Metal expansion joint of connecting rod ≠ common tie rod: structural disassembly and core function
Don't be in a hurry to place an order. Many people regard the metal expansion joint of the connecting rod as a "bellows with several rods". This cognition is still far from the truth. The core of a real connecting rod metal expansion joint lies in those few rods-not decoration, not stiffening ribs, but rigid restraining elements.
Bellows body + connecting flanges at both ends + tie rods evenly distributed along the circumference (usually 2, 3 or 4) + spherical washers and locking nuts. The key lies in the cooperation between the tie rod and the spherical washer: the tie rod only limits the displacement of the bellows during axial tension or compression, but does not limit the lateral displacement and angular displacement. That is to say, when the pipeline expands and contracts thermally, the bellows can expand and contract freely, but it will not be excessively elongated to crack the corrugations, and it will not be pushed by external pressure to become unstable.
What is the difference between it and ordinary tie rod expansion joint? The poles of ordinary tie rods (such as large tie rods) are directly locked with nuts, and the poles themselves also assume the guiding function. The lateral displacement of bellows is realized by the hinge structure. The tie rod of the metal expansion joint of the connecting rod is equipped with spherical washers at both ends, which allows the rod to swing at a slight angle-which gives the bellows the lateral and angular compensation ability. Is that the truth? Therefore, it is particularly suitable for those pipes that absorb both axial displacement and have a small amount of lateral misalignment or deflection.
Which operating conditions must have a connecting rod structure? Actual combat scenarios from power stations to air-cooled islands
And guess what? Two days ago, I met a friend of a dry power plant project. He asked me: Is it okay to just buy a general-purpose corrugated expansion joint on the steam pipeline? I replied directly: Look at the direction of the pipeline. As long as the pipeline turns several corners, the equipment interface is slightly misaligned, or the space is limited and the hinge expansion joint cannot be installed, the metal expansion joint of the connecting rod is the "just needed product".
- Main steam pipeline of power station: High temperature and high pressure, large axial thermal displacement, but the pipeline often goes close to the steel beam, and there is no space to install large tie rods or hinge types. At this time, the metal expansion joint of the connecting rod is used, and the tie rod limits the maximum axial displacement, while the bellows absorbs the lateral deviation by themselves. It is used in many power plant feed water pump outlet pipelines and turbine bypass pipelines.
- Air-cooled island vacuum pipe: This place has drastic temperature changes, and the pipe is large in diameter and thin in wall, which is sensitive to displacement. We have one on our siteDouble hinge expansion joint for air-cooled island vacuum pipelineIt's designed for this-but if you only need one-way compensation and compact space, the connecting rod metal expansion joint is more economical instead. Its tie rod prevents the bellows from being deflated by vacuum while allowing the pipe to expand and contract naturally.
- Hot air duct in cement industry: The hot air temperature is often above 400℃, the thermal insulation layer of the pipeline is thick, and the installation error is not easy to control. With the connecting rod structure, the bellows will not be tightened even if the field flange is welded by a few millimeters.
To put it bluntly, as long as one end of the pipe is a fixed point and the other end is an equipment or pipe frame, and the axial displacement is the main contradiction, but the lateral/angular displacement can't be ignored-the metal expansion joint of the upper connecting rod is correct.
The three most easily overlooked pits in model selection: displacement, pressure and guidance
After working for more than ten years in expansion joints, I have seen more rollover scenes than successful cases. Let me list you three must-step pits in the selection:
Pit 1: The displacement amount only depends on the total compensation, not the direction combination
Many people get the form "Model and Size of Expansion Joint", and when they see the "compensation amount ±50mm", they think everything will be fine. However, the actual situation is that the axial displacement + transverse displacement + angular displacement occur in the pipeline at the same time, and the true stress of the bellows may double after the three directions are superimposed. The tie rod of the metal expansion joint of the connecting rod can only limit the axial maximum value, and the lateral and angular bearing capacity depends on the bellows itself. If your pipe is laterally displaced beyond the limit allowed by the bellows, the tie rod becomes a "suicide tool" — the pole holds up against the flange and the bellows is hard twisted. What to do? Be sure to provide a detailed pipe stress analysis report, at least giving clear displacement in each direction.
