Page 298 - Water Loss Control

Contr olling Real Losses in the Field—Pr oactive Leak Detection 267

Leaking Fire Hydrants, Air Valves, and Scour Valves
Leaks also occur at distribution system appurtenances such as fire hydrants, air valves,
and scour valves. These are relatively straightforward to detect as they are usually vis-
ible or able to be detected by direct sounding.

16.3.2 Leak Detection via Leak Noise Sounding
Leak sound frequencies vary depending on the type of leak, the type of pipe, backfill
material and density, and whether a water-filled cavity has formed around the leak. In
general, there are three types of situations that generate leak sound frequencies:

• Friction sound is the sound created by water forcing its way through the pipe
wall and making vibrations along the pipe. This tends to be a high-frequency
leak sound, ranging between 300 to 3000 Hz. In general, high frequency leak
sounds are easy to recognize but do not travel very far along the pipe. Note that
pipe vibration against surrounding material may cause noise overlaying the
leak sound, especially metallic service connection pipelines (see Fig. 16.6).
• Fountain sound is the sound of water circulating around the leak site and tends
to be lower frequency, in the range of 10 to 1500 Hz (see Fig. 16.7).
• Impact sound is the sound of a leak impacting
on the walls of the hole around the leak and
the sound of the impact of rocks, which Sound generated by leaks
often are thrown around the leak. This is continuous, unlike transient
sound also occurs in the range of 10 to 1500 Hz ambient noise.
(see Fig. 16.8).

Friction sound

Leak

FIGURE 16.6 Friction sound.

Fountain sound

Leak

FIGURE 16.7 Fountain sound.