Where exactly must the flue be fitted with expansion joints? Teach you judgment step by step

Find out first: Why should the flue be fitted with expansion joints? What happens if you don't pretend?

Two days ago, a customer called in a hurry, saying that the flue in their factory had just been running for three months, and the interface began to leak, and even cracks could be seen. I asked him if he installed the expansion joint in the first place. He froze for a moment and said, "Isn't that thing dispensable?" Alas, this is the typical lesson.

The flue, first running high-temperature smoke and second running corrosive gas, will expand when the pipe is heated, and it will expand by several centimeters. If the expansion joint is not installed, the pipeline will carry it itself hard, and the consequence will be two words: stress. Stress saved to a certain extent, weld cracking, flange deformation, equipment suffocation, all of which are enough for you to drink a pot. So, what flue needs expansion joints? The answer is: as long as the flue has temperature change, vibration and installation error, it basically has to be installed. But which kind to install and where to install has to be judged step by step.

I'll walk you through it step by step below. If you follow it, you won't overturn.

Step 1: Look at the temperature – Which expansion joint to choose for high temperature flues?

Temperature is the number one deciding factor. You first figure out the maximum operating temperature of the flue, and then get in your seat.

• Below 100℃: Ordinary working conditions, such as hot water and low-temperature flue gas, userubber compensatorOrRubber PTFE compensatorIt's cheap and easy to install. However, note that rubber is afraid of oil and acid and alkali, so don't use these things in the medium.
• 100℃~400℃: This is the most common interval, such as steam pipes, boiler flues. PreferredUniversal corrugated expansion jointOrCorrugated expansion joint for power station industryThe material is 304 stainless steel, which is temperature-resistant enough. If the pipe is under high pressure, it can be put onExternal pressure single axial expansion jointWith better stability.
• 400℃~800℃: High-temperature flue gas comes, such as kiln outlet and catalytic cracking flue. At this time, ordinary stainless steel can't bear it, so it has to be usedHigh temperature axial expansion jointAdd thermal insulation inside, or simply chooseMetal corrugated expansion joint (the kind used in cement industry)。
• Above 800℃: Extremely high temperatures, metal bellows can use metal expansion joints (inner castable), orNon-metallic expansion joint (fabric fiber expansion joint)。 This thing has a temperature resistance of 1200℃, and is particularly resistant to thermal shock. It has weak pressure bearing capacity and can't be used in high-pressure systems.

Tips:Never use ordinary rubber parts for high-temperature flues, which will burn in a few minutes. In addition, if it is a square flue, skip this step directly and look at the third step, because square flues mostly use non-metal or metal rectangular expansion joints.

Step 2: Look at the medium-how to choose corrosive flue gas?

After reading the temperature, then see if there is "poison" in the smoke. Common corrosive media include sulfur dioxide, hydrogen chloride, hydrogen fluoride, organic solvent vapor, etc. And guess what? Many customers only look at the temperature and ignore the medium. As a result, the expansion joint is perforated in one or two years.

To deal with corrosive smoke, there are several tricks:

• Weak corrosion (e.g. low SO₂ concentration)Expansion joints made of stainless steel 304L or 316L, such asUniversal corrugated expansion joint, moderately priced. However, note that stainless steel is afraid of chloride ions (such as hydrochloric acid). If there is a lot of chlorine, don't use stainless steel.
• Strong acids and bases (sulfuric acid, hydrochloric acid, sodium hydroxide): directly withPTFE-lined hoseOrPTFE compensator。 Tetrafluoride can withstand almost all strong acids, but the upper temperature resistance limit is only about 200℃, so it can be used in high temperature and strong acid environmentMetal expansion joint + lining with PTFEThe composite structure of.
• Particulate dust (such as flue gas from cement plant and power plant): There are hard particles in the flue gas, and high-speed flushing will wear the bellows. At this time, you must addguide tube(a protective sleeve inside the bellows), withMetal rectangular expansion jointOrLarge diameter thick wall expansion joint。 Note that the direction of the guide tube should be consistent with the direction of the airflow. If it is installed backwards, it is equivalent to not being installed.

Many times, corrosion and high temperatures coexist. What to do? PrioritizationNon-metallic expansion jointBecause its skin material (fluororubber, silicone rubber composite cloth) can be both temperature-resistant and corrosion-resistant, and the replacement cost is low. The disadvantage is that it needs to be checked regularly, and the skin should be changed once every 2~3 years.

