Slug Catchers in Natural Gas Production - NTNU

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Karam – Slug Catchers in Natural Gas Production position the operational and the transitional points which will assist in determining if the flow is stratified or not. Two flow pattern maps are shown in Figures 6 and 7; the first illustrates a map for a 20 inch diameter horizontal slug catcher while the second is for a 26 inch diameter horizontal slug catcher. The two maps show that an increased diameter will provide a better stratification of the flow in the catcher. The volume needed to handle the entering liquid flow has to be decided upon after determining the minimum diameter of the slug catcher. The latter has to be increased in order to accommodate the accumulating liquid and avoid carryovers. The accumulating liquid will destabilize the flow in the catcher and stratified flow is consequently not maintained with such a pre-determined minimum diameter. The operational liquid holdup, H oper , can be calculated by solving the combined momentum equation for the stratified flow conditions. It depends on the liquid and gas average flow rates. The transition equation can be used to determine the maximum superficial liquid velocity knowing the superficial gas velocity. Thus, the transitional liquid holdup, H tran , can be calculated. The available volume to accommodate the liquid in the slug catcher is represented as the difference between the operational and the transitional liquid holdup. Thus, the length of the slug catcher for a specific diameter is calculated using equation (18). [ ] (18) Larger slug catcher dimensions result from such calculations due to two assumptions considered. The first consists of having a lower accumulated liquid volume than what is calculated in equation (14) since the liquid continues to be under the gas bubbles in the liquid film during production. As for the second, the liquid in the slug catcher is represented by H Loper before slug production while this amount drops as gas pockets and film are produced. The overestimation of the dimensions of the slug catcher can be considered as an advantage as it is a safety factor in production. The set of calculations is applied to one finger, but is valid to more than one finger knowing the liquid distribution among the fingers. 4.3 Components and specifications The design of the slug catcher follows a series of computational steps using the equations stated previously. As a first step, data from the field are required such as temperature, pressure, °API, inlet Page 16 of 56
Karam – Slug Catchers in Natural Gas Production flow rates of the gas and the liquid, diameter and roughness of the pipes. Afterwards, the operational point is to be plotted on the flow pattern map generated for the designated diameter of the inlet pipeline to the slug catcher. The operational point should be in the slug flow region of the map otherwise a slug catcher is not required. The flow characteristics are also calculated using the equations stated previously in this paper. The time difference in the slug arrival is mainly determined by the nature and the operating way of handling the system. Pigging can affect greatly the regularity of the slug emergence to the slug catcher aside from the natural slug flow. The slug catcher, in this case, should be designed based on the interval of pigging, the volume of slug to be produced from each sphering phase and a contingency volume. If pigging is not to be performed frequently, the maximum sphere-generated volume, SGV, of liquid should be determined by a computer program to size the slug catcher. In normal flow, the size of the catcher, according to Shell (1998), should be designed in a way to handle the difference between the volumes of the steady-state holdup generated by the fluctuating liquid flow in case of no pigging. Some complications should be accounted for in the sizing process. For long pipes, the pigging activities should be controlled as to limit the size of the slug catcher since the slug-sphered volumes (SGV) might be very large. There should be a comparison in the cost of having a more frequent pigging activity and a smaller slug catcher and that of a large slug catcher with occasional pigging (Mokhatab, Poe, & Speight, 2006). Sizing slug catchers with very rough elevation profiles of pipelines needs a specific computer program to simulate the transient flow. This is due to the terrain slugs that will form. By-pass pigging was also considered to cut the size of the slug catcher as it reduces the rate of the slug arrival and extends the arrival period of the slug ahead of the pig (Shell, 1998). The gas and liquid flow rates heading to the fingers’ inlet are considered in the design taking into account an even distribution among the different fingers. An even distribution is retained by the use of Tee-junction shaped splitters receiving the inlet flow perpendicularly. The splitters’ main function is to divide and further divide the flow into 2, 4 and 8 equal and parallel streams going downwards through the runs. The runs are constantly adjusted to keep the flow velocity constant and the flow distribution equal through back pressure induction. The inlet manifold is located perpendicularly to the splitters and should be of a large diameter so that the phases are evened before proceeding to the downcomers. Each inlet manifold can take up to eight downcomers which will be mounted to a constrictor. Page 17 of 56
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