The methods for carrying out a hydraulic test are governed by the safety regulations that apply to the pipeline.
For France, the applicable regulation is the "multi-fluids" order of 4 August 2006 (amended by the order of 20 December 2010). The test durations and acceptance criteria are described by methods guide GESIP 2007/06
An hydraulic test has two aims: on the one hand to ensure that the structure has the required strength to support the operating conditions and on the other hand to ensure its perfect tightness.
An hydraulic test is carried out in several steps:
- Filling with water, pressurizing.
- Period of thermal stabilization so that a thermal balance is established between the fluid contained in the duct and the surrounding soil. According to the diameter, this period can last several days.
- Inspection for the absence of air: too much air can mask a leak.
- Test (in the presence of the DRIRE).
The pipeline to be tested is filled with water and pressurized to a pressure higher than the maximum working pressure (MWP) and less than the design pressure (DP). The pressure is usually recorded for two hours and no pressure variation is permitted unless it can be explained by a significant temperature variation. For overhead parts that can be visited, a visual inspection is carried out at the same time.
Leak tightness test:
This test is used to demonstrate the tightness of the structure, i.e. that there are no leaks.
The test takes place over a period of several hours or even a full day. During this time the conservation of the weight of water contained in the pipeline is checked: pressure variations should be capable of being justified by temperature variations. The pressure must be measured to an accuracy of about 10 mbar.
In order to measure the temperature variations of the pipeline, several sensors are installed appropriately along the structure. The measured temperature values are weighted by the lengths of the sections to which they are allocated.
Verification of the tightness by calculation involves:
- The geometrical characteristics of the conduit (diameter, length, thickness)
- The mechanical characteristics (modulus of elasticity, Poisson's ratio, coefficient of expansion)
- The physical characteristics of the test water (coefficient of compressibility, coefficient of expansion)
- The pressure variation over a given time period (usually 6 or 24 hours)
- The temperature variation over the same time period.