By the PICA Corp Engineering Team  |  Updated September 2026  |  Est. reading time: 9 min

Most utilities do not have an inspection problem. They have an allocation problem. The budget covers three miles of detailed condition assessment and the network is four hundred miles long, so the question is never whether to inspect. It is which three miles.

Pre-screen NDT answers that question cheaply. It is a fast, low-resolution survey run on a live pipeline that maps where something is wrong, so the expensive high-resolution work goes to the right place. What follows is how it works, what it finds, what it cannot tell you, and when it is worth running at all.

What is pre-screen NDT?

Pre-screen NDT is the first tier of a staged pipeline condition assessment program. The tool goes in while the pipe stays in service, collects data over a long run, and comes out. Analysis produces a positional map of anomalies along the alignment. That map is the deliverable.

What it is not is a structural assessment. Pre-screening measures symptoms, not remaining strength. A leaking section is definitely a problem. A section that is not leaking may still be three years from a rupture. The tier narrows the search, and it is honest about that limit.

PICA delivers pre-screening with the Navigator multi-sensor acoustic sphere, developed with parent company Russell NDE Systems and documented in a technical paper on the multi-sensor autonomous inspection device, or MSAID, presented at the ASCE conference.

The three problems pre-screening solves

You cannot take the pipe out of service

This is the constraint that kills most inspection programs before they start. A transmission main feeding a hospital district or a single-feed community cannot be dewatered for a survey, and the bypass pumping that would allow it often costs more than the inspection. Pre-screening sidesteps this: the sphere goes in through a 4-inch or greater air release valve port, check valvet or tap and travels with the water. The main never leaves service.

The assessment budget covers a fraction of the network

Detailed electromagnetic inspection of large-diameter concrete pressure pipe runs $100,000 to $200,000 or more per mile once you account for dewatering, access, and mobilization. At that rate, a $600,000 capital line item buys three to six miles. Spending it on segments chosen by age alone is a guess. Spending it on segments a screening run already flagged is a decision. Same logic as inspecting before you replace, applied one tier earlier.

Inside the Navigator: how a multi-sensor sphere collects data

The Navigator is a free-swimming sphere between 3.25 inches across. It carries no tether, no camera, and no power cable. It goes in, floats through, and comes out downstream carrying a memory card.

Four sensors, one pass

A high-sensitivity acoustic sensor listens for the pressure signature a leak makes as water escapes a pressurized wall. An accelerometer records motion and supports positional tracking. A pressure sensor logs the hydraulic gradient. A magnetometer picks up magnetic features in non-metallic pipelines and baseline magnetic flux in metallic ones.

Running all four simultaneously matters because the sensors cross-check each other. An acoustic event that lines up with a pressure discontinuity and a magnetic anomaly is a stronger finding than any signal alone.

Buoyancy, launch, and retrieval

Buoyancy is adjustable on site. The sphere can float at the surface, run neutral, or travel along the invert, which is what lets one tool work in wastewater, raw water, and fire water lines of different densities. Insertion needs a 4-inch or greater air release valve port, check valve or tap. Travel velocity ranges from 0.3 to 12 feet per second, with 1 to 4 feet per second producing the cleanest data, across pipelines from 6 to 78 inches at pressures to 300 psi.

Data is stored on board and analyzed by PICA within 72 hours of deployment. There is no live feed and no real-time decision to make in the field.

What pre-screening finds, and what it cannot tell you

A run returns leak locations, gas and air pocket locations, an elevation profile and pressure gradient, deposits restricting flow, and magnetic features in non-metallic pipe or baseline magnetic flux in metallic pipe.

Gas and air pockets deserve more attention than they get. Trapped air throttles capacity, creates the conditions for surge and column separation, and in wastewater force mains concentrates hydrogen sulphide against the crown where it drives corrosion. Utilities frequently discover through screening that a capacity complaint they had blamed on a pump was an air pocket the whole time.

Now the limits. Pre-screening cannot measure remaining wall thickness, cannot count broken prestressing wires or bars, and cannot characterize loss of pre-load in concrete pressure pipe. A main can pass an acoustic screen cleanly while carrying 40 percent wall loss, because corroded pipe that has not yet perforated is silent.

