Leakage detection of transient hydrogen-natural gas mixture using reduced order modelling

Abstract

Early detection of gas leakage and its location in a pipeline is crucial in the effort to avoid impending disasters such as pipeline rupture. Existing studies mainly use sensors to detect and determine the onset of leakage, but these sensors, depending on their types are expensive to install. They could also give rise to false alarms and their handling needs skilled operators. As such, mathematical modelling has been adopted to be a viable alternative that is highly sensitive to pinpoint the leak location even for small leaks and to minimize the occurrence of false alarms at low cost. The present investigation focused on the development of a mathematical model for transient non-isothermal flow of hydrogen-natural gas mixture in a pipeline. This mixture is considered as hydrogen is often added to natural gas to enhance the latter’s burning capacity, and because hydrogen needs to be transported in the same pipeline as natural gas due to its storage problem and to reduce transportation cost. The mathematical model developed took into consideration the effect of the mass ratio of gas mixture, the transient condition due to the sudden closure of valves during leakage, the surrounding temperature and the inclination angle of pipeline. The gas mixture was assumed to be homogeneous and the transient pressure wave was created by the sudden or instantaneous closure of a downstream shut-off valve to ensure the attainment of minimum pressure at the downstream end within a short time. The boundary conditions imposed were under the assumption that a reservoir exists at the upstream and a sudden closure valve was at the downstream. The governing equations consist of non-linear partial differential equations of unsteady, compressible and non-isothermal one dimensional flow. They were numerically solved using the reduced order modelling (ROM) technique, which had not been previously applied on non-isothermal models involving gas mixtures. The transient pressure wave analysis was adopted to calculate the leak location and leak discharge. Specifically, the iron pipeline was taken to be 0.4m in diameter, 600m long, mass flow Qo=55kg/s at a static temperature T=15°C and an absolute pressure P=35bar. Numerical results on the effects of inclination angles, mass ratio of gas mixture and temperature change on the transient pressure and celerity waves due to the inclined pipeline show that the leakage occurs at about 200m. It is observed that the leak position is closer to the reservoir and the amount of leak discharge is higher than that of isothermal flow. An increase in the mass ratio Ø leads to an increase in the pressure and celerity wave, while the leak location and amount of leak discharge decrease. It is found that the mass ratio of hydrogen to natural gas should not be more than 0.5 to ensure that leakage does not occur before the estimated leak position. It is also observed that an increase in the inclination angle θ increases the pressure drop and leak discharge but the celerity wave and the leak location do not seem to be affected.