You can use CAESAR II to compute the support loads, forces, and stresses in the vent piping system when the relief valves fire simultaneously.
Venting steam stagnation properties are given. On the main window ribbon, click Analysis > Piping > Dynamic Analysis to open the Dynamic Analysis dialog box. Click Relief Load Synthesis to compute the maximum thrust load magnitude at the vent pipe exit. This dynamic load acts downward at the vent elbow nodes 65 and 100. Venting lasts for approximately one second, and the opening and closing time for the relief valve (as provided by the manufacturer) is 8.0 milliseconds. A static load case is run first to perform spring hanger sizing at nodes 20 and 22. The static load case #3 is the operating case and is used to set the nonlinear restraints for the dynamic analysis.
CAESAR II Gas Thrust Load Calculations
The spectrum table name is arbitrarily selected as Relief and is defined as having a Frequency range and a Force-Multiplier ordinate. A # sign precedes the name in the spectrum definition because the shock table is to be read from an ASCII file. The spectrum definition is shown as it appears in the Dynamic Analysis dialog box is shown below:
The DLF Spectrum Generator builds the ASCII file, Relief, which contains the relief valve spectrum table. You must define the filename, maximum table frequency, number of points, and the time-history waveform. In the example shown below, a maximum frequency of 33 Hz and 20 data points are used to generate the table.
The points in the time history waveform are entered as shown below. These points represent the opening of the valve, its one-second vent time, and its closing.
The resulting DLF Spectrum is shown below.
The Frequency versus Dynamic Load Factors is written to the file Relief. Click Save to File, and then click OK to close the Spectrum Table Values dialog box.
The thrust loads act at points 65 and 100. These loads are defined on the Force Sets tab and are entered as shown below.
There is only a single load case defined on the Spectrum Load Cases tab as follows:
Only one static/dynamic combination case is defined. It is the combination of the sustained static load case with the dynamic load case. This is defined on the Static/Dynamic Combinations tab as follows:
Only one item needs to be set on the Control Parameters tab. It defines the static load case for setting the nonlinear restraints. In the following example, the setting is 3 (line 1). Alternatively, you can set the modal combination method (line 11) to ABS instead of SRSS to produce conservative results.