Step 3: Look at the size and displacement-what about large flues and square flues?

Flues are not only round pipes, but also many square, rectangular, and even shaped cross sections. Then the question arises: What is the difference between selecting expansion joints for circular flues and rectangular flues?

circular flue: The size is not large (within 500mm in diameter), useUniversal corrugated expansion jointJust do it. If the diameter is more than 1 meter, it is called a large-diameter flue, and it must be usedLarge diameter thick wall expansion jointOrCompound hinge transverse expansion joint。 Why? Because the diameter is large, a bellows alone absorbs the displacement, and the stiffness is too large, which is easy to become unstable. The complex structure can disperse the amount of displacement.

Rectangular/square flueNine times out of ten, this is the choiceMetal rectangular expansion jointOrRectangular non-metallic expansion joint。 Metal rectangular parts are suitable for high temperature and high pressure, but the shell is thick and heavy. Non-metallic rectangular parts are lightweight, corrosion-resistant and low-cost, but the bearing pressure generally does not exceed 0.1MPa. If your flue is square to round, with many turns, give priority to non-metallic ones, which have a large degree of freedom of installation.

In addition, the displacement type should also match the expansion joint. Simply put:

• Only doaxial expansion and contraction(the pipe goes straight): withAxial expansion joint, such asStraight pipe pressure balanced expansion jointCan eliminate the blind plate force.
• haveLateral displacement(Pipe Lateral Offset): UseCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。
• haveAngular displacement(at the corner of the pipe): Usehinged expansion jointOrRotary compensatorThe latter is suitable for overhead pipelines.

Note:If your flue is buried in the ground (such as a direct buried pipeline), then don't use regular goods, go aheadDirect buried (fully buried) type expansion joint, it has protective sleeve on the outside, not afraid of earth pressure and groundwater.

Special circumstances: If the flue needs to be isolated for maintenance, don't forget to match the baffle door and expansion joint

This step is a "hidden level" that many designers will miss. What's the situation? There are equipment in your flue system that need regular overhauls, such as fans, dust collectors, desulfurization towers. During maintenance, this section of flue must be separated from the general system, otherwise the smoke will flow back, and it will be dangerous for people to enter.

At this time, the expansion joint alone is not enough, but it has to be fitted next to the expansion jointFlue gas baffle door。 There are many types of flapper doors:

• Single-axis double-flapper door: Most common construction, manual or electric operation, sealing is average. For occasions where isolation is not strictly required.
• Double-sealed single-axis circular baffle door: Two seals, with blowing and scavenging in the middle, can achieve zero leakage. It is used in desulfurization and denitrification flue to prevent toxic gas leakage.
• Electric plug-in insulation door: Plug-in type, completely cut off the channel, sealing by pressing the rubber strip. Suitable for low pressure flue, completely isolated during maintenance.
• Manual plug-in insulation door: Same as above, just manual driven, cheap and suitable for small calibers.

Expansion joint in front (near heat source), baffle door in rear (near service equipment)。 In this way, the expansion energy saving absorbs the displacement of the pipeline, and the baffle door can be cut off. Never do the other way around, otherwise the baffle door is deformed by heat and will not be closed tightly.

Summary: Give you a three-step judgment process, just follow it

Okay, after so much verbose, let me give you a fool's process. Next time you encounter the flue expansion joint selection, go through it in order:

1. Measure temperature, medium, caliber:Take note of these three parameters. The temperature is ≤400℃ and there is no strong corrosion, and the general-purpose corrugated expansion joint or metal expansion joint is preferred; Temperature> 400℃ or strong corrosion, preference is given to non-metallic expansion joints or PTFE/superalloy lined parts. The large square flue jumps directly to the non-metal or metal rectangular expansion joint.
2. Calculate the displacement amount and direction:Use axial type for large axial displacement, compound hinge for large transverse displacement, hinge or rotational compensator for angular displacement. If the pipe diameter is large (> 1m), choose double or straight pipe pressure balance type.
3. See if you need to quarantine:If there is a maintenance need, install a flue gas baffle door (double seal or plug-in plate type) next to the expansion joint. Pay attention to the order: the expansion joint depends on the heat source, and the baffle door depends on the equipment.

If you follow this process, there will be no problems in nine of ten projects. The remaining one, with a high probability, forgot to add the guide tube or chose the wrong material. Tsk, then I can only flip back and read the second step again.