What lies beneath

A construction company who did not want to be named was about to hand over a multi-million pound out-of-town retail park. The 18-month project had gone without a hitch, and the team was weeks ahead of schedule when disaster struck.

The foreman reported that the 1000m cold-water distribution pipe that serviced flower beds and walking areas was losing pressure despite the constant running of the pumping station. This pipe was leaking in one or more places.

Finding the leak presented a big problem. The pipe was below 5ft of recently laid concrete so the time and cost implications of excavating along its length would be enormous. Even on the assumption that the leaks were located at one or more of the six joints, it would be necessary to dig six holes at a cost estimated to be £12,000, just to check. And it wasn’t certain that the leaks were at the joints. The pipe could have been damaged while being transported to site or during installation and could be leaking from anywhere.

The leaking water was seeping away into the earth so it wasn’t causing any damage. However, ignoring the problem wasn’t an option as the pipe was probably losing thousands of litres of water a day, which would mean horrendous water bills for the support services division of the company, which won the contract to manage the retail park once it opened.

It may have looked like tearing up the new floor was the only option, but there are better ones – leak detection techniques that use gas, sound or thermal imaging to pinpoint the leaks before ripping up finishes.

Water with gas

A gas detection technique was used in this case because the depth of the pipe would have made acoustic detection difficult. Technicians drained the system and pumped a specialist gas through the pipe. As the gas escaped through the leaks, it permeated through the concrete to the surface, where the technicians waited with detectors.

They began at 6pm and by 11pm, the technicians had completed their survey, having pinpointed four separate leaks.

The next day the contractor excavated four 2m-wide holes at the marked points, discovering four leaks, caused by incorrect fitting. The subcontractor was recalled to repair the pipe.

This technique relied on tracer gas, which is a heavy gas mixed with a light gas, with the light gas rising for detection. If the detectable gas is toxic or explosive then only a small concentration (5%) is used for safety reasons.

Acoustic detection is the most commonly used method for precise leak location in domestic and commercial properties

Concrete is a relatively porous material. The gas first starts to reach the surface approximately one hour from a depth of 5ft. It takes longer for concentration levels to build up enough to pinpoint the leak.

Gas detection is not always the right method. The less porous the material, the longer it takes gas to permeate. For example, with tarmac it can take several hours before the gas levels are high enough to pinpoint the leak at 5ft. Frozen ground can also be a problem, particularly in clay soil. In this instance, the technicians would drill small bore holes to help accelerate the permeation. Some floor coverings, such as marble, make the tracer gas method ineffective.

In these cases it is possible to detect a leak using the acoustic method. Acoustic detection is the most commonly used method for precise leak location in domestic and commercial properties, particularly for the detection of leaks in mains and feed water supplies. The principle behind this method is to detect the noise produced by a leak, known as “body-sound”.

The first stage of acoustic leak detection is pre-location, where body-sound levels are pre-determined above ground using a contact microphone. As the technician scans an area with the microphone, minor variations in sound levels are picked up. These variations are displayed on a dial, with the highest reading indicating the approximate location of the leak. Pre-location narrows down the search area before pipe mapping and sound profiling are used to pinpoint the leak within a few centimetres.

To further reduce the search area, the pipes are mapped using electro-acoustic location equipment, which detects the electromagnetic field generated by metallic pipes and cables. In the case of plastic pipes, a metallic cable is inserted into the pipe via the stop-tap to enable detection. Using this method, the technician is able to accurately map pipe location and depth in a given area.

Nowhere to hide

Sound profiling is the final stage of acoustic detection, which is used to precisely locate the leak to within a few centimetres. A ground microphone amplifies the body-sound of the leak to an audible level, which the technician follows to the source of the leak. Sound profiling can detect leakage at depths of up to 10ft. The accuracy of this method is particularly beneficial when the suspected leak is located beneath an expensive floor covering.

One of the most effective methods for precisely locating leaks in hot water pipes and under-floor heating systems is thermal image detection. Using an infrared camera to produce a heat diagram of the area being examined, a leak detection specialist can quickly and accurately identify the source of the leak. Sections of pipe that have missing or damaged insulation can give off similar heat readings to hot water leaks but a skilled technician is able to distinguish between them.