1.1 – Piping Vibration (under construction)

1.   Vibration & pulsation study shall be carried out under the following situations:

1.   Reciprocating Pump
2. 1. Motor kW > xx  kW
   2. Running speed > xxx rpm
   3. Variable speed drive
   4. Production unit is dependent on  this pump (i.e. without a standby pump)
   5. Piping modification, machinery configuration, operation conditions are being modified.
3.  ii.  Centrifugal Pump

1. 1.  Reed critical frequency (RCF) of Vertical Turbine Pump to be calculated for new installation. Speed range of pump shall avoid RCF with a separation margin.
   2. If delta P across a throttle valve is greater than xx%.
   3. If pump ESD, water hammer study is yet to be carried out.

2.  Pipe supports that are adequate to support pipe under static load may not be adequate to support pipe under dynamic loads.  Typically, piping static stress study requires flexible piping to cater for thermal growth, whereas vibration study requires stiffer piping by installing more supports. 

Vibration = Dynamic Flexibility x Dynamic Force

Note:  Dynamic flexibility is frequency dependent.  High dynamic force, whose excitation frequency coincides with mechanical natural frequency (MNF), will result in resonance.  In order to avoid resonance, the excitation frequency of a high dynamic force should offset from MNF by a separation margin(xx%).

3.  Fatigue failure in piping can be caused by vibration which may arise under the following situations:

1.     Flow-induced vibration (FIV):   high flow in gas service with deadleg.
2.     Acoustic-induced vibration (AIV):  downstream of a pressure-reducing device   (e.g.  relief valve, control valve, orifice plate) in a gas service.
3.    Flow-induced turbulence (FIT):  high flow in liquid or gas service with flexible piping system that is inadequately supported.
4. Flow Pulsations: high pressure system with reciprocating compressor/pump, 

4.  Flow-Induced Vibration (FIV):

• Caused by low frequency waves (<XXHz). 
• Beam mode vibration that is perpendicular to pipe axis.
• Severity is determined by momentum flux (r v²), failure susceptibility can be categorized into 3 groups(negligible, medium, high)
• For system that is categorized under high failure susceptibility, the likelihood of failure (LOF) will need to be calculated.  If LOF >0.3, then  corrective action will need to be taken to mitigate vibration.

5.  Acoustic-Induced Vibration (AIV): 

• Caused by acoustic waves are in the range of XXX ~ YYYY Hz.
• Due to large dP across a pressure reducing valve, orifice plate or relief valve caused by choking of gases or flashing liquids. (Note: As a rule of thumb, upstream pressure must be at least 2X of downstream pressure for a gas to choke.)
• Mode of vibration is along the pipe circumference. Pipe failure typically occurs in thin walled pipe due to large hoop stress.
• Acoustic energy attenuates from the source.  Below xxxdB, circumferential vibration is no longer a concern.
• Mitigation measures 

1. 1.   Use larger pipe size to reduce fluid velocity.
   2.   Use heavier pipe schedule.
   3.   Use contoured fittings (i.e. avoid weldolets) to reduce stress concentration.
   4.   Use full wrap repad (i.e. avoid conventional circular repad).

6.   If a piping model has n nodes and x restraints, it will have (3n – x) degrees of freedom and (3n-x) modes of vibration.  However, only lower modes of vibration are associated with structural vibration as higher modes are associated with axial vibrations.

7.  May experiment various stiffness factors for friction  (e.g. XXX to YYYY, etc.) to see its effects on modal analysis results.  Generally speaking, piping natural frequency varies insignificantly when stiffness factor for friction increases beyond xxx value.  (Note:  stiffness factor for friction shall not be taken into credit for seismic analysis under the Code.)

8.   In “Lumped Mass” model, Caesar II ignores rotational inertia and off-diagonal terms in the mass matrix, and will give less accurate piping natural frequency estimate, this model was used in the early days when PC computing power was weak.  “Consistent Mass” model considers every terms in the mass matrix and it should be used as a default to calculate more accurate piping natural frequencies.  In order to avoid resonance in piping that may eventually lead to fatigue failure, 

F_x / F_n < 0.75 or

F_x / F_n > 1.25

where F_x = excitation frequency of rotating machinery

               F_n = mechanical natural frequency of piping

9.  Piping vibration is commonly overlooked by EPC contractors in Day One piping design.  Post-construction, in-house contractors typically propose to use U-bolts to clamp vibrating pipe to mitigate vibration.  This counter-measure is usually ineffective as U-bolt cannot totally eliminate vibration, only certain hold-down clamp can meet the required support dynamic stiffness required for vibration service.

10.  Properly designed anti-vibration pipe supports shall have adequate mass or stiffness to dynamically restrain the piping  so that a vibration node (i.e. zero amplitude) is formed at the support location.

11.  Mechanical natural frequency (MNF) of a piping span shall be 2.4 times the reciprocating compressor max. rated speed as per API. Additionally, it is good practice to ensure that MNF of a piping span is at least xx Hz to be considered dynamically fixed.