Well production collection and preparation systems play an important role in the separation of oil, gas and water. Proper control of the parameters of gas separators and settling tanks is a key task for their efficient operation. In this paper, the determination of the performance of separators and settling tanks is considered, and special attention is paid to the boundary conditions for maximum efficiency of the separation process. At the moment, the calculation of the performance of gas separators and various settling tanks is carried out according to formulas derived in the twentieth century, and the scientific world does not question their reliability or the reliability of their conclusion. And in order to either prove the reliability of these calculations, or to refute them, an analysis of work was carried out to determine the performance of separators and settling tanks. At the beginning of the work there is a description of how productivity is calculated in the modern world. After that, a discussion is held about the reliability of these calculations, where in the course of reasoning, logical arguments are given about the erroneous conclusions of past calculations. So it was found out that the main assumption in the previous calculations was the use of the absolute velocity of the body in determining the resistance force of the medium, while it was necessary to use the relative velocity of the body, namely the velocity of the body relative to the medium. The following are the new boundary conditions, the solution of this problem, and the conclusions obtained.But the most interesting are the further discussions in the work, during which answers to the questions are given: how and at what speed will the oil particle move in the gas environment of the gas separator at different ratios of forces acting on the particle. Thus, it was concluded that the relative velocity of a body floating freely in a liquid or gaseous medium always tends to a certain constant value, which is numerically equal to the critical velocity of the medium, that is, such a speed at which a particle of oil would be stationary relative to the earth with a moving medium. In this case, the absolute velocity of the body (particles of oil, water, gas) will be equal to the vector difference between the velocity of the medium and the critical velocity of the medium. Combining the obtained conclusions, a new theory about the motion of bodies in a moving medium is created.