end
crank delivery side
crank crosshead
drive cylinder
9
Basic principles - single & double acting

Single-acting compressor p head end

Cylinder head
0° 180° 360°
drive cylinder °crank angle or t [sec]

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Double-acting compressor p crank end head end
end

head end
crank

“crank” “cross head”

drive cylinder 0° 180° 360°

°crank angle or t [sec] 10
Start of cycle

suction valves closed

suction

end

end
TDC

head
crank
0°
TDC delivery

delivery valves closed

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 180° 360° crank angle

11
HE suction valve opens

head end suction valve opens

suction

end

head end
40°

crank
delivery

delivery valves closed

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 40° 180° 360° crank angle

12
CE delivery valve opens

head end suction valve open

suction

end

head end
105°

crank
delivery

crank end delivery valve opens

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 105° 180° 360° crank angle

13
HE suction & CE delivery valve closed

BDC head end suction valve closes

suction

end

head end
180°

crank
delivery
BDC
crank end delivery valve closes

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 180° 360° crank angle

14
CE suction valve opens

crank end suction valve opens

suction

end

head end
crank
235°
delivery

delivery valves closed

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 180° 235° 360° crank angle

15
HE delivery valve opens

crank end suction valve open

suction

end

head end
300°

crank
delivery

head end delivery valve opens

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delivery pressure
indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 180° 300° 360° crank angle

16
CE suction & HE delivery valve close

crank end suction valve closes

suction

end

end
360°
TDC

crank

head
0°
delivery

head end delivery valve closes

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delivery pressure
Indicator pressure

indicator pressure
head end
crank end

suction pressure

0° 180° 360° crank angle

17
Basic principles - real versus ideal pt-diagram

60 bar
0.1 sec

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p
end

head end
crank

“Crank” “Crosshead”

drive cylinder
0° 180° 360°
°crank angle or t (sec)
18
Multistage compressor

• Compression in 2 - 8 stages
- to achieve higher pressures - up to several hundred bars in high
pressure and ultra-high pressure compressors
• Cooling between the compression stages:
- to avoid exceeding the permissible temperature for compressor
materials and lubricating oil
- to save energy costs

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1st stage cooling 2nd stage cooling 3rd stage cooling
gas gas
intake
delivery

3 bar 8 bar 22 bar

example:
1 bar 3 bar 8 bar 19
Multistage compressor

• Energy saving: Multistage compression with inter-cooling

p
single stage compression
to high delivery pressure

compression in the 2nd stage

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energy savings through 2-stage
pressure

2nd stage
compression with intercooling
minus intercooler losses

compression in the 1st stage

1st stage

volume V 20
Energy balance

TRANSFERRED
heat transferred to cylinder cooling

to intercooler to gas

HEAT
to lubricating oil
1st 2nd 3rd
and environment
stage stage stage

energy supplied
at the crankshaft

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frictional losses
energy for

LOSSES
leakages, heating of suction gas
isothermal
intercoolers
compression
ventilation losses (losses through
pressure drop in pipes valves)
adiabatic losses
21
Crank mechanism - components

crankshaft big-end bearing crosshead

connecting rod
crosshead guide

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crosshead bearing
main bearing crosshead connecting pin
crankcase small-end bearing
22
Crank mechanism – crankshaft, crankcase

• The crankcase houses the crankshaft and forms the bearing structure
to the base frame or foundation.

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23
Crank mechanism - connecting rod

• Connecting rods convert the rotary motion of the crankshaft

into the reciprocating motion of the piston
- the “upper” (piston end) connecting rod eyes are usually closed,
the “lower” ones usually split (except on overhung cranks or
assembled shafts)
• Materials:
- forged steel, nodular graphite cast iron, cast steel. In special
designs may be fabricated from sheet steel or aluminium.

