TNO Sci Science ence and Indu Industr stry y
The PULS PULSIM IM appro a pproach: ach: ‘Pr ‘Preve eventi ntion on is i s bett b etter er than cure cure’’
API 618 Analysis for reciprocating compressors Reciprocating compressors are frequently used in many applications, because they can compress gas in a wide range of pressure ratios, with a large range of flow rates. However, However, a reciprocating compressor also generates pulsating flow, which can create a number of problems. These include severe vibrations, pipe failure due to fatigue, reduction in efficiency of the compressor,, 'hammering' of compressor, compressor compressor, check and safety valves and errors in flow measurements. three-step approach approach in the analysis, according to the design approach 3 of the API Standard 618, 2nd edition of 1995. This includes items M2 to M8 of the standard’s appendix M. The three steps are outlined below. Step 1: Prestudy or dampercheck In order to prevent costly and time-consuming changes to the damper design, an acoustic prestudy (also called dampercheck) of the damper is carried out at an early stage of the project. This analysis involves a detailed simulation of the internal cylinder gas passage, the pulsation damper and the pipe system is replaced by an infinitely long line. A calculation and optimisation is carried out related to the following areas: the pulsa-
0 5 2 / 5 0 0 2 5 0
Even when pulsations are within allowable
PULSIM solves the one dimensional flow
tion levels near the compressor valves, the
levels, high vibration and cyclic stress levels
equations by using the method of character-
in- or outlet of the dampers and the pulsa-
can occur. Pulsations and vibrations can be
istics in the time domain, up to a frequency
tion induced forces on the damper. In addi-
controlled and life cycle costs consequently
of at least 32 times compressor speed. The
tion, PV charts are calculated in order to pre-
reduced if the problems are solved during
general purpose finite element program
dict the effect of pulsations on power
the design stage of the project. Pulsation and
ANSYS has been coupled with PULSIM and is
consumption and capacity. Furthermore,
mechanical response analyses have proven to
used to determine the mechanical response
there can be an optimisation of the interac-
be effective tools in meeting such objectives.
of the system to the calculated pulsation
tion between the pulsations and the dynamic
TNO Science and Industry has developed the
induced forces. In terms of dynamics, these
mechanical behaviour of the compresso compressorr
digital simulation program PULSIM
tools can optimise the system in such a way
valves, in order to avoid compressor valve
(PULsation SIMulation) in order to calculate
that all pulsation related problems are con-
problems.
the acoustical behaviour of a pipe sys tem.
trolled. TNO Science and Industry uses a
TNO Sci ence an d Industr y
API 618 Analysis for reciprocating compresso rs
Process Industry
Step 2: Pulsation analysis of the piping
same as a frequency component of the
Step 2 involves the precise modelling of the
pulsation-induced shaking forces. It is
complete pipe system, including T- joints,
particularly difficult for compressors with
check and control valves, and such equip-
variable speed to achieve a mismatch of
ment as heat exchangers and separators. The
acoustical and mechanical natural
pulsations and pulsation-induced shaking
frequencies.The mechanical ANSYS model is
forces are calculated and compared with al-
built up of beam type elements and includes
lowable levels. If these are exceeded, possi-
all important components which influence
ble modifications are investigated. Those
the mechanical natural frequencies and
which are frequently advised necessitate the
cyclic stresses. These include the nozzle and
following changes: Installation of orifice
flange flexibility, supporting beams and pipe
P.O. Bo x 155
plates, increasing of the diameter of pipe
racks and stress intensification factors of
2600 AD Delft
parts, relocating of closed valves and in-
The tea m of the ‘Flow and St ructural Dynamics’ department: your partner in solving questions about the dynamics of your insta llation. Mode lling, mea suring and optimising of dynamic flow and vibration phenomena is our specialty. We use model ling techni ques like PULSIM to enhance the reliability and safety of your process installation.
TNO Scie nce a nd In dust ry
The Nethe rland s
stalling additional volume. It is very rare www.tno.nl
that a redesign of the pulsation dampers is necessary subsequent to the dampercheck. The final aim of the pulsation study is the
Bert Egas
prevention of the following areas of diffi-
Tel. +31 15 269 21 34
culty:
Fax +31 15 269 21 11
• Unacceptable levels of pulsations and
E-mail:
[email protected]
vibrations and cyclic pipe stresses causing fatigue failure • Increased power consumption and
www.pulsim.com
Solid model plot
capacity loss • ‘Hammering’ of compressor valves, relief valves and check valves due to pulsations • Inaccuracies of flow measuring devices.
Time function and spectral analysis
120 mm/s pp
Vibration mode with maximum vibration level in mm/s peak to peak
Time function and spectral analysis
T-joints and nozzles. The mechanical model
This normally involves advice concerning
is excited by the pulsation induced forces
the modification of existing supports, the
calculated by PULSIM and includes the right
installation of additional pipe supports
phase. Subsequently, the vibrations and
and/or the increase of the stiffness of the
cyclic stresses in the pipe system are calcu-
structures on which the piping is mounted.
lated. If a full design approach 3 study is carried out, this also involves the detailed
Results of the three step approach
modelling of the compressor manifold (cylin-
If a full design approach 3 analysis is car-
ders, crosshead guide and distance piece).
ried out, we have experienced that 90 to
Then the so-called compressor manifold
95% of the systems can be started without
Step 3: Mechanical response study
vibrations are calculated. If the calculated
any pulsation related problems. This elimi-
Even if the pulsations are within allowable
vibration and/or cyclic stress levels exceed
nates the need for subsequent costly and
levels, unacceptable vibration and cyclic
the allowable levels, modifications are inves-
time consuming corrective measures.
stress levels can occur if a mechanical
tigated which lead to recommendations
natural frequency is either close to, or the
abouthow to achieve acceptable levels.
Pulsation as f unction of compressor sp eed
