Friends who do pipeline design all understand that thermal expansion and contraction in pipelines is a trouble that can't be avoided. As soon as the steam pipeline heats up, the pipeline of tens of meters can jump out for several centimeters. If it is hard and left untreated, the flange surface will be torn, the bracket will be dislocated, and the equipment interface will be deformed. All of them are paying for lessons. To put it bluntly, the compensation method of metal expansion joint is one thing: find out where the pipe is going, and then choose the right type to absorb this displacement. Let's break this up and talk about it today.
Differentiate three kinds of displacement: axial, transverse and angular, and compensation mode follows the displacement
The displacement of the pipeline looks complicated, and there are three basic forms when it falls on the expansion joint.
Axial displacementThat is, the expansion joint is stretched or compressed along the axis of the pipe. This kind of displacement occurs most often in the temperature change of straight pipe section, and it is also the best one to deal with. The universal corrugated expansion joint is what it does. The bellows directly absorbs the expansion and contraction amount, which is simple and rough but effective.
Lateral displacementIt is the relative misalignment of the two ends of the expansion joint perpendicular to the axis, like the meaning that two sections of pipe are not aligned and hard joined. In this case, with a single corrugated expansion joint, the bellows will be damaged by bending. The correct way is to absorb the lateral displacement by converting the lateral displacement into angular displacement by two hinge groups.
Angular displacement, is the angular deflection of both ends of the expansion joint around a certain point. The thermal expansion of Z-shaped or L-shaped pipe segments usually carries an angular component. To deal with this condition, single hinge expansion joint or double hinge combination can do it, provided that the rotation center of the hinge matches the actual rotation angle of the pipe.
Before selection, take the pipe system stress analysis diagram to see which displacement vector dominates at each node. Don't take an axial expansion joint to absorb the lateral displacement. It is equivalent to forcing the bellows to do the work it can't do, and the fatigue life will fall like a cliff.
Don't read the wrong parameters when selecting the model: How to match the displacement, pressure level and temperature range
Two days ago, I met a customer who asked me with the working condition table, saying that the working pressure of the pipeline was only 0.6MPa and the temperature was 220℃. Why was the expansion joint quotation given by the manufacturer ridiculously expensive? As a result, looking at the displacement he mentioned, the axial compression was 60mm and the lateral yaw was 35mm. Where is this the work that ordinary general-purpose types can do?
The selection has three parameters that must be aligned.
displacement amountTo distinguish the relationship between rated displacement and fatigue life. The displacement capacity of the bellows is changed by the number of cycles. For the same expansion joint, the design life of 1000 times and 10000 times, the allowable displacement differs by 30% to 40%. Your pipeline starts and stops once a day, which is 365 cycles a year. You have to select it at least 5,000 times. Don't take the peak displacement to set the maximum value on the sample.
Pressure ratingNot only the working pressure, but also the test pressure and impact load. The bellows is subjected to the circumferential stress and axial thrust generated by internal pressure. If the pressure level is selected low, the bellows will bulge, and if the pressure level is selected high, the stiffness will increase and the compensation ability will decrease, which is all troublesome.
Temperature range, which is the easiest to overlook. The allowable stress of 304 stainless steel bellows at 400℃ is only about 45% of normal temperature. At the same working pressure of 1.0MPa, single-layer bellows is enough at room temperature, and Inconel alloy or multi-layer bellows structure may have to be replaced at 400℃. Throw the temperature to the manufacturer's column and fill in "high temperature", which is equivalent to pushing all the selection risks to others, and the final things you get may not be suitable for your working conditions.
Installation and adjustment are exquisite: tie rod nut, pre-tension and guide bracket cannot be missed
Whether the expansion joint is installed correctly or not is more critical than the type selection. On the spot, no matter how good the expansion joint is, it will be useless.
tie rod nutThis item is shipped as a shipping protector in a locked state to hold the corrugated knuckle, but many installation teams treat it as a permanent piece and don't release it after installation. The tie rod is not loose, and the expansion joint is a rigid body. The displacement is all held on the bellows, and it will crack after several rounds of thermal cycles. After the universal corrugated expansion joint and the double hinge transverse expansion joint arrive, the tie rod nut is retracted to the limit position, so that the corrugated pipe can expand and contract freely. As for the screw of the expansion joint, does it need to be removed? Sub-situation: the tie rod that bears the internal pressure thrust cannot be disassembled, which is the force-bearing part; The transport protection screw must be removed, which is a temporary fixture.
