When I met many customers who did pipeline design, I asked, "How long is the telescopic rod? Give me a price."
Hey, asking this is a layman. The metal expansion joint telescopic rod looks like a pole, but if you only look at the length in the selection, there will be problems on the spot nine times out of ten. Today, I will break the door into pieces and explain it clearly.
What the hell are telescopic rods used in metal expansion joints?
Bellows generate great thrust under pressure. DN500 pipeline, 1MPa pressure, light internal pressure thrust can reach about 20 tons. If there is nothing to hold this force, the bellows will be straightened, and it will be a matter of time before the weld cracks and the corrugation deforms.
The role of the telescopic rod is to carry this thrust down. It is fixed to the connector at one end, and the other end limits the amount of displacement of the bellows through nuts and washers, so that the bellows can only expand and contract as far as the design allows. To put it bluntly, it is a safety device that "covers the bottom" of the bellows.
Telescopic bars are not intended to completely lock the bellows. It allows for axial displacement but limits excessive stretching or compression. If the selection is wrong, either the restriction is too dead to lead to insufficient compensation, or the restriction is too loose to protect.
Tie rods, hinges, telescopic rods-don't confuse the structural parts
Many peers call the tie rod and the telescopic rod mixed, but in fact, the structure is completely different.
The tie rod is generally used on the general corrugated expansion joint, which functions to restrain the internal pressure thrust and does not transmit the transverse displacement. It is common to have several poles evenly distributed around the bellows, fixed at one end and adjustable with nuts at the other.
Hinges are used in pairs, such as double hinge transverse expansion joints, which use hinges to absorb lateral displacement while withstanding internal pressure thrust. The hinge structure is complex and allows angular displacement, but the telescopic rod can only move axially.
A telescopic rod is strictly a variation of a tie rod, but it has a more defined stroke design, usually with a limit nut that allows precise control of how much the bellows is stretched and compressed. When choosing, you should distinguish the needs: what you want is axial compensation, so choose the telescopic rod; To compensate horizontally, you have to use hinges; Simply prevent internal pressure thrust and blind plate force, ordinary tie rod is enough.
Selection Parameters of Metal Expansion Joint Telescopic Rod: How to Determine Stroke, Thrust and Material
Three parameters must be confirmed at the same time during the selection.
The first is the itinerary.The stroke is not simply to measure how long the pole is, but to see the effective compensation amount of the bellows. For example, if an axial expansion joint needs to absorb 30mm compression, the limit nut of the telescopic rod must reserve at least 30mm clearance. If it is left small, the bellows will be held back, and the compensation will not be in place; If you leave it too large, it may exceed the allowable limit of the bellows, resulting in instability.
The second is thrust.Thrust calculation should calculate internal pressure thrust, elastic reaction force and additional force generated by pipe deformation. The formula is not complicated: internal pressure thrust = pressure × effective area of bellows. However, in actual selection, many people only calculate the pressure, but forget to calculate the impact force generated by the flow of the medium. Especially for steam pipelines, the instantaneous thrust of the water hammer may be several times that of normal working conditions. If the strength of the telescopic rod is selected according to the average value, the water hammer may bend the rod.
The third is the material.The material of the pole itself should match the pipeline medium and ambient temperature. Ordinary carbon steel is fine at room temperature, but if it is the corrosive working condition of desulfurization flue gas baffle door, the pole has to be made of stainless steel, or at least anti-corrosion treatment. There are also high-temperature occasions, such as high-temperature axial expansion joints, where the working temperature exceeds 400℃, and the allowable stress of carbon steel will be greatly reduced. At this time, alloy steel has to be used, and the additional stress caused by thermal expansion has to be considered.
Length is just the result, not the cause. First set the stroke, thrust and material, and then push back the length and diameter of the pole.
Three of the most common errors at installation sites, each of which can cause a telescopic rod to fail
Screw the limit nut to death during installation. Some people think that it is only firm when it is tightened. As a result, as soon as the corrugated pipe expands and contracts, all the force acts on the pole, and the nut collides with the flange head-on, and the pole quickly breaks from fatigue. The correct way is to leave enough clearance according to the drawing requirements so that the nut only plays a limiting role, not a locking role.
Ignore the orientation of the telescopic bar. The limit direction of the telescopic rod must be consistent with the displacement direction of the pipeline. In some sites, the left and right installation is reversed, and as a result, the lateral displacement does not bend the rod. You say it was wrong or not? There are markings on the poles when they leave the factory, a thing to take a look at before installing.
The multi-head telescopic rod is unevenly stressed. It is common that four poles are arranged diagonally. If the nut adjustment amount of each pole is different, the force will be biased. The bellows is crushed to death on one side and stretched on the other. Over time, it will inevitably become partially unstable. When adjusting, use a torque wrench to ensure that the preload force of each pole is consistent.
How to match and maintain telescopic rod and corrugated pipe so that the expansion joint can be used for full life
Telescopic rods are not installed and done once and for all. During daily inspection, focus on whether the nut is loose and the pole is corroded. Especially for outdoor installation, rainwater accumulates in the gaps between poles and nuts. After a few years, it has rusted like iron bumps, and it can't be twisted even if you want to adjust the stroke.
Another, check the position of the limit nut regularly to see if there is any change. If it is found that the clearance of the nut is larger than the initial one, it means that the bellows may be permanently deformed; If the gap is small, it means that the displacement of the pipeline exceeds the design value, and both cases must be checked in time.
Also, the telescopic rod and bellows work together, don't replace one of them separately. For example, if you change a thicker pole, the stiffness changes, and the stress state of the bellows also changes, which may affect the fatigue life. Conversely, if you replace the bellows with non-metallic expansion joints, the structural form of the telescopic rod may no longer work — non-metallic expansion joints usually don't have a metal telescopic rod because they absorb displacement differently.
When selecting the model, don't just look at the parameters on the product sample. It is best to tell the manufacturer the actual working conditions. For example, the corrugated expansion joint used in the power station industry often has complicated working conditions, including thermal cycle, vibration and ash and powder wear. At this time, the detailed design of the telescopic rod-such as the connection method of the rod end and the nut locking structure-needs special consideration.
The metal expansion joint telescopic rod is really not as simple as a pole.