Temporary fixation of non-metallic compensators: why, how, and when to dismantle?
Two days ago, I met a customer. After the non-metallic compensator (that is, the fabric fiber expansion joint) was installed on the spot, it was directly tested by steam. As a result, the skin was torn before the ripples began to expand and contract. Reason? The temporary fixed tie rod carried when leaving the factory was not disassembled at all. This kind of thing is actually not new in the industry, but every time something goes wrong, it can stop production and replace parts, or damage the whole pipeline system. So today, let's break it down,Temporary fixation of non-metallic compensatorHow to manage it.
What exactly is temporary fixing protecting against? -Structural weaknesses and transport/installation risks of non-metallic compensators
The main body of the non-metallic compensator is fabric, rubber, PTFE and other flexible materials (such as this station'sNon-metallic expansion joint (fabric fiber expansion joint)Andrubber compensator), itself has little axial stiffness. To put it bluntly, it is a soft connector, which absorbs thermal displacement by the deformation of the corrugations. But precisely because it is soft, if you are not careful when transporting and lifting, the flexible segment will be stretched, twisted and even squashed.
A rectangular non-metallic expansion joint leaves the factory with only a few millimeters of gap between the skin and the frame. If there is no temporary fixing, the self-weight or bump of the pipe during movement can make the skin bear loads outside the design range. The installation phase is more dangerous – pipe matching deviations, welding stresses, support adjustments, these forces are transmitted directly to the compensator. The core function of temporary fixation is to "lock" the compensator at the installation length, and then release it after the pipeline system is completely in place, the bolts are tightened and the support is stable. Otherwise, the compensator is already internally injured before it even starts working.
Three Mainstream Ways of Temporary Fixation: Tie Rod, Limit Bolt and Auxiliary Support
Common practices in the industry are divided into three categories, each with its own applicable scenarios:
- Tie rod fixation: Most prevalent. Thread several full-length screws between the flanges at both ends of the non-metallic compensator, screw on the nuts, and firmly clamp the compensator. Note: The diameter and number of tie rods should match the caliber of the compensator. For example, the compensator of DN800 must use at least 4 M20 tie rods. Make sure the ends are parallel when adjusting the nut, otherwise the compensator will be found to be crooked after installation.
- limit bolt: Suitable for large size rectangular non-metallic expansion joints. Drill threaded holes directly in the flange and screw in bolts against the opposite flange to limit axial displacement. The advantage is that it is easy to disassemble, but the bolt head is easy to scratch the skin, so it is necessary to add gasket protection when installing.
- Auxiliary support: For vertically mounted or long-span non-metallic compensators, the tie rod alone may not be able to support its own weight. At this time, it is necessary to add a temporary bracket or jack under the compensator, transfer the weight to the pipe support, and then remove it after the equipment is in place. Never weld the auxiliary support to the pipe, leave a space for disassembly.
Also, something likeHigh temperature axial expansion jointOrCompound hinge transverse expansion jointFor this kind of product with its own tie rod, the tie rod nut itself is a part of temporary fixation when it leaves the factory, but on non-metallic compensators, the tie rod usually needs additional configuration.
The easiest pit to step on at the installation site: treat temporary fixation as permanent fixation
I've seen this problem more than once. The installation team found it troublesome, or simply forgot, and left the temporary tie rod and nut on the compensator all the time. And the result? After the system heats up, the compensator wants to expand but can't move. All the stresses are applied to the pipe support and flange bolts, and the flange gasket is squeezed and deformed, finally leaking. The skin of the non-metallic compensator is more brittle and tears directly.
Some people think that "keeping the tie rod can increase the strength", so they simply don't dismantle it. For some metal corrugated expansion joints (such asUniversal corrugated expansion joint), the design does allow the tie rod to be kept as a limiting device, but the non-metallic compensator is completely different-its elastic coefficient is low, and once the tie rod is not disassembled, it becomes a rigid constraint, and the compensation function fails completely. Remember: the "Remove after temporary fixed installation" label attached to the compensator at the factory is not decorative.
When to remove temporary fixings? — — Before heating up, pressing down or running?
This is a technical job, and it is troublesome to dismantle it early or late. According to JB/T 12235-2015 "National Standard for Non-metallic Expansion Joints" and industry experience, the best time is divided into three steps:
Step 1: After the plumbing system is installed, all welding and bolting are completed.At this time, both ends of the compensator have been completely fixed in place, and there is no relative displacement. Loosen the temporarily fixed nut so that the tie rod is in a relaxed state, but don't take the tie rod away yet-in case of an accident during pressing or piping at the back, it can be re-locked.
Step 2: Before the pressure pipe is subjected to hydraulic testing (pressing).Be sure to remove the temporary fixings first! Why? When pressing, the pressure in the pipe rises. If the compensator is still pulled, the internal pressure will try to bulge the skin outward, while the tie rod limits the stroke, and the local stress will concentrate on the flange weld. Once there was a project that I forgot to dismantle, and the compensator flange collapsed directly when it was pressed. Therefore, the tie rod, limit bolt and auxiliary support must be removed before pressing.
Step 3: Before the system heats up and debugs for the first time.If the pipe does not need to be pressurized (such as a low pressure flue gas system), then remove it after the pipe insulation is completed and before the inlet medium heats up. At this time, after disassembly, verify whether the dimensions of the compensator in the free state are within the allowed installation tolerances of the drawings (generally ± 5mm).
After welding is complete, before pressing or heating。 Don't wait until the last few minutes before running, when the pipes are in place and the tools can't reach in.
An easily overlooked problem: inspection and reset after demolition
The tie rod was removed, but the compensator itself may have been "hidden injured" by short-term stress. So two things must be done after dismantling:
- Visual inspection: Check the skin surface for bulging, wrinkles and scratches, especially for degumming between the fabric layer and the rubber layer. ForRubber PTFE compensatorOrPTFE compensator, also check that the PTFE lining is cracked.
- Reset Verification: If the compensator is excessively compressed or stretched during installation, the flanges at both ends need to be pushed and pulled manually to allow them to return to their designed length. For example, the original designed compensation amount is ±50mm, but it is in a stretched state of 60mm after installation, so it is necessary to reset it by adjusting the pipe bracket or re-matching it, otherwise one end of the compensator will soon be insufficient after operation.
In addition, it is mentioned in the product information of this siteHow to adjust the tie rod nut of expansion jointAndDo you need to remove the screw of the expansion jointThese two frequently asked questions and answers are actually applicable to more proportions of metal corrugated expansion joints. However, for non-metallic compensators, the tie rod is temporarily used, and you don't need to screw it back after removing it. Unless you're using one that comes with a permanent limit rodLarge tie rod expansion jointThat belongs to the metal corrugated category, and the structure is completely different. Don't get confused.
At the end of the day,Temporary fixation of non-metallic compensatorIt's not an optional question, it's a must-answer question. If done right, the compensator can absorb expansion and reduce vibration safely and steadily, and it will be used for ten or eight years without any problem; If you do it wrong, you can repair it on the spot or scrap the equipment. Next time you see those humble screws on the package, be careful.