Improperly marked, underground natural gas lines are some of the most dangerous utilities. This is illustrated by high profile disasters like the San Bruno explosion in California. When unmarked or improper gas line marking is performed, it can pose the threat of fire, explosion, or suffocation, potentially causing millions of dollars in damage and loss of life. Conversely, when gas line utilities are properly protected, they are as safe as any utility underground.

Proper Gas Line Marking is Important Because Lines Come in Many Different Sizes

From half-inch interior residential gas plumbing to 24-inch transmission lines, gas lines can have huge disparities in size. Furthermore, some lines are highly pressurized, which means if they are broken they can quickly spew a high volume of natural gas into the surrounding environment. Other lines can be low or medium pressure, which means that if they are punctured, they release a lower volume of gas. Most transmission lines are high pressure, and pressure in the lines tends to be reduced the smaller the pipe carrying the fuel.

gas line marking

Under Pressure

It’s a little counter-intuitive, but depending on the situation, a low or medium-pressure pipe can be more dangerous than high-pressure lines. This is because, like any fuel that burns, natural gas requires oxygen to burn. If the gas mixture has too much fuel and not enough oxygen (the upper explosive limit has been reached), the fuel cannot burn.

A high-pressure line is more likely to push so much fuel into the surrounding area that there is no way a fire can start, while medium and low- pressure line breaks tend to catch fire much more easily. This does not mean high-pressure lines are safer if broken, because human beings also need oxygen, and a high amount of natural gas in the air can reduce the amount of available oxygen to breathe.


Gas Pipes Also Come in a Variety of Different Types

There are copper interior plumbing pipes, high-density polyethylene services and distribution mains, and steel services, mains, and transmission lines. There are even PVC gas lines, though these are no longer considered safe to install. These different types of pipe require different methods of locating.

Interior Gas Line Locating

Interior gas lines are usually metallic, so there are a couple of obvious ways to locate them. One method is to use standard radio detection (RD) locating by directly connecting to the pipe at a point where it is not underground, and following the signal to the intended scope of the work. This method can have hazards, as has been discussed in our blog post How Does A Utility Locator Work?, namely that sending a signal down a bare copper pipe is a good way to bleed signal into any other surrounding utilities or even into the reinforcement in any nearby concrete.
Another way to locate these lines is to use ground-penetrating radar (GPR). Ground-penetrating radar can detect any metal object in or under a concrete slab, and usually subsurface gas lines inside a residence or commercial building can be found this way. The hazard of locating using GPR is that the radar does not differentiate between two different metallic objects, so it can be difficult to know what you are locating: a half-inch water line will look exactly like a half-inch gas line, if they are both metallic. However the upside is that usually you would not want to break a water line OR a gas line, or any other utility, and the GPR will usually give you a good picture of whatever is underground.

gas line marking

Locating Steel Gas Lines

Though polyethylene pipe makes up most new construction, steel is still the most ubiquitous type of natural gas line pipe. Luckily, steel lines are usually pretty easy to locate. RD locating is usually the best way to locate these types of lines, by direct connecting to the steel line at a riser, and tracing the line to the intended scope of the locate.
Some larger steel transmission lines will also have a tracer wire buried along with the pipe, since the larger diameter of the pipe contributes to more signal loss. These tracer wires can be connected to with gator clips directly at a test station or marker post, then located using radio detection.
However, sometimes steel natural gas pipes are highly corroded or for some other reason will not transmit an adequate signal, and in these cases it is best to use ground-penetrating radar. Typically, locating steel natural gas lines is pretty easy to do, as compared to some other types of gas lines.

Locating Polyethylene Natural Gas Lines

High-density polyethylene (HDPE) lines usually have a guide wire buried along with them, because they do not transmit a signal using radio detection locating. Direct connecting to this tracer wire will usually yield a quality locate of the line, and tracer wire has the added benefit of much less signal loss over a long distance due to the small diameter of the wire and fact that it is less affected by tees in the pipe. Tracer wires often also have a protective plastic coating, which insulates them and reduces signal loss.
But tracer wire is far from perfect. Sometimes people will remove tracer wires without knowing what they are used for, and sometimes they are broken, damaged, or installed incorrectly. All of these factors can make a tracer wire unable to transmit a signal to the intended area to locate, and in these cases it is best to attempt to use GPR to locate the line. Depending on soil type and the diameter of the pipe, this can be done safely and accurately.

If Natural Gas Lines Cannot be Located or Found by Potholing

gas line marking

It is important to remember that if pipes are unable to be located, there is no safe way to excavate without potholing to find the line underground. In these cases, the customer may want to consider potentially hand-digging or using vacuum excavation for that portion of the project area. Remember to call 811 before any excavation, so that public utilities have a chance to mark their lines. Even if many lines are located by both public and private locators, which we covered in our recent blog, “How Does A Utility Locator Work?” , you should be potholing those utilities to verify their depth and exact location. If the lines are owned by your local utility company, ALWAYS notify them if their lines are unmarked or unable to be found BEFORE you begin excavating, and follow all of their directions.

If you have lines that you need located, whether private or public, Encompass Inspections has years of experience locating every type, size, and material of natural gas line, and can be counted on to give you an accurate picture of where the lines are. On the off chance that the line cannot be located, we can instruct you as to the necessary next steps to excavate around these hazardous lines. Visit our website or contact us for all your subsurface locating needs.