If the flue expansion joint is broken, don't replace it in a hurry? Follow these 5 steps to check the reason
Two days ago, I met a customer and called in a hurry, saying that the flue expansion joint in their factory was leaking, and asked me to send a new one quickly. I said don't worry. You should check it according to the steps I said first. Maybe you can save a lot of money. And guess what? After checking, I found that the tie rod bolt was not adjusted properly, and it was fine after a few screws.
There are actually only a few reasons for the damage of flue expansion joints, and you can't escape the four King Kongs of temperature, corrosion, installation and vibration. Today, I will break up this set of troubleshooting methods and crumble them to tell you, and you can just follow it.
Step 1: Look at the medium and temperature-is temperature excess the culprit or accomplice?
First of all, you have to find out what medium is running in the pipeline and what the temperature is. Some flue temperatures can soar above 600℃. If you use an ordinary general-purpose corrugated expansion joint, it will definitely not be able to bear it. At this time, it is necessary to use high-temperature axial expansion joints or corrugated expansion joints used in power station industry, which have much higher temperature resistance grade.
How to check? Take the thermometer gun to hit the surface of the expansion joint. If the surface temperature exceeds 15% of the design value, it is basically the temperature. Some media contain sulfur, which can also accelerate corrosion at high temperatures. Remember: temperature excess is not necessarily the culprit of direct damage, but it is definitely an accomplice-it makes the material soft and less strong, and what could have been carried for 10 years will be wasted in two years.
If the temperature remains high all the time, it is recommended to replace it with non-metallic expansion joints (fabric fiber expansion joints) or PTFE-lined hoses, both of which have good high temperature and corrosion resistance.
Step 2: Check corrosion traces-How to judge acid dew point corrosion?
The favorite problem in the flue is acid dew point corrosion. Especially in the flue that burns coal or heavy oil, the sulfur dioxide in the flue gas becomes sulfurous acid when it meets water. As soon as the temperature is lower than the dew point, it directly condenses on the inner wall of the expansion joint, and that piece will be corroded into small pits one by one.
You crouch down and look, shining a flashlight on the inside of the bellows. If rust spots are found to be pieces with irregular edges, it is basically acid dew point corrosion. If the entire piece is uniform rust, it may be ordinary oxidation. The former needs to change corrosion-resistant materials, such as polytetrafluoroethylene compensator or anti-corrosion expansion joint matched by desulfurization flue gas baffle door.
Never wipe the corroded area with a cloth! Acid may get on the skin. Take a photo and send it directly to the manufacturer to ask how to choose materials more reliably.
Step 3: Keep an eye on the installation details-are the tie rod bolts screwed correctly?
Many flue expansion joints are broken, purely because the installation master's hand slipped. The expansion joint leaves the factory with a set nut on the tie rod bolt, which is used for pre-stretching. After installation, this set nut must be loosened to allow the expansion joint to expand and contract freely. Many people forgot this step and screwed the positioning nut to death directly. As a result, when the pipe was hot, the expansion joint couldn't unfold at all, and the bellows was forcibly cracked.
How to do it? Find a wrench and loosen the nut on the tie rod so that the screw can rotate freely. If the tie rod is double nut to top, loosen the outer nut first, then loosen the inner side, and finally leave a gap of about 5mm. For details, please refer to our previous question and answer on "How to adjust the expansion joint tie rod nut".
In addition, is the installation direction reversed? The direction of the arrow of the expansion joint is pointing to the flow direction of the medium, and it is easy to cause problems if it is reversed.
Step 4: Observe the fatigue crack – are the shocks and displacements out of limit?
If no problem is found in the first three steps, the high probability is fatigue damage. When the flue expansion joint is in the environment of vibration or repeated displacement for a long time, micro-cracks will gradually occur at the root of the bellows, and finally expand into penetrating cracks.
This kind of crack is difficult to see with the naked eye. You can use the method of penetration detection: spray some cleaning agent to wipe the surface clean, then spray red imaging agent, wait for 10 minutes, and the crack will show a red lever. If a crack is found, it means that the life of this expansion joint has reached, and the fatigue is irreversible, so it can only be replaced with a new one.
How to avoid it? Install limiting devices, or replace them with double hinge transverse expansion joints and straight tube pressure balance expansion joints, which have stronger displacement absorption capacity. In situations like air-cooled island vacuum pipes, there are also double-hinged expansion joints to cope with complex displacements.
Step 5: Comprehensive Judgment-Is it to replace or repair?
The final step is to make the decision. Based on the results of the first four steps, you judge:
- If it is just that the tie rod is not adjusted properly and the installation direction is wrong: fix it! Just transfer it back.
- If it is slight corrosion (area less than 10%): You can temporarily plug the leak with metal repair agent, but it is recommended to replace it at the next shutdown.
- If it is large area corrosion, fatigue crack, temperature seriously exceeds the limit: change, don't hesitate.
Don't buy randomly when you change it. You have to match the model according to the medium, temperature, pressure, and displacement. For example, in the flue desulfurization section, it is recommended to use rubber compensator or rectangular non-metallic expansion joint, which has good acid resistance. If it is the main steam pipeline of the power station, it needs a metal rectangular expansion joint or a high-temperature axial expansion joint. If you are really unsure, just take the nameplate of the old part to the manufacturer for refurbishment, or let the manufacturer send you a sample.
The investigation of the cause of flue expansion joint damage is not to have you become an expert, but to let you have a bottom before reporting for repair, so don't be fooled into spending money. When it is time to order, take a look at the product information in our station. Metal hose, PTFE-lined metal hose and flue gas baffle door all have detailed parameters.
Okay, that's it. Next time the expansion joint is broken, follow these five steps first.