API 674 – pump pulsations study

Typical scope of work and activities

The objective of the API 674 pulsations study is to prevent large pressure pulsation levels. Excessive pulsations in a piping system can reduce the service life of a pump, produce significant noise and also produce large shaking forces which can lead to pipe failure.

The pulsation characteristics of a piping system depend on factors such as:

  1. Complexity of the system layout
  2. Number of pumps
  3. Operating speed
  4. Liquid properties
  5. Pump type
  6. Pump size (power)
  7. Number of plungers
  8. System operating conditions
  9. Piping layout

The analysis will be conducted based on Analysis Approach 2 (Acoustical Simulation) as described in the API 674 2010 edition.

From a project execution perspective, four (or five) different phases will be distinguished:

  1. Initial recommendations
  2. Modelling of the system and identifying all operating conditions
  3. Pulsation analysis
  4. Recommendations based on pulsations analysis
  5. Mechanical review
  6. Recommendations based on mechanical review
  7. Reporting

Phase 0 – Initial recommendations

  • Dampener as close to the pump as possible.
  • Large equal tee for dampener connection.
  • On-ground supporting to ensure rigidity of supports.
  • Avoid systems lengths on the order of resonance at low harmonics.

Phase 1 – Modelling of the system and identifying all operating conditions

We always start the project by modelling the relevant piping system using our own pulsation software package BOSpulse. The pump(s) are modelled in detail as well as any pressure reducing components such as dampeners/accumulators or orifice plates. All downstream piping through which the pulsations will travel is modelled. The modelling of downstream piping can end at for example a closed valve, injection points into a gas filled pipe or when reaching a large diameter vessel.

A number of simulation cases are set-up based on the pump rpm and the different operating conditions. Simulation cases will include a change of pump rpm from +- 10%. This is done to account for any geometrical uncertainty in the model as well as uncertainty in the liquid properties and does not represent an actual change in pump rpm.

Preferably, the mechanical model is built during this phase as well. However, this might not always be possible at the start of the project as this requires more information about the system such as support locations. Optionally, the structural steel is also modeled.

Phase 2a: Pulsations Analysis

The simulation cases as identified during the previous phase are analyzed using BOSpulse. The results are interpreted and are compared to the allowable values as described in the API 674. Three limitations are set:

  1. Maximum peak-to-peak pressure pulsations level. A limit is set to the peak-to-peak pressure pulsations to prevent large shaking forces which can lead to the failure of the piping.
  2. Maximum pressure. This limit is to prevent pressure from exceeding the pipe design pressure as well as prevent any pressure relief valve from opening prematurely.
  3. Minimum pressure. This limit is critical near the suction side of the pump and should allow for enough margin between the minimum pressure and vapour pressure to prevent cavitation from occurring.

Phase 2b: Recommendations based on pulsations analysis

If API 674 criteria are not met, we will advise with the client about possible solutions. These solutions vary per system but can include:

  • Change positions of dampener/accumulator.
  • Change type of dampener/accumulator.
  • Addition of orifice plate.

Phase 3a: Mechanical review

During this phase, the mechanical model is built (if not already done in Phase 1) first build and then analyzed. If the structural steel is not included in the mechanical model all supports are assumed stiff relative to the piping. It is up to the client to ensure that this assumption is valid for all supports.

The goal of the mechanical review is to determine the maximum cyclic stress amplitude that can be produced by the shaking forces resulting from the pressure pulsations. This is done by selecting the maximum shaking forces as calculated during the pulsations analysis and applying a harmonic stress solver to the mechanical model using the stress software program CAESAR II.  The maximum cyclic stress amplitude is compared to the endurance limit for the material to prevent fatigue failure.

Phase 3b: Recommendation based on mechanical review

If the stress amplitude exceeds the endurance limit of the material, we will advise the client about possible solutions to resolve this issue. Recommendations include:

  • Locally changing the support functionality. This is not always possible due to thermal stress issues.
  • Addition of a re-routing of the piping to enable changing of support functionality without thermal stress issues.

Phase 4: Reporting

In agreement with the client, the reporting will be presented in the form of an elaborate presentation or a written report. The reporting will include the model details, input parameters, hypothesis, a description of the simulation cases, and the results. If applicable, the recommended changes will be described including the consequences of these changes to the problems identified. These recommendations for improvement are always an integral part of our deliverables.

Additional information required

The following additional information is required:

  • Detailed isometrics
  • Pipe information
  • Pump details
  • Flow characteristics of inline components such as valves or orifice plates
  • Flow conditions