The metal expansion joint looks inconspicuous in the pipeline system, but if something really goes wrong, it can be shut down for maintenance at the least and media leakage at the worst. Therefore, don't think about fooling the pressure test process. Some people think that pressure testing is to check for leaks. Just fill it with water to see if it leaks or not-that's really not the case.
What exactly is the pressure test verifying?
The core of the metal expansion joint pressure test is two things: the pressure bearing capacity of the bellows and the strength of the end structure. Bellows is the "heart" of expansion joint, and its wall thickness, wave height and wave pitch directly determine how much pressure it can carry. And the end-whether it is a connector, flange or flanged structure, must ensure that it does not deform or break off under the action of pressure.
The pressure test of ordinary pipes is to verify the tightness of welds and joints, but the expansion joints are different. The expansion joint is to produce displacement when working, and the bellows will produce stress concentration under the action of pressure. Therefore, the pressure test depends not only on whether it leaks, but also on whether the bellows has any signs of instability under pressure, such as whether the wave pitch is uniform and whether there is any local bulge. This is why some expansion joints of the same diameter can withstand the design pressure, and some have problems when they hit 80%-the molding quality of bellows and the rigidity of end structure can be known at a try.
Preparation before pressure test: Don't rush to take over the tube
First, confirm the model of the expansion joint clearly. There is a difference between the pressure testing methods of the general corrugated expansion joint and the external pressure single axial expansion joint-the external pressure medium is outside the bellows, and the observation focus during pressure testing is completely different. The installation direction must also be accurate, and the status of the guide bracket and pull rod/screw should be checked once, and the locking should be tightened, and the loosening should be loosened. For expansion joints with guide tubes, attention should be paid to whether the direction of the guide tubes is consistent with the flow direction of the medium during pressure test. Installation backwards not only affects the flow rate, but also may deform the guide tubes by pressure difference.
For the convenience of transportation when the expansion joint leaves the factory, the tie rod is usually in a locked state, limiting the displacement of the bellows. If the tie rod nut is not adjusted before the pressure test, the pressure state of the bellows is different from the actual working condition, and the test result has no reference value. The function of the expansion joint tie rod is to bear the internal pressure thrust. If the tie rod is in wrong state during pressure test, the bellows may be straightened directly as soon as the pressure comes up.
How to set the pressure test pressure? There is a basis, don't pat your head
Design pressure, test pressure, working pressure-these three numbers must be clearly sorted out first. The design pressure is the maximum pressure that the pipeline system can achieve under normal working conditions, and the test pressure is the pressure value to verify the strength of the product, which is usually higher than the design pressure.
The pressure setting of hydraulic test is generally 1.25 to 1.5 times of the design pressure in the industry. For example, the expansion joint with a design pressure of 1.0MPa is qualified only when the hydraulic test is between 1.25MPa and 1.5MPa, and the pressure is held for 10 to 30 minutes. The pressure does not drop, there is no leakage on the surface, and there is no visible deformation. The risk of air pressure test is much greater than that of water pressure, and the pressure value is generally 1.1 to 1.15 times of the design pressure, and strict safety measures must be taken. Why is the coefficient of air pressure lower than that of water pressure? Because gas is compressible, once it breaks, it releases much more energy than liquid. Therefore, if you can use hydraulic pressure, it is not recommended to use air pressure.
The specific medium to choose depends on the site conditions. The hydraulic test is simple, safe and low-cost, but it can't be used in some occasions. For example, the pipeline system is not allowed to enter water, or the residual moisture of the equipment will affect the subsequent operation. At this time, the air pressure or air tightness test will be considered.
Pressure test operation process: Pressure increase should be slow, and observation should be careful
The process of pressure testing pays attention to the word "stable". The pressure boost should be carried out in stages. For example, the test pressure is 1.5MPa, which can be divided into 3 to 4 times. After each pressure boost, stop for a while to observe whether the pressure gauge pointer is stable. If the pressure is boosted too fast, the bellows has no time to deform evenly, and the local stress is too large, the originally qualified products may also be "tested" for problems.
How to set the holding time? The national standard GB/T 12777 clearly stipulates the pressure test of metal expansion joints, which is generally held for 10 to 30 minutes. During the pressure holding period, pay attention to: whether the pressure gauge pointer is slowly descending, whether there is any leakage on the bellows surface, and whether there are water droplets seeping out at the weld. There are three criteria for qualification-no leakage, no visible deformation, and stable pressure gauge pointer. These three conditions are indispensable.
After the pressure test is completed, it's not immediately over. After the hydraulic test, the internal water should be drained clean, especially the trough position of the bellows is easy to accumulate water, and the residual water may freeze and swell the bellows in a cold environment. Then blow dry with compressed air or dry naturally, and then apply anti-rust oil for protection. These post-processing details seem trivial, but if something goes wrong, they will be a lesson.
Common reasons for failed pressure tests
The most common reason for the sudden instability of the bellows during pressure test is that the bellows is not stiff enough or the wave pitch design is unreasonable. It is not uncommon for bellows to experience column instability or plane instability under pressure, and obvious buckling deformation in shape. Weld cracking is more intuitive-cracks appear in longitudinal welds or circumferential welds, usually because the welding quality is not up to standard, and defects such as weld failure to penetrate, slag inclusion and pores are directly exposed under pressure.
End deformation is also frequently encountered. The thickness of the neck flange at the flange connection is not enough, or the wall thickness of the connection pipe is selected thin, and the end structure bulges outward as soon as the pressure is applied. This kind of problem is more because the strength check of the end structure is not enough during the selection design.
The failure of these pressure tests can be avoided by product selection at the root. When selecting bellows, confirm the design pressure, temperature, medium and displacement in advance, and let the manufacturer design the wave height, wave number and layer number according to the actual working conditions. Especially in high-pressure applications, multi-layer bellows is preferred instead of single-layer thickening scheme-multi-layer structure has more advantages in pressure bearing capacity and fatigue life.
If the factory pressure test is qualified, it does not mean direct use on site
Many customers get the expansion joint, and as soon as they see the factory pressure test report, they install it directly on the pipeline. There is a misunderstanding here to make it clear: Passing the factory pressure test is the performance of the product under standard test conditions, but the field working conditions are often much more complicated than the test conditions.
Pressure fluctuations in the pipeline system, temperature changes, support settlement, water hammer effect – none of these factors are present at the time of the factory pressure test. For example, when you buy a car, 4S stores will do PDI testing for the car, but on the road, road conditions, driving habits and oil quality, each of which affects the actual performance of the car. The same is true for expansion joints. The factory pressure test verifies that "the product itself is qualified". Whether the site can operate safely depends on the installation quality, the stress analysis of supporting pipelines and the level of operation and maintenance.
Therefore, the pressure test of the metal expansion joint has to be grasped at both ends: the pressure test is strictly according to the standard before leaving the factory. After installation at the site, it is recommended to do the pressure test with the pipeline system if conditions permit. This minimizes the risk of leakage and instability. Remember one sentence: pressure testing is not a formality, it is to buy insurance for the pipeline system.