Reciprocating Compressors

1. Reciprocating Compressor
System – Process Design
Prepared by: Ankur Srivastava
Chemical Engineer
Email: ankurcheme@gmail.com

2. What is a Reciprocating
Compressor-1?
A reciprocating compressor
is a positive-displacement
machine in which the
compressing and displacing
element is a piston moving
linearly within a cylinder.
Figure in the next slide
shows the action of a
reciprocating compressor.

3. What is a Reciprocating
Compressor-2?

4. Pressure Volume
Diagram
Explained

5. Reciprocating Compressor Sectional View-1

6. Reciprocating Compressor Sectional View-2

7. Reciprocating
Types and
Classification-
1
• Reciprocating compressors are classified as either high
speed or slow speed
• Typically, high-speed compressors run at a speed of 900 to
1200 rpm and slow-speed units at speeds of 200 to 600
rpm
• Figure in the next slide shows a high-speed compressor
frame and cylinders
• The upper compressor is called a two-throw machine
because it has two cylinders attached to the frame and
running off the crank shaft
• The lower compressor is a four-throw machine because it
has four cylinders attached to the frame
• The number of throws refers to the number of pistons

8. Reciprocating
Types and
Classification-2

9. WHY
Reciprocating?
• Broadest pressure range in the compressor family –
vacuum to 3000 barg
• Multiple Services on one compressor frame
 On a multi-stage frame each cylinder can be
used for separate gas service
 Example – One cylinder dedicated for propane
refrigeration with balance cylinders dedicated
to product gas
• Lower capital cost
• Can handle wide variations in capacity with much more
ease than any other type
• Complete skid mounted units allow easy transportation
and installation and relocation
• In general, higher efficiencies compared to centrifugal type
for the same operating conditions
• Especially suited for low molecular weight applications
such as hydrogen and hydrogen rich gases

10. Application
Limits-1
• Flow
 Can handle very low flows without significant
loss in efficiency
 High capacity is limited by cylinder size, stroke
length and speed
• Pressure
Very high pressures up to 3000 bara are
practically applied in the compression of
ethylene for producing LDPE (Burckhardt)

11. Application
Limits-2
• Discharge Temperature
 Discharge temperature generally
restricted to 135⁰C
 For hydrogen rich services (molecular
weight less than or equal to 12) and
non-lubricated cylinders the discharge
temp. shall not exceed 120⁰C
 Compressed air applications allow
higher discharge temperatures

12. Application
Limits-3
• Compression Ratio (Pd / Ps) is limited by
the following;
 Maximum Discharge Temperature
 Allowable Rod Load
 Cylinder Volumetric Efficiency
 Typical compression ratios for a single
stage are 1.2 to 4.0

13. Application Limits -4
• Horsepower
 In gas processing
applications power ratings
of more than 7.5 MW
are rarely found
 Special machines with power
rating as high as 30 MW are
available for other
applications

14. Application
Limits-5
• Rotative Speed
 Low to moderate speeds typically at 300-700 rpm with
motors
 Moderate to high speeds typically at 600-1800 rpm
with motors or gas engines (field gas compression,
gas plant, pipeline)
 Low to moderate speeds in accordance with API STD 618
 Moderate to high speeds in accordance with ISO
STD 13631

15. Reciprocating
Terms and
Formulas

16. Piston Displacement
Definition
• Piston displacement of the reciprocating
compressor is the volume swept by the
piston inside the cylinder in unit
time and it is same as the capacity of the
compressor.

