Pigging is a maintenance procedure used extensively in the oil and gas industry to clean and inspect pipelines carrying refined and unrefined products. The process is made possible by launchers and receivers. Receivers (or traps) are vessels installed on the pipeline that allow for unloading of pigs without interruption of flow. There are a number of factors that should be considered when designing receivers. Some of these are discussed below.
Mechanical design characteristics -A pig receiver must be designed and using the same codes as the pipeline it is installed on. The American Society of Mechanical Engineers (ASME) governs pipeline design and construction in the U.S. Specific codes that apply to the design and installation of receivers include but are not limited to ASME B31.4 (liquid pipelines), ASME B31.8 (gas pipelines), ASME B31.3 (process piping), and ASME B31.11 (slurry pipelines).
Dimensions – A typical pig receiver consists of a major barrel, which is larger than the pipeline size, a minor barrel, which matches the pipeline bore, and a reducer barrel connecting the two. In a pig launcher, the shape of the reducer barrel should ideally be eccentric, with the flat side on the bottom to facilitate easy loading of the pig. However, this is not essential in a pig receiver where the reducer barrel can be concentric. In many instances, operators will elect to oversize pig receiversin order to ensure that larger/longer pigs can be accommodated if needed.
Location – Careful consideration should be made regarding the placement of pig receivers on the pipeline. This includes positioning the receiver so that there is sufficient space to operate the closure door and also to conductroutine maintenance of receiver valves and instrumentation.
Routine Inspections are Key
As is the case with any pipeline-related piece of equipment, pig receivers should be inspected periodically to ensure that they are in good working order. This is especially critical in high-pressure lines, as well as in applications where the receiver will be exposed to highly corrosive conditions.