What to do if the bellows breaks? Close the valve first and then check. Don't rush to replace it with a new one

What to do if the bellows breaks? Close the valve first and then check. Don't rush to replace it with a new one

Two days ago, I met a customer. The corrugated expansion joint of the steam pipe was broken, and the steam on the spot was like a sauna. What was his first reaction? Take a wrench to screw the flange bolt and try to crush the leak to death. And the result? As soon as the bolt was tightened, the crack was directly torn open, and the whole section of the pipe almost collapsed. Alas, we have seen too many of these things.

To be honest, 90% of people did the first step wrong when it came to bellows rupture. Either scrambling to block, or rushing to call for a new one. Today, let's make it clear how to rectify this situation.

First, don't panic when you find a leak-90% of people make the first step wrong

As soon as the leak comes out, people instinctively panic. But you have to understand: Bellows rupture is usually a local failure and will not blow up instantly. The right first step is —Valve closing, pressure relief, isolation。 Whether it is a gas bellows or a metal hose for steam, first close the upstream and downstream valves, discharge the medium, and let the system pressure return to zero. Don't screw bolts or touch damaged parts under pressure. The pressure will tear big cracks and even hurt people.

After closing the valve, don't rush to dismantle it. Ask yourself three questions: What media is leaking? What's the temperature? How high is the pressure? Gas leakage should be prohibited from any fire source, high-temperature steam must wait for cooling, and corrosive media must be protected. If you want to use this information later, write it down first.

2. Investigation of the cause of rupture: fatigue, corrosion, or forgot to remove the tie rod during installation?

When many people see that the bellows is broken, they directly decide that it is a quality problem. Tsk, not necessarily. I've seen more than half of the early ruptures, which are good things done when installed-such asExpansion joint tie rod nut not adjusted in place, or more commonly: the tie rod is not removed at all.

You asked what will happen if the tie rod is not removed? The expansion joint should have absorbed the displacement, but it was locked by the tie rod. The thermal expansion force of the pipe was fully pressed on the bellows, so it was strange that it didn't crack. So troubleshooting step one: check the tie rod and screw. Someone asked in the question and answer of this site, "Does the screw of the expansion joint need to be disassembled?" The standard answer is: the transport tie rod of the axial expansion joint must be disassembled or loosened after installation, unless it is a tie rod with limit function. How many scenes have you seen where the pipes are connected to SAIC and the tie rods are still screwed?

In addition to installation issues, there are several reasons for the crack:

• Fatigue failure— — The pipeline starts and stops frequently or vibrates, the bellows expands and contracts repeatedly, and fine cracks appear, which slowly develop into perforations. Such cracks are usually distributed transversely along the trough.
• Corrosion perforation— — The medium contains chloride ions or acidic gas, and the material of the bellows is wrong. For example, the gas pipeline is made of ordinary stainless steel, which is quickly pitted through. PTFE-lined hoses or PTFE compensators are more resistant in such situations.
• Stress corrosion cracking— — High temperature + chloride environment, stainless steel bellows will crack in a few days. Common in power stations or chemical industries.

Look at the crack shape. Fatigue cracks resemble hair strands, subtly parallel; Corrosion holes are irregular round holes; Stress corrosion cracks are like branches bifurcating. If you are not sure, cut a section and send it to metallographic analysis, and you will pay dozens of dollars to report it.

Third, selection and replacement are not patting the head-how to pick in high temperature, high pressure and large displacement occasions

After finding out the reason, here comes the key point: what kind to change? Don't go to the hardware store and just buy one. The selection must match the actual working conditions, otherwise it will be a short-lived ghost.

