Listening for a leak - really?

Cast-iron Transmission Main Inspection Along Eastern Parkway

May 15, 2018

Have you passed a Louisville Water crew and wondered what lead them to their work on a pipe? Believe it or not, sound allows our crews to detect leaks within our lines, allowing us to make small repairs before a main break occurs. Journalist Gil Longwell featured Louisville Water Company's use of a listenting tool called a Sahara in a Municipal Sewer and Water article, which is below: 

Has your municipality considered acoustic pipe inspection? The potential time and cost savings make it a worthwhile investment. Here, learn the basics of this up-and-coming technology.

Editor’s Note: This is the first in a three-part series on acoustic pipe inspection. Watch for more on this technology, including site preparation, data analysis and more.

Simply put, acoustic pipe inspection, or AI, uses sound to locate and characterize pipe based on the sound of flowing water. And, it’s quickly becoming a buzzword for infrastructure owners and managers thanks to its potential cost savings.

AI systems can help prioritize maintenance needs, which lets utilities avoid needless cleaning of relatively clean pipes or repairs to pipes with only minor issues.

The process

All AI processes involve sound and a pipe, which is often where similarities end. AI can be active or passive, internal or external, use automated and remote data collection or the inspection can be continuously monitored in real time. It can locate leaks and quantify the scale of a leak at first reading. Some AI processes require full-bore flowing liquid while others require less-than-full bore gravity flow. Some AI processes include video capture.

According to the EPA’s white paper, “Condition Assessment of Wastewater Collection Systems,” passive devices can be permanently mounted to pipes or appurtenances, or they can be inserted into a pipe and allowed to move with the flow. Other AI devices listen from one or multiple places on the ground surface with no direct connection to the pipe.

Although AI is not new, the techniques are evolving.

Being a good listener

“Is that a leak I hear?” might be the most basic question posed by a technician listening to a real-time inspection or during a recorded playback.

“Not all leaks are water-losing leaks,” says Keith Coombs, a P.E. with Louisville Water Company Infrastructure Planning. “Some may be water unexpectedly moving through a valve that was not fully closed.” (Watch a video of Louisville Water Company using the Sahara tool to inspect pipes.)

Although still unanticipated water movement, this situation is handled quite differently from a leak inside the pipe to the exterior. A leaking valve often doesn’t command the same urgency for remediation as a catastrophic pipe rupture.

It’s unlikely an AI team would be deployed to the site of a rupture, but on the other hand, that rupture might be avoided as a result of AI screening and prioritized maintenance scheduling.

SmartBall and Sahara
SmartBall and Sahara devices — manufactured by Pure Technologies and described in the EPA white paper — travel with flow after being inserted into a pipe.

SmartBall is an untethered device that will only move forward if the surrounding liquid is moving. The ideal flow velocity is about 1 foot per second, and it can travel in excess of 10,000 linear feet. The only limitation is battery life. Inserted through a 6-inch, double-sealed connector, it is suitable for pipes 6 inches or greater in diameter, regardless of material.

Sahara, a tethered device, also requires flow, yet it is always under control and can be retrieved at any time. The Sahara’s travel rate is influenced by the current’s velocity and the limits of the tether cable’s control mechanism and the cable’s bottom and sidewall friction. Inserted through a 4-inch access port and suitable for pipes 6 inches in diameter or larger, Sahara requires a faster flow rate of about 1.5 to 2 fps. Traverse distance is limited by a combination of cable length, pipeline geometry and velocity.

The SmartBall cannot be recalled to a specific spot for a second look, yet the Sahara can do two complete listening scans: one while outbound and one during inbound recovery. Because inserting and retrieving these devices might require significant preliminary work, these technologies are most often deployed in an ongoing series of inspection campaigns that coordinate mobilization and operating strategies. Sahara data can be evaluated in real time, while SmartBall data must be downloaded after retrieval and before evaluation.

Partially filled pipes, which typically carry sewage or stormwater, have different sound conveying properties so a different AI technique is required. Instead of listening to water movement, the SL-RAT from Infosense evaluates sound traveling between manholes in the air-filled portion of the pipe.

The SL-RAT has two components: One transmits a blast of sound with known frequency and volume characteristics, and the other listens for that blast. Positioned above grade at adjoining open manholes, the transmitter and receiver are linked wirelessly and acoustically. Each component can easily be carried to off-road manhole locations. Because CCTV equipment requires a specialized truck to haul the crawler and its support infrastructure — as well as to introduce and retrieve the crawler — the SL-RAT deployment is simpler and faster.

By measuring the time interval from when the transmitter shouts and the receiver hears, and by measuring the volume and clarity at the receiver, the receiver calculates and grades the pipe on a scale of zero to 10. The inspection also factors in pipe diameter, pipe material and segment length.

The segment’s rating is generated in a few minutes while the crew is still in the field. It is determined, in part, by comparing the received sounds characteristics to a sound library that includes hundreds of pipe sounds ranging from completely unobstructed to fully clogged.

Apples and oranges
These technologies are not equal, and they do play different roles. Both are applicable for pipes that cannot be taken out of service, although the SL-RAT can be successfully used on empty or dry pipes but not on pressurized pipes.

The SL-RAT helps prioritize segments for more detailed inspection, while SmartBall and Sahara conduct more comprehensive inspections. Because of this, it’s not appropriate to compare their deployment methods, costs or the reports they produce.

According to the EPA study, the SL-RAT delivered useful actionable information for about 14 cents per foot. A comparison number for a CCTV inspection of the same lines ranged from $1.68 for in-street locations to $2.03 per foot for off-road locations.

Pure Technologies reports that variable client needs, the broad range of system characteristics and get-ready costs preclude generic linear-foot cost projections.

SmartBall and Sahara require significant planning and might require system modifications while the SL-RAT requires no more site preparation than opening a pair of adjoining manholes.

Whether deployed as a complete inspection tool or the first step in a systematic process to prioritize cleaning, maintenance and repair campaigns, all of these products are tools worthy of investigation by municipalities and utilities.

For more information and videos showing these products in operation, visit and