API 618 Study - Pulsation, Vibration and Compressor Manifold
Reciprocating compressors generate pressure pulsations inherent in the reciprocating motion of the pistons. These fluctuations propagate in pipe networks in the form of dynamic waves, inducing oscillating stresses on closed ends, elbows and changes of direction.
When these stresses coincide with the natural frequencies of the network or compressor manifold, resonance phenomena can occur, resulting in high vibration levels and cyclic stresses that can lead to fatigue failure.
Mastering these mechanisms is central to API 618 compliance and to the reliability of installations incorporating reciprocating compressors.
Pulsation, induced forces and risk of fatigue
The pulsations generated by the piston’s reciprocating motion produce periodic pressure variations in the cylinders. These fluctuations propagate through the pipes in the form of standing waves, the amplitude of which depends on the lengths, diameters and volumes connected. This propagation results in :
- unbalanced pulsating forces in anti-pulsating lines and bottles
- dynamic loads on supports and connections
- cyclic mechanical loads likely to excite the system’s eigenmodes
When excitation frequencies approach the natural frequencies of the network or manifold, vibration amplitudes can be significantly amplified. The consequences observed in operation include cracking, weld failures, premature degradation of supports and unplanned shutdowns.
The challenge for industry is twofold: to prevent fatigue failures, and to guarantee compliance with API 618 requirements right from the design stage or when modernizing an installation.
SIM Engineering’s scope of intervention and global approach
SIM Engineering is involved in the entire dynamic system made up of :
- the reciprocating compressor
- anti-pulsation bottles
- nearby pipes and their supports
The approach adopted combines pulsation analysis and structural dynamic study of the manifold. This global vision enables us to address both the source of the problem (pulsations and induced forces) and the mechanical response of the structure.
Pulsation study: control of pulsations and unbalanced forces
The analysis begins with a pulsatory study to characterize the generation and propagation of pressure fluctuations in the network.
The calculations show :
- assess pulse levels
- determine the unbalanced forces applied to the lines
- identify critical operating configurations
Where necessary, optimizations such as :
- installation or resizing of anti-pulsation cylinders
- adjusting pipe lengths and diameters
- addition or optimization of restrictor orifices
This phase reduces pulsatory amplitudes and limits the dynamic excitations transmitted to the system.
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Dynamic study of the compressor manifold: modeling and structural calculations
The second step is to analyze the mechanical response of the complete compressor manifold. Representative modeling is performed using finite elements in ANSYS, or a combined AutoPIPE and ANSYS approach, depending on the complexity of the system.
The model integrates :
- the compressor
- anti-pulsation bottles
- nearby pipe sections
- support conditions
By calculating the natural modes of the compressor and its manifold, we can identify the natural frequencies and associated modal shapes. This step is decisive in assessing the risk of coincidence with pulsating excitations.
Dynamic loads taken into account
Dynamic analysis integrates all stresses representative of actual operation:
- cylinder pressure forces
- unbalanced pulsating forces in pipes and cylinders
- unbalanced forces and moments on moving equipment
This comprehensive approach guarantees a consistent assessment of expected vibration amplitudes.
- cylinder pressure forces
Analysis of vibration amplitudes and cyclic stresses
Based on the loads applied and the dynamic characteristics identified, vibration amplitudes are calculated for the various operating regimes.
The cyclic stress calculation is then used to :
- identify critical areas
- assess the risk of mechanical fatigue
- compare results with API 618 criteria
This cross-analysis of pulsations and structural dynamics makes it possible to objectify the level of compliance and precisely qualify the industrial risk.
- identify critical areas
Results and recommendations
The conclusions of the study lead, where appropriate, to prioritized technical recommendations that may include :
- optimization of anti-pulsation devices
- modification or addition of supports
- local reinforcement of critical areas
- adjustment of geometric configurations
- adaptation of operating conditions
The aim is to sustainably limit vibration amplitudes, reduce fatigue stresses and ensure the mechanical robustness of the manifold and associated networks.
- optimization of anti-pulsation devices
An integrated API 618 approach to industrial reliability
The combination of a rigorous pulsation study and a complete dynamic analysis of the manifold provides a coherent response to the requirements of API 618.
By integrating advanced modeling, analysis of induced forces and assessment of cyclic stresses, SIM Engineering provides independent expertise aimed at preventing failures and securing installations.
This global approach enables us to anticipate resonance phenomena, control vibration risks and make reciprocating compressor networks part of a sustainable industrial performance approach.
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