HOW TO LOOK FOR A NATURAL GAS LEAK—Carl Schwab

This approaching January 13, 2011 will be the second anniversary for out neighborhood conversion from #2 oil to NG, natural gas.

December 22, 2010, National Grid trucks appeared on our street "loaded for bear". What I mean with the backhoe and Bobcat guys and two levels of supervision. Being the curious sort, I  collared one supervisor and inquired what they were up to--------I told them I lived nearby.

He informed me that they had a report of a possible gas leak and were just here to check it out. I had the time so I just watched (and also learned <G>). I never got a totally straight story about how the leak was reported. One version was National Grid had detected it by a sensor on one of their surveillance trucks that have a super sensitive NG detectors. The other version was someone in the neighborhood said they smelled NG.

Immediately it was evident they suspected a 1" high pressure line that brought high pressure NG from the 6" street main to a curbside cutoff valve. So the first step was to confirm that the leak was coming from this line section before they started digging things up. To do this the second level supervisor used a probing tester. This tester worked as follows. First they have a hardened, sharpened 3/8" diameter rod about 5' in length with a reciprocating mass weight that the man operated to quickly go down to about a 3 foot depth. Once the hole is made then the sensor probe is inserted and reads out in parts per million of NG. This process took about 2 to 5 minutes per test hole. A second person records the data and makes a crude map plot of the suspect area. After the first half dozen test holes they had strong indication where they should probe further to refine the location of the leak. All in all I counted about 25 test holes. I was surprised to see they could detect NG at 36" below the ground surface up to 60 feet away---the tester is that sensitive.

Pretty quickly the supervisor was convinced that the leak was on the 1" line but about 3 to 5 feet off the 6" main. So now the backhoe, shovels and digging started. The backhoe operator quickly got rid of the first 30" down and the shovels got down to the 1" line. The supervisor decided to open things up back to the 6" street main so that the "T" could be examined. Soap solution testing quickly showed that an electric activated thermal splice was the culprit and showed damage possibly from its initial installation. The "T"
was activated to close the opening into the 6" side wall of the street main and the 1" line spliced with electrically activated thermal splices to remove the section of line with the defective splice.

Repair complete--------now the Bobcat and shovels plus a pneumatic compacting hammer came into play and in short order had things closed up and the repaired line pressure tested to 187 psi and held for 20 minutes, as regulations require.

The last detail that surprised me was they flushed out the soil layer surrounding the leak for some 45 minutes. The supervisor said experience showed that this flushing greatly reduced the nuisance callbacks by people claiming they still could smell NG.

Quite an interesting operation.

BTW our conversion is completing its 2nd year of operation this January 13, 2011. FWIW our standardized monthly rate is now set at $109 per month. This computes to $1308 per year and is less than the $1523 we paid the first year.

So we continue happy.

Comments

[Devremülk]

i like to read your posts. thanks for this one.

[Dick LaRosa]

I've seen similar operations on the next street. Thanks for explaining the flushing. I couldn't figure out what they were doing.
They installed our new gas lines during a very cold winter. The excavated soil froze into big chunks that didn't fit back into the tunnels. They took the excess away at night and paved over the street. We now have a network of tunnels that can fill with gas if there is a leak underground. Good reason to be sure the basement wall is sealed.