To design a new (or to evaluate an existing) relief disposal system, it is necessary to model the response of the disposal system to the relief stream that would flow into it in each of the common contingency cases identified in the analysis outlined above in Section 1253. Hydraulic modeling software packages are available from several vendors for performing this analysis.
The analysis method may be broken into two parts. The first is the description (and input to the software) of the physical lay-out of the existing or proposed relief stream collection system. Typically this process starts with an isometric sketch of the collection piping, fittings, and equipment that includes all relief devices, control valves, and depressuring valves that discharge into the collection system. For each piping segment in the collection system, an entry is made in the software model giving the pipe size, length, elevation change, insulation, pipe roughness, and the resistance factor for all fittings (valve, elbows, tees, diameter changes, etc.) in the segment. Typically, an identification number is given to each pipe segment thus entered, as well as to the “nodes,” or intersections of two or more segments. Any separation equipment (such as knockout drums) is also entered, and flow resistance factors are also included for flare tips and seals that may be present. Sources of data for flow resistance data for various fittings and other equipment in a relief header system are listed in Figure 1200-35.
When the physical layout of the collection system has been identified and entered into the modeling software, the second part of the hydraulic analysis can proceed.
For each contingency identified in “Disposal System Design Basis” on page 1200-77, flow data is entered for each relief device, control valve, and depressuring valve identified to be flowing in that contingency. The input flow data includes the flow rate, pressure, temperature, and composition of the stream arising at each “source.” The model is then run; the software uses the known pressure at the ultimate discharge location and the flow and resistance data to calculate the pressure developed, the fluid temperature, and the fluid phase at each node in the system, working backward from the discharge point to each system inlet.