Improperly Depressuring Piping to Remove a Hydrate Causes Injury
The latest Safety Alert from BSEE (the United States Bureau of Safety and Environmental Enforcement) is entitled Improperly Depressuring Piping to Remove a Hydrate Causes Injury.
The report does not include a P&ID, so it is difficult to follow the sequence of events. However, some general comments to do with the removal of hydrates from piping are provided here.
Hydrates are solid crystalline compounds that can form when water combines with hydrocarbon gases under the pressure and temperature conditions commonly found in pipelines. Hydrates often look like dirty snow or ice (they are sometimes referred to as ‘dirty snowballs’). Being solid, hydrates can create major operating and safety problems because they can plug heat exchanger tubes, oil and gas production flow lines and instrument lines.
Because hydrates form at low temperatures they will generally disassociate back to the original hydrocarbon and water if they are warmed up. Hydrates can also be removed from process lines by adding methanol to the flowing stream. The methanol dissolves the hydrate solid.
If a pipeline becomes plugged with hydrates it is important not to apply too much pressure to the obstruction, otherwise, when the plug does move, it will become like a missile within the pipe. If the plug hits a pipe elbow or a valve body severe damage can result. (There have been fatalities caused by this effect.) Similarly, if a solvent such as methanol is added to the line the hydrate plug can break free (which seems to have been what happened in this case, and it appears as if the person involved in this incident could have been hurt much more seriously).
This BSEE Safety Alert has five recommendations. From a process safety point of view, two of the recommendations are important.
The first recommendation ― bleeding pressure upstream and downstream of the blockage ― means that if the hydrate plug does break loose, it will not move at high speed down the line. Depressuring the system in this way removes the hazard.
The fourth recommendation reads,
Use gas dehydration systems to remove water vapors to reduce the risk of hydrate formation inside of piping.
This recommendation is in line with our current theme which is Safety in Design, i.e., design the system so as to prevent the creation of hazards.
