School of Mechatronic Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China
Pigging is a routine operation in the oil and gas industry. In this paper, the governing equation of pig speed was combined with the gas flow equations. The transient equations of gas flow are solved by the method of characteristics (MOC). An experiment was carried out to test the proposed pigging model. The measured speed of the pig coincides with the calculated speed well. The process of a pig carrying a brake unit to pass over a hilly gas pipeline is simulated. The results indicate that the brake unit would lead to a sharp increase of the pressure on the tail of the pig, because the pig is dragged by the brake unit and thus prevented to accelerate together with the gas column in a downhill gas pipeline. This way, the pig speed in a downhill gas pipeline is much lower by using a brake unit, but the speed of pig still can hardly be controlled in the desired range. Furthermore, response surface methodology (RSM) is used to study the maximum speed of pig with/without a brake unit in downhill gas pipeline. Based on the results of the RSM simulations, two equations are present to predict the maximum speed of a pig in a downhill gas pipeline.