Crews tackling installation or replacement of natural gas mains need to purge the lines with an inert gas to eliminate the combustion hazard. Purge testing before work begins requires a gas detection device that won’t be overwhelmed or damaged by the high gas levels found in this type of work. It’s also important to realize that many gas detection sensors require oxygen for accurate measurements.
Nitrogen is the gas most often employed to displace the natural gas when servicing main lines and create an inert service area. After the work as been completed, a service valve on the line with either a stand pipe or diffuser attached is cracked slightly to allow the venting gas or nitrogen to escape.
Measurements are taken at the stand pipe using a gas detector such as the GX-2012 from RKI Instruments. This monitor is ideal for purge testing. It’s also capable of all the standard confined space gas detection requirements, but is also specifically designed for oxygen-depleted (inert) atmospheres. The GX-2012 utilizes a durable thermal conductivity (TC) sensor which is able to measure high gas levels without damaging its LEL (Lower Explosive Levels) detection capabilities.
For gas purging applications a T-fitting adapter can be used to sample the air from a wider area and avoid over-saturating the sensors. Make sure the monitor is operating in % Vol only mode. This is also referred to as “purge mode.”
For an existing main, purge the line with Nitrogen (N2) gas until readings verify Oxygen levels are 0.5 or less, and gas readings are below 2%. Once the purge is verified, lines can be opened and the service performed. Working on the lines will result in air being introduced into the main. It will therefore be necessary to re-purge with N2 . Once this is done, recheck the line, again in % Vol mode, and verify the O2 reading is 0.5% or less. Finally, open the upstream valve and charge the line with gas. Use the GX-2012 to make sure the new reading is 98% or more.
New main lines simply need to have the initial purge and verification followed by opening the upstream valve, and rechecking to make sure the final reading is 98% or higher.
If you have questions about gas line purge testing, please don’t hesitate to contact us either online at www.pksafety.com, or at 1-800-829-9580. We’re available Monday through Friday 6:00am – 5:00pm.