Experimental Investigation and Modeling of Rupture Disk Vent Line Systems in Two-Phase Gas/Liquid Flow
Rupture disk devices in technical plants prevent unsafe over pressurization of vessels, tanks and other apparatus. Once the burst-pressure is reached, the busting disk is activated making an opening to the relief line. The flow resistance in the relief line must not be too high during relief – this may otherwise increase the pressure upstream of the rupture disc device further.
Rupture disk devices in chemical industry are typically subjected to mixtures of gas and liquids. The pressure loss is hard to calculate. Currently there is no validated model for sizing rupture disk devices including the relief line upstream and downstream.
Suitable sizing methods are being developed at the CSE 2 Phase Flow Loop so as to characterize flow in bursting disk devices. A new model is being developed especially for gas-liquid flow so as to size rupture discs and their associated vent-lines (upstream and downstream) reliably. The model shall be validated with experimental results.
I: How can the rupture disk discharge area be determined experimentally?
The Minimum Net Flow Area (MNFA) or rather the open rupture disk free discharge area A0 should be calculated after a complete activation of the rupture disk device with appropriate allowance for any structural members which may reduce the net flow area (American Society of Mechanical Engineers, 2017). It is however taken to be the minimum diameter of the rupture disk holder because there is no prescribed method to calculate this area as intended even though an open rupture disk device typically has a rest fragment by design which restricts flow significantly. The open rupture disk free discharge area is requisite in determining the dischargeable mass flow rate through a rupture disk device installed in a vent-line accurately subject to working standards such as the AD 2000-Merkblatt Part 1 (Verband der TÜV e.V., 2006) or the API 520 Part I (American Petroleum Institute, API, 2014).
Two analytical methods to determine the rupture disk discharge area experimentally with low-velocity flow have so far been developed. The suitability of the methods to determine the flow area is validated experimentally with diverse test objects. The flow area determined this way is a more suited estimate of the rupture disk flow area for calculating the dischargeable mass flow rate through a rupture disk device per working standards and with new alternative methods that are under study.
II: How can the pressure drop across a rupture disk be modelled and pressure profile in a rupture disk relief line be predicted better?
Proper sizing of rupture disk vent-line system involves fluid dynamic coupling of the rupture disk device and the entire vent-line with all its fittings. Sizing requires correct consideration of the fitting’s and piping’s losses coefficients to determine the irreversible pressure loss and dischargeable mass flow rate. A fitting’s minor loss coefficient is typically determined under low-velocity and incompressible flow. It is however not precisely applicable for all plausible flow cases that occur in practice especially for high-velocity compressible gas flow, two-phase flow or flashing liquids without proper consideration of compressibility. Experiments done at CSE test facilities so far show that rupture disk irreversible pressure loss, determined assuming a constant minor loss coefficient, with classic methods may be underestimated significantly even for gas flow.
An analytical method to predict the pressure drop in a rupture disk and a method to better predict the pressure profile in a relief line with a rupture disk installed have been developed. Experiments show that these methods predict the pressure profile with less uncertainty as compared to classic methods. The method is also applicable for compressible gas flow as is typically the case during pressure relief in a complex rupture disk relief line. These methods developed are more suited predict the pressure drop across a rupture disk device and the pressure profile in a rupture disk relief line. These are some of the new alternative methods that are under study at the CSE Center of Safety Excellence, in Pfinztal Germany within the scope of BurstDisk2phase research project.