Dec 2014, AutoPIPE V8i 09.06.01.10
This piping system to be analyzed is small bore piping carrying oil at high operational pressure from a pump to several terminal points on engine . The single mostconsideration is to avoid coincidence of natural frequencies in the piping system to pulsation frequencies from fluid media and mechanical excitation frequencies fromterminal points at pump and engine. Considerable pressure fluctuations are expected in the system so susceptibility to vibration excitation from the following sources is to be considered and there is a stipulated load limitation on the vibration induced stress from non‐resonant shaking forces.
a. A transient water hammer type of excitation surge at startup and shutdown from 0 psig to 3500 psig for 10000 life cycles.
Most clients are using the fluid transient for the startup and closure of the valves e.g. using Flowmaster or PipeNET. Although a fluid transient (shutdown) can be simulated quickly with AutoPIPE’s fluid transient module for any gas, steam or liquid.
Sample Startup time history event from PipeNET
Sample Shut]down time history event
Case (a) needs to be checked with a fatigue curve. If ASME polished bars fatigue curves are used then the stress should be multiplied by a factor of 2 to account for the difference between SIF and SCF, otherwise ASME NB nuclear code can be used with detailed fatigue design. ASME B31.3 can also be used with proper stress range factor and code SIF values.
b) High pressure fluid induced pulsations with low frequency . 3100 psig +/] 363 psig within frequency range 10Hz to 85Hz. This is expected during normal operation and has to be evaluated for infinite cycles.
The system appears to have a reciprocating pump and so a pulsation evaluation and shaking forces should be evaluated for cases b and c. Static pressure changes as per Tony Paulin are also needed which can be easily done in AutoPIPE but they are not usually as critical as the dynamic pulsation pressures.
Step 1: Define two operating cases
Operating Case 1 ] 3100 ]363 = 2737 psig Operating Case 2 ] 3100 + 363 = 3463 psig
Step 2: Define a code combination range between the two cases OP1]OP2.
Step 3: After that the user have two options: to calculate the equivalent number of cycles according to equation (2) of paragraph 102.3.2, or refer the attached document from Paulin Research. For the code infinite cycles means 10E8 Cycles @ f=0.15
c) Low pressure fluid induced pulsations with high frequency 3100psig+/‐73 psig within frequency range 70Hz to 150Hz. This is expected during normal operation and has to be evaluated for infinite cycles.
Covered under b)
d) Mechanical vibration induced at terminal points over a frequency range of 10 Hz to 85Hz. This is expected during normal operation and has to be evaluated for infinite cycles.
This can be simulated using harmonic displacements or velocities at terminal points or pump/motor connections for multiple pump harmonics. Note: Imposed harmonic displacement, velocity or acceleration can be applied at any support or anchor. Harmonic forces can be applied at any unrestrained point in the model.
If the system is designed to avoid resonance then static imposed displacements can also be used for this evaluation (which is similar to ZPA in harmonic analysis).
It is recommended to use the ZPA option when using harmonic loads as this will give you more accurate nozzle reactions. Also the cut‐off frequency should be at least 1.5 times the highest harmonic load frequency. In addition use the Tools/model options/Edit to
add intermediate mass points (Mass points per span = A) based on the cut]off frequency. This provides more accurate captured modal mass for the harmonic analysis.
If pump nozzle flexibility exists then scale the harmonic displacement in order to get the desired movement which was lost due to the pump nozzle flexibility.
Bentley Technical Support KnowledgeBase
Bentley LEARN Server
Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article tothe "Comments" area below. THANK YOU!