The Design Institute of Emergency Relief Systems (DIERS) was formed in 1976 as a consortium of 29 companies to develop methods for designing for the design of emergency relief systems to handle runaway reactions. DIERS spent 1.6 million to investigate the two-phase vapor-liquid onset/disengagement dynamics and the hydrodynamics of emergency relief systems. Of particular interest to DIERS were the prediction of two-phase flow venting and the applicability of various sizing methods for two-phase vapor-liquid flashing flow.
Step 1
In this example we will take a typical fire case scenario and design for single-phase to determine the required capacity and required orifice size and compare that against the existing relief device size to determine if it is adequate.
Step 2
We will then use DIERS methodology to perform a two-phase test to determine if two-phase flow is likely based on an article by Harold Fisher and Harry S. Forrest, "Protection of Storage Tanks from Two-phase Flow Due to Fire Exposure".
Step 3
If two-phase flow is likely then we will use DIERS methodology to determine the two-phase venting requirement based on using the two-phase homogeneous sizing method describe by J.C. Leung from the article, "Simplified Vent Sizing Equations for Emergency Relief Requirements in Reactors and Storage Vessels.
Step 4
Once the two-phase relieving capacity is determined then we will use DIERS methodology to determine the maximum fluid flux(Gmax) using direct integration of the VdP integral for the relief nozzle based on an article by Larry Simpson, "Estimating two-phase flow in Safety Devices", which will set the basis for the design of the relief orifice size using the equation, A = W/(Gmax), where A is the required orifice size, W is the two-phase relief rate, Gmax is the maximum fluid flux that typically occurs at sonic conditions. This method will also be employed to determine the sizing of the inlet and outlet piping based on the Simpson article, " Navigating the two-phase Maze" We can then compare the two-phase relief orifice size with the existing and single-phase size to determine if the existing relief device size is adequate for both single and two-phase relief.
Step 5
Our final step will be to evaluate our two-phase design to determine if we can reduce the required orifice size for two-phase by using the coupling equation to determine the relieving capacity using different flow regime models, such as bubbly or churn-turbulent based on and article by Harlow Fisher and Michael Grolmes, "Vapor-Liquid Onset/Disengagement Modeling for Emergency Relief Discharge Evaluation. And we will also evaluate different techniques to move the two-phase design into the single-phase region by utilizing the alpha vs. Psi curve.
