ADS Environmental Services Eureka Digital QR 775020 A0 - Manual (Page 12)

ADS Eureka Digital System User Manual
8
Methods for Locating Leaks
Previous methods of leak location relied on an initial detection of a noise,
which was then traced along ground level to identify the location of the
maximum noise. This location was assumed to be directly above the
location of the leak. The equipment used for this method included
listening sticks and stethoscopes, but these were difficult to use if the noise
proved too faint or loud for accurate location. Background noise was
another problem; electronically-filtered amplifiers were introduced in an
attempt to overcome such problems.
A correlator operates by comparing the noise detected at two different
points in the pipeline. Noise travels from the leak in both directions along
the pipeline at a constant velocity (depending on several factors).
Therefore, if the leak occurs at an equidistant location between the two
sensors, these sensors will detect the noise at the same time. Conversely,
if the leak does not occur at an equidistant location, then the sensors will
detect the same noise at different times. The correlator measures this
difference (delay) in time.
The following diagram provides an illustration of this principle:
Assume the sensors are located on the valves A and B (convenient access
points for underground pipes). However, the leak occurs closer to valve
A.
By the time a particular noise from the leak has reached A, the same noise
heading towards B has travelled only as far as X. The distance from X to
B causes a delay (t) before the noise arrives at B. Therefore, the correlator
detects the delay (t) between the arrival of the noise at A and B. Assume
the velocity of sound is V and the distance between the sensors is D. As
the distance from X to B = V * t (velocity x time), then D = (2 * L) + (V *
t). This equation may be rearranged to provide L, the distance from the
sensor closer to the leak site:
2
) * ( t V D
L


The sound velocity can be calculated based on the pipe diameter and
material, and the distance between the sensors can be determined by
careful measurement. Therefore, the correlator can calculate and display
the location of the leak as a direct distance from the closer sensor. The
correlator also may be used to measure the actual velocity of sound in the