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Vibrations are a common and potentially harmful problem in piping systems and their supporting. Proper mitigation of vibrational issues is important and DRG can support you with that.
Vibrations can be caused by a number of different things. The varying flow from reciprocating pumps, compressors or process conditions are often the cause of a pulsating flow within the piping system. Excessive pulsation amplitudes can lead to mechanical vibrations and thereby fatigue failure of the piping or supporting.
To avoid these problems, proper system design is key and here DRG can be your perfect partner.
All lines in the systems are screened for potential FIV issues. Inputs to this study include line sizes, geometry and review of all operating conditions the system will experience. The outcome of the study will indicate a Likelihood of Failure (LOF) and shows the necessity of mitigation measures in order to prevent Flow Induced Pulsation issues. The screening method is conducted with reference the Energy Institute Guideline methodology.
Based on the screening of the system, re-designs can be proposed by Dynaflow in order to reduce or eliminate the LOF for the system. This can include redesign of equipment, increasing wall thickness or other system specific changes. The re-designs are always proposed in close consultation with the client in order to come up with a design that is optimal from both a technical as cost perspective.
For some systems, initial screening methods might prove overly conservative or simple redesigns are not possible. For these systems, Dynaflow offers detailed analysis of the specific location of concern. Through this analysis, Dynaflow can eliminate conservatism made in earlier simplified analyses possibly eliminating the need for changes to the system completely or create problem specific re-designs.
For critical systems, Dynaflow can conduct vibration measurements and assessment during start-up of the system. This is recommended for critical systems or for systems that are susceptible to vibrational issues and that have not been analyzed during the design phase of the system.
It is almost unavoidable that some vibration occurs in piping and equipment while in operation. Pressure pulsations are intrinsically present because compressors and pumps drive the flow. Small imperfections in equipment can introduce vibrations in the piping and pipe fittings. Furthermore, valves always generate a certain level of flow disturbance, resulting in the presence of small pressure fluctuations. However, a minor level of vibration is often acceptable.
When a vibration is observed on-site, DRG is often asked to perform an on-site inspection and even vibration measurements. Based on interpretation of the measurement results, DRG can determine on a substantiated basis the necessity of actions to mitigate the vibration. If an analysis is required, the measurement results serve as input to build and fine-tune the analysis model.
Vibration measurements is a service that can, and often is, combined with DRG’s other vibration services.
The effectiveness of the excitation to cause the vibrations (and cause fatigue failure) is determined by the mechanical response of the system on the excitation. The mechanical response is determined by several aspects including:
Piping geometry
Piping weight/size
Type of supporting
Stiffness of supporting steel
Excitation mechanism including a large range of possible sources including:
Pulsations from reciprocating equipment
Pulsations from screw compressors
Flow-Induced Turbulence (FIT)
Flow-Induced Vibrations (FIV) / vortex shedding
Acoustic-Induced Vibration (AIV); for example high gas flow through (pressure reducing) valves or orifice plates
Slug flow / multi-phase flows
Machine vibrations
Acoustically Induced Vibrations (AIV) refer to the vibrations occurring downstream of pressure reducing equipment in gas flow systems. The vibrations are caused by high frequency acoustic energy generated by, for example, chocked valves, orifice plates or pressure relieve valves and can lead to failure of the system due to fatigue within minutes of operating the system. The vibrations are high frequency, shell type mechanical modes and are typically not visually observable. AIV issues are therefore best identified and prevented during the design phase of the project.
Systems that especially susceptible for AIV are gas systems with high flow and high pressure drop:
Choked valves: Recycles systems, control valves, pressure safety valves
Any type of equipment with large pressure drop: orifice plates, silencers, diffusers.
Flow Induced Vibrations (FIT) / Flow Induced Turbulence (FIT) refer to vibrations occurring due to high kinetic energy due to turbulent mixing with boundary layer separation. The vibrations typically occur at frequencies between the 0.5 Hz to 50 Hz.
The vibrations can cause fatigue failure in the system after continued operations. While it is best to eliminate the issues during a design stage, FIV/FIT issues can also be mitigated during operations if identified during start-up/commissioning.
Systems that especially susceptible for FIV/FIT are gas, multi-phase or liquid systems with high flow/line velocity. Also, piping systems supported on racks or other elevated structures are more susceptible to FIV/FIT issues. Computational Fluid Dynamics can be used to quantify these vibrations.
Flow Induced Pulsations can cause vibrations and fatigue failure in piping systems. The pulsations arise due to the coupling of vortex shedding (typically from a dead end side branch) with acoustic resonance modes in the system. The vibrations are beam type vibrations and can cause fatigue failure in the system after continued operations. While it is best to eliminate the issues during a design stage, Flow Induced Pulsation issues can also be mitigated during operations if identified during start-up/commissioning.
Systems that especially susceptible for Flow Induced Pulsations are gas systems with high flow/line velocity together with equal diameter/relatively long dead end side branches.
An external flow over a cylindrical shape might give rise to vortex-induced vibration. For example, a pipe, a chimney, or a vertical pole of a wind turbine exposed to wind, or a heat exchanger tube array subjected to the flow of the process fluid, can develop vortex-induced vibration.
Depending on the fluid velocity and diameter of the cylindrical shape subjected to the flow, the Strouhal number of the fluid can lie close to the natural frequency of the cylindrical structure. Under these circumstances, frequency locking will result in dangerous resonance of the structure or pipe, with only very limited structural damping.
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