Calculate the thermal displacement first: temperature change is the starting point, installation temperature is a pit
Metal expansion joint calculation method? The first step is always thermal displacement. The formula is simple enough to recite: Δ L = α × L × Δ T. The linear expansion coefficient α is 0.012 mm/m·℃ for carbon steel, 0.016 for stainless steel and 0.018 for copper. Just note it. The pipe length L is dead, but how to take Δ T? A lot of people are stuck here.
Does the installation temperature use the ambient temperature or the actual pipe temperature? I'll tell you, directly based on the local extreme temperature difference. For example, outdoor pipelines in the north, -30℃ in winter and +40℃ in summer, Δ T is 70℃. Don't worry about what month it is when it is installed, leave an allowance. The expansion amount of the pipe is considered small, the expansion joint is hard, and the bellows is directly torn. A while ago, a customer used a general-purpose corrugated expansion joint on a steam pipeline. The installation temperature was 20℃, the actual steam was 160℃, and Δ T was only 140℃. As a result, the actual operating temperature of the pipeline fluctuated to 180℃, and the expansion amount exceeded 20%. The bellows cracked after half a year.
The wrong calculation of pressure and thrust causes the expansion joint to be scrapped directly
The thrust generated by the pressure is F = P × A, and this A is the effective area of the bellows. However, the effective area calculation methods of different structures are completely different. For example, the effective area of the general corrugated expansion joint is equal to the circular area corresponding to the average diameter of the corrugated pipe; However, the effective area of the external pressure single axial expansion joint should be calculated according to the inner diameter of the external pressure cylinder, not the bellows itself. There's a big pit here: the blind plate force created by pressure, and the pipe can twist like noodles if it is not withstood by the tie rod or the main fixing bracket.
When calculating pressure thrust, don't forget that the medium may still have impact force. Water hammer effect when steam pipe starts, instantaneous pressure may be twice the working pressure. You calculate the thrust according to the steady-state pressure, but the fixed bracket is not designed enough, and the expansion joint tie rod breaks directly. Last year, in a chemical plant, the straight pipe pressure balance expansion joint was not counted as a water hammer, the fixed bracket was pushed and displaced by 30mm, and the bellows was unstable and scrapped.
Compensation amount, stiffness and fatigue life-how to break the iron triangle?
These three parameters are constrained by each other. The greater the amount of compensation, the softer the bellows (lower stiffness), but the fatigue life will be shortened. The national standard requires that the fatigue life is not less than 1000 times, which is the basic line. In practice, we generally design corrugated expansion joints and high-temperature axial expansion joints for power station industry according to 10,000 times. How to calculate? Reference to EJMA standards, combining wave pitch, wave height and wall thickness.
A bellows with the same caliber DN300 and a wave pitch of 40mm is 30% more stiff than a wave pitch of 50mm, but the fatigue life may double. The wall thickness increases by 0.3mm, the stiffness increases obviously, and the compensation ability decreases. It takes repeated iteration to find the balance point. Two days ago, someone took a double hinge transverse expansion joint and asked for a compensation amount of 100mm. I saw that the wall thickness was only 1.5mm, the wave number was 8, and the fatigue life was only 500 times. It is recommended that the wave distance be widened, the wall thickness be 1.8mm, the wave number be increased to 12, and the life time be mentioned only 8,000 times.
Don't use one algorithm for all models
The calculation ideas of straight pipe pressure balance expansion joint and double hinge transverse expansion joint are completely different. The pressure balanced type counts the thrust cancellation generated by internal pressure-it has two bellows itself, one absorbing displacement and the other canceling the pressure thrust. However, the hinge type should consider the coupling of angular displacement and lateral displacement, not simple superposition. For example, the transverse expansion joint of compound hinge, the angle change between two hinge groups will produce axial and lateral displacement at the same time, and it is necessary to draw a displacement vector diagram to calculate it clearly.
Large diameter thick wall expansion joints are more troublesome. The increase of wall thickness leads to the increase of stiffness and the decrease of compensation ability, but the pressure thrust is large. You have to assume a wall thickness first, calculate the compensation amount, check the fatigue life, and adjust the wall thickness or wave number if it fails, and iterate repeatedly. Some designers try to save trouble and directly use empirical formulas. As a result, the stress of the pipeline exceeds the limit and the vibration is abnormally large.
Guide tube and tie rod nut are not decorations
The function of the guide tube is to reduce the erosion of the medium, especially for high temperature and high speed steam. But it affects the effective area and the pressure loss – the inner diameter variation must be taken into account in the calculation. Many people directly calculate according to the inner diameter of the bellows, ignoring the space occupied by the guide tube. As a result, the actual pressure loss is more than 30% greater than the calculated value. The adjustment of the tie rod nut directly determines the direction of pre-deformation, and if it is installed backward, the expansion joint will fail in advance. How to adjust the expansion joint tie rod? The principle is: when cold tight, the tie rod nut compresses the bellows, so that it has a pre-stretch in the installed state, so that when the working temperature rises, the bellows can be both compressed and stretched, and the service life is longer.
Actual Combat Checklist: Don't wait for an accident to regret it
After getting the medium temperature, pressure, pipe diameter and direction diagram given by the customer, follow this order: first draw the displacement vector diagram to clarify the axial, transverse and angular displacements; Calculate thermal displacement and pressure thrust; Then the structure type is selected according to the working conditions-high temperature axial type for high temperature, large diameter thick wall expansion joint for large diameter, and transverse type of compound hinge for absorbing multi-directional displacement; Finally, check the fatigue life, which is not less than the design life.
Two days ago, a customer used the double straight pipe bypass pressure balance expansion joint on the steam pipeline. As a result, the lateral displacement was not counted. After the thermal expansion of the pipeline, the lateral offset of 15mm was produced, and the expansion joint was hard, so the bellows leaked after three months. You say it was wrong or not? The lateral displacement is only 15mm, and it can be solved by adding a double hinge transverse expansion joint. Why save that money?
Metal expansion joint calculation method? To put it bluntly, there are three parameters: thermal displacement, pressure thrust and fatigue life. If one is wrong, it will wait for rework. Selection is not a chance, it is calculated.