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Reciprocating problems with sustainable solutions
Pulsations & Reciprocating equipment, such as pumps and compressors, are a common cause of vibrational issues in piping systems. This equipment causes a pulsating flow in the system which can cause the system to vibrate. A number of API standards from the American Petroleum Institute can be used to ensure that the vibrating system does not exceeds any limits that can cause failure in the system. API 674 concerns reciprocating pumps, API 675 positive displacement pumps, API 618 reciprocating compressors and API 688 positive displacement compressors.
To avoid these problems good system design is key and here DRG can be your perfect partner.
The API 674/675 code for reciprocating pumps provides two different design approaches. Depending on the requirements of the client, most suitable design approach can be conducted.
Design Approach 1 of API 674 includes a Damper sizing to pulsation level criteria C.1.5 to C.1.7. And a simplified mechanical analysis to show separation margin between acoustical excitation frequencies and mechanical eigen-modes.
Design Approach 2 of API 674 includes Acoustic Simulation and Piping Restraint Analysis. This design approach extends upon Design Approach 1 and includes the acoustic simulation of the pump, damper and connected piping system. Limits are set on pulsation levels (C.1.5), minimum pressure (C.1.6) and maximum pressure (C.1.7). The piping restraint analysis can include a piping span table or can be extended to a forced mechanical response study depending the client requirements.
Often, pipe stress analysis of the piping on the skid or complete system is included. This typically provides an additional cost and time benefit for the client because necessary changes to mitigate either static or dynamic problems can be considered in the same analysis.
DRG assists its clients in sizing and the design of Damper / Pulsation Suppression devices. The acoustical performance can be determined at an early stage in the design of the system. And will prevent unnecessary and costly changes to the damper at a later stage in the project.
The acoustical performance will be determined through modelling of the pump or compressor and damper in BOSpulse without explicitly including the final connected piping system. Both gas filled (pre-charged) dampers and liquid filled dampers can be considered for the analysis.
Mechanical performance from both a static and dynamic perspective are included on client request. The mechanical design is done using Finite Element Analysis according to the applicable code (ASME VIII division 2 or EN13445).
Acoustic analysis is conducted using the pulsation analysis software package BOSpulse. This package combines the experience of DRG in the field of acoustic simulations with user-friendly modelling. The complete system can easily be modeled based on various sources of input (GA’s, isometrics, pcf files, input files from Caesar 2).
The acoustic analysis typically covers all possible operating conditions including changes temperature, pump speed, pumps operational and changes in feed/discharge lines and pressures. Upon request, pumps running in parallel can be included in the study.
At the start of the analysis, a pre-study is conducted of the damper and pump neglecting the effects of the connected piping. This allows for an initial verification of the damper performance and can prevent project delays due to changes to the PSD at a later stage in the project.
The acoustic analysis covers all piping starting from the pump up to an appropriate boundary condition and will include damper and potential other inline equipment such as filters. The ends of the model (appropriate boundary conditions) will be determined in consultation with the client and can include large diameter vessels, injection locations or long gas transportation lines.
Required changes for conformance with the API 674 / API 675 or API 618 / API 688 limits are discussed in close consultation with the client.
Depending on the results of the acoustic analysis and an initial review of the supporting of the piping several approaches are possible. Either separation margin between acoustical excitation and mechanical eigen-modes will be checked for (typical for Design Approach 1) or a force mechanical response is included. The separation margin check will be conducted using either span tables or modal analysis of the system. For the forced mechanical response, the shaking forces calculated will be analyzed using the ANSYS plugin in BOSpulse to evaluate the vibration levels and fluctuating stress levels (typical for Design Approach 2 of API 674 or Design Approach 3 of API 618).
During commissioning or after changes to the operating conditions, vibration levels during operations can become critical. In these situations, DRG supports its clients using a high pace vibrational assessment, root cause analysis and mitigation. A client specific approach is proposed typically including simultaneous conditions of vibrational measurements, acoustic simulations and mechanical simulations.
Conducting these in sync, results in a quick evaluation of:
Vibrational assessment. Is the system safe to operate or at risk of failure with continued operation.
Determine the root cause of the vibrations whether this is of acoustical or mechanical cause.
Propose most (cost) effective way to mitigate the vibrations using both the acoustical and mechanical simulations and prediction of impact on the system.