Pre-stretchingThis is the most particular step in the compensation method. When the installation temperature of the pipeline is lower than the operating temperature, the bellows works in a compressed state, and it is pre-stretched by half a displacement, so that it can be balanced between the cold state and the hot state, and the force is uniform throughout the whole life. The pre-stretching amount is usually 50% of the rated displacement. For the specific value, see the installation instructions provided by the manufacturer.
guide bracketThis thing manages the working trajectory of the expansion joint. The axial expansion joint needs the first guide bracket to be no more than 4 times the nominal diameter away from the expansion joint, and the second guide bracket to be no more than 14 times the nominal diameter away from the first guide bracket. Otherwise, the pipeline will be swung laterally as soon as it is hot, the bellows will deflect, and the compensation amount will be completely chaotic. The transverse expansion joint of compound hinge should ensure that the hinge rotation plane is consistent with the bending direction of the pipe, and the bracket setting should allow the pipe to move freely in a limited direction and lock in other directions.
Different working conditions and different types: How to choose general type, double hinge type and pressure balance type
To understand the selection and installation, you must also be able to select the type according to the working conditions.
Straight pipe section, low displacement, low pressure,Universal corrugated expansion jointIs the most economical option. This kind is widely used in steel mill pipelines and general thermal pipelines in workshops.
Pipe systems with limited space but large thermal displacement have to rely onCompound hinge transverse expansion joint。 It uses two groups of bellows and a hinge group in the middle, and its transverse displacement absorption capacity is much stronger than that of single expansion joint. The double-hinged expansion joint used on the air-cooled island vacuum pipeline is a typical example of this type of structure.
The pipe system is connected to sensitive equipment such as steam turbine and compressor, and the axial thrust will damage the equipment interface. At this time, usePressure balanced expansion joint。 The straight pipe pressure balance expansion joint cancels the internal pressure thrust through the area difference of two sets of bellows, and the force felt by the equipment end is basically zero. WhileCurved pipe pressure balance type expansion joint is suitable for L-shaped or Z-shaped pipe systems that need to absorb both axial and transverse displacements at the same time. The cost is complicated structure and high cost, but the equipment safety in exchange is worth it.
Large diameter or thick-walled pipes are preferredExternal pressure single axial expansion jointThe bellows is external and the guide tube is lined, which is not only wear-resistant but also convenient for maintenance. Use if the medium is highly corrosivePTFE compensatorLiner, for dust and smoke environmentNon-metallic expansion jointMore resistant to dust accumulation.
Compensation failure troubleshooting: the expansion joint is not moving, first check that the bracket is still cold and tight
The on-site feedback expansion joint is not moving, so don't rush to remove it and repair it.
The first step is to see if the guide bracket is stuck. The support roller rust, the sliding surface is filled with welding slag, and the insulation material falls into the guide groove, which will lock the pipeline. The expansion joint itself is fine, it's the bracket that keeps it in check.
The second step is to check the cold tightness state. If the pre-stretched tie rod is not locked in place in the cold state, as soon as the pipe heats up, the expansion joint directly pushes to the extreme position, and there is no margin to absorb the subsequent thermal expansion. This is a common occurrence on initially operating pipelines because the cold tightness values are not corrected to the actual ambient temperature during installation.
The third step is to see if there is any foreign object stuck in the bellows. The pipeline is not washed thoroughly, the welding slag accumulates in the trough, and the expansion joint can't move if it wants. At this time, you can judge by listening to the sound: the normal expansion joint will have a slight creep sound when the temperature changes, but it will be quiet when it is stuck.
First outside and then inside, first stent and then body. More than 70% of the "failure" cases of expansion joints were finally found to be problems with the peripheral accessories, and the bellows itself was good. Do you think the money was spent unjustly?
The core logic of the compensation method of metal expansion joint is not complicated, but every step has to be confronted with the actual working conditions. It is much more useful to look at a few more pipeline stress analysis diagrams and talk to the on-site installation master than to turn over samples.