Nonmetallic Compensator Welded Pipe: Complete Interpretation of Connection Process and Sealing Key
How to weld non-metallic compensator to metal pipe? How to fix if the welding is not good and it leaks? This problem is actually quite bad-the non-metallic compensator itself is made of fabric fibers or rubber, but the ends must be connected to the pipe through metal connectors. This welded pipe is the vital gate that connects the past and the next. Once there is a problem with welding, it will smoke and burn, and the whole pipeline will be stopped. Today, let's break it down and talk about it.
Take it apart first and see: What role does the welded pipe really play in the compensator?
From our stationNon-metallic expansion joint (fabric fiber expansion joint)Andrubber compensatorThe main body is of flexible material, but a length of metal short tube or flange is usually pre-installed at both ends. This section of metal pipe is a welded pipe, which is used to butt weld to the pipe on the spot. The problem is that heat is conducted along the metal tube during welding, and once it overheats, the non-metal layer may burn or delaminate. Two weeks ago, a customer reported that after welding, he found that the edge of the compensator was smoking-this is a typical overheating damage. The fibers or rubber in the flexible material were baked by high temperature, and the sealing performance was directly scrapped.
Therefore, the design and construction of welded pipes must put "heat insulation" and "temperature control" in the first place. Think about it, the thermal conductivity of metal pipes is so high, and the welding arc temperature is thousands of degrees. Without some protective measures, the non-metallic layer can't bear it at all.
How to do the welding process correctly? Core Principles: Fast, Cold, Jump
Quick welding, cold welding, segment welding.
- Quick welding: Small current, short arc, reducing heat input. The current is adjusted to 20% ~30% lower than normal welding. Don't be greedy. As soon as the arc is elongated, the heat can't dissipate.
- Cold welding: Wrap the root of the metal tube with a damp cloth, or stick a piece of copper close to the back of the welding spot to absorb heat. The damp cloth should be wrung out without dripping water to prevent steam from escaping into the non-metallic layer. Copper blocks work best, but be careful not to burn your hands when operating.
- Segment welding: Jump welding, first weld a section of 50mm, wait for natural cooling to room temperature before welding the next section. Don't go on a continuous lap. No one can bear the heat accumulation.
The selection of welding electrode is also particular. If the pipe is made of carbon steel, use carbon steel welding electrodes; It's stainless steel, so use stainless steel welding electrodes. Don't mix it, especially don't use acid welding electrode to weld on alkaline steel. That weld will crack for you in three days. Don't forget to do the airtightness test after welding-standard referenceNational Standard for Non-Metallic Expansion Joints JB/T 12235-2015。 The test pressure is usually 1.5 times the design pressure, and it is considered qualified if it is kept for 5 minutes without leakage.
Design level: The length and wall thickness of the welded pipe are not determined by patting the head
The welding pipe is too short, the heat dissipation is poor, and the welding heat is easy to channel to the non-metallic layer; If it is too long, the stress concentration will become larger, and the weld is prone to fatigue cracking when the pipeline expands and contracts thermally. Generally, it is recommended that the length be controlled at 150~200mm, and the wall thickness is consistent with the pipeline. For example, the wall thickness of flue gas pipes is generally 4~6mm, and the same wall thickness is used for welded pipes, so there is no need to thicken them-thickening will increase the welding heat input, which is not worth the loss.
In some cases, it is necessary to install a guide tube. We have talked about the function of the guide tube in the FAQ. Simply put, it is to prevent the medium from washing the welded joint, especially for dusty airflow or high-speed steam pipelines. When installing, pay attention to leaving a gap of 3~5mm between the guide tube and the welded pipe, otherwise the hard top will tear the weld during thermal expansion.
Application scenario: High-temperature flue gas pipeline and cement industry are the main forces, but there is a big pit
Welded pipe connections are more common in high-temperature flue gas pipelines and cement industries. Why? The flange connection is prone to air leakage, and the bolts will slack at high temperatures-no matter how tight you tighten it, the bolt preload will drop as soon as it is baked at 500℃. Welding once and for all, as long as it is well welded, there is basically no leakage. Two days ago, an engineer from a cement factory told me that they used flanges before, and they had to tighten the bolts once every six months. They changed the welding connection, and there was no problem in three years.
If the pipe vibrates heavily, the welded joint may fatigue crack. Don't think everything will be fine if you die by welding. Vibration is the natural enemy of metal welded joints. Once the stress concentration point cracks, the whole pipe has to stop. In this case, you have to change toLarge tie rod expansion jointOrMetal hoseTo compensate for the vibration. The expansion of the large tie rod saves energy and absorbs lateral and axial displacement, while the metal hose is good at coping with small-amplitude high-frequency vibration-this is all available in our product information, and it can be selected according to the working conditions.
Finally, let me talk about the practical details: before installing the welded pipe, adjust the pre-drawing amount first
Many teachers welded as soon as they came up, and only after welding did they find that the tie rod of the expansion joint was not adjusted. That would be blind-the pre-tension amount of the expansion joint is used to absorb the thermal displacement of the pipe, and the stroke has been limited after welding and adjustment. The correct method is: before installation, pull the expansion joint to the installation length and tighten the tie rod nut for positioning. Refer to what we said in our Q&AHow to adjust the tie rod nut of expansion jointGenerally, it is tightened according to the installation length, and then loosened the nut after welding. If it is not loosened, the compensator will not be able to expand and contract after the pipe heats up, and the non-metallic layer will be torn in minutes.
After welding, check the surface of the welded pipe for cracks or pores. Don't be too troublesome, scan it with a magnifying glass or penetration detection, 5 minutes, so you don't have to rework it later.
Non-metallic compensator welded pipe this matter is not complicated, but the details determine success or failure. From the process parameters to the installation sequence, every link is accurate, and it is not difficult to not leak for ten years.