17. Piston
Displacement
Formulas

18. Volumetric
Efficiency -
Definition

19. Volumetric
Efficiency -
Formula

20. Suction Capacity – Definition & Formula
• It is the actual volume flow
across the cylinder taking into
account the reduction in flow
due to volumetric efficiency

21. Discharge
Temperature

22. Power-1

23. Power-2

24. Reciprocating
Capacity Control

25. Requirement
& Methods
• Capacity control is required for two reasons
 To adjust the suction mass-flow to match the
process demand
 To save energy
• Following four methods are employed
 Speed Control
 Suction Valve Unloader
 Clearance Pockets
 Spillback Control

26. Speed
Control
Most speed control reciprocating
compressors are integral design with a
gas engine
Speed range is 75% to 100% for
integral gas engine type thus providing
a capacity turndown of 75%
Speed control is often augmented by
spillback control and suction valve
unloaders and / or clearance pockets

27. Speed Control – Electric Motor
Variable Speed Drive

28. Suction Valve
Unloader
• Valve unloaders are mechanisms which hold
open or bypass one or more of a cylinder’s inlet
valves at each end of double acting cylinders.
This provides complete unloading of one or both
ends of the cylinder.
• Common Unloader Options
 Three step unloading (100%, 50%, 0%)
capacities for single cylinder compressor stage
 Two step unloading (100%, 0%) capacities for
single cylinder compressor stage
• Actuator sizing may limit the use of valve
unloaders to services with inlet pressures at or
below 70 barg

29. Clearance
Pockets
• Cylinder delivered capacity can be reduced by increasing the
cylinder clearance volume. This is done by adding a clearance
pocket. The fixed volume clearance pocket is a volume
chamber separated from the normal cylinder end clearance
volume by a valve or plug. Opening the pocket reduces the
cylinder inlet volumetric flow by trapping additional gas in the
now larger clearance volume at the end of the piston stroke.
This trapped gas is not delivered through the discharge valves
& the cylinder volumetric efficiency is reduced. Capacity
reduction of 50-60% easily achieved
• Fitted to the outboard or head end of the cylinder than to the
crank end or packing box end
• Valve unloaders combined with clearance pockets can achieve
five step unloading which provides nominal cylinder capacities
of 100, 75, 50, 25 and 0 percent
• Clearance pockets can be arranged for local/manual
operation, manual/pneumatic operation, or
automatic/pneumatic operation

30. Combination – Suction
Valve Unloaders and
Clearance Pockets

31. Spillback
Control
• Simple controls
• Low initial cost
• Practical and easy to install
• Capacity Range: 0 to 100%
• Can be stand-alone or in combination with suction
unloader valve, clearance pocket
• Inefficient since power is wasted in recompressing the
recycled gas
• Generally used in small compression requirements
• Often used as a safety backup for other capacity
controls
• Finds use during compressor start-up and shutdown
operations

32. Schematic –
Spillback
Control

33. Reciprocating
Compressor
Curve

34. Performance Map Multistage

35. Datasheet
Preparation
• Don’t reinvent the wheel. Use the API 618 (latest) datasheet
provided in Annex-A, which is also available in MS-Excel format to
fill up the process data
• What process data needs to be filled in?
 Define the suction and discharge conditions (P,T, Flowrate)
and gas composition
 Define all possible options of suction and discharge conditions,
gas composition that are present and likely to be there in the
future
 In oil & gas applications it is often observed that during
summer and winter the suction/discharge conditions of
produced gas changes. Define all the cases based on ambient
conditions
VENDOR SELECTS MACHINE BASED ON THE COMPLETE OPERATING
ENVELOPE. AND THE CONSULTANT / CLIENT IS RESPONSIBLE TO
PROVIDE THAT.

36. Process
Simulation
• HYSYS simulation model
• Based on the Compressor performance
data
• Forms the basis for material and energy
balance.
• Defines the cooler heat duty.
• Generates property data for the
calculations.

37. Process Simulation PFD – Aspen HYSYS

38. System Components

39. Lubrication
System

40. Suction
Scrubbers
• Vertical Knock out vessels.
• Limit liquid carry over to the compressors.
• Internals – SMS / SV / SVS
• Design based on Shell DEP for 2 phase
separation.
 DEP 31.22.05.11 Gas/liquid separators
- Type selection and design rules
• High Liquid Level in scrubber initiates a
trip interlock of the compressor

41. Coolers
(Interstage
and After)
• Cool the hot gas between stages
(interstage coolers)
• Cool the final discharge gas (aftercooler)
• Majority oil & gas applications use Air
Cooled Heat Exchangers

