Why does someone always take a wrench to screw the tie rod nut of the expansion joint at the installation site? The existence of those screws and nuts is not for you to play casually. Adjusting the distance, to put it bluntly, is to leave a "safety margin" for the thermal expansion and contraction of the pipeline. Today, let's not go around the bend, but directly dismantle the physical logic and practical operation behind this action.
1. The essence of adjusting distance: what are pre-stretching and pre-compression fighting against?
When the temperature rises, the pipe elongates, and the expansion joint bellows is compressed; The opposite is the case with a temperature decrease. The compensation amount calculated during design is the theoretical value, but the installation temperature is often not the design temperature, so it is necessary to forcibly pull the bellows to a certain initial position with a tie rod nut-this is called "cold tightening" or "pre-stretching".Universal corrugated expansion joint、High temperature axial expansion jointThat's what we have to do. The greater the rigidity of the bellows, the more difficult it is to pull it up. This force directly determines how many turns the nut is screwed. Refer to this siteStiffness and Calculation Formula of BellowsAdjusting the distance is actually "borrowing force"-borrowing a little deformation in the cold state in exchange for a safe space in the hot state.
2. Different models have completely different postures
Don't think that all metal expansion joint adjustment distances are screwing nuts. Such asLarge tie rod expansion joint (double hinge transverse expansion joint)The tie rod mainly limits the lateral displacement and adjusts the relative position between the bellows groups; WhileStraight pipe pressure balanced expansion jointIt relies on the internal pressure balance structure, and the pull rod only undertakes auxiliary positioning. Conversely,Sleeve type pipe expansion jointThere is no pull rod at all, and the adjustment distance depends on the limit bolt of the sliding sleeve.Metal Corrugated Expansion Joints in Cement IndustryAndCorrugated expansion joint in power station industryThe direction of displacement compensation is different, and the adjustment mode is also different. The selection is wrong, and the tune is also white.
3. Practical operation: How to adjust the tie rod nut? Loosen first and then tight, or tight first and then loose?
Two days ago, a customer asked, "Does the screw of the expansion joint need to be removed?"-Whether it is removed or not depends on whether the adjustment distance of your unit is pre-stretched or pre-compressed. If the medium temperature is higher than the installation temperature and needs to be pre-stretched, then before installation, the tie rod nut should be screwed out so that the bellows can be pulled apart by a certain distance (usually about 50% of the design compensation amount); Pre-compress if the medium temperature is lower than the installation temperature. After adjustment, do you keep the tie rod nut or remove it? For restrained expansion joints (such as large tie rod types), tie rod nuts must be retained for thrust transfer and guidance; For non-constrained types (such as general-purpose types), the tie rod should be loosened or removed after installation, otherwise the expansion joint cannot be freely deformed. Function of expansion joint tie rod, this siteIn Q&AThere are more detailed instructions.
4. What happens if you adjust it wrong? The consequence is more than a leak
The adjustment distance is too large, the bellows is overstretched, the stress at the root of the corrugation is concentrated, and the fatigue life plummets-it may crack in less than a year; If the adjustment distance is too small, the bellows will be excessively compressed in the hot state, and instability (bulging or lateral bending) will occur, which will directly jam the pipe. Special reminder:High temperature axial expansion jointIf the cold tightening is not in place, a huge thrust will be generated during operation, and the fixed bracket will be crooked. This siteQuestions and answers on the service life of expansion jointsLi mentioned that 90% of failure cases are related to improper installation adjustment. So how do you judge that the adjustment is correct? The actual length of the bellows assembly can be measured on site, and the deviation is controlled within ±2mm according to the installation length value calibrated by the factory.
5. The boundary of the adjustment distance should be clearly thought out in the selection stage
Don't wait until the scene to find out that there is not enough room for adjustment. Such asLarge diameter thick wall expansion jointThe rigidity of the corrugated pipe is large, and the pull rod cannot be twisted by manpower. You have to use a hydraulic wrench or simply chooseExternal pressure single axial expansion joint。 Another exampleSpecial hose for vacuumAndPTFE-lined hose, cannot be used as expansion joints to adjust axial displacement, they mainly solve vibration and deflection. The larger the adjustment distance, the better. The design should be combined with the layout of pipeline brackets and the spacing of guide brackets. Bottom line: Find out whether this thing is "active adjustment" or "passive let" before installation, and the difference is big.