Why is the position of flue expansion joint always a headache?
To be honest, who in our business hasn't been tortured by flue expansion joints? Two days ago, a customer of a power plant called to complain that their flue system was only in operation for half a year, and the expansion joint leaked. When I removed it, the bellows tore directly. I asked him where the expansion joint was installed, and he said it was installed behind the fan outlet elbow. Alas, here's the problem-the wrong location is chosen, and no matter how expensive the expansion joint is, it can't handle it.
The flue is filled with high-temperature smoke, which is often three or four hundred degrees, and the amount of thermal expansion is not a small number at all. A 50-meter-long flue, whose temperature rises from normal temperature to 400℃, has an axial elongation of more than 250 mm. Such a large displacement must be digested by the expansion joint. But if you randomly find a place to plug it, the displacement direction is not correct, or the compensation amount is not enough, the result will be leakage, deformation, and even the whole pipeline system collapse.Flue expansion joint arrangement positionThis matter, to put it bluntly, is a systematic project, which requires calculation, thought and experience.
Did you put the few "golden" layout points that are common in flue systems right?
Let's talk about a few recognized reliable positions first, and you can compare them to see if there are any omissions.
- Near boiler/chimney outlet:This is the starting point of the heat source, and the thermal displacement is the largest. An expansion joint that can absorb axial and radial displacement must be installed. For selection, for high temperature zoneMetal rectangular expansion jointOrHigh-temperature axial expansion joint, specifically see the cross-sectional shape of flue.
- Fan inlet and outlet pipes:The fan vibrates greatly, and the expansion joint not only absorbs thermal displacement but also isolates vibration. The position should be as close to the fan flange as possible, leaving room for maintenance. Recommended useUniversal corrugated expansion joint orCompound hinge transverse expansion jointAvoid the hard connection from transmitting stress to the fan.
- Equipment connection:For example, desulfurization tower, dust collector, flue gas baffle door and other equipment import and export. There are often special-shaped parts and valves in these places, and rigid connections can easily lead to flange leakage. InInstall one front and one back of the desulfurization flue gas baffle doorNon-metallic expansion joint (fabric fiber expansion joint)Is not only corrosion resistant, but also absorbs multi-directional displacement, and is convenient for maintenance.
- Front and rear straight sections at pipe turns:The elbow itself can absorb a portion of the thermal displacement, but limited. Set a fixed bracket to guide the displacement to the straight section, and install expansion joints on both sides of the elbow. Note: At least two expansion joints are needed at the elbow to cooperate, and one single one will die miserably-this will be talked about later when we talk about the pit.
What is the difference between the position selection of metal expansion joint and non-metal expansion joint?
This problem is easily confused by many engineers. In fact, the "temper" of metal and non-metal in the flue is completely different, and the arrangement logic is naturally different.
Metal expansion joint(e.g.Metal rectangular expansion joint、Corrugated expansion joint for power station industry、External pressure single axial expansion joint) is resistant to high temperatures and pressures, but has limited ability to compensate for displacements – particularly lateral and angular displacements. So that metal expansion joint is more suitableStraight pipe sectionOr when the lateral displacement is constrained by the guide bracket. If the flue direction is complicated, it must be usedCompound straight pipe bypass pressure balanced expansion jointOrCurved tube pressure balance expansion jointTo balance the blind force. But in the final analysis, the "fault tolerance" of metal is low, and the position deviation of a few centimeters may cause stress concentration.
Non-metallic expansion joint(e.g.Non-metallic expansion joint (fabric fiber expansion joint)、Rectangular non-metallic expansion joint) is different-it can withstand large multidirectional displacements, and it is corrosion resistant and insulated well. The disadvantage is that it cannot withstand high pressure. So that that non-metallic expansion joint is particularly suitable for mounting onCorrosive flue gasAreas (such as desulfurization systems),Large section rectangular flueas well asEquipment import and export。 And guess what? Many power plants attach non-metallic expansion joints directly to the side of the flue gas baffle door, and directly remove the expansion joints when inspecting the baffle door, which saves time and effort.
