Surge Peak Pressure Magnitude Impact due to Vapour Formation and Collapse: Simulation Study, Mitigation and Testing

Abstract

A surge study was conducted for the addition of a new LNG storage tank in PETRONAS Malaysia LNG in Bintulu, Sarawak. With the new LNG storage tank being the largest, furthest, highest elevation and with the biggest LNG loading pump capacity, a thorough surge study is essential to ensure safe operations of the existing loading lines network. During emergency situations, Emergency Shutdown (ESD) system may be activated where the ESD valves will be closed within 15 seconds to return the loading lines network into safe conditions. However, with the rapid closure of ESD valves, there is a sudden velocity change to the LNG liquid in the loading lines. This results in a vapour formation and collapse phenomenon which causes a surge pressure peak and resultant transient force onto the piping. The surge study started with a simulation study, where it was found that the initial surge pressure may surpass the loading lines piping design pressure. Then, the simulation carries on evaluating the possible mitigation measures that could be applied to the loading lines network to reduce the surge pressure to safe operating limits below the piping design pressure. Once satisfied, an actual ESD test was conducted where the ESD situation is replicated and the actual surge pressure was recorded. This test serves to reaffirm the simulation study and possibly quantify the magnitude of surge pressure experienced in real ESD situations. The mitigation measures of a 20 second pump delay timer and kick back valves to open during ESD activation was applied prior to the ESD test. During the test, the surge pressure peaks at approx. 20 barg, which was 4 times the normal loading pressure. This is as expected from the surge simulation considering mitigation measures are applied. If the mitigation measures were not applied, the surge pressure peak may go up to 10x the normal loading pressure, enormously exceeding the loading lines design pressure and may be catastrophic. Additional findings and discussion on the test results are presented in this manuscript. In conclusion, the surge pressure magnitude experienced due to sudden velocity change may peak numerous times above the normal pressure even though only for a short time. This situation is applicable to LNG loading lines but is also possibly applicable to other liquid lines with sudden fluid velocity change within the piping. It is important to ensure that the necessary mitigation actions are taken prior to putting the lines into service.