Let's start with the name: What does this "unconstrained" mean?
Having been in this business for a long time, you will find an interesting phenomenon: many owners take drawings and ask, "Is the unconstrained expansion joint the kind without a tie rod?"-Yes, but not entirely right.
The so-called "unconstrained type" refers to the absence of external constraint structures such as tie rods, hinges and universal hinges on both sides of the bellows. Common general-purpose corrugated expansion joints and external pressure single axial expansion joints all belong to this category. They rely on the flexibility of the bellows itself to absorb the axial displacement, and the internal pressure thrust is directly transmitted to the fixed brackets at both ends, without being "digested" by devices such as pull rods and hinges.
What about the one with the tie rod? The function of the tie rod is to restrain the internal pressure thrust, so that the fixed bracket can carry less force, at the cost of compressing the effective displacement of the bellows. The hinge type is to limit the displacement in a plane, and the hinge bears the thrust. So you see, "unconstrained" does not mean that the bellows can move how it wants, but that it has no constraint device in structure, and the displacement reaction force is completely borne by the external pipe support.
The applicable scenarios are also clearly distinguished. For pipelines with long straight sections that need to absorb large axial displacements, general corrugated expansion joints are preferred. For high-temperature and high-pressure steam pipelines, the external pressure single axial expansion joint is more suitable-the external pressure structure makes the inner wall of the bellows not contact the medium, and the flow rate washes the bellows much less.
Core Force Logic: How much force does the fixed bracket have to carry?
The requirements of unconstrained expansion joint for fixed bracket are one order of magnitude higher than those of hinge type. This is not to scare people, it is determined by mechanical logic.
How to calculate internal pressure thrust (blind plate force)? Estimation by blind plate force formula : F = P × A. P is the working pressure (MPa), A is the area of the circle corresponding to the average diameter of the bellows (mm²), and the calculated force unit is N. For example, the DN500 pipe has a pressure of 1.0 MPa, an average bellows diameter of about 550 mm, and a blind plate force of about 1.0× (π/4×550²) ≈ 23.7 tons. This force is supported entirely by the fixed brackets at both ends.
Why don't I just make the bracket stronger? The problem is not so simple. In addition to carrying the blind plate force, the fixed bracket also has to bear the thrust generated by the thermal expansion of the pipeline, the friction from the sliding bracket, the wind load and the earthquake load-the superposition of these forces has strict requirements on the structural form and rooting position of the bracket. The stiffness of the bracket is not enough. As soon as the pipeline expands, the bellows will be hard-pulled, which will cause the fatigue life to be discounted, and the bellows will become unstable and ruptured.
"Unconstrained" does not mean "casual installation". On the contrary, it means that the forces of the pipeline system are all digested by external structures. If you don't do force analysis before the selection, there will be problems in the later period.
The Three Points Where Model Selection Is Most Prone to Rollover
Temperature, fatigue life, guide tube configuration, these three points contract almost 90% of the problems on site.
In terms of temperature, steam ducts and high-temperature smoke ducts are completely different things. The temperature of the main steam pipe of the power station is above 550℃, and the bellows material must be Inconel 625 or similar superalloy. Ordinary 304 can't withstand high-temperature creep at all. The temperature of flue gas pipelines in cement industry fluctuates greatly, and there is sulfide corrosion. If the materials are not selected well, they will be worn in half a year.
Fatigue life is more straightforward. Every time a bellows expands or contracts, a plastic strain occurs at the root of the corrugation. The core parameter of the fatigue life calculation formula given in EJMA standard is the single wave displacement. In order to save costs, some people reduce the wave number, the displacement of a single corrugation bears a larger amount, and the fatigue life decreases exponentially-this "money-saving" method is equivalent to gambling on safety.
Deflector configurations are also often overlooked. The function of the guide tube is flow diversion and wear resistance, but there is one detail that many people don't know: the guide tube should open a gap. Without opening a gap, the annular space between the bellows and the guide tube forms an air chamber, and the pressure pulsation will make the bellows vibrate, and the weld will crack over time. In the working condition of high flow rate, the material of the guide tube should be thickened or cemented carbide surfaced, otherwise the bellows will be worn out and directly exposed to the dusty airflow.
