Pulsation Study

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Acoustic, Mechanical Vibration, API 618, API 688, Design approach 3, Reciprocating Compressor

A compressor pulsation study was conducted by INERTANCE. The three (3) stage reciprocating compressor has been running for 25 years, since these compressors had been initially commissioned after construction. With consideration of improved process demand, it was necessary to revamp the process and pipe condition for these compressors, and this was due to perform new pulsation study. This pulsation study was performed as per Design Approach 3 procedure in accordance with API 618, API 688, and this page shares the analysis status of Design approach 3.

Contents

  1. Scope of this analysis
  2. Modeling Status
  3. Pulsation Induced Vibration
  4. Cyclic Stress
  5. Vibration for Small Bore Connection
  6. Key words
  7. Related Services  

1. Scope of this analysis

Design Approach (DA) 3 is to perform the mechanical analysis with forced mechanical response analysis in addition to Acoustic study. For this analysis, mechanical analysis for compressor cylinders and crank case is not included, because these compressors on crank case & cylinder were running by exist operating condition without high vibration issue, and it is planned to run continuously with same operating speed (300 rpm) after revamping construction.

The scope of service for this compressor pulsation study as Design approach 3 below activities;

  • Review for Existing Pulsation Study
  • Mechanical Modeling
  • Forced Response Analysis
  • Pulsation Induced Vibration   
  • Vibration Criteria
  • Cyclic Stress  

2. Modeling Status

3D finite element method to study the deflected shape and natural mechanical frequencies of the piping system was used to avoid any coincidences and review the vibration amplitude by pulsation induced shaking forces. The calculated vibration and alternate stresses amplitudes have been compared with API criteria.

Three (3) individual compressor cylinder and surge volume was modelled and the related piping system was analyzed as shown in below figures. This model includes the knock-out drum, the riser pipe to gas relieving point as PSV to flare header and tie-in point up to process boundary. For interstage piping system, single compressor cylinder and piping system was taken into acoustic model, because the piping system of each interstage configuration has similar pipe geometry and identical stationary equipment. See below the mechanical model status for 2nd interstage piping system.

Mechanical Model for 2nd Interstage, compressor pulsation
[Mechanical Model for 2nd Interstage]

3. Pulsation Induced Vibration

The mechanical excitations caused by reciprocating compressor are typically the strongest at frequencies of 1x and 2x of compressor speed. In the area surrounding the compressor, there should not be a mechanical natural frequency coincident with these excitation frequencies. The acoustical simulation should also predict the most significant acoustical unbalance forces in piping lines at frequency 1x of compressor speed, when cylinder is single-acting, and 2x of compressor speed, when cylinder is double-acting. If possible, in piping system, there should not be a mechanical natural frequency coincident with these excitation frequencies. 

Pulsation Case Study

Therefore, the forced harmonic analysis was performed. As three (3) compressor are operated in a double acting compressor type, the highest significant shaking forces generally occur at compressor speed (1st harmonic) and at 2x compressor speed (2nd harmonic). Forces above 20th component are negligible and consequently are not included in this analysis.

As results of case study for Design approach 2, the maximum level of pulsation magnitude was calculated as per cylinder phase differences. The worst case conditions were considered to export the pulsation induced shaking forces, which was used for input data the mechanical model. These kinds of procedure would be overly conservative to assume the dynamic vibration amplitude, stress and reactions. 

However, with consideration of revamping construction, it is not always possible by means of installing additional pipe supports to shift all the mechanical natural frequencies out of these identified frequency range by cylinder operation. This was due to perform the forced response analysis directly with skipping the separation margin assessment by natural frequency of piping system on this analysis. Accordingly, to achieve & verify a reliable & robust piping system, the vibration amplitude by application of highest shaking force, which was sourced from acoustic case studies, was shown in below figure. Below figure indicates the color contour map of highest vibration amplitude at 2nd interstage piping system. It was found that the maximum expected vibration amplitudes satisfy the API recommendation for all frequency range.

Vibration for 2nd Interstage, compressor pulsation
[Vibration for 2nd Interstage]

When we compared with vibration amplitude for 2nd interstage for both condition between existing and revamping operation, they have similar vibration amplitude for all frequency range. And, it was understood that the high critical vibration issue did not reported during existing operation yet, hence, no additional support is necessary to control mechanical vibration.

Vibration Amplitude for 2nd Interstage
[Vibration Amplitude for 2nd Interstage]

4. Cyclic Stress

Pulsation and mechanically induced vibration shall not cause a cyclic stress level in the piping and curve volume in excess of the endurance limits of materials. For carbon steel pipe with an operating temperature below 370 degC, the peak-to-peak cyclic stress range shall be less than 26,000 psi, and it is recommended to apply the stress criteria as 6,500 psi (45 MPa) considering stress concentration factors 4.0.

It was checked the dynamic stress status, so that we can confirm the design reliability of piping system. It was assessed that the estimated stress (0.5 MPa) was in line with API criteria (45 MPa) as shown in below figure.

Stress for Piping System
[Stress for Piping System]

With consideration of tolerance such as construction and manufacturing tolerance, it is recommended to perform the visual inspection and field start-up vibration measurement after finishing revamping construction based on current piping system.

5. Vibration for Small Bore Connection

It should be noted that vibrations on small connections like instrument installation and small bore connection (SBC) like drain and vent lines are not included in this analysis, because these pipes are not significant from pulsation point of view and consequently they are not simulated in the acoustical study. In addition to above, these installations are always carried out during construction and revamping period according to the best possible arrangement at Site. Hence, they could not be considered in the acoustic and dynamic study for design purpose.

It is highly recommended to make stronger all the SBCs, located at least on surge volumes and piping system in vicinity with compressors, using below instructions;

  • SBC shall be braced in two planes directions
  • Mass of unsupported valve / instrumentation shall be minimized.
  • Unsupported length shall be made as short as possible.
  • SBC diameter shall be maximized.

6. Key Words – Pulsation Study

  • Reciprocating Compressor
  • Compressor Pulsation
  • Mechanical Vibration
  • API 618, API 688
  • Cyclic Stress
  • Small Bore Connection
  • Pulsation Shaking Force
  • Forced Response Analysis

7. Relates Services

INERTANCE has a full capability regarding related noise and mechanical vibration engineering as listed in below, and feel free contact us at (mountain@inertance.com).