42. Pulsation
Suppression
Devices-1
• Necessity
 Because of their cyclic mode of operation,
reciprocating compressors produce pressure
pulsations in their inlet and discharge piping
• Pulsation suppression devices can be plain
volume bottles, volume bottles with baffles,
orifice systems, and proprietary acoustic filters
• During detail engineering, piping system design
is acoustically simulated , using analog and digital
computation methods, to determine pulsation
levels and ensure satisfactory piping vibration
levels, compressor performance, and valve life

43. Pulsation
Suppression
Devices-2:
Volume Bottles
• Simplest Pulsation Suppression Device is a Volume Bottle (unbaffled)
• Mounted on or very near a cylinder inlet or outlet
• Volume bottles are supposed to be part of the connected piping vibration
analysis for reciprocating compressors
• Volume bottles are sized empirically for an adequate volume to absorb the
pulsations
• Approximate volume bottle sizing can be done using the chart above

44. Pulsation
Suppression
Devices-3:
Volume
Bottles

45. Suction,
Discharge and
Check Valves
• Suction Line and Discharge line valves
provided for system isolation
• In many cases the isolation valves are
automatic valves with operation available
both from field and plant control room
• Check valve prevent backflow and are
installed in the discharge line as close as
possible to the discharge connection as
feasible
• In all applications involving flammable
hydrocarbons and possibility of fire
exposure the valves shall be of fire safe
design

46. Pressure Relief
Valves
• Required at the discharge of each
reciprocating compressor stage
• Relief Valve setting not to exceed MAWP
of the cylinders
• MAWP for cylinders should be at least
10% or 1.7 bar, whichever is greater, over
the rated discharge pressure

47. Blowdown
Valve
• Required to empty the system of gas
during a planned maintenance or
emergency shutdown
• Most blowdown valves are automatic
remotely operated valves
• Compressors handling hazardous and
flammable gas have blowdown valves
connected to the plant flare system
• In absence of a flare, blowdown valve
piping should be routed to a remote
location for atmospheric blowdown based
on safety studies

48. Suction and
Discharge
Piping
• Threaded piping not to be used
• For hydrocarbon service minimum line
sizes should be 2 inch
• Pipe wall thicknesses to be consistent with
Schedule 80 pipe
• In rare circumstances, where 1- or 1.5-
inch pipe is used in pressure services, the
wall thickness shall correspond to
Schedule 180.

49. Process
Calculations
• Blowdown calculation
• Pipe sizing calculations
• Hydrate calculations
• Gas Blowby Calculations

50. Blowdown
Calculation
• Intent: Reduce the pressure of the
equipment to 50% of design pressure
within 15 minutes during a fire
emergency.
• Typically done using Dynamic
Depressuring Utility in Aspen HYSYS
• Relief valves are not a depressurisation
device.
• Ball valve + Orifice combination OR
control valve

51. Pipe sizing
calculations
• Importance of pressure drop and machine
performance.
• Tools used.
• Cooler header sizing.
• Avoiding loops in suction.
• Provision of drain boots.

52. Hydrate
calculations
• Hydrates are ice-like non-stoichiometric
crystal structures composed of water
molecules encaging natural gas molecules.
• Solid formation, chokes piping.
• Flow problems.
• Formation depends on P,T conditions and
composition.
• Predicted by HYSYS

53. Gas Blowby
calculations
• Caused by loosing liquid level in the
scrubbers.
• High pressure gas flows into the low-
pressure system potentially over
pressurising it
• Calculations are done to ensure that the
downstream system is adequately
protected
• The control valve is considered to be fully
open during this case
• The highest operating pressure of the
upstream system is considered for sizing

54. Safeguarding
philosophy
• Process shutdown.
• Emergency shutdown.
• Other shutdowns.

55. Process
Flow
Diagram

56. Literature
Reference
Compressors: Selection & Sizing, 3rd
Edition, Royce N. Brown
A Practical Guide to Compressor
Technology, 2nd Edition, Heinz P. Bloch
Compressor Handbook, Paul C. Hanlon
Reciprocating Compressors, Operation &
Maintenance, Heinz P. Bloch & John J.
Hoefner