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24
Crank mechanism – crosshead, crosshead pin

• Crossheads accommodate the piston rod and form an

articulated joint with the connecting rod by means of the
crosshead joint pin.
- the crosshead runs in the slide ways of the crankcase or in its
own crosshead slide ways
- the running surfaces are usually cast iron shells babbitted with
white metal, screwed on to the crosshead
• Materials:

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- crosshead housing made of cast iron,
- crosshead joint pin made of hardened steel.

crosshead
joint pin

25
Rotating and oscillating components

big end bearing piston rod

crankshaft (rotating) connection rod (oscillating) piston & rings
(rotating) (2/3 rotating) (oscillating)
(1/3 oscillating)

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crosshead crosshead crosshead bearing
(oscillating) joint pin (oscillating)
(oscillating)

26
Crank mechanism - kinematics: rod ratio λ

length

r
l

ius
r rad
λ=
l BDC TDC

8
TDC cmax with rod length = TDC

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cmax
piston speed

BDC
0
0° 180° 360°

cmax cmax with usual designs

The rod ratio (usually 0,2...0,25) must be taken into account

in the design of valves and capacity control systems
27
Crank mechanism - components, masses, forces

• Oscillating mass forces and mass moments of inertia

- are generated by the acceleration and de-acceleration of the
reciprocating masses of the crank mechanism
- they generate inertia forces in the stroke direction and forces
of inertia around the drive’s centre of gravity
- partly avoided by suitable cylinder arrangements or
compensated by balancing shafts
• Rotating masses

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- centrifugal forces of the rotating parts are balanced by
counter weights applied on the cheeks of the crank shaft

28
Crank mechanism - rod forces

• The gas acting on the piston and the moment of inertia

generate forces which are transferred to the connecting rod
via the crosshead and the joint pin

Rod force = gas force + inertia forces

indicator diagram force diagram
crank end head end gas forces inertia forces
35
indicator pressure [ bar ]

+200

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30

tension
rod force [ 1000 N ]
+100
25

20 0

compression
15
-100
10
Rod force
5 -200
TDC BDC TDC TDC BDC TDC
crank angle crank angle 29
Crank mechanism – 'rod load reversal'

• When designing the cylinder units the compressor manufacturer

makes sure that load reversals take place.
• This load reversals ensure that the lubricating film at the
crosshead joint pin is maintained. Loss of load reversals can
cause damage to pin and bearing as well as further
consequential damage.

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+200

tension
• As a valve supplier we must take this +100
into account:
rod load
- Changes in load conditions due to 0
reversal
retrofitting of capacity control systems

compression
or high valve leakage can lead to loss -100
of rod load reversals.
-200
TDC BDC TDC
30
Crank mechanism – 'rod load reversal'

• The rod is only under compression load, when one cylinder

is unloaded. There is no tension load and thus no rod load
reversal.
- Lubrication of the connecting pin is not guaranteed !
crank end unloaded no reversal of rod force!
head end gas forces inertia forces
200

no tension
35
indicator pressure [ bar ]

30

rod force [ 1000 N ]

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100
25

20 0
crank end

compression
15
-100
10
Rod force
5 -200
TDC BDC TDC TDC BDC TDC
crank end crank end
31
Cylinder - assembly

diaphragm cylinder suction duct

suction valve with unloader valve cover

piston rod valve cage

distance piece suction valve

suction valve nest

piston
piston rings

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cylinder cover

discharge valve nest

piston rod discharge valve
packings
valve cage
valve cover
cylinder
diameter cylinder liner discharge duct cooling water channels
32
Valve types for Recips

• Closing Elements
Plates - rings - stripes - reeds - poppets - cones - balls ....
traditionally made of steel, today increasingly of fibre-reinforced plastics.
Closing elements of HOERBIGER Valves are:

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steel plates profiled plastic rings plastic plates

poppets reeds flapper plates

33
Suction valve and discharge valve

Actuator

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Unloader

discharge Valve
suction Valve exploded view
34
Installation in the valve nest

• Requirements in Cylinder Design:

• easy valve installation and
removal
• low pressure losses in valves
and valve pockets
• small clearance volumes

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• the valve cage or the jack bolt
arrangement must have
adequately sized openings for
the gas flow.

