1. Why do we have to do a pre-displacement? Can you not set it up?
Two days ago, I met a customer who had a directly buried (fully buried) expansion joint on the steam pipeline. After installing it according to the drawings, the flange collapsed directly after half an hour of startup. The problem is that the pre-displacement is set to zero-which is equivalent to having the compensator directly carry the heat expansion on the cold pipe. To put it bluntly, pre-displacement means that the compensator "eats" part of the displacement in advance when installed, so that it is in the middle position at the working temperature, so as to avoid stretching one side to the limit and compressing the other side to death. The thermal pipeline design code clearly states that after calculating the thermal elongation, the pre-displacement is generally about 50%. However, the real working conditions are complicated, and 50% is only a starting point. If you don't set the pre-displacement, it is equivalent to leaving the bellows at the extreme position in the cold state. As soon as the temperature rises, it will either break or crush. So don't ask "can you not set it up", ask is waiting for rework.
2. How to calculate the pre-displacement? The formula is not complicated, but don't slap your head with the data
Δ X = α × L × Δ T × K. α is the linear expansion coefficient, carbon steel is about 0.012mm/m·℃, L is the length of the pipe section, Δ T is the difference between the working temperature and the installation temperature, and K is the pre-displacement coefficient (usually 0.5~0.7). But don't think you'll be done with a formula. The sensitivity of the rotary compensator and the large tie rod expansion joint to the pre-displacement is completely different. The rotation compensator itself absorbs the displacement by rotation, and if the pre-displacement is too small, the rotation angle will exceed the limit; However, the pressure balance expansion joint of straight pipe has stricter requirements for axial displacement, and if the pre-displacement is larger, the bellows may be pressed out of plastic deformation. Therefore, after calculating the theoretical value, we have to check it again according to the article "Stiffness and Calculation Formula of Bellows" in the product information of this site, combined with the allowable displacement of the specific model. Otherwise, the number you calculate may just be the fatigue limit point of the bellows, and it will be wasted once it is used.
3. Different compensators, the "character" of pre-displacement is far different
Metal corrugated expansion joints and rubber compensators are not the same thing. The elastic modulus of the rubber compensator is low, so it is easy to bulge if the pre-displacement is set large, and it can't protect if the pre-displacement is set small. What about PTFE-lined hoses? The PTFE layer itself is afraid of stretching, and the safety margin on the stretched side must be prioritized for pre-displacement. Let's talk about the corrugated expansion joint-high-temperature and high-pressure steam pipeline used in power station industry. The pre-displacement amount often needs to be more than 60% of the calculated thermal displacement, because the temperature difference impact during startup and shutdown will bring additional dynamic displacement. On the contrary, the metal corrugated expansion joint in cement industry has much dust and slow temperature fluctuation. It is enough to set the pre-displacement to 40%, but it is easy to get stuck by dust accumulation if it is set too much. You see, it is also called a compensator, and the pre-displacement "character" in different scenarios can be twice as different. So don't conquer the world with a single coefficient.
Fourth, at the installation site, how to "adjust" the pre-displacement?
Armchair is over, talk about work. The pre-displacement is typically performed by a tie rod or screw. For example, for the expansion joint of a large tie rod, the screw will be locked in the pre-stretched position when leaving the factory. When installing, compress or stretch the expansion joint to the pre-displacement required by the drawing, and then lock the tie rod nut. There is a pit here: many people read the question and answer "How to adjust the tie rod nut of the expansion joint" on this site, thinking that just twist it a few times. In fact, you have to measure the bellows length variation with a dial gauge or vernier caliper, accurate to the millimeter. For example, the universal corrugated expansion joint requires a pre-compression of 10mm. When you screw the screw with a wrench, you must monitor the flange spacing at both ends at the same time. Don't rely on the feel. In addition, if there is a compensator near such equipment as electric plug-in plate isolation door, the mechanical impact when the isolation door is opened and closed should be considered in the pre-displacement, leaving an additional margin. Otherwise, when the door was closed, the pre-displacement would be directly knocked out.
5. The most terrible rollover cases, guess what?
One power plant uses a curved tube pressure balance expansion joint, and the pre-displacement direction is set reverse-what should be compressed is set to stretch. As a result, the bellows was squeezed into twists and scrapped directly after operation. Second: a chemical plant selected an external pressure single axial expansion joint, and the pre-displacement completely copied the theoretical value. It was not considered that the ambient temperature during installation was 35℃ (the designed installation temperature was 20℃), which was equal to the actual pre-displacement being about 15% less. Two months later, the compensator end weld cracked. Third (this one is the most outrageous): some people also calculate non-metallic expansion joints (fabric fiber expansion joints) according to the pre-displacement coefficient of metal. The creep characteristics of fabric fibers are completely different from those of metals, and excessive pre-displacement leads to early fatigue tearing of the fabric layer. Therefore, don't be lazy. The product manual of each compensator has a recommended pre-displacement range and correction factor, but some people don't look at what is in their pocket. Is that the truth?
6. Summarize an iron law: Pre-displacement setting is a "live", but it has to be done according to the rules
There is no universal formula, but there is a basic logic: first calculate the thermal elongation-confirm the displacement capacity boundary of this type of compensator according to the product data of this site-consider the installation temperature correction-accurately adjust with tools in the field-finally make cold state marks and record. If you are really unsure, go directly to the manufacturer to set the parameters. For example, pressure balance expansion joint, compound hinge transverse expansion joint and other complicated structures, the pre-displacement setting is often linked to pipeline stress analysis, not a matter of screwing a few screws. Remember, if the compensator is selected correctly and the pre-displacement is set wrong, the whole system will still be played out. Therefore, the next time someone asks you "How to set the pre-displacement of the compensator?", you will throw these six words to him: calculate, correct, repair, adjust, remember and ask. One step less, and it may collapse.