Design Approach 3 (API 618, 5th ed.)

Pulsation + Mechanical Analysis

Today’s higher speed compressors can experience high vibration on the compressor components and piping system. Mechanical (dynamic) analysis is often required to avoid these problems.

A Design Approach 3 (DA3) study applies to larger or critical compressors. The study includes a pulsation solution (DA2) plus recommendations to avoid mechanical resonance and vibration.

In some cases, a Forced Response analysis may be required to verify that vibration and stress amplitudes are below guideline.

DA3 Scope

  • Includes Design Approach 2 requirements: Recommend a pulsation solution; assess static and dynamic pressure drop; Piping Restraint Analysis
    > more on DA2
  • Generate accurate Finite Element Analysis (FEA) model of vessel and piping system
  • Calculate Mechanical Natural Frequencies (MNFs) for vessels and piping system.
  • Recommendations to shift MNFs above 2.4 times run speed (to avoid resonance at first and second order of run speed)
  • Recommendations to shift MNFs at higher frequencies to meet API 618 separation margin criterion
  • Option: Assess vibration and stress levels using a Forced Response analysis when resonance is likely (e.g. insufficient separation margin)
  • Option: include “super element of compressor frame” in mechanical model to improve accuracy of cylinder motion boundary condition.

Application & Technical Support

> Compressor risk assessment (short survey to assess vibration requirements)

> Information required for DA3 study

> Background on API 618 (5th edition) pulsation and vibration studies

> Application Support for Reciprocating Compressors

Features of BETA’s DA3 Pulsation/Vibration Study 
This chart defines the requirements of API 618 and the standard features in a BETA study. 
> more

DA3 Limitations/Options

  • An accurate FEA model is critical to the succes of a vibration analysis. Ensure your supplier has field proven accuracy and follows these specifications.
    > more
  • DA3 does not include dynamic analysis of the skid and/or foundation design. The dynamic stiffness of the compressor package is an important component of a vibration solution [refer to skid design, or foundation design].
  • Other options for the compressor package may include:

> Torsional vibration analysis

> Piping flexibility (thermal) analysis

> Compressor Design Optimization


 
 		 

 FEA Model
Mechanical vibration analysis requires an accurate FE model (above) to identify MNF's, resonance, and vibration amplitudes.

 

 Mechanical Design
After analyzing the system, BETA will recommend changes (shown in red) to avoid resonance and vibration problems.

  
 
  • Standard analysis considers pulsation and vibration up to 150 Hz. Compressor cylinder gas passage acoustic resonance and shell mode vibration in piping and vessels are outside the scope of this analysis.
  • Off-skid piping will be included as long as the required information is available during the start of an analysis. If the information is not available during the course of the on-skid acoustical analysis we will use our proprietary method for performing a sensitivity analysis of the on-skid components to the off-skid components. BETA can revisit its work if off-skid details are made available after study is completed (additional scope)
  • Piping stresses are only calculated if the Forced Response Study option(s) is selected. This is defined as:
      • Step 3.b.1 for Compressor Manifold System Vibration & Dynamic Stress Analysis (formerly study M.6);
      • Step 3.b.2 for Piping Dynamic Stress Analysis (formerly study M.7). >more information on forced response analysis
    •  Vibration problems resulting from cylinder motion at frequencies above two times run speed are outside of the scope of this analysis unless the Forced Response option (Step 3.b.1) is selected - see above comment

     Customer Benefits
    Avoid piping and vessel vibration failures due to resonance (acoustical and mechanical).

    • Avoid transmitting pulsation problems to nearby facilities.
    • Avoid resonance in piping system due to many mechanical forces inherent in compressor
    		 
     


    Tips for Improved Compressor Designs

    • Involve BETA early in the design process. We can provide input on the design layout to speed up the process and avoid re-work or changes.
    • Specifications: To avoid vibration and reliability problems, include a specification for pulsation/vibration study with the compressor package bid documents.
      > specification
    • For existing compressors, we recommend a site visit to assess how proposed changes may affect the piping/compressor design.
    • Units with high rod loading (>80% rated) and high HP/cylinder (>700) will typically require a forced response study. (Step 3.b.1)
    • Operating Conditions. Include all anticipated operating conditions in design scope. This can avoid future vibration problems.
    • Pressure Drop. Ensure your pulsation consultant is compliant to API 618 (5th edition). This includes accurate predictions of static and dynamic pressure drop throughout the compressor package.
      > article on dynamic pressure drop
    • Performance. Pulsation effects can significantly affect performance. We recommend that a Performance Analysis be included in the DA2 report (which includes pulsation effects).
    • Check to see if a Torsional Vibration Analysis or other studies are required. 
    		 

    Advantages
    Beta Machinery Analysis is a global leader in pulsation analysis because of these factors:

    • Responsive application advice and ongoing customer service.
    • Most advanced acoustical/mechanical modeling software in the market
    • Studies are compliant with API 618 (5th edition)
    • Timely and practical recommendations
    • Mechanical vibration modeling technique are field verified
    • BETA has full time field engineers to assist with start-up checks, or onsite support.