• Temperature: Ordinary general-purpose corrugated expansion joint temperature resistance is only a few hundred ℃, more than 500 ℃ can be usedHigh temperature axial expansion jointWith insulation inside. If the medium exceeds 1000℃? Forget about it, use non-metallic expansion joints (fabric fiber expansion joints), lined with refractory fibers.
• pressure: High-pressure steam pipes (such as above 10MPa) must be made of multi-layer stainless steel bellows, which can't be carried by a single layer. At this timeStraight pipe pressure balanced expansion jointOrExternal pressure single axial expansion jointIt is more reliable and can balance the internal pressure and thrust.
• displacement amount: Large displacements don't expect to rely on one expansion joint alone. The pipeline has many turns and large thermal expansion, so considerCompound hinge transverse expansion jointOrRotary compensatorThey can absorb angular displacement or torsion and reduce the burden on the bellows.
• Medium corrosivity: Acid, lye? choosePTFE-lined hoseOrPTFE compensator, PTFE corrosion resistance is almost universal. Flue gas pipes contain sulfur?Desulfurization flue gas baffle doorAddNon-metallic expansion jointCombinations of are common.

Cement plant clinker conveying pipeline, temperature 400℃, pressure 0.5MPa, axial displacement 80mm. useHigh temperature axial expansion jointAdd the guide tube, note that the direction of the guide tube must follow the flow direction of the medium (it is said in the question and answer of this site that "the direction of the arrow of the expansion joint refers to"), and it will be blown directly if it is installed backwards.

Fourth, after changing it, it's over? These installation details will last you three to five years longer

If you choose the right one, it will still be useless to install a rollover. Let's talk about some of the easiest pits to step on:

• Check the tie rod before installation: The transport rod must be loosened or removed, but the limit rod must be adjusted to the predetermined position. TakeLarge tie rod expansion jointFor example, you have to adjust the nut to the design length according to the drawing, otherwise the compensation amount will not match.
• Cold-tight pre-deformation: When the pipe temperature is high, the installation should be pre-stretched or pre-compressed. For example, in steam pipelines, the expansion joint is stretched for a section at ambient temperature, and the bellows returns to the neutral position when the pipeline is thermally expanded, doubling its life. The amount of pre-deformation has to be calculated, don't screw it by hand feel.
• Don't install the guide tube backwards: The function of the guide tube is to protect the inner wall from being washed, and the arrow points to the flow direction of the medium. Installed backwards, the medium directly collides with the bellows and wears out in less than a year.
• Don't be lazy about supporting hangers: The bellows cannot bear excessive weight, and a guide bracket must be set nearby, so that the pipe can only move axially, and cannot twist or sag.

And guess what? Many factories put on new ones and leaked less than two years after running. Looking back, it was checked that the support and hanger were not fixed properly, and the pipeline sag and the expansion joint was fractured. Details determine longevity, don't believe it.

5. Several heart-breaking questions: Can you weld? How many years will it last? When should I change it?

Q: Can the bellows be welded if it is broken?
A: Basically not. The wall of the bellows is thin (1~1.5mm), and it penetrates as soon as it is welded. Moreover, the material near the welding bead becomes brittle, and cracks appear again soon. The only exception is that only a small trachoma is broken and the medium is low pressure and normal temperature. It can be made up by argon arc welding with a small current, but this is an emergency measure and must be replaced afterwards. Formal practice: change components directly, don't repair them.

Q: How many years does it generally last?
A: There is no way to give dead numbers. For example, the national standard requirement of gas bellows (household use) is not less than 8 years, and the industrial design life is usually 10~15 years. However, it depends on the actual working conditions-it may take less than 5 years under high temperature and high pressure, but it can be used for 20 years at low temperature and normal pressure. In the question and answer of this site, someone asked "the service life of expansion joint", and the core is one sentence: look at the number of fatigue times and working cycle. If it starts and stops once a day, the expansion joint with less wavenumber may crack 10,000 times; If it runs steadily all the time, it will last much longer.

Q: When should I take the initiative to change?
A: Don't wait for a leak before replacing it. During the inspection, it is found that there are corrosion pits, discoloration (overburning), surface cracks on the surface of the bellows, or the guide tube falls off or the tie rod is loose and deformed, so it is time to arrange replacement. Also, during the overhaul, take a measuring tool to check whether the wave pitch of the bellows is uniform. If a certain wave pitch is compressed or stretched to the limit value, it means that it has reached the end of life.

After so much, the core is one sentence: don't panic when the bellows breaks, isolate it first, then check it, then select it, keep an eye on the installation details, don't weld it blindly and don't make do. Whether the pipe in your hand is used for a long time or not depends on whether these gates are strictly guarded or not.