Pit 2: Pressure Level "Pats the Head" by Experience
Tsk, the most outrageous case I have ever seen: using a connecting rod metal expansion joint with a nominal pressure of 1.0MPa to connect a 2.5MPa steam pipe, as a result, the bellows burst and almost had an accident. When selecting the model, the pressure should not only look at the design pressure, but also considerPressure thrust-The internal pressure of the bellows will generate a large axial thrust, which must be borne by the tie rod. If the pressure is high, the pipe diameter is large, and the diameter and number of tie rods are insufficient, the pole will be broken directly. In the product information on our site,Straight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion jointIt is specifically solved to solve this problem, but the connecting rod structure itself can also bear part of the thrust, and the specific calculation has to be confirmed by the manufacturer.
Pit 3: Ignoring pipe guide brackets
This step goes wrong in 99% of on-site installations. The metal expansion joint of the connecting rod requires that the pipeline has enough guide brackets at both ends of the expansion joint, otherwise the pipeline will swing laterally when it expands and contracts in the axial direction, which will not bend the connecting rod. Many customers buy it back and weld it directly on the pipe, regardless of the spacing between brackets. As a result, the bellows leaked half a year later and the product was poor. The practical requirements are: the distance between the guide bracket and the end of the expansion joint should not exceed 4 times the pipe diameter, and there should be no free cantilever in the middle.
Installation site rollover case: Is the tie rod nut adjusted or not? Can the screw be removed or not?
This question ranks in the top of our FAQ (How to adjust the tie rod nut of expansion joint、Do you need to remove the screw of the expansion joint）。 Direct conclusion:
- Tie Rod Nut: Must be loosened when installed in place.Many people think that the tighter the nut, the better, but as a result, the bellows are locked, and there is no room for compensation when the pipe expands thermally. Correct practice: When installing, tighten the nut gently so that the spherical washer contacts the flange, and then retreat half a turn to one turn in reverse direction to allow the tie rod to be in a free state. If the pipe needs to be pre-stretched or pre-compressed, adjust the nut position to the design value.
- Screw: Absolutely not disassembled!Individual "smart" installers felt that the screw was in the way and dismantled it directly. Results After the pipeline was put into operation, the bellows bulged and deformed under pressure. The tie rod was originally a protective element, and it was equivalent to suicide if it was dismantled. The screw must be retained unless the design explicitly requires removal as a transport protection (which is rare).
They screwed all the nuts of the metal expansion joint of the connecting rod to death. As soon as the pipe heated up, the bellows was hard pressed by the tie rod and could not be extended. As a result, all the stress was transferred to the weld joint, and the weld cracked. You say it was wrong or not? So be sure to read the instructions before installation, don't rely on your feeling.
How to distinguish it from hinge type and large tie rod type? A comparison table makes it clear
Many people confuse connecting rod metal expansion joint, hinge type expansion joint and large tie rod type expansion joint. The names all have "rods", but the structure and function are nowhere near. Direct to dry goods:
| Type | Tie rod/hinge structure | Compensation capacity | Typical applications |
|---|---|---|---|
| Connecting rod metal expansion joint | Spherical washers at both ends of tie rod, swingable | Axial + a small amount of lateral + angular | Main steam, air-cooled island vacuum pipe and high-temperature hot air pipe of power station |
| Large tie rod expansion joint | Fixing nuts at both ends of pull rod, rigid connection | Axial only (transverse hinge fit) | Straight pipes requiring large axial compensation |
| hinged expansion joint | Hinge pin, allowing only one in-plane angular displacement | Angular (not axial) | L-shaped and Z-shaped elbows requiring orientation angle compensation |
| Compound hinge transverse expansion joint | Two hinges + intermediate connector | Large lateral displacement | Pipeline requires horizontal and lateral misalignment compensation |
Select the connecting rod in the axial direction, select the hinge in the pure angular direction, select the large tie rod for the huge axial direction, and select the compound hinge in groups for the transverse displacement.Don't get confused.
Regardless of which you choose, you must submit the actual working condition parameters (temperature, pressure, medium, displacement direction, pipe diameter, wall thickness) to the manufacturer completely. Our product line in our site is very complete, fromUniversal corrugated expansion jointToSpecial air-cooled island partsAll, but only if the parameters are accurate, we can match you with the most suitable model. Don't use "probably" or "probably" to choose a model, that is a problem with money.