Tools & Techniques

Issue No. 42–January, 1993,

Taking Measurements
When you're looking for section leaks, all of your calculations and graphing techniques need to be extremely accurate. That's why it's important to actually wheel off your distance measurements. Don't rely on cable records for this information. You need to know the true distance (as close as possible) between measurement points. So, don't use wall-to-wall measurements. Measure from valve-to-valve.

Supplemental Air Sources
Stabilized readings are as important to the leak locating effort as are accurate distance measurements. For this reason, you should never use air pipes as supplemental air pressure delivery sources. The pressure in the pipes varies too much. Also, don't take pressure readings immediately when you use nitrogen tanks. Allow the system to stabilize for a day before you take new measurements.

Here's a tip! Never use a tank itself as the measurement point. Ideally, you want the first measurement point to be at least a manhole away from the tank. If this is impossible, try to get two valves on the splice case. Use one for the tank's air chuck and the other for the pressure reading. If you don't set it up this way, and there's a big leak close to the manhole, your calculations will be way off the mark. For example, with a big leak nearby, the actual pressure at the splice case could be 8 or 9 PSI, while the tank pressure reads 10 PSI.

What You Should Know About Using Helium and the B-Gas Indicator
Here are some important tips to consider when using helium and the B-Gas Indicator to locate section leaks in the underground:

• When releasing helium into the cable, check to make sure that it is actually entering the cable. The best way to do this is to route the helium through a 0-20 Standard Cubic foot per Hour (SCFH) portable flow rater. Crank up the regulator on the tank until the flow rater shows 20 SCFH, then give it another tweak more.

• Test the duct with the leaking cable approximately every five minutes. If you don't start testing at the onset, after about 20 minutes the helium will be dispersed throughout the cable and you won't have any immediate clues regarding the location of the leak.

• With the B-Gas Indicator, check all the ducts around the one with the leaking cable. If you detect the helium coming out of several ducts (especially the top and side ones), you can guess that the cables were hit by a back hoe. Go up on the street and look at the physical condition of the road. Does it look like it was recently cut into or repaved in sections? Perhaps a water main was put in, or something of that nature, where the use of digging equipment resulted in damage to the duct structure.

• If you detect helium, but only in one duct, the cable was probably damaged when it was pulled in. Look closely at the cable in the manhole for signs of sheath damage.

• If you know that there is a slight incline in the duct structure and you cannot sniff any helium, go to the next closest utility hole on the uphill side and try again. Since helium rises, you might be able to detect it at this location even though the leak may be much closer to where you put the helium in.

Tool #1: Air Flow Calculation

The first item in the tool bag is the Air Flow Calculation. It will help you determine if there is only one leak between air sources (i.e. air pipe manifolds in most systems). First measure the flow rate at the manifold. Then, using pressure readings, pneumatic resistance and distance measurements (see "Taking Measurements" at the start of this article), calculate the air flow at each manhole. The flow rate will be the same from section to section until you pass the leak. If the flows are not the same, you've got more than one leak.

But if you calculate the flows starting at both air sources (air pipe manifolds) and they do not change until you reach the section with the leak, you know you've got only one leak in the section. This eliminates a lot of your problems. When identifying a section leak, it's just as important to identify which sections don't have leaks.

Tool #2: Five Part Calculation

In the past, the most common method of looking for a section leak in a dual feed pneumatic section was to draw a graph. But there's a more accurate way of zeroing in on the leak. It's called Calculator Graphing. This five step procedure uses pressure readings and valve-to-valve distance measurements to indicate the location of the leak. In addition to the requirements of being in a dual fed section with no changes in pneumatic resistance, you must also have at least four accessible pressure measurement points on the cable, and the leak must be between the inside two measurement points (see figure below).

Just a warning! This calculation is no better than guesswork if you don't take precise pressure readings and distance measurements. That's what gives it the advantage over mechanical graphing.

Tool #3: Direction of Flow Indicator

Although the Direction of Flow Indicator is most often used to check for leaking plugs or leaks in interlaced cables, it's also ideal for indicating the direction of flow in a cable. If your calculations lead you to a manhole but you are unable to determine which way the air is flowing, you can place two valves on the cable and use the Direction of Flow Indicator to point the way. While we suggest this procedure with some hesitation (because of the possibility of creating leaks in the system), it sometimes is the only way to obtain the information you need.

Tool #4: Helium and the B-Gas Indicator

Helium and the B-Gas Indicator are excellent tools to use if you suspect a big leak. From the manhole closest to the suspected leak location, place helium in the cable and use the B-Gas Indicator to "sniff" the duct structure. The helium will eventually work its way through the cable, escape through the damaged sheath, and be detectable in one or more of the ducts.

It's a good idea to use the gas detector about every five minutes to check for any signs of the gas. This will give you an idea of how far out the leak is from the manhole. If the helium comes out of a duct right away, you'll know that the leak is somewhere near the manhole. However, if you're unable to detect anything right away, the leak may be a considerable distance away. Be patient. It can take a few hours for the helium to flow through a cable before you can detect it.

By the way, if you're looking for a leak in a buried section, you first have to identify the cable path (see Tool #5), then place a number of small holes (1/2" to 1" in diameter) in the ground above the cable near the estimated leak location. Place these holes approximately every three to four feet (or as far apart as the cable is deep) at a distance of about 50 feet on both sides of the estimated leak location.

Tool #5: Cable Locator

Before you begin pounding holes in the ground to sniff for helium, you need to use a cable locator to identify the path of the cable. This equipment will indicate—within approximately 12 to 18 inches—what line the cable takes. As you walk off the cable's path with the cable locator, make a mark approximately every 6 to 8 feet. It's a good idea to start at the utility hole closest to the suspected leak location and trace the path at least 50 to 60 feet beyond the leak.

Tool #6: Ultrasonic Duct Probe

The Ultrasonic Duct Probe is one of the most important tools in determining where to dig for a section leak. To improve your chances of finding the leak with this tool, you can place a nitrogen tank on the cable. Set the regulator on the tank to 15 PSI instead of 10 PSI so that there will be plenty of air coming out of the leak for the probe to detect.

Some technicians will put a nitrogen tank on the cable and set it to 10 PSI. But if they're close to the leak, by the time the nitrogen enters the cable, there may only be 3 PSI there. If your calculations indicate that the leak is relatively close by, crank up the delivery pressure to 15 PSI. You need high delivery pressure for the Ultrasonic to work properly.

Tool #7: Sheath Fault Locator

If a pressurized cable has a detectable sheath fault (a shield to earth fault), it is also leaking air. The Sheath Fault Locator can be used to identify the location of the fault and the leak. The equipment consists of three components: a Transmitter, a Receiver and an Earth Contact Frame.

A tone is placed on the cable using the Transmitter. Then the cable is traced to the fault with the receiver, using the Earth Contact Frame. When locating the fault, it is important to remove the ground bonds at both ends of the section. This will help ensure measurement accuracy.

For instructions on how to use the Sheath Fault Locator, refer to the operating instructions that are supplied with the equipment.