Mistaken Installation Error Halves Expansion Joint Life – Have You Ever Encountered It?
A few days ago, a friend who was doing power plant maintenance called me and said that the non-metallic expansion joint they had just changed for half a year began to leak again. When I removed it, the corrugated layer was worn out. I asked him if he left enough error margin when installing it. He was stunned for a long time and said, "It was just aimed at the welded one". Alas, this kind of thing is really uncommon-if you don't understand the error range of flue expansion joint installation, no matter how good the product is, it won't last long.
Don't believe it, the expansion joints replaced in our factory are most likely not the quality of the product itself, but the installation error exceeds the standard. Axial, radial, angular, any direction is deviated, the bellows has to bear extra stress, and the fatigue life is directly cut in half. Today, let's talk about this matter and explain those easy-to-step pits one by one.
What are the installation errors of flue expansion joints? Axial direction, radial direction and angular direction must not be missing
Let's talk about the most basic-there are three types of installation errors:
- Axial error: It is the displacement along the centerline of the pipe, such as compressing or stretching the expansion joint too much during installation. Metal corrugated expansion joints are designed with a pre-stretch amount. If you install them backwards or too tightly, it can't compensate for thermal expansion normally.
- Radial error: The center line of the pipe is misaligned, one higher and one lower, or left and right. For metal rectangular expansion joints, this error is particularly headache, because its corrugated structure is sensitive to lateral displacement, and hard pulling and hard pulling easily leads to wave crest cracking.
- Angular error: The axes of the two pipes are not parallel, forming an included angle. Angular displacement will cause one side of the expansion joint to be over-compressed and the other side to be over-stretched. Over time, accidents will inevitably happen to the riveted part or weld.
These three kinds of errors tend to be superimposed in actual installation. Think about it, how can you be so perfect when you are counterparts on the spot? So the key is to control the total error to be within the allowable range, rather than pursuing the so-called "zero error".
Is the allowable error margin the same for metal rectangular expansion joint vs non-metal expansion joint?
It's totally different. Rectangular metal expansion joints (such as the corrugated expansion joints used in the power station industry in our website) have high rigidity and high elastic modulus, and the installation errors they can withstand are actually relatively loose. Take the axial error as an example, which is generally allowed to be within ±10mm. There are also specific values in radial and angular directions, which depends on the product specifications.
But the same cannot be said for non-metallic expansion joints. Non-metallic expansion joints (also called fabric fiber expansion joints) are mainly composed of flexible materials such as rubber, polytetrafluoroethylene or silicone cloth. They are soft and have strong compensation ability, but they are more demanding on the installation accuracy. Why? Because it does not have the rigid support of metal bellows, after the installation error is large, the fabric layer will be unevenly stretched and soon tear at the weak point. According to the national standard for non-metallic expansion joints (JB/T 12235-2015), the axial error is generally controlled within ±5mm, the radial error does not exceed 3mm, and the angular error does not exceed 1°. You see, much more demanding than metal.
The customer installed a rectangular non-metal expansion joint behind the flue gas baffle door, but the pipe was not aligned, so it was pulled through with bolts. And the result? With less than two months of operation, the fabric layer ripped directly from the flange root. This is the difference between failing to figure out the margin of error.
Before installing the expansion joint, you must confirm these key data
Before installing, be sure to figure out these things out:
- Design thermal displacement of pipeline: How many millimeters can the pipe run, is it axial or lateral? This data is the basis of type selection and the bottom line of installation error.
- Design compensation amount of expansion joint: What is the rated compensation amount of this expansion joint in your hand (such as universal corrugated expansion joint or compound hinge transverse expansion joint)? The instructions given by the manufacturer are all available. Don't forget to take the safety factor into account, generally leaving a 20% margin.
- Actual counterpart size on site: Last time, we helped a cement factory install metal corrugated expansion joints in cement industry, and the center distance between flanges at both ends of the pipeline was 15mm. Logically, adjusting the gasket or re-piping should be used. As a result, the installation team saves trouble, and the pre-stretching of the expansion joint should be directly used to fill the pit. We resolutely refused to let it, and finally reworked the flange end face on the spot to finish it.
Let me tell you a real case. Last year, a desulfurization flue gas baffle door was installed in the desulfurization flue of a power plant, and a large-diameter thick-walled expansion joint was connected in series behind it. In the field measurement, it was found that the pipeline had a deviation of 8mm along the axial direction and 4mm in the radial direction. The designer calculated that if it is compensated by adjusting the tie rod nut of the expansion joint (that is, the role of the tie rod of the expansion joint) during installation, the actual displacement of the expansion joint will exceed 30% of the design value during hot operation. Finally, we suggest adding a compensator on both sides of the flue, one with axial type and one with transverse type, so as to solve the problem. Therefore, the key data must be confirmed on the spot, and don't rely only on drawings.
What should I do if I find that it exceeds tolerance during installation? Don't panic, I'll teach you a few tricks to adjust on the spot
If you find that the installation error is beyond the allowable range, put down the tool first, don't force it. There are several commonly used field adjustment methods:
- Adjust expansion joint tie rod nut: Many expansion joints (such as large tie rod expansion joints) come with tie rods and nuts, and the axial and angular positions of the expansion joints can be fine-tuned by loosening or tightening the tie rod nuts. Note that the adjustment amount cannot exceed the design stroke of the tie rod, otherwise it will destroy the stress state of the bellows.
- Add gasket at flange: If the error is mainly radial or angular, wedge-shaped gaskets can be added between flange surfaces. The gasket material should match the pipeline medium. Corrosion-resistant polytetrafluoroethylene gaskets should be used in desulfurization systems, and metal winding pads can be used in ordinary flue gas systems.
- Re-pipingThis is the dumbest but most reliable trick. If the error is more than 10mm, do not make do, the cut pipe is re-welded and cut. Don't feel bad about that half-day effort, otherwise it will cost more to change the expansion joint later.
Does the screw of the expansion joint need to be removed? After installation, all screws and supports for temporary fixation must be removed, otherwise the expansion joint will not work properly. In 2019, there was a chemical plant because the screw was not removed, which caused the expansion joint to get stuck and the pipeline to crack, and the lesson was heavy.
To summarize: Where is the bottom line for error control in flue expansion joint installation?
In the end, the bottom line of controlling errors is two words- -Retention。 Allow sufficient margin during design, control according to standards during installation, and check regularly during operation. Different types of expansion joints allow different error ranges. Metal rectangular expansion joints can be looser, while non-metallic expansion joints must be strict. Axial, radial and angular errors should be measured, and we can't only stare at one direction. Remember: For every 1mm of installation error exceeding the standard, the service life of the expansion joint may be shortened by 10%.