What is the difference between reducing diameter expansion joint and ordinary expansion joint?
Two days ago, a buddy who made a heating network called me and asked me, saying that the pipe changed from DN500 to DN350. Should I install an expansion joint in the middle? What kind of fit? I asked him, do you want to hard join with reducer fittings, or do you want to absorb the thermal displacement by the way? He was stunned and said that he had never thought about it.
This is the meaning of the existence of variable diameter expansion joint-when the pipe diameter changes, the flow velocity of the medium changes, and the thermal stress distribution also changes. Ordinary equal-diameter expansion joints have the same diameter at both ends, and the force of bellows is symmetrical; The metal reducing diameter expansion joint is larger and smaller, and the stress state in the cone angle area of the bellows is completely different. It has to do two things at the same time: get the geometric changes in the caliber transition done, and then absorb the thermal displacement. You can also realize the function by adding a large head in the middle of two standard expansion joints, but it takes up space, has two more welds and two more leakage risk points.
Design difficulties brought about by caliber change: How to balance stiffness, pressure resistance and stability
The design difficulty of reducing diameter expansion joint is not the expansion joint itself, but in that reducing diameter section. The diameter changes from small to large, the flow rate of the medium drops sharply, the pressure recovers, and the axial thrust bearing by the bellows is also different. The most headache in design is the contradiction between stiffness and stability-if the stiffness is low, the bellows will easily become unstable; If the stiffness is high, the compensation amount is not enough.
Check the column stability of the bellows according to the pressure and thrust for the small-diameter end, and check the plane stability for the large-diameter end. Different calibers correspond to different critical pressures of instability, so they can't be calculated uniformly according to the big end to save trouble. In order to save materials, some manufacturers make the cone angle too large, and as a result, the trough bulges after several months of operation. The cone angle is generally controlled within 15 degrees. Beyond this range, it is necessary to add a reinforcing ring or adopt a multi-layer bellows structure.
Key Points of Type Selection under Different Working Conditions
The selection of pipeline diameter reduction is relatively simple, mainly depending on the direction and size of thermal displacement. Axial displacement is the main, and general corrugated expansion joint is selected; If there is a lateral displacement, it is necessary to consider the double hinge transverse type expansion joint or the double tie rod type. The pump valve interface is a little troublesome-the pipe vibration at the pump outlet is constant, and the medium flow rate is high, so the expansion joint here not only has to compensate for displacement, but also has to bear fatigue. Two days ago, in a water plant project, the reducing diameter expansion joint at the pump outlet cracked after less than a year of use. When it was removed, the guide tube was flushed out of a gap.
In high temperature and high pressure scenarios, I give priority to external pressure single axial expansion joint or straight tube pressure balance expansion joint. The external pressure type bellows is protected by sleeves, so it is not easy to accumulate dust and is not directly washed by the medium. The pressure balance structure can offset the blind plate force, and the thrust difference generated by the reducing diameter end can also be eliminated, so the load requirement for the fixed bracket is much lower. Those projects in the cement industry and power station industry, which run high-temperature air ducts and steam pipes, basically take this path.
Installation and pre-deformation: the easiest details to overlook
Installing this piece, the reducing diameter expansion joint is more demanding than the ordinary expansion joint. Because the center of gravity of the reducing section is deviated, if only one end is hoisted when hoisting, the bellows may be sprained. After installation, the adjustment sequence of the tie rod nut is also particular-first adjust the nut on the large end side, and then adjust the nut on the small end side, so that the force on both sides can be uniform.
Another is pre-deformation. The actual operating temperature of the pipe is higher than the installation temperature, and the bellows must be pre-stretched or compressed a certain distance. Many people don't know about this piece, including some installation teams who have worked for several years. The pre-deformation quantity of the reducing diameter expansion joint cannot be calculated only according to the axial stiffness, but the thermal expansion of the reducing diameter section should also be superimposed. If you act in the opposite direction, it will not be called compensation, it will be called adding chaos.
One more detail – the guide bracket. Guide brackets must be provided at both ends of the reducing diameter expansion joint, and the guide brackets at the small diameter ends should be closer to the expansion joint. The reason is simple: the small end has great stiffness, and it is easy to transmit all the displacement to the bellows.
Common failure modes and countermeasures
One is that the overall column is unstable, and the bellows bend over like a column; The other is plane instability, with troughs bulging to one side. The countermeasure of the former is to increase the wall thickness or wave number, while the latter has to add a reinforcing ring. How to judge? Look at the wave distance-when the wave distance is enlarged, it is the tendency of column instability, and when the wave peak is deflated, it is the plane instability.
Fatigue cracks are basically at the root of the bellows trough. The stress is the greatest here, and the welding heat-affected zone is a weak link. There are nothing more than two countermeasures: one is to choose a bellows structure with high fatigue life, such as multi-layer bellows instead of single-layer thick wall; Second, don't let the flow rate at the small diameter end be too high, and the abrasion and corrosion are superimposed, and the fatigue life will fall off a cliff.
The problem of deflector wear is easily ignored by many people. The small end side of the guide tube of the reducing diameter expansion joint is the easiest to wear out. Because when the medium flows from a large bore to a small bore, the flow rate becomes faster, and if there is particulate matter in the medium, it is like sandblasting. When selecting the type, the material of the guide tube must follow the medium-if the abrasion is strong, the guide tube should be made into a replaceable type, or a cemented carbide bushing should be added. Moreover, there should be enough gap between the guide tube and the bellows, so that the thermal expansion does not push the guide tube to death on the bellows.
In the final analysis, the selection of variable diameter expansion joint is a trade-off. The difference in stiffness, pressure and thrust, and flow velocity caused by the change of caliber must be taken care of. Don't expect a standard model to take all conditions, and don't think of the reducing expansion joint as simple as an ordinary expansion joint with a reducing tube. Before you choose, you have figured out the working condition parameters. The expansion joint is essentially calculated, not determined by patting your head.