Robust optimal decentralized observer for multi-phase flow measurement

Abstract

In this paper, a non-fragile optimal observer is proposed for the decentralized multiphase flow measurement based on the interconnections between the two subsystems, that is, gas and liquid, constituting the whole system. Due to the dynamic model of system and presence of disturbances and slowly varying quantities, a non-fragile decentralized observer is designed and the states of the condensate and gas sub-systems were separately estimated. Lyapunov-based stability conditions are converted to linear matrix inequality (LMI) and observer gains are optimally selected from solution set such that the effect of the disturbance on the states’ estimation error becomes minimized. The estimation is conducted using the real-time measurements including lines pressures, single-phase gas flow, and single-phase liquid flow in the refinery outlet. To check the stability and performance of the system against the changes, the Lyapunov theory has been used. Finally, the estimation results are compared with real-world data from the industry showing the high accuracy of this method as the estimations were consistent with the operation data. In all stages, the investigations were based on the data collected from the actual process in the South Pars Gas Complex (SPGC), Iran. Additionally, the Extended Kalman Filter (EKF) based on the simplified drift flux model (DFM) was used to estimate the states then both methods’ results are compared and using the HYSYS simulator with the real process data, it is found that both observers are capable to identify the states with some differences in performance and DFM model is sufficient for estimation of parameters and states of the multiphase flow entering the gas refinery. As a result, these techniques not only can be substituted for the existing system at the gas refinery, but also can be as a backup for available measurement systems.