A flow line or gathering line is a pipe that that carries oil, gas or water from the wellhead to a well-test manifold or production facilities. In the example below, the flow lines collect the production from four separate producing wells before combining it in a manifold. Flow rates are measured before it is put into a group line which transports the production fluids to a production facility.
A flow line is sized based on the maximum flow rate that the production well sees which is a function of pump size, stroke length and time. In heavy oil applications, a flow line may be insulated to retain the heat of the formation in order to prevent plugging. If the line is too large, the velocity could be slow enough where separation might occur or particulate may settle out in the pipe, which causes corrosion issues.
Flow lines may sit directly on the dirt, be buried, on supports or placed on sleepers. Most modern installations will place the flow line on supports or sleepers. The added cost of a support against the added cost of premature leaking due to external corrosion is negligible. This is especially true in a well that does not undergo constant surveillance where a line leak may take days to discover.
In the United States, flow line design, construction and inspection generally falls under the jurisdiction of ASME B31.4. However, there may be areas that require the line to be designed and built under ASME B31.3.