AirTalk Issue 6September, 2001
by Jack Price, Verizon, AOM Midtown Manhattan
Bob Menendez, Verizon, AOM Northern Manhattan
When it comes to a subject as old as air pressure, sometimes you think there can't be anything new. In Manhattan, we've got some of the most congested manholes and difficult-to-access cables imaginable. So, when System Studies Field Engineer, Rich Rethorn, said he had a new leak locating tool that would make our jobs easier, he didn't have to fight hard for our attention.
We have been using helium detectors successfully for years, and they're great. But until recently, we didn't have anything similar to compare with them. What we've since learned about hydrogen gas and the new Hydrogen Leak Detectors from System Studies has not only dispelled some of our misconceptions about hydrogen, but also given us a new and better tool for leak locating.
First of all, the word "hydrogen" should not be equated automatically with "explosion." The hydrogen gas mixture used with this leak detector equipment is a standard ready-mixed, commercially-available, green label mixture of 5% hydrogen (H) and 95% Nitrogen (N). This mixture, which is totally non-flammable and non-explosive, is effective both in outdoor (manhole) environments and indoors, if need be. It is also one of the least expensive tracer gasses available.
While this tool is extremely easy to use, let us briefly describe the more technical attributes of the technique. Because of its high molecular velocity, low viscosity and low density, the hydrogen tracer gas is ideal for locating leaks in cables. It is also one of the lightest gases known, in contrast to the heavier, denser helium. For example, when helium escapes from a cable, it doesn't disperse very quickly. So, when your leak detection equipment begins to pick up a trace of helium, it may just be some of the residual gas that has settled away from the leak. The hydrogen tracer gas, on the other hand, quickly rises and disperses into the atmosphere. When you start to detect it, you could be within inches of a leak. Detection is enabled immediately on entry to the manhole when a leak is present.
A Tool for Every Application
With Rich's help we tested two hydrogen leak detectors: 1) a medium sized, over-the-shoulder unit (P/N 9800-3708) that can be used with a duct probe, duct fan, surface probe and other accessories, and 2) a more maneuverable handheld unit (P/N 9800-3707) that's essentially a light weight version of the bigger unit.
We quickly discovered that both detectors are excellent for locating leaks in manholes, but the larger 3708 unit has special accessories that make it ideal for finding section leaks in underground ducts or direct buried cables from street level. One of the things that appealed to us most was the fact that you don't have to drill or pound holes in the pavement in order to use the larger hydrogen detector successfully. Because hydrogen is one of the smallest molecules and the lightest of all gasses (about 15 times lighter than air), it has the ability to rise through surfaces even as dense as concrete. The detector can sense the escaping gas, thereby effectively reducing the search area to within a few feet or less of the leak.
A surface probe and wheel unit are available as attachments for effective use in connection with street level detection. The surface probe consists of a three foot wand with a vacuum pump, battery, and three-inch rubber boot, which seals off the sensor probe to detect the tracer gas. The wheel attachment for the surface probe has a rubber mat approximately 12 inches square that is helpful when surveying larger areas. A duct probe attachment and duct fan round out the accessories for the field unit, making it a versatile and effective all-around tool. The blower prevents gas from saturating the duct, by forcing it forward until the leak can be clearly identified.
Because of its relatively small size (approximately 12" high x 2.75" wide x 1.25" thick) and its convenient belt-mount clip, the handheld detector is best suited for leak locating in tight, confined spaces, such as manholes. We found it to be very practical for identifying leaks quickly in quadruple-racked cables, especially leaks in cables closest to the wall.
In many of our manholes, it is nearly impossible to access some of these cables to take readings. That's where the hydrogen detector is most useful. A short time after placing the tracer gas in a cable at an air pipe manifold (½ to 2 hours, depending upon the size and gauge of the cable), we can often identify the leak. And not just one of the many small cable leaks prevalent in the pressurized cable system, but THE leakthe one causing the most damage to the system. These large leaks are impossible to find using an ultrasonic leak detector, for example, which only identifies the inaudible high frequency sound produced by smaller leaks.
Another thing that we liked about the handheld hydrogen detector is that it has an audible "ticker" that works like a Geiger counter, and it reads all the time. When leak locating in hard-to- reach places, it is impossible sometimes to see a gauge face, meter or indicator scale. The hydrogen detector produces a distinctive ticking sound that increases in volume as you approach a leak.
In manholes where there is extensive interlacing, the handheld hydrogen detector also saves valuable time and effort compared to other leak locating tools. For example, rather than having to rely just on the flow direction indicator to chase air flow from cable to cable, you can use the hydrogen detector to quickly rule out cables that aren't leaking or, in some cases, to find the leaking cable right away.
Success in the Field
Despite the many capabilities of the larger detector for locating section leaks in ducts and buried cables, we were pleased most with the way both detectors worked in the manholes. That's where we typically find 95% of our leaksoften the result of the high traffic in the manholes. After identifying the highest flowing leak at a manifold location, for example, we would hook up the hydrogen tracer gas to the manifold port, turn off air feed to the cable, and force gas into the cable at about 15 pounds per square inch (psi). Depending upon the size of the cable, it generally takes a half hour to an hour for the gas to reach the leak and exit the cable. And when it does, the Geiger counter-type device starts ticking like mad. So, there's no doubt when you have found the leak.
Because the hydrogen detectors from System Studies give us the ability to determine easily and accurately which cables are leaking, and which ones aren't, we've now got another valuable tool to help us keep our high revenue-generating cables dry. Combined with the PressureMAP monitoring and dispatching system we've been using successfully over the years, our chances of finding those big leaks before they cause cable failures in our Manhattan environment are much better than ever before.
For more information about the Hydrogen Leak Detectors and accessories, contact System Studies at 800-247-8255 or 831-475-5777. You can also send us some email.
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