Method for Detecting the Smallest Leaks
From Network Survey
The smallest leaks include damage to pipes in which the noise of the medium flowing out cannot be measured with acoustic location methods. This may be the case if there are hairline cracks where the amount of medium flowing out is too low to be measured or if there aren't any measuring points for noise sensors or if the points are too far away. To solve such problems, Sewerin offers a service using the tracer gas method. The application range includes leak detection in pipes and all kinds of containers, primarily in cases where the pressure test yields negative results.
If a constant, but low pressure drop is determined by measuring devices during a pressure test of recently laid lines, for a water filled pipe it must be assumed that an inaudible dripping or other damage that cannot be acoustically registered has taken place.
Because these and similar difficulties cannot be determined acoustically or with other measuring technology, pipes that are pressurised with air can be damaged as can pneumatic tube systems or tube systems for cables under heavy load.
A nitrogen/hydrogen gas mixture is used for the tracer gas method described here. After the gas mixture flows out at the damaged spot and moves up through the soil to the surface, it is detected. Thus, it is called a 'tracer gas'.
A recently laid pipe for which the pressure test yielded negative results is used to explain the method.
First, water must be emptied from section of pipe to be tested and, if there are low points, foam scrapers must be used to dry them. Then, to fill the pipe, it is sealed at both ends (e.g. with dummy flanges). A shut-off valve with a 1' internal female thread is installed on both the gas filler side and the ventilation side, respectively. The required amount of gas, which depends on filling pressure and pipe volume, is contained in pressurised gas canisters. The pressure of the tracer gas while filling must be at least as great as the lowest value to which the pressure fell during the test. This ensures that the tracer gas will also flow through the leak because there are leak sites that close again below a certain pressure (particularly with sleeves sealed by rubber joints).
The tracer gas has important, essential characteristics:
- Not explosive
- Not poisonous
- Environmentally safe
- Lighter than air
The sensitivity in the ppm range means that even 10 ppm (parts per million) represents a maximum level in the most sensitive measurement range of the gas leak detector.
To pinpoint the location exactly, bar holes are made in the length of the pipe section in the area of a determined gas indication. The gas concentration will be measured here. It is expected that the leak will be found below the bar hole with the highest concentration.
In practice, the leak detection procedure depends on the respective specific local conditions: for this reason, performing detection on the pipe line only makes sense if the gas has had time to diffuse to the surface. Based on experience, a leak of approx. 400 l gas should have occurred. The waiting time, which can last several days for very small leaks, can be determined based on the drop in pressure. It is important to note that about 10 times as much gas flows out of a leak than water. The soil on the site influences the waiting time as well. The rate at which the gas flows upward is slowed by the amount of moisture in the soil and its cohesiveness. Asphalt or concrete surfaces also provide an obstruction. Bar holes must be bored through the solid surface at intervals of approx. 2 metres.
The tracer gas method can be applied with any hollow body that is filled and/or ventilated from two positions, e.g.:
- Gas pipe lines
- Water pipe lines
- Heating systems
- Wastewater systems
- Sprinkler systems
- Telecommunications cables
- Pneumatic tube systems
- Tube systems for cables under heavy load
- Pressure tanks
The method cannot be used or used only in certain conditions for the following:
- Laying casing pipe
- Closed insulation of the medium pipe (district heating)
- Marshy areas or lines that are inaccessible
Using the tracer gas method successfully depends on many factors. For this reason, each time before the method is used, an expert technical consultation is required. In a telephone briefing, inquiries regarding all of the necessary details will be made, solutions discussed and a binding schedule agreed upon.
This provides the basis for successful leak detection, which essentially depends on the expertise and experience of the respective personnel.