Section 1: Arrow ≠ Direction of Expansion-What exactly is that arrow engraved by the manufacturer indicating?
Nine out of ten installers recognized the arrow on the metal expansion joint as the "expansion direction". And the result? It was installed backwards, the pipe blew up, and it was scrapped in advance. The arrow engraved on the bellows by the manufacturer isDirection of media flowWhich is not the direction of expansion deformation. You see what Article 10 of the question and answer says: The arrow indicates the flow direction of the medium (steam, water, flue gas, etc.) in the pipe, and the purpose is to ensure the correct installation direction of the guide tube and prevent the medium from washing the root of the bellows. The expansion joint itself is passively absorbing displacement, and it does not "expand" itself in a certain direction. Think of the arrow as the direction of expansion? That's as outrageous as treating a road sign as a destination.
For example, a steam pipe is fitted withUniversal corrugated expansion jointWhen the worker looked at the arrow to the right, he decided that the bellows could only extend to the right. After the pipeline heats up, the pipeline extends axially, and the expansion joint should be compressed axially. As a result, the worker pre-adjusts the tie rod to make it "ready to expand to the right", but the actual direction is exactly the opposite-the bellows is forced to stretch, and it leaks after several rounds of start and stop. So remember: the arrows are only responsible for directionality and have nothing to do with the type of compensated displacement.
Section 2: There are three true "expansion directions": axial, transverse and angular
The "direction" metal expansion joints really have to deal with is the type of pipe displacement. According to the displacement mode, there are three types:
- Axial displacement: The pipe extends or shortens along its own centerline. Corresponding products such asHigh temperature axial expansion joint、External pressure single axial expansion joint、Direct buried (fully buried) type expansion joint。 This type of expansion joint bellows is directly subjected to tension or compression, and a guide tube is installed inside to protect the corrugation.
- Lateral displacement: The displacement of the pipe perpendicular to the axis, such as the lateral bending of the L-shaped pipe. Corresponding products such asCompound hinge transverse expansion joint、Double hinge expansion joint for air-cooled island vacuum pipeline。 They absorb lateral offsets through two sets of bellows cooperating with a hinge mechanism, and the bellows themselves do not bear axial forces.
- Angular displacement: The pipe is angularly deflected at a node, such as a turning point in a Z-shaped pipe. Corresponding products such asCurved tube pressure balance expansion joint、Compound straight pipe bypass pressure balanced expansion joint。 They counteract pressure thrust with balanced bellows and absorb only angular displacement.
Not all expansion joints can be used casually. You install the high-temperature axial type at the elbow that needs lateral compensation, and the bellows is stressed laterally, and the weld cracks quickly-this is not a problem of "installing in the reverse direction" at all, but the wrong type selection.
Section 3: What happens if you play backwards? — — A case of steam pipeline
Tell me the truth. A DN600 steam pipeline in a thermal power plant in northern China has a design temperature of 350℃ and a length of 80 meters. The natural compensation is insufficient, so the design choosesAxial expansion joint。 To save trouble, the installation team installed the expansion joint near a tee. After the pipeline heats up, transverse thermal displacement occurs at the tee, and the expansion joint bellows is forced to bear lateral bending. How do you guess the bellows would react? Two months later, fatigue cracks appeared on the bellows crest, and steam hissed and sprayed out. Stop the furnace for maintenance, and take it down to see: the bellows is twisted into a twist shape.
If the lateral displacement must be absorbed by the expansion joint, then chooseCompound hinge transverse expansion jointOrLarge diameter thick wall expansion joint mating hinge. If you choose the wrong direction type, no matter how good the craftsmanship is, it will be useless. Many problems on the spot are not that the product quality is not good, but that the installer has mistaken the meaning of "expansion direction"-thinking that the arrow will compensate wherever it goes, and as a result, the axial type is regarded as a jack-of-all-money.
Section 4: How to Tell Which Direction Your System Needs Compensation? Three-step method
Don't rely on your feeling, follow the process:
Step 1: Look at the direction of the pipeline。 Draw a diagram of the pipe system and mark the position of the fixing bracket and the guide bracket. If the straight line segment is long, axial compensation is mainly considered; Lateral and angular displacements should be analyzed if there are L, Z or space turns.
Step 2: Calculate the thermal elongation。 Δ L = α × L × Δ T. α is the coefficient of linear expansion (about 12×10⁻⁶/℃ for carbon steel), L is the length of the tube, and Δ T is the difference between the operating temperature and the installation temperature. The calculated value is the amount of compression that the axial expansion joint needs to absorb. At the same time, use stress analysis software (CAESAR II, etc.) to calculate the additional thrust of the pipe to the expansion joint, and confirm whether it exceeds the allowable value.
Step 3: Select the corresponding model。 Large axial displacement and high pressure? chooseExternal pressure single axial type expansion joint, external pressure structure stability is good, suitable for high temperature and high pressure. Buried underneath? chooseDirect buried (fully buried) type expansion joint, with outer guard tube and anti-corrosion layer. Laterally displaced and space constrained?Compound hinge transverse expansion jointIs the standard scheme. Need to balance pressure thrust?Curved tube pressure balance expansion jointOrStraight pipe pressure balanced expansion jointCan eliminate the thrust of the main fixed bracket and reduce the civil construction cost.
Don't forget to calculate the thrust of the expansion joint on the fixed bracket。 Pressure thrust, spring stiffness thrust and friction resistance are all indispensable. Otherwise, if the expansion excerpt is selected correctly, the bracket will be pushed down and still leak.
Section 5: Be sure to check the displacement marks on the design drawings before installation
The product label says the model, nominal diameter and pressure rating, and it won't tell you "which direction should this expansion joint be installed". What can really guide the installation isDisplacement direction and displacement amount marks on design drawings。 For example, if an axial expansion joint is drawn on the drawing, and "axial compensation ±40mm" is marked next to it, then when you install it, you should ensure that the axis of the expansion joint is parallel to the center line of the pipeline, and reserve the cold tightness (pre-stretching or pre-compression).
Tie rod nuts and screws。 Q&A#12 says: Tie rod nuts are adjusted to put the expansion joint in the correct pre-deformed position when installed, not to limit orientation. Q&A Article 13 says: The screw (transport screw) must be removed after installation, otherwise the expansion joint cannot deform freely, which is equivalent to being locked. Many installers don't dismantle the screw, thinking that the iron bar is for reinforcement. As a result, as soon as the pipeline heats up, the bellows has nowhere to go, and it bulges and bursts directly.
Alas, say a thousand words ten thousand, metal expansion joint expansion direction this pit, the root of which lies in insufficient education. Arrows are not directions, displacement maps are. Next time someone points to the arrow and asks "which way to load", throw this article directly to him.