The selection of expansion joint of negative pressure pipeline is much more troublesome than that of positive pressure. Under positive pressure, the bellows is spread open, and the material is tensile. Under negative pressure, it is equal to pressing inward from the outside. Bellows are typical thin-walled structures, which will deflate when pressed. Two days ago, I met a customer who made a vacuum furnace. The pipeline with a diameter of 400 was pumped to-80kPa. Less than a month after installation, the bellows was deflated. When I removed it, the troughs were all wrinkled together. The manufacturer said that the product was qualified, and the customer said that the working condition was fine. Finally, it was found that the external pressure stability was not counted at all during the selection.
Essential Difference between External Pressure and Internal Pressure on Bellows
Internal pressure acts on the inner wall of the bellows, generating circumferential tensile stress, which pushes the wave peak outward. The external pressure is just the reverse, the circumferential direction becomes compressive stress, and the trough is squeezed inward. Once the compressive stress exceeds the critical value, the bellows becomes unstable. There are two kinds of instability: plane instability is when the wave distance becomes denser and several waves are crowded together; Column instability is when the entire expansion joint bends over like a noodle. The bellows of the same specification usually withstand external pressure only one-third to one-half of the internal pressure. This is why the negative pressure working condition must be checked separately, and the positive pressure selection table cannot be directly set.
The critical pressure of corrugated pipe instability is related to wave height, wave pitch, number of layers and material yield strength. The larger the wave height, the easier the instability, and the more layers are easier to slip between layers than single layers. Therefore, when selecting bellows under negative pressure, we can't only look at the nominal pressure, but have to let the manufacturer provide the calculation book of external pressure stability. If anyone doesn't count for you and says, "We all do it according to standards," then this job will be replaced as soon as possible.
First distinguish what kind of displacement the pipeline needs to absorb
When selecting expansion joints for negative pressure pipelines, the first step is not to check the pressure, but to look at the displacement type. Axial displacement with external pressure single type axial type expansion joint, its bellows is outside, internal pressure thrust is borne by the shell, stability is naturally better than ordinary axial type. The lateral displacement has to be a compound hinge transverse expansion joint, and the two bellows are connected with a hinge in the middle. Both angular displacement and lateral displacement can be absorbed, but the hinge itself does not absorb axial force, so the pipe support has to be calculated separately as thrust.
If the pressure thrust in the negative pressure system is too large and the guide bracket can't stand it, then a straight pipe pressure balance expansion joint has to be used. It has a balanced bellows inside, which cancels out the blind plate force, and there is no need to set a main fixing bracket in the middle of the pipe. This is commonly used in vacuum pipes of air-cooled islands, or a combination of double hinge expansion joints. But the price of the pressure-balanced type is there, which is two or three times that of the ordinary axial type. If there is no demand, don't go hard.
Vacuum degree and temperature are a double test
The medium temperature directly affects the permissible stress of the corrugated pipe material. Under negative pressure conditions above 300℃, 304 can't be used, and 316L or Inconel is required. When the temperature rises, the yield strength decreases, and the permissible external pressure decreases. The same expansion joint of DN300 can carry-100kPa at room temperature, but may only carry-60kPa at 350℃. This conversion factor cannot be ignored.
The guide tube should also be reconsidered under negative pressure conditions. The purpose of installing a guide tube in positive pressure pipeline is to reduce the medium washing the inner wall of the bellows. The medium flow rate in negative pressure pipeline is usually not high, but the pressure difference between the inside and outside of the guide tube is large. If the wall thickness of the guide tube is not enough and it is deflated and attached to the bellows, the bellows will directly bear all the external pressure, and its life will be sharply shortened. In the special vacuum hose and air-cooled island vacuum pipe double-hinged expansion joint products, the guide tube is made of reinforcement ribs, which is the reason.
Installation details make or break
Pre-stretching and cold tightening are subtle in negative pressure pipes. The cold tightness is properly designed, and the axial displacement falls within the allowable stroke of the bellows in the hot state; However, negative pressure pipelines are often accompanied by a sudden drop in temperature, and the cold tightness is miscalculated. The bellows is in a compressed state in the cold state, and then the external pressure is superimposed, which is equal to a double blow.
Tie rods and nuts are load-bearing parts under negative pressure conditions, not transportation protective parts. The axial pressure thrust generated by negative pressure is greater than that of positive pressure, because the effective area of the bellows multiplied by the external pressure difference is the thrust value. The function of the tie rod is to transmit this part of the force to the pipe support to prevent the bellows from being overcompressed. It must not be removed after installation. A common problem is that there is a tie rod on the construction site, thinking that it is for transportation and fixation, so it is screwed off at hand. As a result, once the pipeline is vacuumed, the bellows is directly crushed to death.
The truth of failure in after-sales feedback
After-sales returns of failed parts, the bellows is sucked deflated the most, followed by weld cracking. Weld cracking is often not a welding quality problem, but a trough stress concentration caused by external pressure, superimposed fatigue cycles, and cracks initiated from the bottom of the trough. In most cases of sudden fatigue life drop, the actual displacement exceeds the design value-the design is given ±20mm, and the actual displacement is displaced to ±35mm on the spot because the distance between the guide brackets is too large.
Negative pressure compensation calculation and pressure-thrust balance are neglected in model selection. The amount of compensation is small, and the bellows is damaged; The thrust was not calculated, and the bracket was pushed away. Those expansion joints that leaked after one year of use, when removed, eight out of ten are not products that smash the brand, but the working condition parameters are wrong.