Why is the brace said to be the "invisible partner" of the expansion joint?
Anyone who has done pipeline engineering knows that no matter how well the metal expansion joint itself can be carried, it is useless if the bracket is not strong. The bellows expands, contracts, twists and vibrates at high temperature, all by the bracket to restrain and guide. Without a reasonable bracket, the bellows will deflect, become unstable and even crack in minutes. Two days ago, a customer called and said that there was a machine in their factoryUniversal corrugated expansion jointIt leaked after less than half a year of use. When I disassembled it, the gap between the guide frame was 5mm, but it should actually be 2mm. As a result, the bellows was forcibly twisted. You say it was wrong or not?
The stent is like the "invisible partner" of the expansion joint, inconspicuous but indispensable. Only by choosing the right bracket and installing the bracket can the expansion joint really play its compensation function. Otherwise, even if you pay a lot for itLarge diameter thick wall expansion jointOrHigh temperature axial expansion jointStill scrapped in advance.
Three basic roles of scaffolds
The bracket is not just welded to an iron frame. According to function, there are three main categories:
- Fixing frame-Hold the pipe firmly at a certain point and keep it from moving. It takes the thrust of the pipe and distributes the displacement to the various expansion joints. LikeStraight pipe pressure balanced expansion jointOn both sides, fixing frames must be set, otherwise the pressure thrust will collapse the system.
- guide frame-Only allow the pipe to slide in the axial direction and limit radial offset. When the gap is large, the pipe will sway, and when the gap is small, it will get stuck. Generally, the guide clearance is controlled at 2~4mm, and the high-pressure steam pipeline should even be 1.5mm.
- sliding frame(Also called support frame) -Holds the weight of the pipe and allows it to slide freely when the temperature changes. The sliding surface should be treated with low friction, such as polytetrafluoroethylene backing plate, otherwise the friction force will break the bellows.
Each of these three roles is in charge of its own business, and one is indispensable. Look at those cases of running and leaking, nine times out of ten, the fixing frame is loose, the clearance of the guide frame is wrong, or the sliding frame is stuck.
Actual selection: How to build a bracket according to the direction of the pipeline and the type of expansion joint?
Before choosing a brace, find out what expansion joint you have in your hand. Different products, bracket requirements are much different.
- Universal corrugated expansion joint-Most commonly, it mainly absorbs axial displacement. It is necessary to set a fixed frame at both ends and a guide frame in the middle, and the spacing is calculated according to the stiffness of the pipeline. For example, the steam pipe of DN400 usually has a distance of no more than 4 meters between the guide frames.
- Large tie rod expansion joint-It carries a tie rod itself, which can absorb lateral displacement. The function of the tie rod is to bear the internal pressure thrust, so the fixing frame can be simplified, but the guide frame cannot be saved. Do you have to remove the tie rod nut during installation?Refer to the FAQ of this site "Does the screw of the expansion joint need to be removed"— — For the large tie rod type, the transport screw needs to be disassembled after installation, but the tie rod body cannot be disassembled, otherwise the constraint will be lost. A lot of people are planted here.
- Direct buried (fully buried) type expansion joint— — Buried underground, the bracket is made into a fixed pier or sliding pier. This type of product requires more guidance because backfill soil will hold the pipe down. We usually recommend setting a fixed pier at each end of the directly buried expansion joint and setting a guide support every 6 meters in the pipe trench.
- Compound hinge transverse expansion jointOrCurved tube pressure balanced expansion joints-they can absorb multi-directional displacement by themselves, and the bracket arrangement should match the hinge direction. For example, the hinge axis must be perpendicular to the displacement direction, and the fixing frame must be arranged on the hinge side.
You see, the selection is not a formula, it has to be combined with the direction of the pipe (horizontal, vertical, or many elbows) and the medium temperature. For example: the cement industryMetal Corrugated Expansion Joints in Cement IndustryThe pipe is often thick and hot, so the bracket has to be reinforced, and the sliding frame uses roller type instead of skateboard type, otherwise the friction will be too great.
Installation Minefield: Should the tie rod nut be removed? How much guide clearance is left?
"How do you adjust the tie rod nut of the expansion joint?" – this is the most asked question in the field. Simply put:The tie rod nut (locking nut) for transportation must be removedOtherwise, the expansion joint cannot be freely expanded and contracted. However, the working tie rod (such as the large tie rod type) is kept, and the amount of pre-stretch should be adjusted well during installation. How to tune it? refer toHow to adjust the tie rod nut of expansion jointThat question and answer, I won't go into details here.
How much guide clearance is left? According to different media:
-Steam and hot water pipeline: 1.5~2mm
-Flue gas and air duct: 2-3mm
-Corrosive media (e.g. desulfurization flue gas, withDesulfurization flue gas baffle doorMatching): 3~4mm, also consider the thickness of anti-corrosion coating.
Note: If the gap is too small, it will easily get stuck, and if the gap is too large, it will lose its guiding function. Don't pat your head by your feeling, but follow the design drawings.
Another minefield is the distance of the fixture from the expansion joint. Many drawings say "the fixing frame should be located on one side of the expansion joint", but how close is it? Specification requirements: The end face of the fixing frame away from the expansion joint shall not exceed 1 times the nominal diameter (DN). For example, the expansion joint of DN500, the distance of the fixing frame cannot exceed 500mm. Far away, the bending moment of the bellows will be greater.
They loadedExternal pressure single axial expansion jointWith a fixture distance of 1.2 m (DN400), the bellows showed bulging after two weeks of operation. After checking, the bending moment exceeded the standard and the position of the fixing frame was wrong.
Routine inspection and maintenance: Before leakage, bracket status is the first alarm signal
Many operation and maintenance personnel only stare at the expansion joint body to see if there is any leakage. In fact, the state of the bracket exposed the problem earlier. Think about it, when the pipe expands and contracts, if the bracket is loose and offset, the bellows will be eccentrically loaded, which will leak for a long time.
How to check? Three steps:
- See if the retainer is displaced. Ranging with a level ruler or laser, record the initial position. Measure once every six months. If the offset exceeds 5mm, it means that the bolts of the fixing frame are loose or the foundation sinks.
- Touch guide frame clearance. Use a gauge to measure, if it is more than 1mm larger than when installed, it means that the fixing frame does not control the direction, or the guide rail is worn.
- Listen to the sliding frame sound. Is there a "crunch" rubbing sound when running? If any, it means that the sliding surface is poorly lubricated or stuck. Hurry up to clean and grease.
Also, don't forget to check the anti-corrosion of the bracket. Used inDesulfurization flue gas baffle doorOrNon-metallic expansion jointThe bracket next to it, corroded very quickly. We met a power plantRectangular non-metallic expansion jointOnly half of the support was corroded, but no one noticed it until the pipe collapsed.
The bracket is cheap, but it is the "skeleton" of the entire plumbing system.Support for metal expansion jointOnly when you choose the right, install it and check it diligently can the expansion joint work peacefully for more than ten years. Otherwise, the money for replacing the expansion joint is enough to buy dozens of brackets, so why bother?