Mounting and bracket fitting are hidden thresholds
I have encountered several projects, and the quality of the bellows is fine, but the result is still leaked-after checking, it is all the fault of the installation details.
The requirements of unconstrained expansion joint for fixed bracket have been mentioned earlier, and then talk about sliding bracket. The spacing and friction of the sliding brackets directly affect the transmission of thermal expansion of the pipe. If the friction force is too large, the actual displacement absorbed by the bellows is smaller than the design value, and the stress distribution of the pipeline will change. The places that should be expanded and contracted can't be contracted, and the places that should not be stressed can't be carried. Too little friction won't work, and the pipe will swing irregularly.
Do it cold tight or not? How to set pre-deformation? These two issues must be clearly resolved before installation. Cold tightening (pre-tensioning/pre-compression) is done to keep the bellows in optimal working condition at operating temperatures. For example, the operating temperature of the pipeline is 200℃, the installation temperature is 20℃, and the thermal elongation is assumed to be 60mm. The cold tightness is about 30mm. The actual displacement of the bellows during operation is plus or minus 30mm, instead of eating hard in one direction from 0 to 60mm. The setting of pre-deformation directly affects the fatigue life. Don't rely on patting your head, but according to the cold tightness diagram given by the design institute.
Bellows cannot withstand torsion. Uneven pipeline matching and uneven tightening force of flange bolts will cause torsional stress in the bellows. The damage of this stress to fatigue life is hidden, which could not be seen at that time, and cracks appeared in the bellows after several months of operation.
When not to choose unconstrained type?
Unconstrained expansion joints are not all-purpose. Its biggest shortcoming is that it can only absorb axial displacement and a small amount of lateral displacement.
Space-constrained pipe systems, such as pipes in pipe galleries, are densely arranged, and the axial displacement is not enough straight pipe section length to release. At this time, the unconstrained type is not easy to use. If you hard-install a universal corrugated expansion joint, the lateral displacement is slightly larger, and the side of the corrugated pipe will bulge.
When the transverse displacement or angular displacement needs to be absorbed, it is more reliable to directly attach the transverse expansion joint of the compound hinge. The double hinge structure converts the displacement into angular displacement through the cooperation of two groups of bellows and hinge connecting rods, which requires much less installation space. Rotary compensator is another way, which absorbs heat and elongates by the rotation of the sleeve, which is suitable for large displacement, low pressure and buried pipelines.
How to rule it out? Remember the three "haven't": Are there enough straight pipe sections? Is there enough space for axial displacement? Are there any lateral or angular displacement needs? If the first two answers are no, or the third answer is yes, change the form of constrained compensation as soon as possible.
Maintenance and longevity: No tie rod can be adjusted, but more worry-free
The routine maintenance of unconstrained expansion joints is simpler than those with tie rods-no tie rod nuts to adjust, no hinges to lubricate, just stare at the bellows itself.
Look at whether there are corrosion pits, cracks and scratches on the surface of the bellows; Second, see whether there is obvious deformation, especially whether the wave pitch is uniform; Third, see if there is any abnormal vibration or displacement. Slight oxide scale on the surface is normal, but if there is penetrating corrosion or circular crack, it is ready to be replaced.
Regarding the service life, the industry is often asked "How many years will it last for expansion and energy saving"-there is no standard answer to this question. The design life is usually calculated by the number of fatigue times. The allowable cycle times given by EJMA standard range from several thousand to tens of thousands, but the actual life depends on the operating temperature, the number of pressure fluctuations, the corrosiveness of media and the installation quality. It is normal for the main steam pipeline of a power station to have a design life of 30 years, and the expansion joint may be changed two or three times in the middle.
- There is no penetrating corrosion on the surface of the bellows, and the local pitting depth does not exceed 10% of the wall thickness;
- The wave pitch is uniform and there is no obvious tensile or compressive deformation;
- No abnormal vibration or noise, and the displacement indicator (if any) is within a reasonable range;
- The guide tube does not fall off and the weld seam does not crack;
- No leakage at the flange connection.
After checking these items, you can basically judge the state of the expansion joint. If you want to accurately assess the remaining life, you still have to find a manufacturer to review the fatigue life and benchmark the original design with the operating parameters. Don't be too troublesome, the cost of replacing an expansion joint is not high, and no one can afford the cost of an accident.