5 Jan 91
Gas Distribution System.
It has been decided to install cathodic protection on the gas
distribution piping in a post housing facility. Figure 2-11 shows
a portion of the piping.
The water distribution system has recently been replaced with
nonmetallic pipe. On this basis, it has been decided that the gas
piping can be protected with impressed current from a deep anode
groundbed without causing interference to the water pipe. (Note:
Some county, state, and federal agencies such as the EPA may have
regulations which prevent the use of deep anode beds because they
can provide a conduit for mixing of waters between aquifer layers.
In such cases, regulations have required cementing of the annulus
between the deep anode bed casing and the augered hole to prevent
this water migration. The system designer should check with the
applicable agencies before committing to a deep anode design.)
a. Design data.
resistivity at a depth of 50 and 200 ft to be 2000 ohm-
Piping consists of:
28,000 ft of 1 1/4-in. diameter pipe
1,200 ft of 4-in. diameter pipe
3,600 ft of 6-in. diameter pipe
Design cathodic protection anode for a 20-year life.
Experience with similar pipe in the same general soil
type has shown that a design current density of 2 A per
sq ft of bare pipe is conservative.
Piping is welded steel, poorly coated; considered to be
bare for protection purposes and, therefore, the
coating efficiency is 0.0 (CE = 0.0).
Piping is isolated at each house and at the tie-in to
the main supply line.
The cathodic protection circuit resistance should not
exceed 1 ohm.
120/240 V AC, single phase power is available.