35
Correct installation in valve nests

1. Correct valve assembly 2. Don’t mix up suction valves with

delivery valves.
D (API 618 describes
valve designs which
are impossible to
mix up).
S

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3. Correct 4. Valve

delivery valve pocket

delivery valve pocket

positioning firmly seated

of valve in in valve nest:
valve nest Danger of
breaking
valve seat
if valve can
move about.

36
 Incorrect installation

• Jackbolt failure and

consequential damage

37

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Incorrect installation in the valve nest

• Valve Installation - Causes of LOOSE VALVES

- Wrong assembly sequence of valve cage, cage clamping bolts
(jacking bolts) and valve cover bolts
- Correct sequence:
1. tighten the valve cover bolts to the specified torque, with valve
cage clamping bolts (jacking bolts) withdrawn
2. tighten the valve cage clamping bolts to the specified torque
- Missing or defective gaskets

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- Height of valves or valve cages too low after overhaul
• Gas tightness in valve nest, if valves are installed
without gaskets
- Adequate quality of sealing surfaces of valve and valve nest,
grinding-in may be required
- For this reason the contact surface of HOERBIGER valves
are finished to N5 or N6 quality (HN150).

38
'Valve losses'

• Total Gas Flow Losses in Indicator Diagram

A certain percentage of the indicated power is lost due to:
Discharge losses discharge
30 pressure

25
Area bordered
pressure [bar]

20 by the red line:

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Indicated work
15 of cylinder end
suction
10 pressure

5
Suction losses
0
0% 20% 40% 60% 80% 100%
displacement [%]
39
'Valve losses'

• The areas exceeding nominal delivery pressure

show the different losses at the delivery side.

losses
losses in in valve
valve nest
pipe losses and

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pulsation losses
nominal
discharge
pressure pv diagram

pulsating pressure
in the pressure chamber

40
Shape of valve nest and 'valve losses'

• The shape of the valve nest in the cylinder has a considerable

influence on total gas flow losses. In the calculation of total flow
losses with the help of the TKK, the valve nest shape can be
taken into account with Pocket Factors.

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• Two examples for considerable flow losses→high pocket factors.
41
Common pocket shape and 'valve losses'

• Low losses in the cylinder nests can be achieved by minimal

restriction of gas flow into cylinder,
- by wide passage areas
- by pockets or recesses in the cylinder head.

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42
Shading of valve nest by cylinder head

• Narrow ports and gaps are restricting the gas flow too much.

end of contact area

retracted piston retracted piston
of piston rings

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Cylinder head, Cylinder head protrudes
retracted until here into cylinder

43
Piston - types

• Types

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- Plunger piston (single-acting)
- Disk piston (double-acting)
- Differential piston
- Step piston
- Tandem piston
• Materials
- Cast iron, steel, and aluminium

44
Cylinder rings - types

Support the weight of the piston and half of the

rod weight, but should not seal gas pressure.

Rider Rings

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Piston Rings

Seal the compression chamber

between the piston and the liner.
45
Piston rod - design

• Double-Acting Compressor:
- The piston rod connects the piston to the
crosshead and transmits the piston force.
The packings and oil wipers slide on the
hardened rod surfaces (e.g. tungsten
carbide coating).
• Materials:
- Case hardening and tempering steels. piston rod

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piston

'Tailrod' (not shown): If the piston rod extends through the

cylinder head for additional guidance 46
Packings - types

Pressure Packing Intermediate Packing Oil Wiper Packing

Seals the inboard Seals the first distance Keeps off crank case
compression chamber piece against the second oil from the distance
against the distance distance piece piece
piece

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47
Packings - Pressure Packings Ring Sets

• Pressure reduction is carried out in steps by a certain number of ring

sets which depends on the height of the pressure difference. A gas
tight packing system is created by combining various ring types.
• For ease of installation and to maintain sealing after wear (self-
adjusting action) the rings are split. To seal the cuttings, at least two
rings per set are used and positioned offset from each other, e.g.:

