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How to evaluate DLF values consider by AutoPIPE during the dynamic analysis.
Some dynamic loads can be considered with an equivalent static approach like Blast, Relief, etc. However, in a piping system acted upon by time varying (dynamic) loads, the internal forces and moments are generally greater than those produced under the static application of the same magnitude load. This amplification (difference between max dynamic and static displacement) is often expressed as the dynamic load factor, DLF, and is defined as the maximum ratio of the dynamic deflection at any time to the deflection which would have resulted from the static application of the load.
The Static Equivalent approach simulates dynamic events with Static Load times the DLF (ex. Static Earthquake). A piping system’s DLF will range between 1 and 2 depend on the type of load (ex. speed of application) and response of the system (ex. stiffness). This is true only for an instantaneously applied load and so is a maximum conservative value.
Therefore, the DLF does not get applied to the dynamic analysis. If an equivalent static approach is used to represent a dynamic load, the DLF could be multiplied by the static response to make an assumption about what the dynamic response would be. In AutoPIPE, we have to ability to apply these loads dynamically, so we recommend doing so.
1. The maximum dynamic response to an impact load is 2 x the static load response. The ratio of dynamic to static response is termed the dynamic load factor or DLF.
2. Common impact loads on piping systems include :
Relief Valve Discharge
Water or Steam Hammer
Slug Flow
3. Static Equivalent Load Method = Static Load X DLF
4. A typical force response spectrum starts at a DLF = zero (very low frequency), rises to a maximum of DLF = 2 and then down to a DLF = 1 at high frequencies. (see spectrum below)
5. B31.1-2004 Relief Valve DLF curve having only single DOF on a rigidly supported pipeline.
6. The low frequency (flexible) response is generally ignored, but the drop from a DLF of 2 to a conservative 1.1 is set by the opening time of the valve.
7. The valve manufacturer can provide valve opening time then a more realistic force response spectrum can be generated as shown below.
8. The overall duration changes the response curve at very low frequencies i.e. very long duration gives a steeper initial curve.
1. Define time history load TEST.TIH starting at time 0 and load = 89000 lbs as shown below:
2. Convert Time history load file to force spectrum [Load/Convert to Force Spectrum]
3. Notice at 20.4 to 510 Hz, the force is about 176000 lbs giving a DLF of about 2. At very high frequency, >2000 Hz, it is 105480 with DLF= 105480 / 89000 = 1.1
The rise time of 0.0004 sec appears unrealistic.
4. If changed to 0.01 sec the force spectrum becomes:
Dynamic "Time History" Analysis
Bentley AutoPIPE