Experimental investigation on speed excursion of PIG due to friction variation in natural gas pipeline

Natural gas pipeline pigging with speed excursions may lead to potential loss of inspection data and possible dangers due to high acceleration; however, the operation relies heavily on a rule of thumb. Most pigging operations in gas pipelines are performed at normal operating pressures with regular flow rates, and the PIG velocity is generally in the range of 2–5 m/s. However, pigging in low-pressure and low-flowrate gas pipelines is very difficult due to speed excursion, which rapidly increases the PIG velocity. Mitigating speed excursions is an important challenge during the gas pipeline pigging process, but fundamental studies on this phenomenon have not been conducted. This study is the first to conduct a lab-scale experiment on speed excursion due to friction variation during gas pipeline pigging to investigate the mechanism of speed excursion and the relationship between the main variables and speed excursion. Based on the differential pressure results, mechanism of speed excursion was derived as 5 phases of speed excursion process: Stable behavior, build-up phase, pre-speed excursion phase, speed excursion phase, recovery phase. In the results of relationship between main variables and speed excursion, it was found that the flow velocity has a linear relationship with the speed excursion, but the excursion ratio has an exponential fit curve that rapidly increased at low flow velocity. These result means that low-flow pigging produces relatively low-speed excursions, but can be a very risky operation due to the rapid increase in excursion ratio. Both build-up time and recovery time also showed an exponential fit curve that increased rapidly at low flow velocity. These result indicate that pigging behavior is significantly unstable due to the long build up time and recovery time during low-flow pigging. When the linepack length changed, higher speed excursions occurred as the linepack length increases even at same friction conditions. When the wall thickness change ratio increased, the increase in friction led the speed excursion increased from about 46% to 110% showing the same trend of linear fit curve, and the rate of increase was higher as the flowrate decreased, and these results means that PIG behavior is more unstable due to increased friction, especially in the low flowrate. The results indicated that the linepack length is the main factor in pigging behavior. In particular, linepack should also be considered as an important variable that affects speed excursion, and behavior in long-distance pipeline pigging.