Increasing use of underground piping and cabling is making it very important for construction crews to be able to locate and follow the route of underground pipes to prevent digging into a pipe by mistake. Live underground electric cables and piping containing oil or gas products could cause serious injury to people and damage to property in the event of an accident.
Underground piping locators help construction crews to know where the underground pipes are laid and how to avoid cross boring into them. (Learn more in " The Science of Getting it Right: Locating Underground Utilities.")
Laying Pipes Underground
Laying pipes underground is common practice and has gained popularity for a number of reasons.
Architects, engineers and landscape companies have become focussed on the aesthetic appeal of a new construction. It has become important to hide unsightly industrial equipment from the public eye.
Safety concerns are another reason for laying pipes underground. It is safer to have hazardous materials buried underground rather than on the surface where a pipe bursting can create an immediate explosion risk. The routing from source to destination is also more direct, making the pipelines shorter and therefore safer.
Types of Locators and How They Work
The simplest, but least reliable method for locating an underground pipe or cable is called passive location. A receiver is used to search for naturally emitted signals from the underground source. Live electrical cables emit a magnetic field associated with the current flowing through them, and some low-frequency radio waves present in the environment enter the ground and travel along buried pipes. A receiver can be used to pick up these signals and identify the path of underground pipes.
However, most underground pipe locators make use of transmitters and receivers. Using this technology, a signal is generated in the pipe by the transmitter, which is then measured with a receiver in order to locate the pipe. (Also see " A Look at Pipe Penetrating Radar.")
Direct connection involves connecting the transmitter to a pipe using cables. An earth rod is hammered into the ground and one cable is attached to the earth. The other cable from the transmitter is attached directly to the pipe, or a valve connected to the pipe. The transmitter generates a radio signal at a specific frequency which travels along the length of the pipe. The receiver is tuned to this frequency and measurements are taken at ground level to determine the location of the pipe.
Direct induction makes use of a magnetic field to energize the pipe at a specific frequency rather than using electrical cables. As current passes through a coil inside the transmitter, a magnetic field is generated. A transmitter clamp is placed around the pipe to transmit the signal into the pipe. As in the direct connection method, a receiver is tuned to the transmitter frequency and measurements are taken at ground level to determine the position of the pipe.
For non-metallic pipes, it is possible to push a duct rod containing copper wire into the pipe. Direct connection or induction can then be used to energize the copper wire, and the receiver can pick up the signal and thus the position of the pipe.
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How to Use Underground Pipe Locators
In order to trace the route of an underground pipe, the area is divided into grids so that it can be systematically checked and marked.
A transmitter is connected to the pipe by direct connection or induction described above. The receiver is then set to match the frequency of the transmitter.
Once the equipment is set up, the operator walks along the grid lines with the receiver, stopping when a signal from the underground pipe is received. The operator then moves the receiver around the location to find the maximum signal strength (which occurs directly above the underground pipe). (Learn about " The View Underground: Ground Penetrating Radar.")
A marker is inserted into the ground at that point to show the location of the pipe, after which the operator continues with the grid sweep, stopping to mark each new spot where the receiver picks up a signal. The complete set of markers on the surface after the grid sweep shows the route of the pipe underground.
Pitfalls of Using Underground Pipe Locators
Although the principles of underground pipe locators are easy to grasp, there are practical limitations of the technology in the field, which inexperienced operators may miss.
Underground pipe locators use the conductivity of the pipes to transmit signals, therefore they do not work on non-metallic materials without the use of a duct rod containing copper wire.
It is also important to note that If there are many pipes under the surface, transmitted signals may leak into the other pipes and thus confuse the results of the receiver. If these pipes are laid alongside one another, it may not be possible to separate them from each other. This is compounded by the background noise of some industrial sites. A high level of many frequency signals on the site may interfere with the functioning of the underground pipe locator.
As more pipelines are laid underground, the chances of accidentally digging into a buried pipeline grow higher. It is important for personnel involved in any construction activity to identify the location of existing pipes before breaking ground to prevent injuries and damage to property.
Technology to identify underground piping has developed over the years. Direct connection and direct induction are the primary technologies in use. Equipment is portable and easy to use, but it is important to understand the limitations and to work with experienced contractors so that costly errors are not made.