National Grid is an international electrical and gas company, who play a vital role in delivering gas and electricity to millions of people across the UK and parts of the USA. John Howlett, the site supervisor at Hurst substation, kindly put together a few details on how they use the PHD-4 helium leak detector to locate underground pipe damage.

"We have high-voltage cable circuits buried around London at 275,000 volts. The longest of these circuits is 14 miles long, and feeds power into the city from power stations situated in Kent. To allow these cables to carry more current, we run cable cooling pipes adjacent to these cables."

"These cooling pipes have water pumped from cooling station pump buildings along one mile sections, and the water is cooled in these stations using cooling radiators. With the cooling enabled, we can get an extra 25% loading on the cable."

"We have two types of water pipe: aluminium coated with PVC, and polypropylene. The two main causes of leaks are third-party damage, and damage caused by vibration. The pictures above and below are from two different locations where damage was found on both types of pipes. Both sets of damage were caused by a third-party using mole techniques when installing new water services."

"When a leaking pipe has been identified, the water is removed from this section of the pipe, and helium is injected from the pumping station. When helium is detected at the far end of this section, the pipe is filled along its route. The PHD-4 is then used to locate the leak from the road surface above by walking along the pipe section route."

"Due to its small molecular structure, the helium is able to rise to the surface via the easiest path, which is usually a drain, a break in the tarmac, or between the kerb and road surface. Once the helium is detected, the surrounding area is excavated and, with the circuit switched out, the leak is identified and repairs carried out."

"For the aluminium pipe, a copper sleeve is made to cover the damage, and is plumbed onto the aluminium at both ends. The whole repair is then wrapped with PVC tape and covered in a heat shrink PVC sleeve to insulate it from earth. On the polypropylene pipe, the damaged section is cut out, and a new piece is inserted and fusion welded in place at both ends. The pipe is then filled with water (circulated to expel any air) and returned to service."