Session: 03-05-01 New Inspection Technologies and Facilities II

Paper Number: 133966

133966 - FFS Approach to Assessment of Vibrations Caused by Compressor/Pump Pulsations 

Abstract: 

Vibrations in pipeline systems often result from the periodic flow from pumps and compressors. Such vibrations can occur not only in main transportation lines, but in supporting systems throughout plant facilities. Equipment used to transport fluids generally operates in a periodic manner, making use of reciprocating pistons, rotating impellers, or similar means. The periodic operation generates corresponding fluctuations in the velocity and pressure in the piping, known as pulsations.  Vibration problems arise when the pulsation frequency coincides with, or is at least close to, an acoustic resonance of the fluid in the piping, a structural resonance of the piping or related structure, or both simultaneously. When this occurs, the result can be high amplitude vibrations, which can lead to noise generation, instrument malfunctions, and loss of containment failures due to the accumulation of fatigue damage. Preventing or mitigating pulsation induced vibration can improve system reliability, reduce unplanned downtime, and increase safety for personnel and the environment.

During the design stage, standards like API 618 and API 674 are used to provide guidance to reduce the likelihood of pulsation induced vibrations from occurring. The stated objective of API 618 is to provide a purchase specification to facilitate the procurement and manufacture of reciprocating compressors for use in petroleum, chemical, and gas industry services. It provides piping vibration limits for use in analyses that are performed during the design phase. An API 618 vibration analysis is typically performed by compressor manufacturers or vendors based on specifications supplied by the purchaser.

Predictive vibration modeling is similar to a weather forecast, in that it is very useful for planning purposes, but occasionally surprises occur. The analysis can be sensitive to factors that are not fully known in the design stage. Once in service, facilities may change, equipment may be added, flow rates may be increased, or fluid compositions and temperatures may vary. As a result, pulsation driven vibrations may occur despite predictive analysis.

In operating facilities, excessive vibrations are often discovered due to a failure event or operator observations. When vibration is encountered in an existing facility, a question arises about what criteria should be used in the assessment. Operators often fall back on design standards like API 618 and API 674, but they are not necessarily relevant to existing equipment. For reciprocating gas compressors, API 618 provides limits on calculated pressure pulsation amplitudes, shaking forces, and displacements for commissioning of new piping, but may be unnecessarily conservative for assessing vibrations in operating facilities.

While criteria entirely based on predictive modeling is appropriate in the design stage, fitness for service (FFS) methods are appropriate for assessing vibration in operating facilities. The same pulsation analysis approach used in the design stage should be performed and supplemented with measured vibration and pressure data. At the outset of the vibration study, the additional insight from physical inspection and measured data provides: 1) verification that the vibrations are sufficient to cause concern of failure, 2) evidence that the vibration is in fact due to pulsations, 3) insight into potential differences between the modeled and the actual system, and 4) validation and potential “tuning” of the predictive model. Acceptance criteria to be used for vibration severity can be based on FFS approaches like the current Energy Institute guidelines or on the soon-to-be-published ASME/API 579 Part 15, Assessment of Piping Vibration. Pulsation simulation is then repeated with a goal of identifying potential mitigation strategies. After mitigation is performed, a vibration survey should again be performed to verify that the mitigations were successful, and that the equipment is now operating below the limits of acceptable vibrations.

This paper provides a brief introduction to vibrations generated by pump and compressor pulsations, a discussion of the benefits and limitations of design criteria vs FFS standards such as ASME/API 579 Part 15, and further description of the FFS approach for existing facilities.

Presenting Author: Larry Matta Stress Engineering Services

Presenting Author Biography: Dr. Matta has been a Staff Consultant at Stress Engineering Services for 15 years. At Stress, his work includes vibration and acoustic analysis, external loading of pipelines, unsteady flow phenomena, and investigating failures, fires, and explosions. He received his BS in Aerospace Engineering from Penn State, and his MS and PhD from Georgia Tech. He is a licensed engineer in Texas and is a certified fire and explosion investigator.

Authors:

Larry Matta Stress Engineering Services
Lyle Breaux Stress Engineering Services, Inc.
Gyorgy Szasz Stress Engineering Services, Inc.