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Vibration, dynamics and noise

formerly BETA Machinery and SVT



Small-Bore Connections (SBC) Assessment

for all machinery types and applications

Small Bore Connection (SBC) Vibration Analysis

Small-bore connections, also called branch connections, to the main process piping represent the most common vibration problem on rotating and reciprocating machinery and associated process piping.

Small-bore connection design assessment and field vibration testing are strongly recommended to avoid piping integrity risks. 

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1 Background: Small-Bore Connections; Definition, Risks and Consequences
Field audit of small-bore connections       Cracked vessel due to SBC (blind flange)
Field audit of small-bore connections Cracked vessel due to SBC (blind flange)

Piping vibration and fatigue can account for up to 20% of hydrocarbon releases; and a large portion of those are due to failure of small-bore connections per Energy Institute, 2008, Guidelines for the avoidance of vibration-induced fatigue failure, (AVIFF). Hydrocarbon emissions can lead to fire, explosions, injuries, property and environmental damage, and penalties.

A small-bore connection (SBC) is generally defined as a branched connection on mainline piping that has a nominal diameter 2” (DN 50) and smaller, including connections that have a branch pipe to mainline pipe ratio of less than 10%, and excluding connections that have a ratio of greater than 25% (see Figure below). Note that “mainline piping” could also describe equipment like a vessel or cooler to which the SBC is attached.

Small-Bore Piping and Connection Definition Small-Bore Piping Definition

Small-bore piping (SBP) is defined as the piping attached to the small-bore connection, extending until the effect of the mainline piping vibration is negligible (typically, the nearest support or brace), as shown in the illustration below.

The SBP of most concern is that which contains production fluid at operating pressure. Auxiliary lines, like pneumatic air, crankcase vents, etc., are not as critical. Wood uses the term SBC to refer to this entire class of piping vibration problems, including branch connections, blind flanges, PSVs, site glasses, RTD connections, drain lines/valves, etc. 
Multichannel systems used to assess SBC vibration risks

Multichannel systems used to assess SBC vibration risks

SBCs are highly susceptible to problems due to their geometry and mass. Even very low amplitude (and acceptable) vibration on the main process piping can cause the branch connections to vibrate excessively and break due to fatigue failure. This is due to the local resonance of the SBC (see our Training video, Module 1 for examples of resonance). At certain frequencies, base vibration can be amplified by 20 to 30 times, causing branch connections to fail.

The consequences of a vibration-induced failure can be catastrophic. A small crack will release process fluids causing safety risks (explosion, fire, or toxic chemicals), environmental risk to the nearby area, and production downtime. In recent cases, SBC failures have been responsible for the shutdown of a pipeline and production facilities for weeks. Given the risk of these piping failures, owners must take an active role in avoiding them.

2 Approach and Scope
2.1 Approach

To address SBC integrity risks, the owner must specify the appropriate engineering assessment. Ideally, the evaluation occurs in three stages, as shown in the table below.

SBC assessment activities 
Defines the appropriate scope
and requirements for dynamic design

Screening Assessment of Piping System: Pulsations, Flow Turbulence, Acoustic-Induced, Mechanical excitations, Surge, Other Transient Events

Detailed design  
Review and evaluate SBC designs 
SBC Analysis: Quantitative Approach, Identify Fatigue Risks, Finite Element (if required), Recommend Modifications 
Onsite testing, documentation, modifications (steady state and transient flow evaluations)
SBC Vibration Assessment (Baseline): Steady State Conditions, Transient Operations, Extrapolate Results to Other Operating Conditions 
Finite Element Analysis (FEA) of Small-Bore Connection

Finite Element Analysis (FEA) is used to calculate stress and confirm acceptance (or failure) of small-bore piping design

Qualified dynamic engineering experts are required to perform this work because it involves stress analysis, fatigue failure analysis, FEA, and other advanced dynamic analyses. 

Warning: One common misconception is that a SBC assessment is similar to a standard vibration monitoring program such as condition based monitoring or route bases vibration check, which is often performed by vibration technicians. While this program is useful for monitoring degradation in bearings, it is not applicable to SBC engineering design and evaluation.

The scope of the SBC assessment will vary based on the application (liquid, gas or multiphase), operating frequency, location, and process conditions.  These variables will affect the type of analysis conducted at site. 

2.2 Scope (Design and Field Analysis)

The following standard services are available for address SBC piping integrity risks. Contact Wood for more information on the detailed scope, or if a customized solution is required.

New projects (or modified systems) - includes design and field activities
Service Description Summary Remaining risks
SBC-D1 Design SBC Review

Basic approach: review available drawings and provide “best practice” recommendations. No field measurements.

Medium  Many remaining risks since resonance is possible (and no field verification/testing)


Standard SBC Integrity Assessment

Design evaluation of proposed SBC with recommended changes.
Shop and field testing to verify vibration levels are acceptable.

Low  Common risks are significantly addressed. A small amount of residual risk remains. 


Comprehensive SBC Integrity Assurance

Wood is directly involved in SBC design to ensure a safe system that avoids resonances at key frequencies.
Shop and field testing to address steady state and transient situations.

Minimal  Maximum assurance for the designed operating system.

Existing operations
Service Description Summary Remaining Risks


Standard SBC Field Assessment

Ideal for evaluating SBC integrity and the associated piping on the machine system. Includes impact testing for MNFs, vibration screening, speed sweep, and detailed FEA (if required) to resolve problem locations.

Low  For standard service, there are practical limits to testing all locations. Not all operating conditions checked. Plant piping (off-skid) SBCs not evaluated.


Comprehensive SBC Integrity Audit

For critical applications where more stringent testing is required. Includes transient and steady state testing, fatigue analysis, and assessment of pipe strain. Can include station piping (away from the rotating/reciprocating machine).

Minimal  Common risks are significantly addressed. A small amount of residual risk remains.

To mitigate this risk, the owner (or its Engineering Consultant) must specify a SBC vibration study.

3 Wood Advantages
  • Over 50 years of field experience in SBC evaluations.
  • Multichannel data acquisition system (>100 channels) for SBC testing – a key advantage for larger piping systems.
  • Design experts in dynamics, pulsation control, vibration mitigation on reciprocating and rotating assets.
  • R&D programs focused on developing innovative solutions to evaluate and mitigate SBC vibration, including research for the Gas Machinery Research Council (GMRC).
  • Preferred supplier to the largest gas and liquid operators and pipeline companies.
  • Global support, including field engineers certified for offshore facilities.
4 Related Information
5 Related Services
6 Keywords
  • Branch connections
  • Small-bore connections (SBC)
  • Small-bore piping
  • Small-bore failure
  • Small-bore fatigue
  • Small-bore vibration
  • Branch connection failure

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