Pipeline Pigging

4.5.8.4 Pig Launching And Receiving Procedures

Pig launching and receiving procedures are often supervised by senior operations staff and fully monitored by all pipeline users but the actual procedures laid down for each pig launching/pig receiving facility will vary.

4.5.8.5 Pigging Problems

The pig launcher-receiver is probably the only high-pressure vessel on the facility, in hydrocarbon service, which is regularly opened to the atmosphere and then pressured as a normal operating procedure.

If the launcher/receiver is incorrectly purged and pressured, an explosion becomes a major possibility. To reduce the chances of such an incident, the relative procedures are commonly backed up by an “interlock-system”, which prevents the movement of valves and door closure devices until certain criteria have been met within the system.

Figure 4.33 illustrates the logic of a simple interlock system.

In the last decade at least two launchers have been involved in major explosions in Britain.

When pigs are launched into a pipeline there is always the possibility that the pig will stop or reduce the flow of fluid through the pipeline. The most common incidents and their causes are:

The pig fails to launch (this only becomes apparent alter the launch procedure is at its final stages. The possible causes are:

1. The pig is too small (wrong pig or under- sized) and the flow cannot pick up the pig in the launcher barrel.

2. The pig is too large, wrong pig or oversized and it is jammed in the exit to the launcher.

3. The pig is too far back in the launcher.

The pig indicator, Figure 4.35 should show that the pig has launched. They are, however, not always reliable.