How to choose flue expansion joint? Step-by-step teaching you to avoid 5 pits
Two days ago, a customer complained to me, saying that the flue expansion joint in their factory cracked after less than half a year, the weld collapsed, and the smoke went straight out. Only when I asked did I know that when I selected the model, no one cared what medium was in the flue. I only looked at the same size and installed it. And the result? High-temperature corrosion plus displacement is incorrectly calculated, and the whole system has to be stopped for replacement, resulting in a loss of more than 100,000. You say it was wrong or not?
So today let's break it up and talk about itHow to consider expansion joints in flueThis thing. Follow the following five steps to ensure that you go from entry to mastery, and avoid those minefields that hurt people without paying for their lives.
Step 1: First, find out the "temper" of the flue-temperature, pressure, medium, none of them can be missed
Many people come up and ask, "Is there any expansion joint of DN800?" This is equivalent to going to the hospital and directly telling the doctor, "Prescribe me a medicine"-the doctor has to know what your illness is first! So is the flue. You have to figure out three things first:
- Temperature:What is the highest degree? Is it a constant heat or an instant shock? For example, the exhaust temperature of the flue of the power plant may be above 400℃, so you have to chooseHigh temperature axial expansion jointOrCorrugated expansion joint for power station industry； If the preheater is exported from the cement industry, the temperature is higher,Metal Corrugated Expansion Joints in Cement IndustryThat's the right answer. Remember, the temperature resistance of non-metallic expansion joints generally does not exceed 300℃. If it exceeds this number, don't hold it hard.
- Pressure:Positive or negative pressure? What is the pressure difference between the front and rear of the smoke baffle door? There's a lot of pressure to considerStraight pipe pressure balanced expansion jointOrExternal pressure single axial expansion jointOtherwise, the bellows will bulge for you.
- Media:Are there any corrosive components in the smoke? The wet flue gas after desulfurization contains sulfide, which can not be carried by ordinary metals. At this timePTFE-lined hoseOrRubber PTFE compensatorThat would come in handy. If there are still dust particles, the guide tube must be fitted-which is more detailed in Step 4 later.
Can't get accurate data? Be conservative, choose according to the highest temperature +1.5 times the safety margin, which is better than blowing up.
Step 2: Depending on the flue shape and space, lock in the expansion joint type – metal rectangular, non-metal or round?
This step is actually quite simple, just look at the cross-sectional shape of the flue. Circular flue with circular expansion joint, square/rectangular flue with rectangular. But the question is: Where are there so many standard sizes readily available?
Metal rectangular expansion jointSuitable for large section flue, can withstand higher pressure, but also high cost. If the flue space is limited, or the amount of displacement is large, such as a short distance between two devices but a large thermal elongation, then considerNon-metallic expansion joint (fabric fiber expansion joint)It is soft, has a strong ability to absorb displacement, and is cheap. The disadvantage is that it is not resistant to high pressure and easy to age. It has to be replaced once every three or five years.
If the direction of the pipeline is complicated and there is an angular change, such as turning from horizontal to vertical, then chooseCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 Remember: Don't expect a straight cylinder to expand and save energy to handle displacement in all directions. That's not all-purpose glue.
By the way, it is often used in desulfurization systemsDesulfurization flue gas baffle doorStrictly speaking, this thing is not an expansion joint, but it is often used in conjunction with expansion joints to play an isolation role. If you need to cut off smoke from your pipes, considerElectric plug-in insulation doorOrManual plug-in insulation doorWhich is used in combination with an expansion joint.
Step 3: Calculate the displacement, don't just rely on feeling-how do axial, lateral and angular displacements correspond to products?
"The pipeline expands and contracts by heat. Isn't it enough to elongate it by a few millimeters and compress the expansion joint by a few millimeters?" Tsk, that's a beautiful thought!
Axial displacement(tube elongation and shortening),Lateral displacement(Pipe left and right misalignment),Angular displacement(tube deflection angle). Each has a corresponding product structure:
- Pure axial displacement:chooseUniversal corrugated expansion jointOrHigh temperature axial expansion joint, simple structure and affordable price.
