Tapered Metal Expansion Joint: Displacement Compensation for Reducing Diameter Pipe, Stop Plugging Universal Type
From thick to thin, or from thin to thick-this kind of thermal displacement compensation at the reduced diameter position, many people's first reaction is to plug a universal corrugated expansion joint on it. And the result? The stress distribution is uneven, the corrugation is eccentrically loaded, and it leaks within two years. Alas, this is not the equipment is not good, but the selection direction is wrong.
The tapered metal expansion joint, to put it bluntly, is specifically for this job. Its corrugation is not of equal cross section, but the wave height and wave pitch change gradually from the large end to the small end. Each wave bears more uniform stress, and the fatigue life can be increased by more than 30% compared with the general-purpose model. In addition, it comes with its own guide tube-as mentioned in our Q&A, the main function of the guide tube is to prevent the medium from washing out ripples and prolong the service life. Think about it, the general-purpose type is installed at the variable diameter, and the corrugation bears a large force on one side and a small force on the other. Can it not be eccentric?
Universal vs Tapered: What's the Difference?
The universal corrugated expansion joint is an equal section design and is suitable for straight pipe sections. When the reduced-diameter pipeline is installed, the corrugation at the large end is strongly supported, while the corrugation at the small end basically does not move, and the stress is concentrated on one or two waves. Over time, the crack starts there. The design logic of tapered metal expansion joint is that the corrugated shape changes gradually with the pipe diameter, and the force naturally changes gradually. Each wave shares the displacement proportionally, which is the compensation logic at the variable diameter.
"Then can I connect two general-purpose types in series?" Two days ago, a cement factory customer did this-the diameter reduction of the outlet pipe of the rotary kiln was originally strung with two general-purpose types, but it leaked three times in two years. Changed the tapered metal expansion joint, and with the installation experience of metal corrugated expansion joint in cement industry, there has been no problem for four years. For reduced diameter compensation, choosing the right structure works better than the number of stacks.
The two pits where the selection most often rolls over
The pressure level is selected according to the nominal pressure. The large end and the small end of the tapered metal expansion joint are not subjected to the same pressure-the small end has a smaller cross section and greater pressure per unit area. For example, when it is used on steam pipelines of power stations, it is used together with high-temperature axial expansion joints. You have to calculate the thrust according to the small end section. Just looking at the nominal pressure, the small end may have been overpressurized long ago.
The amount of displacement is only counted as axial. The tapered structure itself produces a certain radial displacement-there will be a tiny lateral component during the corrugation graduation. You calculate the compensation amount according to the pure axial direction. When you install it, the actual displacement exceeds it, and the ripples are hard-pulled. Correct approach: Confirm the actual compensation amount of the conical expansion joint with the manufacturer, and they will have test data or simulation calculation.
Installation points: Don't install the direction in reverse direction
The tapered metal expansion joint has a definite directionality-the large end faces the high pressure side or the high temperature side. The direction of the arrow on the expansion joint is the flow direction of the medium, and the direction cannot be reversed. Install backwards, the small end bears great pressure, and the guide tube may also be washed out.
Pre-stretching amount and tie rod nut adjustment are similar to large tie rod expansion joints, but they must be strictly according to the factory data of the manufacturer. Refer to the article on how to adjust the tie rod nut of the expansion joint in our question and answer-when adjusting, twist both sides symmetrically, and put them in place multiple times. Don't screw them to death at once. In addition, if the vibration of the pipeline is large, it is recommended to install metal hoses or rubber compensators at both ends of the tapered section to isolate the vibration. The metal hose can absorb high frequency vibration, and the rubber compensator is more effective for low frequency amplitude, which is selected according to the working condition.
Case: Lessons from Rotary Kiln in Cement Plant
The aforementioned cement plant customer used to string two general-purpose corrugated expansion joints at the reducing diameter. Every time he stopped for maintenance, he had to weld and repair the cracks, and worked three times in two years. To put it bluntly, the general-purpose type is the general-purpose type. If the working conditions are not suitable, it will be a matter of time before problems go wrong. After changing the tapered metal expansion joint, the manufacturer redesigned the corrugation parameters according to the reducing diameter angle, medium temperature and pressure, and the guide tube was also thickened with a layer of wear-resistant lining. Now it's been four years, and even the sealing surface hasn't been touched. Is it worth the money? Just calculate the loss of discontinuation.
Conical metal expansion joint is not universal, but the thermal displacement compensation of reducing diameter pipe, it is the most appropriate scheme. Calculate the pressure level clearly, check the displacement with the manufacturer, and don't install the reverse direction during installation-these three steps are right, and there will be no problem in using it for ten or eight years.