How to arrange the bracket of the expansion joint? This question is asked almost every few days. Most of the people who asked were that the pipeline stress calculation had just been completed, and the result was stuck in the bracket step-the bellows model was selected and the compensation amount was calculated. As a result, the bracket layout was wrong, and the expansion joint was selected for nothing.
Fixed brackets and guide brackets. The fixed bracket is responsible for dividing the pipe system into several independent pipe sections, each section can digest its own thermal displacement, and prevent force from being transmitted randomly to equipment, valves or weak points. The transverse displacement of the guide bracket tube ensures that the tube goes in a straight line when it expands and contracts, and the bellows is not screwed as a universal joint. With the cooperation of the two, the bellows can honestly do the expansion and contraction it should do. Mixed? The bellows cracked when twisted, the fixed bracket was crooked, and the thrust pushed to the place where it shouldn't have been pushed. There were many accidents.
How to fix the distance of the fixed bracket? Don't slap your head
The fixing bracket is the anchoring point of the pipe system. When the spacing is large, the thermal elongation of the pipe between the two anchor points exceeds the absorption capacity of the expansion joint; The spacing is small, the number of brackets is too large, and the cost can't be suppressed. How does that count? Looking at the type of expansion joint, compensation amount and pipeline stress calculation results, all three are indispensable.
The spacing between the fixed brackets of the universal corrugated expansion joint (axial type) is usually divided according to the natural compensation ability of the pipe system. The natural compensation is not enough, so it is considered to absorb with expansion joints. The hinge-type and compound-hinge transverse expansion joints with tie rods are different-their compensation ability comes from angular displacement and transverse displacement, and the spacing of fixed brackets should be checked according to the length of reverse thrust arm. To put it bluntly, the bellows is subjected to lateral thrust and bending moment in addition to axial force. When the bracket distance changes, the force arm changes.
Someone asked, are there any experience points? Yes, but don't use experience points as a master key. For small-diameter steam pipes below DN300, the distance between fixed brackets is 10 meters and 20 meters. Large-diameter high-temperature pipeline, five or six meters have to be set up. Why? The expansion amount is large, the wave number of the bellows is large, the stiffness is low, and the spacing is slightly enlarged, and the middle pipe section sinks. Calculate the pipe stress, and there are any answers.
The spacing and position of the guide brackets is more specific than you think
The guide bracket is too close to the expansion joint, and the bellows cannot absorb the lateral displacement; Too far, the pipe is unstable. How to handle this sense of proportion? The distance between the first guide bracket and the expansion joint is generally calculated according to 4D (D is the nominal diameter of the pipe), and the second guide is controlled according to 14D. For example: In the pipeline of DN200, the first guide bracket is about 800mm away from the expansion joint, and one is set every 2.8 meters behind it. However, note that this is only the recommended value of the general axial expansion joint-specific to the products of a certain manufacturer, the rigidity, pressure thrust and fatigue life of the bellows are different, so it is best to check the limit value in the product sample.
The role of the guide bracket is to give the tube a sliding constraint so that the thermal displacement goes in the intended direction. The pipe cannot swing laterally at the guide bracket, but it should be free to expand and contract in the axial direction. The most common mistake at the site is that the guide bracket is used as a fixed bracket-the angle steel is welded to death, the axial direction can't move, the thermal expansion and contraction forces are all suppressed on the bellows, and the pipe wall is pushed to bulge. On the other hand, the distance between the guide brackets is enlarged, the pipe shakes laterally, one side of the bellows is stretched and the other side is compressed, and the fatigue life drops directly from a cliff.
The bracket arrangement logic is completely different for different expansion joints
The single axial expansion joint is guided by the pipeline itself, and the bracket arrangement is the most conventional; But what about replacing it with a double straight pipe bypass pressure balanced expansion joint? It comes with its own balance structure, the thrust on the fixed bracket is small, and the bracket can be made lighter. The pressure balance expansion joint of curved tube is suitable for the elbow, and it can balance the internal pressure thrust itself, so the load of the fixed bracket is mainly friction.
It is an iron rule that hinged expansion joints must be used in pairs. Single hinges can only absorb angular displacement, and pairs can form lateral displacement compensation. The bracket must withstand the internal pressure thrust-the hinge structure can balance the internal pressure blind plate force, but the external load must be carried by the bracket. It would be a joke for someone to take the bracket scheme of the universal expansion joint to set the rotary compensator. The rotary compensator absorbs the displacement by the packing seal and the rotation of the rotating cylinder. The bracket layout pays attention to making the pipe rotate around the rotation center, which is completely different from the force logic of the bellows expansion joint.
The Three Most Common Mistakes in Real Cases
First, the fixing bracket is not welded firmly. Two days ago, I met a customer. During the pressure test of the pipeline, the whole bracket was pushed away, the bellows was pulled in a straight line, and the compensation amount was all gone. Check the reason, the welding length between the bracket and the embedded parts is not enough, and the height of the weld seam also shrinks. The internal pressure thrust is hundreds or thousands of kilograms, and the welding joint is not solid, so it is strange that the glue is not opened.
Second, the spacing between the guide brackets is too large. The waste heat power generation pipeline of a cement plant, DN500, and the guide brackets are set at an empirical spacing of 8 meters, nearly double that of 14D. After three months of operation, the bellows was twisted and deformed, and the lateral displacement pushed the bellows out of the pit. When I remove it, I see that the trough is full of fatigue cracks.
The third is to ignore cold tightness or pre-displacement. After the pipe system is cold tight, the bellows will have an initial displacement. The bracket was welded according to the position after cold tightening. As a result, as soon as the operating temperature rose, the displacement direction was opposite to the design, and the initial force of the bracket exceeded the standard. The squeak is considered light, and it is not uncommon for the bellows to be squeezed out of the tie rod limit.
So how to arrange the bracket of the expansion joint? There is no unified formula, but there is a main line: first distinguish the responsibilities of the fixed bracket and the guide bracket, and then determine the spacing and position according to the type of expansion joint, compensation mode and pipeline stress calculation result.