Investigation of Valve Failures Using Statistical Analysis of Proof Test Data

Abstract

This research work examined pressure relief valve (PRV) failures by using statistical analysis of proof test data obtained from the computerized maintenance management system (CMMS) of Liquefied Natural Gas (LNG) facilities site. This study comparatively investigated the 30 months approved inspection interval for the proof test of PRVs by the Department of Petroleum Resources (DPR) of Nigeria. In this research, failure rate and failure behaviour of PRVs were evaluated and analyzed using bathtub curve. Aging effect of PRV using regression analysis and the cause of failures were also analysed. Least square multiple linear regression was the statistical tool used for the analysis with the aid of EXCEL. After thorough analysis, the study depicted drop in failure rate at different times (years) at the bathtub curve. Also, the statistical analysis found that the ageing effect from five spring loaded PRVs evaluated was at most 0.4% per year with 95% confidence and the amount of variance explained per time was minimal (R-Square 7.6%). It was recommended that the inspection interval (30 months) as approved by DPR for proof test of PRVs was optimal with regard to the required integrity condition of the PRVs since test frequencies yielded better result and consequently less risk. Also, further testing and investigation should be pursued to determine if longer intervals between inspections were possible. Furthermore, analysis from the investigation of cause of failure showed that corrosion was a cause of PRV failure and contact area factors such as sticking and cold welding