Version 1
54
considered leaking. If point D is higher than point C, this is an indication of sound being trans-
mitted from another point downstream.
Confirming valve leakage in noisy pipe systems
Occasionally in high pressure systems, stray signals occur from valves that are close by or from pipes
(or conduits) feeding into a common pipe that is near the downstream side of a valve.
This flow may produce false leak signals.
In order to determine if the loud signal on the down-stream side is coming from a valve leak or from
some other source:
• Move close to the suspected source (i.e., the conduit or the other valve).
• Touch at the upstream side of the suspected source.
• Reduce sensitivity until the meter displays a midline ("50 %") reading.
• Touch at short intervals ( such as every 15-30cm and note the meter changes.
• If the sound level decreases as you move towards the test valve, it indicates that the valve is
not leaking.
• If the sound level increases as you approach the test valve, it is an indication of a leak in the
valve.
MISCELLANEOUS PROBLEM AREAS
UNDERGROUND LEAKS
Underground leak detection depends upon the amount of ultrasound generated by the particular leak.
Some slow leaks will emit very little ultrasound. Compounding the problem is the fact that earth will tend
to insulate ultrasound. In addition, loose soil will absorb more ultrasound than firm soil. If the leak is close
to the surface and is gross in nature, it will be quickly detected. The more subtle leaks can also be de-
tected but with some additional effort. In some instances it will be necessary to build up pressure in the
line to generate greater flow and more ultrasound. In other cases it will be necessary to drain the pipe
area in question, isolate the area by valving it off and inject a gas (air or nitrogen) to generate ultrasound
through the leak site. This latter method has proven very successful. It is also possible to inject a test
gas into the test area of the pipe without draining it. As the pressurized gas moves through the liquid into
the leak site, it produces a crackling sound, which may be
detected.
PROCEDURE:
1. Use Contact (Stethoscope) Module.
2. Start Frequency Selection at 20-25kHz.
3. Touch surfaces over ground - DO NOT JAM probe to ground. Jamming can cause probe
damage.
In some instances it will be necessary to get close to the "source" of the leak. In this situation, use a thin,
turdy metal rod and drive it down close to, but not touching, the pipe. Touch the contact probe to the
metal rod and listen for the leak sound. This should be repeated approximately every 1-3 feet until the
leak sound is heard. To locate the leak area, gradually position the rod until the leak sound is heard at its
loudest point. An alternative to this is to use a flat metal disc or coin and drop it on the test area. Touch
the disc and listen at 20 kHz. This is useful when testing concrete or asphalt to eliminate scratching
sounds caused by slight movements of the stethoscope module on these surfaces.
LEAKAGE BEHIND WALLS
1. Look for water or steam markings such as discoloration, spots in wall or ceiling, etc.
2. If steam, feel for warm spots in wall or ceiling or use a non-contact infrared thermometer.
3. Set Frequency to 20 kHz and use Contact (Stethoscope) Module.
4. Listen for leak sounds. The louder the signal the closer you are to the leak site.
PARTIAL BLOCKAGE:
When partial blockage exists, a condition similar to that of a bypassing valve is produced. The partial
blockage will generate ultrasonic signals (often produced by turbulence just down stream). If a partial
blockage is suspected, a section of piping should be inspected at various intervals. The ultrasound
generated within the piping will be greatest at the site of the partial blockage.
To Next Page
Loading...
Need help?
General
