The "skeleton" of metal expansion joints – what does the frame really look like?
To put it bluntly, the metal expansion joint frame is the skeleton of the entire corrugated compensator. Without it, the bellows are like a puddle of mud, unable to withstand the thrust of the pipe. A typical frame consists of an end flange (or connector), an intermediate connector, and a limiting structure. Stainless steel or carbon steel is the main body, and the specific material follows the working conditions. You take apart aHigh temperature axial expansion jointSee thick steel plates on both sides plus a few tie rods in the middle-that is the frame. Another exampleThe metal rectangular expansion joint, the square shell is also a frame, and extra stiffened plates have to be used to prevent deformation.
Function of Frame in Various Metal Expansion Joints
Different types of expansion joints, the focus of frame work is completely different.
- High temperature axial expansion joint: The frame is responsible for withstanding axial thrust while limiting over-stretching of the bellows. Once the frame is not stiff enough, the bellows will be crushed directly.
- Metal rectangular expansion joint: Flue gas ducts are commonly used, and the frame has to resist not only internal pressure, but also deflection caused by negative pressure of air duct. Many cement plant flue gas pipelines burst, all of which are rectangular frame fillet welds tear.
- Compound hinge transverse expansion joint: The frame and the hinge cooperate to convert the lateral displacement into angular displacement, and the rigidity of the frame itself ensures that the hinge does not get stuck.
- Straight pipe pressure balanced expansion joint: There is a balance ring in the middle of the frame, and the pressure and thrust are balanced by the pull rod, and the frame acts as a force-bearing channel at the same time.
You see, the frame is not an ornament, it is the final foothold of all forces.
Three pits that are easy to step on in frame selection
The frame is selected to be thin, and the pressure test will directly deform. Then the question is, what is the easiest pit to step on in frame selection? I have worked for more than ten years and summarized these three.
① Material matching: Frame material and corrugated pipe material must be of the same grade of temperature and corrosion resistance? Not necessarily. Take a counter-example: carbon steel for the frame of the flue gas pipe, stainless steel for the bellows, and the middle welded transition section. But many people want to save trouble, and the frame also chooses stainless steel. The thermal expansion coefficient of stainless steel frame is different from that of carbon steel pipe at high temperature, resulting in shear of connecting bolts. Correct approach: frame material butt pipe, adding flexible transition in the middle.
② Stiffness calculation: Many times manufacturers rely on experience to pat their heads on the frame. In fact, the moment of inertia of the frame section must satisfy the deflection not exceeding 1/500. EspeciallyLarge diameter thick wall expansion jointThe self-weight of the frame is very large. If the stiffness is not enough, the frame will sag after installation, the bellows will be deflected by force, and the life will be directly halved.
③ Installation constraints: The position of the fixed support of the frame and the spacing of the guide frame must be made according to the design. Some people think the frame is strong enough and just weld a few brackets. As a result, the pipe thermal displacement twisted the frame into a twist. EspeciallySleeve type pipe expansion joint, the clearance between the frame and the sleeve should be precisely controlled, and the difference of 2mm may be stuck.
How to improve the service life of expansion joints through frame optimization? Attached Power Station and Cement Industry Case
A factoryThe corrugated expansion joint used in the power station industry is used in the main steam pipeline. The design life is 8 years, but it leaks after 4 years. When disassembled, the thread of the frame tie rod is seriously worn, and the guide tube is also washed out. Solution: The frame tie rod is changed to a rolling support, the flange end plate is thickened, and the wear-resistant bushing is added at the front end of the guide tube. After changing it, it's been the sixth year and there's still no problem.
The cement industry is more typical. The flue gas pipeline at the outlet of the rotary kiln has large temperature fluctuation and lots of dust. Originally usedCement industry metal corrugated expansion joint, the frame is ordinary steel plate welding, half a year weld cracking. Later, it was replaced with an integral casting frame + internal ceramic fiber insulation layer, and the frame welding joint was fully penetrated groove welding, which extended its life to more than three years. When optimizing the frame, focus on two points: First, reduce the thermal bridge effect, and add a heat insulation pad between the frame and the bellows; The second is to reduce the stress concentration, and all right angles are inverted into circular arcs.
FAQ Extension: Synergistic Relationship between Expansion Joint Tie Rod, Guide Tube and Frame
The framework is not alone. Tie rods, deflectors, and even baffle doors all work in tandem with the frame.
Let's start with the pull rod. A lot of people askThe role of the expansion joint tie rod, in fact, the tie rod is a part of the frame-it transmits thrust and prevents the bellows from bearing excessive axial force. Adjusting the tie rod nut allows fine-tuning of the mounting length, but remember: the tie rod is not intended to be forcibly preloaded, it is only limited! Some customers screwed the tie rod to death, and as a result, the bellows burst directly without compensation when the pipe expanded.
Look againThe specific function of the expansion joint guide tube-the guide tube is welded on the inner side of the frame, and its main function is to divert the medium to prevent high-speed airflow from washing the bellows. However, if there is no good expansion gap between the guide tube and the frame, the thermal deformation of the frame will squeeze the guide tube, causing the guide tube to fall off. Common practice: The front end of the guide tube is bolted to the frame, leaving enough sliding at the rear end.
In addition, when encountering flue gas pipeline projects, it is often necessary toFlue baffle door orRound flapper doors (double seals) work together. At this time, the outer dimensions of the frame must match the flange of the baffle door, otherwise it can't be installed on site. Many people only care about calculating the amount of bellows compensation, forgetting the interface coordination between the frame and adjacent equipment-alas, the rework cost is too high.
The frame is the boss of the expansion joint, and the bellows, tie rod and guide tube all listen to its command. Make it solid when designing, and save half of the heart in later operation and maintenance.