What are deflectors used for? What happens if it falls off- -starting with the expansion joint structure
Let's talk about the basics first. The compensator (that is, the expansion joint) usually has a guide tube inside it. This thing looks inconspicuous, but it really doesn't work without it. The guide tube is installed on the inside of the bellows, and it mainly does two things: first, it guides the medium to flow smoothly and reduces turbulence and resistance; The second is to protect the bellows from being directly washed by high-speed media. Think about it, if the thin wall of the bellows is directly attacked by steam or acid with particles, it won't take long to perforate and leak. As soon as the guide tube falls off, the bellows will be worn out and the vibration will exceed the standard, and the whole expansion joint will fail, and the pipeline system will suffer. Two days ago, a customer called and said that a high-temperature axial expansion joint in their factory had been used for less than half a year, and the guide tube fell off, and they were anxious to change the whole machine. I said don't worry yet, take it apart and have a look. The high probability is one of the following five reasons.
Reason 1: The flow rate of the medium is too high or the flow direction suddenly changes, and the guide tube is "blown" down
This is the most common culprit. If the flow rate of the medium in the pipeline is too fast, or the flow direction changes suddenly due to the sudden opening and closing of the valve, the guide tube will be subjected to a huge impact force. Especially in some power station steam pipelines, the flow rate can soar to more than 40m/s. If the guide tube is not reinforced well, the weld will be directly torn. Don't forget, the deflector itself is a cantilevered construction – one end is welded to the end tube and the other is freely suspended. High-speed fluid blows at it like a water gun, vibrating repeatedly, and fatigue cracking is a matter of time. What about that? When selecting the model, you have to calculate the flow rate and pressure drop, not just look at the diameter. For example, the design flow rate of the guide tube used on the double hinge expansion joint of the air-cooled island vacuum pipeline has to be stuck, and if it exceeds it, the tube has to be thickened or changed into a multi-section support structure. Many design institutes have stepped on this pit.
Reason 2: The installation direction is reversed and the arrow is white marked? The guide tube is flushed directly
There are arrows on the expansion joints to mark the flow direction of the medium, but some people don't look at it. The installation drawing saves trouble, or the on-site pipeline is complicated, and there are a lot of cases where the installation is reversed. When the guide tube is designed, only one-way flow is considered-the medium enters from the bell mouth of the guide tube and flows out from the straight tube section. Install it in turn, and the medium directly hits the blind end of the guide tube, which bears all the dynamic pressure like a plug, so it is strange that it doesn't fall off. A more hidden problem is that some manufacturers have opened guide holes or pressure relief grooves on the guide tube, and reverse installation will turn these holes into "attack points" and accelerate local erosion. Therefore, before installation, be sure to check the flow direction mark on the general corrugated expansion joint or external pressure single axial expansion joint, and then test the pressure after installation. You don't know, we have encountered it several times after sales. When we removed it, we saw that the scour marks on the inside of the guide tube were obvious, and the direction of the arrow was reversed.
Reason 3: Welding quality is not up to standard, or fatigue fracture, especially in high temperature working conditions
There is a welding connection between the guide tube and the end tube, and the quality of the weld directly determines the life. There are three common problems: insufficient height of the welding foot, unfused, and the presence of pore slag inclusion. These defects may be fine at normal temperature and low pressure. Once high temperature and high pressure steam is introduced, as soon as the thermal stress of the weld comes up, the crack will expand in a few days. Especially for high-temperature axial expansion joints, when the working temperature exceeds 400℃, the difference of creep strength between the base metal and the welding material will be magnified, and the weld at the root of the guide tube becomes the weakest link. In addition, don't forget that the pipeline system starts and stops frequently, and the alternating stress generated by the thermal expansion and contraction cycle will also make the weld fatigue. According to some data, more than 60% of the failure positions in the heat-affected zone of the weld are in the cases where the guide tube of the expansion joint of thermal pipeline falls off. Want to save trouble? Then honestly ask the manufacturer to do non-destructive testing, such as radiation or ultrasound, and don't save that little money.
Reason 4: Improper material selection of guide tube, loosening caused by corrosion or thermal expansion
This pit often comes out of projects with complex media composition. For example, the desulfurization flue gas pipeline contains a large amount of SO₂ and moisture. If the guide tube is made of ordinary 304 stainless steel, pitting and stress corrosion cracking will come soon. For another example, the material of the guide tube is different from that of the end tube, and the thermal expansion coefficient is different. Under long-term high-temperature operation, there will be a gap at the connection, and the guide tube will shake inside and finally fall off. In equipment such as desulfurization flue gas baffle door or rubber PTFE compensator, the material of the guide tube is best matched with the corrosiveness of the medium-316L should be used, and the nickel-based alloy should not be hesitated. Another point: the roughness of the inner wall of the guide tube also affects the erosion resistance. In order to save money, some small factories use unpolished hot-rolled plates, and rough particles on the face wall adhere, which aggravates local corrosion. Be more careful when making PTFE compensator. Although PTFE guide tube is corrosion-resistant, it has low strength and large thermal expansion. The fixed structure must be specially designed, otherwise it will be squeezed and deformed as soon as the temperature rises, and loosened as soon as the temperature drops.
On-site Remediation and Prevention-How to Avoid Secondary Shedding from Type Selection to Overhaul
What if the deflector has dropped? Don't rush to scrap water-based, you can try on-site repair. But only if the bellows itself is not damaged. If only the guide tube falls off, you can remove the end flange, remove the residue, and reweld or rivet the new guide tube in place. It should be noted that the repaired guide tube should be made of the same material and thickness as the original design, and the welding process should also match. However, if the bellows has been washed and thinned, it is more practical to replace it with a new one, such as choosing a double hinge transverse expansion joint or a curved tube pressure balance expansion joint with its own reinforcement structure.
The flow rate is controlled within 15m/s (depending on the medium), the flow direction is clearly marked and must be checked on site, the welding is carried out according to NB/T 47014 standard, and the guide tube material is at least one grade higher than the base metal in corrosion resistance. During maintenance, open the inspection hole on the expansion joint at least once a year, and use an endoscope to see the wear of the inner wall of the guide tube and the state of the weld. Don't wait until there is an abnormal noise or a sudden change in pressure drop to check, then it is often too late. Every point you attach importance to the deflector, the life of the expansion joint will be three points longer. Is that the truth?