Liquid leak detectors sense when a liquid is leaking from a pipe, tank, or other receptacle. 

 

Basic Information

 

Leak Anatomy

 liquid leak detectors selection guide

Leaks are often conceptualized as being small, round defects which allow liquid to seep out of a pipe or tank. In reality, they almost always have irregular geometric features and may extend for some length beyond where the liquid is perceived as "leaking." For the latter reason, detecting a leak's start and end points is particularly important.

 

Leaks may be initiated by design or construction errors, such as a single improperly-sealed fitting. However, most leaks are caused by corrosion, which begins inside the pipe and causes the pipe walls to deteriorate, eventually leading to a fracture. While corrosion is a fairly natural process, it can be exacerbated by the pipe's construction materials as well as the liquid's pH, velocity, temperature, and compatibility with the pipe.

 

Smaller leaks which are more difficult to detect may be caused by incomplete fabrication. For example, the 50-year-old pipe below was discovered to be leaking by routine testing but exhibited no surface fractures. Common methods for liquid leak detection came up inconclusive, and only after ultrasonic and microscopic analysis was it determined that incomplete welding—which resulted in the infusion of nonmetallic materials—was causing a trace leak.

 

liquid leak detectors selection guide   liquid leak detectors selection guide

A leaking section of pipe (left); the incomplete welding fusion under 25X magnification.

Image credit: Corrosion Testing Laboratories, Inc.

 

Common leak locations within a system, as well as their causes, are listed below.

 

  • Pumps - improper seals
  • General-purpose valves - failure of packing or o-rings
  • Flanges and fittings - gasket failure or loose bolts
  • Sampling connections - leaks from sampling valve outlets
  • Compressors - seal failure
  • Pressure relief valves - faulty valve seats, damaged seals, ruptured gasketing
  • Open pipe connections - failure of caps, plugs, and seals

Why test for leaks?

 

Leak testing is essential in many industries. In water supply, chemical, and heating/cooling applications, leakage rates of 20-30% are not uncommon in some systems; this leakage represents a significant loss of resources and capital. Additionally, in the case of chemical, fuel, and refrigerant systems, leakage may pose a major risk to the environment, system equipment, and adjacent workers. For all of the reasons above, increasing regulation and focus on environmental conservation render the consequences of leaking systems much more severe than in the past.

 

The chart below shows total leakage loss data (by individual utility) in Northern New Jersey in the United States. Note that loss percentages range from around 2% to 33%, with most figures centered between 10 and 30%.

 

liquid leak detectors selection guide

Image credit: R.L Bebach/northjersey.com

 

Types 

 

Leak detection types vary based on the type of vessel to be tested. For example, tank testing can be classified into three broad areas:

 

  • Internal methods monitor the inside of the tank and typically measure volume loss over time.
  • Interstitial methods monitor the area between the tank and a containment barrier.
  • External methods use sensors to detect the presence of the leaked product in the environment outside the tank.

Pipeline leaks are typically detected using any of the methods described in the table below.

 

Type

Description

Advantages

Disadvantages

Gas Detection (tracer)

Involves inoculating system with a small amount of tracer gas; leaks are found by tracing gas outside the pipeline with probes.

Are not dependent on knowledge of pipeline conditions; can be used on underwater systems with correct tracers; can be applied to any system geometry or configuration.

May take up to 30 days after inoculation for accurate measurements; not applicable to most portable water systems.

Fiber Optic

Refractive fiber coatings react with certain contaminants, changing the intensity of transmitted light.

Fiber is versatile and can be used in both point sensors and distributed sensors.

Refractive coating is sometimes unstable.

Acoustic Emission

Acoustic monitoring can detect high-frequency emissions resulting from cracks and fractures; these signals are processed into leak data.

Fractures can be detected in advance of actual leakage; size of potential leak can be estimated based on detected signals.

Background noise from nearby process equipment may interfere with detection; false positives may result from incorrect sensor calibration.

Infrared

Used to detect fluids which have significantly different temperatures than nearby surroundings; thermal imaging is used to detect temperature differences around pipeline.

Very effective and efficient for use in large heating and cooling systems.

Limited to systems with thermal differences between fluids and surroundings.

Magnetic Flux

Uses a magnetic probe inserted into pipeline to detect corrosion, dents, and other anomalies.

Large areas can be covered within a short time.

Pipeline must be completely evacuated in order to launch sensor; pipeline must be large enough and geometrically-suited to probe.

Volumetric Testing

Involves measurement of liquid volume which must be added or removed from system to maintain constant pressure; volume changes indicate either leaks or thermal expansion/contraction of liquid.

Can detect small leak rates, often less than 0.1 gal/hr.

System must be taken out of service during measuring period; temperature compensation must occur to eliminate thermally-induced volume changes; leak location cannot be detected.

Other pipeline leak detection methods not listed above include flow measurement, pressure point analysis, and level monitoring.

 

Design Considerations

 

Liquid leak sensors are typically manufactured for compatibility with specific liquid types. For example, manufacturers may design sensors to detect water leaks, corrosive fluid leaks, or petroleum-based leaks. Sensors may also be configured to effectively detect leaks in foamy, dirty, or opaque liquids.

 

Liquid leak detection products are typically specialized float switches, conductivity sensors, or seal failure detectors which operate according to one of the principles listed above or on the Liquid Leak Detectors Search Form. These sensors output data to a compatible meter in the form of voltage, current, or frequency.

 

liquid leak detectors selection guide

A set of internal leak detectors within a tank. These sensors detect volume changes over time.

Image credit: NetMotion

 

Standards

 

Liquid leak sensors may be used according to various published standards. Example standards include:

 

  • BS 13160 - Leak detection systems (series)
  • API RP 1130 - Computation pipeline monitoring for liquids
  • MIL-PRF-81298 - Liquid dye for the detection of leaks in aircraft fuel systems

References

 

EPA - Introduction to Leak Detection (book)

 

EPA - Refrigerant Emissions: Guide to good leak testing (pdf)

 

Image credit:

Time Mark Corporation