Vibration Induced Fatigue | Materials And Corrosion Control

Damage Description

Affected Materials

Control Methodology

·         Design shall involve the use of S-N curves, where S is the amplitude of the fluctuating stress and N is the number of cycles to failure for the material. Limit the stress to the materials’ endurance limit or that stress below which the material will withstand, without failure, for an indefinitely large number of cycles of stress

·         Install gussets or stiffeners on small bore connections ⁽¹⁾. Eliminate unnecessary connections and inspect field installations.

·         Fittings and unsupported length should be as short possible

·         The mass of unsupported valves/instrumentation should be minimized, e.g., by the use of lightweight double block and bleed valves or monoflange valves

·         Bracing supports should be from the main pipe, thus ensuring that the small bore connection moves with the main pipe.

·         Bracing will not be beneficial in the case of high frequency acoustic excitation.

·         Vortex shedding can be minimized at the outlet of control valves and safety valves through proper side branch sizing and flow stabilization techniques.

·         Vibration effects may be shifted when a vibrating section is anchored.

Monitoring Techniques

·         Check for the audible sounds of vibration originating from piping components such as control valves and fittings.

·         Conduct visual inspection during shutdowns for intermittent vibrating conditions.

·         Measure pipe vibrations using special monitoring equipment.

·         The use of surface inspection methods (such as Penetrant Testing, Magnetic Particle Testing) can be effective in a focused plan.

·         Check pipe supports and spring hangers on a regular schedule.

·         Damage to insulation jacketing may indicate excessive vibration. This can result in wetting the insulation which will cause corrosion.

Inspection Frequency

KPIs

·         Number of visual inspection of piping supports and spring hangers

·         Percentage of braced small bore connections, particularly those associated with rotating machinery.

·         The maximum overall allowable vibration levels for piping with reinforced and without reinforced branch piping are:

o   1 in/s (25.40 mm/s) RMS (define RMS) with reinforced branch piping see Appendix F (2). Where is Appendix F(2)??

o    0.75 in/s (19.10 mm/s) RMS without reinforced branch piping see Appendix F (2). Where is Appendix F(2)??

Reference Resources (Standards/GIs/BPs)

·         ⁽¹⁾ SA Standard Drawing AB-036521

·         ⁽²⁾ SAER-5659