Trapped Liquids in Piping
The July 2024 Process Safety Beacon describes a very serious incident in which one person died, and many others were injured. The Beacon article focuses on the fact that the P&IDs were not correct. The PHA (Process Hazards Analysis) teams did not field verify the drawings; consequently they did not realize that a large section of pipe work was blocked in. Thermal expansion of the liquids in this section caused the pipe to rupture.
Verification of drawings is indeed one of the lessons to be learned from this event. But another question that the incident raises is to do with blocked-in sections of piping. There will always be piping between block valves and other line restrictions. What are the design guidelines in such situations?
There is no single answer to this question. Factors to consider include the pipe diameter, materials of construction, the pressure in the pipe, and whether the contents are liquid or gas.
Codes and standards that can help include:
ASME B31.3 ― Process Piping;
ASME B31.4 ― Pipelines for Liquids and Slurries;
ASME B31.8 ― Gas Transmission;
API 521 ― Pressure-relieving Systems
API 570 ― Piping Inspection;
NFPA 54 ― Fuel Gas Code;
NFPA 58 ― LPG Code;
ISO 15649 ― Piping; and
EN 13480 ― Metallic Industrial Piping.
These documents cover not just piping design, but also maintenance, inspection and the use of relief devices.
P & IDs are good for following a flow path but they show no elevations are actual distances or pipe angles which are all essential to understand restrictions in the pipe, causes of potential blockages and other external influences like ambient conditions, whether the insulation is sufficient. As an Instrument Tech, this has been essential fault finding where ambient Temperature influences the process composition,ie solidifying oils, or condensation when a cold gas liquefies.and gives false flow and pressure valves , the assumption that steam / Boiler Feed water is a clean process , yet in cold pipes can be solified sediment and very difficult to clean if blockages are not identified early enough. This sort of information is lacking since Instrument Engineering has been spread across multiple disciplines in Australia and no one person looking at the whole installation.