Feasibility analysis of a rapid opening valve as a thin film diaphragm replacement for expansion tubes

Abstract

The objective of this thesis was to assess the feasibility of replacing the thin film diaphragm on the University of Queensland’s X2 expansion tube facility with a valve system. The aim of developing this system is to avoid issues associated with the use of the thin film diaphragm, while still producing acceptable test flows. To ensure the test flows are still acceptable, the valve must open in less than 0.5ms, as defined by Burgess [1]. The critical issue introduced by the utilization of a thin film diaphragm is that, upon impact by the shock, the thin film diaphragm is broken into fragments. These fragments can be accelerated into the test section by the flow where they may impact and damage instrumentation. There have been multiple attempts to avoid the diaphragm fragmentation issue. These attempts were compiled and assessed and it was concluded that none satisfactorily avoided the issues introduced by the thin film diaphragm without potentially significantly deviating the expansion tube test flow. To successfully replace the thin film diaphragm, a system which completely clears the expansion tube bore, rapidly, and with minimal disturbance to the surrounding gas, while maintaining a low leakage seal is required. An investigation showed that a gate valve best fulfilled this criterion. A gate valve consists of a flat plate of metal with a hole in it, termed the ‘knife’. To open the valve, the knife is moved such that the hole aligns with the expansion tube bore. The gate valve should be accelerated and decelerated pneumatically, while being sealed using a PTFE encapsulated X-ring pressed against the face of the knife. To achieve the required opening time without knife failure due to excessive stresses, the knife length before the hole should be increased to allow the knife to be accelerated at a lower rate for a longer period of time before the hole begins to clear the expansion tube bore. Similarly, the length of the hole in the knife should be increased so the knife can be decelerated over a longer distance. Analytical equations to approximate knife, connecting rod and piston failure were developed and shown to be conservative using Finite Element Analysis. These equations were then utilised in an iterative, parametrised Python code which both assessed the feasibility of implementing a gate valve system on an expansion tube with a given bore, and optimized the knife, con-rod and piston dimensions. Using this code, it was proven that it is theoretically feasible to implement a gate valve system on UQ’s X2 and X3 expansion tube facilities, as well as the majority of other facilities around the world. The basic parameters for a prototype gate valve to be tested in UQ’s X2 facility have been presented.