Experimental Investigation and Modeling of Rupture Disk Vent Line Systems in Two-Phase Gas/Liquid Flow
In the chemical and petrochemical industry, vessels and pipes are protected against overpressure using safety relief devices, usually rupture disks (also called a bursting disc) or safety valves. In contrast to a safety valve, the opening of a bursting disk is a stochastic process leading to a certain range of flow areas, depending on the manufacturing process of the disc. In general, this area cannot be predicted to the last percent. It determines dominantly the overall pressure loss and, in case of critical flow, the mass flow rate to be discharged through a bursting disk vent line system.
A rupture disk opens making the full vent area available for pressure relief. A small fragment remains attached in the flow area. This so called tongue contracts the flow causing pressure drop. This flow contraction limits the flow through a rupture disk vent line system significantly. To date, tests to determine the rupture disk flow resistance factor are typically performed with low velocity, subcritical, almost incompressible flow with air, nitrogen and water. Test conditions are stationary flow despite the fact that the flow regime during emergency relief varies from liquid only, gas only, gas/liquid two-phase flow or even flashing liquids. Even though a rupture disk is used as a primary relief device, the rupture disk flow resistance coefficients are not precisely applicable for compressible gas, vapor liquid or multiphase service.
There are rupture disk characteristic numbers and methods for sizing short vent-lines. Predicting flow through long and complex rupture disk vent-lines systems by extrapolating these simple methods comes with significant uncertainties. The uncertainties are even larger for two-phase flow. For two-phase flow, there is neither a standardized test section, nor any reliable test results available. Consequently, there is also no precise model to size a rupture disk device in these cases.
Critical flow conditions may occur in a vent-line wherever there is flow contraction in the vent-line. These conditions may occur in the inlet piping, rupture disk, bends or even tees. Critical flow may also occur at the vent line outlet. Changes in density in the fittings in a rupture disk vent line significantly limit the mass flow rate through the rupture disk vent-line system especially for in two phase flow. Frictional losses and flashing effects due to pressure loss significantly limit the mass flow rate. Today the mass flow rate and the pressure drop in a vent-line system is predicted with the use of models for the fittings in the vent-line system assuming fully developed flow. There is however no reliably validated method for sizing rupture disk vent line systems. This is because there is lack of reliable experimental data for validation. Over dimensioning is one option taken today to mitigate high uncertainties. This is not always an option, as it comes with substantial cost effort and also puts the integrity of other systems downstream at risk.
The characteristic numbers of rupture disk devices and the methods used to determine these numbers need be enhanced and harmonized to better capture the prevailing conditions in a vent line in practice especially for compressible flow. This is especially the case for two phase flow and flashing flow. A proper sizing model for bursting disks is indispensable.
BurstDisk2Phase; a research program at CSE Institute, seeks to narrow this gap in research by capitalizing on concerted efforts between the technical plant operators, high quality rupture disk manufacturer, institutions of higher learning and a research team that is guided by experienced leaders with the core aim of promoting safety in technical plants of the future. The HP-Loop will come in handy for improved models in generating the much needed suitable experimental data, deeper understanding and know-how.