Flue expansion joint arrangement positionThe selection is directly related to the temperature and pressure of the medium. For flue gas exceeding 600℃, non-metallic expansion joints basically bid farewell, and obediently use metal and lining insulation layer. On the contrary, when the temperature is low and a large amount of compensation is required, the non-metal is more cost-effective.
I have summarized three of the easiest pits to step on when setting up flue expansion joints
Over the years, I have seen many cases of "rollover". The following three pits can be met in at least seven out of ten projects.
Pit 1: Improper setting of fixed bracket and guide bracket.Some designs only want to install expansion joints on straight sections, but forget to set fixed brackets at both ends of the expansion joints. As a result, the expansion joint not only bears axial displacement, but is also pulled by lateral forces-it is strange that it is not bad. The fixed bracket must be able to withstand the blind plate force of the pipe, and the guide bracket must ensure that the pipe only expands and contracts along the axial direction. Remember:The expansion joint itself does not bear that weight of the pipeSupport is a matter of brackets.
Pit 2: Cold tight pre-stretching is ignored.High-temperature flue is installed with low ambient temperature and high temperature during operation. If pre-stretching is not done, the expansion joint will be over-compressed during operation. The amount of pre-stretch should be about 50% of the thermal displacement (elongate at low temperature and return to the intermediate position at high temperature). This value is wrong, and the life of the expansion joint is greatly reduced. Check it outMetal expansion joint weight tableOr the installation instructions provided by the manufacturer can know the specific tensile value.
Pit 3: Install only one expansion joint at the elbow.The flue elbow itself has asymmetric stiffness, and a large moment will be generated when it expands thermally. If only one expansion joint is installed, it will be forced to withstand bending and twisting, and the bellows will soon crack with fatigue. Correct practice: Install an expansion joint on both sides of the elbow, separated by a fixed bracket in the middle, so that each expansion joint only absorbs one-way displacement.
When encountering a flue with complicated directions, how to judge where to install the expansion joint?
There is no standard answer to this question, but there is a proven methodology. You have to do pipe stress analysis first-it is recommended to use CAESAR II or similar software, model and calculate once, which is much more reliable than empirical guessing. If there is no software, an empirical formula can be used to estimate: first draw the flue path, mark all the fixed bracket positions, then calculate the amount of thermal expansion between adjacent fixed brackets, and then distribute it to the expansion joint. The amount of compensation per expansion joint cannot exceed its allowable value (refer to the product datasheet).
A flue first walks 20 meters horizontally, then 90 degrees elbows up 15 meters, and then folds back 10 meters horizontally. This "Z" direction requires at least one expansion joint to be arranged on each straight section: the horizontal section is used forUniversal corrugated expansion jointAbsorbing axial displacement, for vertical sectionsStraight pipe pressure balanced expansion jointOrCompound hinge transverse expansion jointTo absorb lateral displacement, brackets and expansion joints should also be set before and after the elbow. If space is constrained, consider usingRotary compensatorTo absorb multidirectional displacement, but at a higher cost.
Also don't forget the thrust of the device interface. Such asRound Flap Door (Double Seal)This type of valve has great rigidity, but the pipe thrust may deform the sealing surface of the valve. Therefore, an expansion joint is installed on both sides of the valve, and a limit pull rod is installed at the same time-that is,expansion joint tie rodAction of – Prevent over-stretching.
To summarize: Choose the right location, the flue system can be less faulty
After saying so much, it was actually one sentence:Flue expansion joint arrangement positionIt is not just a matter of installing it, it directly determines how long the flue system will last. Core principle-the fixed bracket sets the boundary, the expansion joint is evenly distributed on the straight line segment, and the elbow and interface are treated separately. Metals and non-metals take their own strengths, don't forget to do cold tight pre-stretching.
If you encounter an uncertain layout on your project, you may wish to look through the product information of this site, such asMetal rectangular expansion joint、Non-metallic expansion joint (fabric fiber expansion joint)、Corrugated expansion joint for power station industry、Desulfurization flue gas baffle doorAnd so on, there are detailed installation instructions. If it doesn't work, go directly to the manufacturer's technical support-don't guess for yourself, after all, the flue burst is no joke.
Did the retaining bracket come loose the last time you checked the flue expansion joint? Hurry up and confirm, maybe it can save you a maintenance cycle.