I am always amazed at the number of facilities that do not understand their hazardous materials and the proper safety techniques in managing the hazards these HAZMATs present. Recently I came acorss a sitution where a contractor was left to “prepare piping for a line break”. The facility utilizes natural gas, as a fuel, within one of their covered processes so they wisely utilize their Line and Equipment Opening (LEO) Permitting process. However, the contractor who has been doing all their NG work for years, argued that compressed air is an “industry practice” (note: an IP is very different from a RAGAGEP!). Some of you may recall the 2010 Kleen Energy plant explosion that claimed the life of my friend and colleague Chris Walters, the site safety manager. Since that day, a lot changed in how we “blowdown/purge” flammable gases, specifically Natural Gas. Here are the code REQUIREMENTS and the reasons why we should NEVER use compressed air in a flammable gas process…
NOTE: NFPA 54 does provide some exceptions based on line/pipe size, length of run, and operating pressure; I do NOT subscribe to this exception as it just opens the door for people to fudge the numbers and use compressed air when they would be prohibited by the code.
We cannot simply push natural gas out of an industrial or commercial line using compressed air. For anything other than small, residential-style piping, codes mandate the use of an inert gas (typically Nitrogen) to prevent the internal atmosphere from crossing through the flammable range (roughly 5% to 15% for natural gas).
Following the catastrophic 2010 Kleen Energy plant explosion, the codes governing these activities were heavily revised. When evaluating our blowdown/purge procedure, we MUST FIRST distinguish whether we are cleaning the line or purging it.
Cleaning is the physical removal of construction debris, scale, water, or dirt using high-velocity gas. Governed heavily by NFPA 56, Standard for Fire and Explosion Prevention During Cleaning and Purging of Flammable Gas Piping Systems, we can use compressed air, water, steam, or an inert gas to clean a line. We are strictly prohibited from using flammable gas.
Purging is the process of displacing one gas with another (e.g., taking a line out of service by displacing natural gas with air). Governed by NFPA 54, National Fuel Gas Code / ANSI Z223.1 and NFPA 56, introducing compressed air directly into a natural gas line creates an internal fuel-air mixture. If internal scale or debris strikes a pipe wall and sparks, or if there is a static discharge, the pipe can detonate internally. To mitigate this, NFPA 54 sets strict requirements based on the pipe’s size and operating pressure:
| Piping Size / Operating Pressure | Purging Requirement |
| Small / Low Pressure (smaller than 2 inches and short runs) | May be purged directly with air or fuel gas, provided it is vented to a SAFE, well-ventilated outdoor location. |
| Large / High Pressure (larger than 2 inches, lengths over 50 ft, or operating > 2 psig) | Inert gas is mandatory. We CANNOT purge directly with air. |
If we are taking a large line out of service, we do not necessarily have to fill the entire pipeline volume with nitrogen. The standard allows for an inert gas slug.
- We inject a calculated volume (a “slug”) of nitrogen into the line to act as a buffer.
- We follow the nitrogen slug with compressed air.
- The nitrogen acts as a physical barrier moving through the pipe, ensuring the natural gas and the compressed air never mix to form an ignitable atmosphere.
Regardless of whether we use a full continuous inert purge or a slug, the execution MUST BE STRICTLY monitored:
- The purge must be routed to a safe outdoor location, away from ignition sources, building intakes, and personnel.
- The vent point must be continuously monitored with a Combustible Gas Indicator (CGI).
- Note: We must use an infrared (IR) or thermal conductivity sensor designed for inert environments. Standard catalytic-bead LEL monitors require oxygen to function correctly and will give false readings in an inert nitrogen atmosphere.
- The purge is only complete when the CGI confirms the combustible gas concentration has dropped well below the LEL before the line is opened to the atmosphere or normal air is introduced.