1. Pressure balanced expansion joint: It does not solve ordinary pipeline displacement problem
Which of us in pipeline design has not competed with thermal expansion? The tube stretches as soon as it is hot and shrinks as soon as it is cold. Ordinary axial expansion joints or corrugated compensators can handle most scenes. But the question is-what to do when there is no place for fixed brackets in the pipe system, or the equipment interface can't bear the blind plate force at all?
That's when you have to ask outBalanced metal expansion joint。 Its job, to put it bluntly, is to swallow the pressure thrust itself and prevent it from being transmitted to the equipment or brackets at both ends. And guess what? Structurally, it uses two or more sections of bellows plus tie rods and end plates, so that the thrust generated by the medium pressure cancels out each other inside the expansion joint. To put it bluntly, this is an "internal friction type" compensator-the internal bellows are opposed to the top, the force is closed inside, and there is zero thrust to the outside world.
For example, a steam turbine inlet or outlet, a pump inlet or outlet, or a compressor pipe, these devices are very sensitive to pipe thrust. With ordinary expansion joints, the thrust directly hits the flange of the equipment, ranging from leakage to equipment displacement. Use pressure balanced type, thrust clearance. Is that the truth?
Second, straight pipe pressure balance type, curved pipe pressure balance type, compound bypass type-what is the difference between these "balance models"?
Straight pipe pressure balanced expansion joint、Curved tube pressure balance expansion joint、Compound straight pipe bypass pressure balanced expansion joint, with "pressure balance" in their names, which one to choose?
- Straight pipe pressure balance type: Used for straight pipes, absorbing axial displacement, the interior consists of two bellows plus a set of tie rods. It is suitable for straight pipe sections where both ends are fixed and the middle needs compensation.
- Curved tube pressure balance type: For L-shaped or Z-shaped pipes, it can absorb both axial and lateral displacement. It has a working bellows and a balancing bellows, like a thong structure (laughs), specifically tackling the thrust at the elbow.
- Double straight pipe bypass pressure balance type: This is more complicated and is generally used for large-diameter pipes or situations where large amounts of compensation are required. It has two working bellows and a balancing bellows, which communicates pressure through a bypass tube, which can compensate for a large amount of axial displacement while balancing thrust.
Two days ago, I met a customer who had to use a straight pipe to install the elbow. As a result, the tie rod was suffocated and the bellows deformed. Selection does not care about the direction of the department, that is to dig a hole for yourself.
Third, when must we go to pressure balance type? How serious are the consequences of using it wrong?
As long as one of the two conditions "the equipment cannot bear the thrust" or "the main fixing bracket cannot be set" appears in the pipe system design, the pressure balance type is the only solution. Otherwise, you use the ordinary axial type, and the thrust is all thrown to the equipment-the pump body is cracked, the turbine interface leaks, and the compressor vibration exceeds the standard. Which one does not start with tens of thousands of repairs?
More seriously, if the ordinary expansion joint is misused on the steam pipe, the blind plate force can push the pipe out of the bracket, which is no longer a problem of losing money. I have seen a chemical plant, just because the thrust is not counted, the DN600 pipe directly bends the pipe gallery column. Tsk, the scene was horrible.
Conversely, not all scenarios have to be used. The pipeline has enough fixed bracket space, and the equipment interface can withstand thrust. Just use ordinary axial or universal corrugated expansion joints. Don't kill chickens with a knife, the pressure balance type is much more expensive than the ordinary type, and don't waste your budget in unnecessary working conditions.
4. Three details that are most likely to overturn during model selection: the number of bellows layers, guide tube and installation direction
Details determine life, which is true in expansion joints. First,Number of bellows layers。 Many people think that more layers are stronger, but they are actually wrong. The number of layers is larger to withstand higher pressures, but at the same time it also reduces the wall thickness of each single layer. It is correct to use multi-layer in high pressure conditions, but it is easy to fracture by fatigue in low pressure and large displacement scenarios. To calculate the stiffness, refer to the Stiffness and Calculation Formula of Bellows for details.
Second,guide tube。 This thing is not standard, but high-frequency vibration or high-speed airflow ducts must be added. Without the guide tube, the medium directly washes the trough of the bellows and wears out in a few months. Moreover, the installation direction must be consistent with the flow direction of the medium-the arrow cannot be pointed casually, and it is pointed against the guide tube for a ride, which not only increases the resistance, but also may generate vortex-induced vibration internally. You say it was wrong or not?
Third,Installation direction。 Especially the curved tube pressure balance type, which has a clear "installation angle" requirement. Torque calculations are completely different for horizontal and vertical installations. If you take the drawings of horizontal working conditions to install vertical pipes, the force direction of the tie rod is reversed, and the balance effect will directly return to zero.
V. Pits often stepped on at the installation site: Should the tie rod be dismantled? Can the direction of the arrow really be pointed casually?
"Does the screw of the expansion joint need to be disassembled?" I answered uniformly: it depends on whether it is a transport tie rod or a working tie rod. Pressure balanced expansion joints usually leave the factory with transport rods or screws, which are used to fix the bellows to prevent transport deformation. These temporary screws must be removed after installation in place, otherwise the expansion joints cannot be moved. However, the working tie rod (such as the tie rod at the pressure balanced end of a straight pipe) is a permanent structural part and cannot be disassembled. If you dismantle it, no one will carry the pressure thrust, and the equipment will suffer.
Arrow Direction。 The direction of the arrow of the expansion joint refers to the direction of media flow, not the direction of installation. You can understand it as "the direction of the water flow". However, some brothers don't look at the arrows and install them casually. As a result, the direction of the guide tube is reversed, or the guide tube is stuck because of the diagonal installation. That little arrow logo was really not randomly printed on it.
Once the installation is complete, remember to check that the tie rod nut is loose. Refer to the question and answer of "How to adjust the tie rod nut of the expansion joint". Just screw it to the pre-tightening force requirement, and don't screw it to death with brute force. If the thread slips, the whole expansion joint will be wasted.
The balance metal expansion joint is a protector if you choose it right, and a time bomb if you choose it wrong. Do you use straight pipe pressure balance or curved pipe pressure balance for this project at hand? Compare the tube drawing, calculate the thrust clearly, and don't pat your head. After all, there is no word "probably" in engineering.