Two days ago, I met a customer, and the equipment was installed. During debugging, it was found that the pipeline offset exceeded expectations. After checking the reason, the tie rod on the tie rod-type metal expansion joint was regarded as a transportation fixed rod and directly disassembled. Disassembled and thrown in the corner of the warehouse. It sounds outrageous, but it's not uncommon at the scene.
The biggest difference between the tie rod metal expansion joint and the general-purpose corrugated expansion joint lies in those tie rods. They are not temporary devices to protect the bellows, but structural elements that participate in the stress. If you dismantle it, the bellows will become a free body without constraints, and the thrust generated by internal pressure will directly act on the fixed brackets at both ends, ranging from the deformation of the brackets to the overall displacement of the pipeline and even the cracking of the bellows.
Structure of tie rod type metal expansion joint: What is the use of tie rod, nut and bellows?
Bellows, tie rods and nuts (some with ear plates or earrings). The bellows is responsible for absorbing displacement – it digests the thermal expansion or mechanical displacement of the pipe through the deformation of the node. The role of the tie rod is to constrain the axial displacement range of the bellows while bearing the internal pressure thrust. The nut is the actuator for adjusting and locking the position of the tie rod.
You note that the tie rod and bellows work together: the bellows provide flexibility and the tie rod provides limitation. Without the tie rod, the flexibility of the bellows becomes a runaway soft spot.
Core Logic of Tie Rod Adjustment: What is Pre-Displacement all about?
Pre-displacement, also called pre-stretching or pre-compression, is the most critical link in the installation of tie rod metal expansion joint.
The pipe is installed in a cold state, but the temperature rises during operation and the pipe will elongate. If the bellows are at their natural length when the expansion joint is cold, the bellows will be compressed after operation. That's fine, as long as the amount of compression is within the allowable range. The real problem is-the bellows get back in place when the pipe cools, and this back-and-forth strain range determines the fatigue life.
Pre-displacement means that the bellows is pre-stretched (or compressed) for about half of the compensation amount in the cold state. For example: The thermal elongation of the pipe is 40mm, and you can pre-stretch the expansion joint by 20mm during installation. In this way, the bellows is compressed from +20 mm to-20 mm, with a total displacement of 40 mm, but both maximum tension and maximum compression are within the allowable range of the material. The calculation of fatigue life is also different – for the same total amount of compensation, bellows with pre-displacement may last two to three times as long as without pre-displacement.
Judging whether pre-displacement is needed depends on the design temperature and installation temperature of the pipeline. When the temperature difference is large, the pipeline is long and the compensation amount is large, pre-displacement is necessary.
Will the tie rod be removed or not during installation? — — Sorting out common misunderstandings
Remove the tie rod as a transport fixture. This is the most pitty. The tie rod of the tie rod type metal expansion joint does have a fixed role in transportation, but its own job is a working member. When the tie rod is removed, the axial thrust constraint of the expansion joint is gone, and the pipe support will bear forces that it should not have.
Do not adjust the nut after installation. The nut on the tie rod is generally locked when it leaves the factory. When installing, the adjustment nut should be loosened to the pre-displacement position, so that the bellows has room for free deformation. If you leave it alone after installation, the tie rod is locked, the bellows can't move at all, and the expansion joint becomes a rigid pipe, with nowhere to release the thermal stress, and it should be bad or bad.
Do not lock after pre-displacement is completed. After the pre-displacement is in place, the position of the adjustment nut should be fixed with a locking nut to prevent loosening caused by vibration during operation. If it is not locked, the tie rod will slowly shift, and the pre-displacement amount will be lost.
Removal of transportation fixtures → Installation of expansion joints → Adjustment of adjustment nuts (pre-displacement) according to calculated values → Lock nuts → Pipeline pressure test.
Tie rod type and pressure balance type expansion joint: choosing the wrong model equals white installation
Also with a tie rod, what's the difference between the tie rod type and the pressure balance type?
The tie rod of the tie rod type metal expansion joint is subjected to internal pressure thrust, but it does not eliminate this thrust-the thrust is still transmitted to both ends through the tie rod. Therefore, it is suitable for use in situations where there is a fixed bracket in the pipeline, and the thrust can be absorbed by the bracket.
Pressure balanced expansion joints (such as straight pipe pressure balanced expansion joints and curved pipe pressure balanced expansion joints) contain balanced bellows in their internal structure, which can offset the internal pressure thrust inside the expansion joints and do not produce blind plate force externally. This product is suitable for use in situations where the fixing bracket is not easy to make or there are sensitive equipment near the pipeline.
The fixed bracket can withstand the thrust, and it is economical and affordable to choose the pull rod type; There is no reliable fixing point, so choose the pressure balanced type.
Daily inspection: the tie rod is loose and the nut is off. How to deal with it?
The most common problem in the operation of tie rod type metal expansion joint is the loose nut. Loose nuts will lead to loss of pre-displacement, uneven force on the tie rod, and even abnormal deformation of the bellows.
Has the nut been displaced, the tie rod bent, and the bellows has an abnormal local bulge. If it is found loose, first return the adjustment nut to the design position, and then tighten the locking nut. If the nut is lost, don't just find a bolt and screw it on-the thread specification and force grade of the tie rod are designed. You take an ordinary bolt instead, it is not strong enough, and something will happen after running for a while.
To judge whether the tie rod metal expansion joint needs to be replaced, look at two points: first, whether the bellows has obvious cracks or corrosion leakage, and second, whether the tie rod has permanent deformation. If either one of these two appears, don't hesitate to change it directly.