One more limit worth stating plainly: the Navigator is rarely the right tool on concrete pressure pipe at 36 inches and above. Large-diameter pipes do not generally develop small, detectable leaks. When they fail, they burst. Acoustic screening earns its keep on smaller diameters where leak signals are distinguishable, and on hydraulic questions across any diameter.

Case study: locating a leak in Jasper, Alberta

The municipality of Jasper suspected a leak on a water line but could not confirm where. Taking the main out of service was not practical, and excavating along the alignment would have been expensive and slow.

PICA deployed the Navigator through augmented fire hydrants for both launch and retrieval, so no excavation was needed and the main stayed in service throughout. Data analysis placed the leak within 10 metres, and the location was identified within 24 hours of the field operation finishing.

What made this worth doing was not the technology. The crew excavated once, at a point they were confident about, instead of digging exploratory holes and restoring pavement at each one. The water loss stopped weeks earlier than it otherwise would have.

When to pre-screen, and when to skip it

Pre-screening is not automatically the right first move. It earns its place under specific conditions.

Run a pre-screen when you have a long run of pipe and no basis for choosing where to inspect closely; when you suspect a leak but cannot locate it; when unaccounted-for water is climbing and you need to know whether losses are distribution-side or transmission-side; when hydraulic performance has degraded and you want to rule air pockets and deposits in or out; or when the main cannot come out of service.

Skip it and go straight to electromagnetic inspection when you already know which segment is the problem and need to quantify how much pipe is left; when the risk you are managing on large-diameter concrete pressure pipe is structural rather than hydraulic; or when the pipe is short enough that full assessment is affordable across its whole length. Timing matters too, and when you run an inspection shapes the quality of the data you get back.

The test is simple. If you cannot yet say which mile to inspect, screen first. If you can, do not spend the money twice.

Why acoustic screening alone isn’t enough

A pre-screening tool answers one class of question: where is something happening right now. Every other question about a pipeline requires a different physics.

Wall loss requires electromagnetic inspection. Remote Field Testing measures actual wall thickness continuously along a run, through liners, scale, and coatings up to about 25-30 mm thick, and detects wall loss on the internal and external surfaces simultaneously without differentiating between them. For concrete pressure pipe it also detects broken prestressing wires, cylinder wall loss, and loss of pre-load on pipe segments. In-service tools cover 2 to 36 inches; dewatered EMIT and RAFT tools cover 36 to 96 inches.

Broken wires in large-diameter concrete pressure pipe that cannot be reached with RFT require Near Field Testing, which reliably resolves five or more adjacent broken wires or bars from 36 to 136 inches out of service. Visible condition, liner damage, and geometric deformation require CCTV, laser and lidar. Localized confirmation at an excavated site uses the Bracelet Probe and handheld ultrasonic probes.

Across the full toolkit PICA inspects pipelines from 2 to 136 inches. No single technology covers that range or that defect list, which is why a screening result is an instruction about where to point the next tool, not a verdict on the pipeline. The utilities that get the most from pre-screening budgeted for the follow-up before the sphere went in the pipe.

What pre-screening costs relative to full assessment

Screening is inexpensive relative to detailed assessment for structural reasons rather than commercial ones. It requires no isolation, no bypass pumping, no dewatering, and no excavation, so the cost drivers that dominate a full survey are largely absent.

For scale on the follow-up work: a full out-of-service Advanced NDT program on large-diameter concrete pressure pipe runs $100,000 to $200,000 or more per mile, and inspection of small and medium metallic distribution mains from 4 to 24 inches ranges from several thousand to $40,000 per mile. Diameter, run length, access, and mobilization move those numbers substantially, so treat them as scale, not quotes.

The comparison that matters to a council or finance department is not screening against assessment. It is either of them against an unplanned failure on the same segment, which typically costs 10 to 50 times more than the inspection that would have caught it. The American Water Works Association and the ASCE Infrastructure Report Card both publish data supporting that argument.

