Leak testing comprises an extensive field of interest, both in terms of installations and techniques. Leakage from:
- pressure vessels,
- storage tanks,
- piping,
- pipelines (of all different sizes and lengths, and containing products),
- complete installations (partly separable by (leaking?) valves), and
- equipment with different degrees of accessibility (buried, under water, insulated)
all require different techniques. The techniques must detect, size and locate the leak. They range from visual (detection of, for example, oil spillage), audible (listen to the hissing noise of escaping gas) to application of tracer products, for example, radioactive gas known as Krypton 85. One should also discriminate between techniques which are used off-line, for example, in the commissioning stage with pressure testing and the continuous leak monitoring systems (mass balance systems on a pipeline).
The requirements for a leak detection system should always be related to the safety and environmental specifications:
- explosion danger
- toxicity
- detection time related to total spillage, etc.
Most of the techniques require highly skilled personnel or well-developed procedures with automatic readings or alarms. The following should give some feeling for the various techniques and their specific advantages and limitations.
Leak Testing – Tracer techniques: With this technique at one side of a containment, an additive is injected. In case of a leak the presence of this tracer is detected at the other side – this can be done visually (fluorescence), by chemical sniffers (a company exists which walk trained dogs along a buried pipeline containing a special additive) or by radioactive detectors. In most cases tracers are used to detect small leaks.
Leak Testing – Acoustic techniques: The simplest and most common technique is listening with the human ear for the hissing noise of outstreaming gas. Working with microphones and selecting the characteristic parts of the spectrum (50 to 70 kHz) is a more sophisticated technique. Also, induced mechanical vibrations may be picked up in the structure, either by accelerometers or acoustic emission transducers, which are in fact resonant accelerometers (a.o. used for leaking valves). For these techniques, insight in attenuation characteristics and sound transmission paths is a prerequisite. Gas leaks may be detected in underwater installations with hydrophones, picking up the specific bubble noise in the 5kHz range.
For pipeline monitoring, a number of systems use the occurrence of dynamic pressure fluctuations inside the pipe. These acoustic signals can be used passively (just registering) or actively (acoustic reflectometry), (see Pipeline monitoring techniques).
Leak Testing – Pressure testing: This is a well-known principle – a (part) of the installation is pressurised and then the pressure is monitored. Any observed drop in pressure should be corrected for possible changes in system temperature. A disadvantage is that no information is obtained on the location of a leak.
Leak Testing – Mass balancing: What goes in must come out! The accuracy of this simple principle depends not only on the precision of the measuring equipment, but also on the fluctuation of the process parameters – the smaller the leaks that must be detected, the greater the chance in varying hold-up conditions such as temperature, pressure and composition. Using proper knowledge and modelling, one may apply corrections for these influences (corrected mass balance).
Leak Testing – Pipeline monitoring techniques
The following techniques can be considered:
Low pressure: When a large leak occurs, the inlet and outlet pressure of a pipeline will drop
Pressure decrease, flow increase: A leak in a running pipeline will cause an increase of the flowrate and a decrease of the pump discharge pressure upstream of the leak
Pressure gradient: A large leak will result in a loss of pressure along a pipeline, that is, the pressure gradient will increase upstream and decrease downstream of the leak. To be detectable, numerous pressure measuring points must be present
Negative pressure wave: A suddenly occurring leak will cause a sudden pressure drop at the leak side, which will move as a negative wave with sonic velocity, both in upstream and downstream direction. With dynamic pressure transducers this can be measured and from difference in arrival times, the leak can be located. The SCADA and Wave Alert systems are based on this principle
Corrected mass balance
Dynamic simulation: The pipeline characteristics are modelled. A deviation between predicted values and measured values indicates a leak.
Ultrasonic leak detection pig: Outstreaming products via a leak produce high frequency noise (30 to 70 kHz). Only in the case of non-evaporating liquids can this noise propagate upstream to the inside of the pipeline. In a gas pipeline the gas will escape with sonic flow rate and sound cannot propagate to the inside of the line. The inside noise can be detected by a microphone built into a pig.
Acoustic reflectometry: At one end of the pipeline an acoustic pressure variation is generated, which propagates down the line. At discontinuities, such as a leak, a reflection (echo) occurs, which can be detected with a pressure transducer. The technique is rather sensitive to dissolved gas or gas pockets and can only successfully be applied to hard liquid transport lines. A great advantage of this method is the ability of locating the leak.
Differential pressure test: This is pressure monitoring of a blocked-in pipeline.
Statistical pipeline leak detection: The system is based on inlet and outlet flow and/or pressure monitoring and statistical techniques, and not on modelling the pipeline’s inventory. When a leak occurs, the inlet and outlet flow or pressures shift in certain directions, independent of the modes of operation of the line. The characteristics of a leak can be identified with statistical techniques. The system runs on a PC. The detection of a leak is very reliable, although it cannot locate a leak accurately.
As mentioned above, no general recommendation for a leak detection system can be given. A selection will depend on all requirements, mode of operation, accessibility, instrumentation, costs, etc., and involvement of a specialist is unavoidable.
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Tangshan Aojie Petroleum Machinery Equipment Make Co., Ltd (Short for: OGEM Solids Control)
Address: NO.2 Jingxi Road,Lunan District,Tangshan city,Hebei province,China
Tel: 86-315-8676484
Fax: 86-315-2648099
Zip Code: 063000
Email: ogem2@ogemsolidscontrol.com
Website: http://www.ogemsolidscontrol.com/
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