What exactly is a metal expansion joint frame? — — Don't just stare at the bellows, the frame is the force bearing
When many people mention metal expansion joints, their first reaction is those turns of bellows. I think the bellows can expand and contract, and the core of the expansion joint is on the corrugation. That's half true. The bellows is indeed the main force that absorbs displacement, but without the "iron frame" to support it, the bellows would have been torn to pieces by pressure and thrust.
The bellows is responsible for absorbing thermal displacement and vibration, and the frame-that is, the structural components of the end tube, ring plate, stiffener, tie rod or hinge-is the "backbone" that really holds the internal pressure thrust and external load. To give an extreme example: on the main steam pipeline of a power station, the medium pressure may rush to more than 10MPa, and the temperature may be 500 to 600 degrees. Without a frame with sufficient stiffness, the bellows can bulge like a balloon and even burst. Once the frame fails, the entire pipeline system is a ticking time bomb.
So,Metal expansion joint frameIt's not a trim piece, it's a line of security. If you only count the fatigue life of the bellows and ignore the strength and stability of the frame when selecting the model, it is leaving a back door for accidents.
How to choose the frame material? Carbon steel, stainless steel or alloy steel? Working conditions have the final say
"Can the frame be uniformly made of 304 stainless steel? Save the trouble of sorting." I said no. If you choose the wrong material, you will either waste money or die.
Carbon steel is a regular. For example, Q235B and Q345R are used in flue gas pipelines with low temperature and non-corrosion, which are extremely cost-effective. But in the cement industry, the air ducts are full of abrasive dust, and the carbon steel frame is worn out in half a year. At this time, wear-resistant lining plate must be applied or low-alloy steel such as 16Mn must be selected.
Stainless steel isn't a panacea either. 304 has good corrosion resistance at room temperature, but it faces intergranular corrosion when it exceeds 400℃; 316L can hold the wet flue gas behind the desulfurization tower, but it still pits in the environment with high chloride ions. For high-temperature situations (such as boiler outlets), you have to use alloy steel, heat-resistant steel like 15CrMoG, or more advanced Inconel alloy frames.
The material follows the temperature, pressure and corrosiveness of the medium。 Don't use carbon steel to deal with high temperature and humidity to save money, and don't blindly pile stainless steel for insurance. I have seen a power plant that used a carbon steel frame with 316L bellows. As a result, the frame corroded, resulting in overall instability, and shutdown and maintenance cost millions.
Three parameters that are most easily overlooked when designing a frame: stiffness, wall thickness, and connection mode
The frame design seems simple, isn't it just a few steel plates welded together? But rollovers are often in the details.
stiffnessIt's the first way. The frame should have sufficient stiffness to restrain the bellows and prevent its columnar instability. Especially for large diameter, low pressure expansion joints, such as rectangular expansion joints behind desulfurization flue gas baffle doors, insufficient frame rigidity will cause bellows to twist. How to calculate? According to EJMA standards, the moment of inertia of the frame must be greater than the critical value, otherwise it is a cartilage head.
wall thicknessDon't smack your head. If it is too thin, the welding deformation will be large, and the vibration will crack during operation; Too thick is self-weight, high cost, and also affects the matching with the pipeline. Generally, the wall thickness of the end pipe is taken as the wall thickness of the connection pipe plus 2-3mm corrosion margin. For large-diameter thick-walled expansion joints, the wall thickness should even be designed according to pressure vessel standards, because its frame itself is a pressure-bearing element.
Connection modeMore cheating. In many on-site failure cases, not the bellows is broken, but the connecting weld between the frame and the pipe is cracked. Why? Because thermal expansion difference is not taken into account. For example, when a carbon steel frame is welded on a stainless steel pipe, the thermal stress is concentrated at high temperature, and the weld is finished first. The solution is to adopt a transition section or flexible connection design. Also, the nut of the expansion joint of the tie rod is tightened too tightly, which limits the displacement, but instead eats all the force on the frame. Refer to the FAQ of our station, how to adjust the expansion joint tie rod nut? Pre-stretch clearance must be allowed.
What if something goes wrong with the framework? The Roots and Countermeasures of Deformation and Weld Cracking
Deformation, cracking, corrosion perforation. Where are the roots?
Deformation is mostly due to insufficient stiffness or deviation of installation. For example, the metal corrugated expansion joint of cement industry used in cement industry, excessive self-weight of pipeline will lead to concave frame. The countermeasures are to increase reinforcing ribs or use hinge structures to disperse loads.
Weld cracking, commonly seen in vibration conditions. On the vacuum pipeline of the air-cooled island, the welds of the double-hinge expansion joint frame are repeatedly subjected to alternating stress, and they will crack in two or three years. Treatment methods: improve the welding quality and do stress-relieving heat treatment; Or switch to integral forged end tubes to reduce the number of welds.
Corrosion perforation, especially the frame behind the baffle door of desulfurization flue gas. After wet desulfurization, the flue gas contains water and acid, and it will rot in one or two years. We have encountered customers with regular carbon steel frames and air leaks for three months. Later, it was replaced with glass scale or stainless steel frame, and the problem was solved. However, note that stainless steel may also be stress corroded in chlorine-containing environment, so it is recommended to use duplex stainless steel or lining anti-corrosion for the net flue gas after desulfurization.
During daily inspection, focus on observing the connection between the frame and the bellows, the weld surface, and the thread of the tie rod. Once you find abnormal deformation or crack, don't hesitate to replace it immediately.
How to match the framework in different industries? Electricity, cement, desulfurization…cases speak
It's better to look at a few actual projects to say a thousand words.
Power industry: On the main steam pipeline, we are equipped with corrugated expansion joints for power station industry, the frame material is 15CrMoG, the wall thickness is thickened, and the end pipe is welded with the pipe by forgings. Because of the high steam parameters, the frame is also designed with an internal guide tube to prevent high-speed steam from directly washing the bellows. Customer Feedback Five years of operation with zero accidents.
Cement industry: The temperature of the kiln tail air duct is high, the dust is large, and the ordinary frame is seriously worn. What we do is the metal corrugated expansion joint for cement industry, the frame adopts 16Mn, and the wear-resistant lining plate is added inside, and the thickness of the ring plate is increased by 30% compared with the conventional. At the same time, the connection mode is made into a flange, which is convenient for maintenance and replacement. After three years, the amount of frame wear is less than 2mm.
Desulfurization system: Rectangular flue for inlet and outlet of desulfurization tower, large size, low pressure and strong corrosion. We recommend rectangular non-metallic expansion joints with external metal frames. The frame is hot-dip galvanized with Q235B, and the exterior surface is painted with epoxy zinc-rich paint. The non-metallic ring belt absorbs displacement and the frame is only subjected to pressure thrust. This saves money and resists corrosion. However, it should be noted that the frame of the non-metallic expansion joint must be provided with drainage holes, otherwise the acid accumulated inside will rot through.
There is another unpopular case-air-cooled island. The vacuum pipe is large in diameter and low in temperature (-40℃ to 80℃), but the dead weight and wind load of the pipe are large. We used double hinge expansion joint of air-cooled island vacuum pipe, and the frame is hinge + tie rod composite structure. The hinge takes on the lateral displacement, and the tie rod limits the axial expansion and contraction. This design avoids bending of the frame by bending moments.
Finally, no matter which industry, the framework design should take into account "redundancy". Pressure, temperature, and corrosion margin are all left alone, so that the metal expansion joint frame can truly become a safe line of defense, not a paper tiger.