Single-acting ring set

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2 sealing rings 1 support ring
radially tangentially for high
split split pressures

Pressure

48
Packings - Components of Ring Sets

Annular springs PTFE rings

(assist assembly) with additives Bronze-rings

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Depending on the gas type and operating conditions, lubricated
packings with metallic or PTFE rings as well as oil-free packings
with PTFE rings are used. For higher compression pressures, oil-
free packings must be cooled to extract friction heat.
49
Liner - Designs

• Cylinder liners are usually made of cast iron, to allow the

piston rings to slide smoothly.
• Some liners however are made of steel or Ni-resist. The
disadvantages of rather poor sliding characteristics and
reduced heat dissipation have to be accepted.

- Dry liners are in direct contact with the

cylinder wall

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- Problem: They tend to hinder cooling.
- Wet liners have coolant circulating
between liner and cylinder wall. These are
used for smaller cylinder diameters
- Problem: keeping the coolant out
of the compression chamber.

50
Distance Piece - Function

• In oil-free compressors, the parts of the piston rod which are in contact
with the medium must not come into contact with the lubricant of the
crank mechanism
• A distance piece with two chambers and an intermediate packing
between them is used achieve 100% separation
- In chamber 1, the piston rod bears an oil catcher.
- In chamber 2, an inert gas buffer may be maintained to avoid any
leakage of (poisonous) gas to the environment.

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two-chamber distance piece

oil wiper
1 2 main pressure packing

piston rod 4.10.1

intermediate stuffing
oil catcher box packing
51
Lubrication - Cylinder Lubrication

• It is important to have a sparing, metered oil supply!

• With excessive lubrication, the valves and pistons build up oil
deposits, oil coke is formed with gases containing oxygen:
- The delivery valves clog up and wear.
- The suction valve plates stick to the valve seats and guards. As a
consequence they open and close too late and suffer from heavier
impacts. The plates and springs break more often.

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52
Cylinder Lubrication, Oil Quantity Estimation

[g/100m2 surface swept by piston]

1.6
3
Specific oil quantity q

1.4
1.2
1 2
0.8
0.6
1
0.4
Differential

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0.2 pressure
0 p2 - p1
0 50 100 150 200 250 300 [bar]

1 Mini lubrication for all gases: PTFE rings and stuffing boxes
Full lubrication for air, CO2, H2, CH4, He, Ar, N2, CO, pure gases:
2
metallic rings, PTFE stuffing boxes.
Running-in for all gases, full lubrication for NH3, C2, H4, H6, C3,
3
C4, C5, C6 parts, impure gases: metallic sealing elements
53
Oil Consumption in Cylinder Lubrication

Calculation of oil consumption per cylinder:

Q = q . D . (s + Lf ) . n . 2π . 60 . 24
100
Q ... Oil quantity in g per 24 hours per cylinder s Lf
q ... Specific oil quantity from diagram
[g/100m² surface swept by piston]
D ... cylinder diameter [m]

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s ... stroke [m]
n ... rpm
Lf ... length of piston [m] which is equipped
with piston and rider rings (approx. 0.8 x s)

Please note:
• Calculation methods of compressor manufacturers differ
• The cylinder lubrication is often measured in droplets per minute
(8000 droplets ~ 0.5 litre ~ 0.4 kg) 54
Peripheral Equipment - Pulsation Dampers

• Pulsation dampers (“buffers”) are part of the gas lines. Mostly

they are cylindrical pressure vessels, sometimes with internals.
They are usually installed immediately before and after the
cylinders to reduce gas pulsations in the lines.
• The amplitudes of gas pulsations are limited by various
regulations (e.g. API 618). Pressure fluctuations can be
calculated by means of computer simulation and design
measures can be taken to dampen them.

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Suction damper

Low pressure “buffer”

with internals
to dampen strong
pressure pulsations
Discharge damper 55
Gas compressors - process gas compressor

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Combined gas engine / compressor
with manual clearance pocket capacity control (USA) 56
 End of the Training File
Reciprocating Compressors

57

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