Frequently asked questions

What is pre-screen NDT in pipeline inspection?

Pre-screen NDT is a low-resolution, non-invasive survey run on a live pipeline to find out which sections deserve a detailed inspection. It does not measure pipe wall thickness or count broken wires. It locates leaks, gas and air pockets, flow-restricting deposits, and elevation and pressure anomalies, then hands these details to the asset manager so the expensive follow-up work can be aimed at real problems instead of guesses. PICA delivers pre-screening with the Navigator multi-sensor acoustic sphere, which travels with the flow while the main stays pressurized.

Can a pipeline be pre-screened without taking it out of service?

Yes. The Navigator sphere is inserted through a 4-inch or greater air release valve port, check valve or tap and travels with the flow at 0.3 to 12 feet per second, with 1 to 4 feet per second being the optimal range. The main stays pressurized up to 300 psi and stays in service. No dewatering, no bypass pumping, and no excavation are required, which is the reason pre-screening can cover long runs that would never justify the cost and disruption of a dewatered survey.

What does a pre-screening run detect?

A Navigator run returns leak locations, gas and air pocket locations, and pressure anomaly when compared to elevation profile for hydraulic modelling, deposits restricting flow, magnetic features in non-metallic pipelines, and baseline magnetic flux in metallic pipelines. Four sensors collect this simultaneously: a high-sensitivity acoustic sensor, an accelerometer, a pressure sensor, and a magnetometer.

How accurate is acoustic leak location on a buried main?

Accuracy depends on pipe material, diameter, flow velocity, and background noise, so no single number applies to every job. On a documented municipal project in Jasper, Alberta, analysis of the Navigator data placed a suspected leak within 10 metres, which was close enough for the crew to excavate once and repair rather than dig exploratory holes along the alignment. Smaller-diameter pipelines generally give cleaner acoustic signatures than very large transmission mains. After additional pipeline details were provided, the leak location was predicted within 1 meter of the actual leak.

How is a pipeline inspected after pre-screening flags a problem?

The follow-up method depends on pipe material and diameter. Metallic mains from 2 to 36 inches can be inspected in service with the SeeSnake and Chimera Remote Field Testing tools; 6-inch and 8-inch cast and ductile iron mains can be reached through a fire hydrant with the HydraSnake. Large concrete pressure pipe is dewatered and surveyed with Near Field Testing from 36 to 136 inches, or with EMIT and RAFT Remote Field Testing tools from 36 to 96 inches. CCTV, laser and lidar document visible condition, and Bracelet Probe spot checks confirm findings at excavated locations.

How much does pre-screening cost compared with a full condition assessment?

Pre-screening is a fraction of the cost of a dewatered survey because it needs no isolation, no bypass pumping, and no excavation. For scale, a full out-of-service Advanced NDT program on large-diameter concrete pressure pipe runs $100,000 to $200,000 or more per mile, and inspection of small and medium metallic distribution mains from 4 to 24 inches ranges from several thousand to $40,000 per mile. Pricing depends on diameter, run length, access, and mobilization distance, so ask for a project-specific figure rather than working from a rule of thumb.

Which pipe materials and diameters can be pre-screened?

The Navigator sphere works in pipelines from 6 to 78 inches in diameter and is compatible with cast iron, ductile iron, steel, asbestos cement, plastic, prestressed concrete cylinder pipe, bar-wrapped pipe, and reinforced concrete cylinder pipe. The sphere itself measures 3.25 inches and operates between 32 and 140 degrees Fahrenheit. It suits wastewater, raw water, fire water, and any water application where skin-contact-certified materials are acceptable. PICA’s service applications reference lists the full material and diameter coverage across every tier.

Can pre-screening replace a full pipeline condition assessment?

No. Pre-screening cannot measure remaining wall thickness, cannot count broken prestressing wires or bars, and cannot characterize loss of pre-load in concrete pressure pipe. A pipeline can return a clean acoustic screen while carrying 40 percent wall loss, because corroded pipe that has not yet perforated makes no noise. Pre-screening tells you where to look closely. Electromagnetic inspection tells you how much pipe is left.