- Large lateral displacement:UsefulCompound hinge transverse expansion joint, or twoLarge tie rod expansion jointUse in combination. Don't be foolish to use a single expansion joint to carry the lateral displacement, and the bellows will be twisted and scrapped in minutes.
- Both axial and transverse:Then go onStraight pipe pressure balanced expansion jointOrCompound straight pipe bypass pressure balanced expansion joint。 Why? Because ordinary expansion joints will generate huge blind plate force when absorbing lateral displacement, which will push the pipe support flying.
Then how to calculate the specific value? My experience is: ask a pipeline stress analysis engineer for data, or use software to simulate. If it doesn't work, use the empirical formula: every 10 meters of flue temperature rise by 100℃, and the axial displacement is about 12mm. Don't ask me if I am accurate. Anyway, you can't go wrong with the calculated number and multiply it by 1.2 times the safety factor to place an order.
Step 4: Don't miss the configuration accessories-deflector, tie rod, baffle door, which is what you just need?
Many customers only stare at the bellows when buying expansion joints and ignore the accessories. But you know what? Accessories are life-saving things.
Guide tube (also called liner tube): Installed inside the expansion joint, the function is to allow the airflow to pass smoothly and avoid direct flushing of the bellows. Especially dusty smoke, without the guide tube, the bellows will be worn out in half a year. For specific functions, you can take a look at the question and answer of "Specific Functions of Expansion Joint Guide Tube" in our station, which is accompanied by a picture, and it is clear at a glance.
Tie rod:It is divided into small tie rod and large tie rod. Small tie rods are used to limit excessive axial displacement and protect bellows; The large tie rod is used to withstand the pressure thrust and prevent the pipe from falling off. The method of adjusting the tie rod nut is in the common questions and answers. Simply put, it is: don't screw the nut before installation, leaving a margin for pre-tensioning/pre-compression.
Flap doors:If the flue needs to be overhauled or switched, matchingFlue gas baffle doorOrRound Flap Door (Double Seal)。 Note that a sufficient distance should be left between the baffle door and the expansion joint, otherwise the vibration generated when the baffle door opens and closes will affect the life of the expansion joint.
Also, is the medium liquid or vapor? That has to be consideredRotary compensatorOrSleeve type pipe expansion joint。 However, the flue is usually free of liquid, so I won't say much.
Step 5: Install and preload. This step is wrong and the front is all in vain-whether the screw is disassembled or not, and what is the direction of the arrow?
"Does the screw of the expansion joint need to be removed?" Article 14 of our FAQ specifically says:The screw cannot be removed before installation, it is used to pre-stretch or pre-compress. Specific operation: First, adjust the nut on the screw according to the designed displacement, so that the expansion joint is in a pre-deformed state, then install it on the pipeline, and then loosen the screw after spot welding and fixing it (some structures need to be dismantled, and others are kept as limits).
In addition, there is often an arrow on the expansion joint cylinder, and the direction is pointing to the direction of the medium flow. Stop acting backwards!The direction of the arrow of the expansion joint refers to the direction in which the medium flows(FAQ#10). If the guide tube is installed backwards, it will not play a protective role, and the airflow directly washes the root of the bellows, which is easy to crack.
Be sure to pay attention to coaxiality when installing. If the flue is slightly misaligned, useSpecial hose for vacuumOrMetal hoseAdjust it. Don't break the expansion joint hard. Consequences of a hard break? You will know by reading the article "The Correct Installation Method of Large Tie Rod Expansion Joint"-it ranges from air leakage to twisting the whole expansion joint into a twist.
Well, after these five steps, you should know how to consider the expansion joint in the flue. If you are still unsure, come to us directly with the parameters, we haveMetal rectangular expansion joint、Non-metallic expansion joint、Round flapper doorWait for a full range of products, help you match one-on-one. Afraid of trouble? Then spend more time figuring out the parameters of the first step, and leave the rest to professionals. After all, the maintenance savings are enough for several barbecue meals, right?