Pipeline insulation can be done by masters who have worked for several years with their eyes closed. However, if you encounter the insulation construction of metal expansion joints and follow the idea of pipelines, there will be problems nine times out of ten. The expansion joint is used to absorb thermal displacement. If you pack the insulation layer, the displacement is "wrapped" to death, the bellows can't move, and the pipeline stress is all held in the system, ranging from leakage to fatigue cracking of the bellows. Therefore, to do this work, you have to figure out one thing first: expansion joints and pipes are not essentially the same thing.
Why can't the metal expansion joint follow the insulation practice of ordinary pipes?
Ordinary pipes are rigid, and the stress of thermal expansion and contraction is digested by the pipe bracket and natural bending. The insulation layer only locks the heat. The expansion joint is different. It is a "soft joint" specially reserved on the pipeline. The bellows absorbs thermal displacement by deformation. You wrap it as tightly as a pipe, and the ripples won't expand or contract properly. Take a steam pipe as an example. For every 100°C increase in steam temperature, the pipe elongates by about 12 millimeters per 10 meters. This displacement is ultimately borne by the expansion joint. If the corrugation is filled with insulation cotton, the wire is tightened, and the displacement is nowhere to be released, the stress concentration of the corrugated pipe will occur locally, and the service life will be greatly reduced. According to statistics, the failure cases of expansion joints caused by improper insulation construction account for a high proportion of bellows leakage accidents.
Understand the types of expansion joints before construction: axial type, transverse type and pressure balance type
Thermal insulation construction of dry metal expansion joint, whether the work in hand is good or not, three points depend on the technique and seven points depend on the recognition of goods.
Axial type expansion joint, the bellows expands and contracts along the pipe axis, and the insulation layer should leave enough axial movable clearance in the corrugated section. Straight pipe pressure balance expansion joint and external pressure single axial expansion joint have similar principles-there are displacements on both sides of the bellows, and there are structures such as guide tube and ring plate in the middle, so heat insulation can't stick the movable parts together. The transverse expansion joint of the compound hinge bears the internal pressure thrust by the hinge and the tie rod, and the bellows only deflects laterally. If the insulation layer is wrapped near the hinge shaft and the rotation angle is limited, there will be a situation of "pressing down and not getting up". Curved tube pressure balance expansion joint is often used in hot air duct, the gap size between bellows and balance ring, which is marked on the manufacturer's drawings, and measured with a ruler during construction.
How does the insulation avoid the corrugated part?
"The insulation layer at the expansion joint should leave an appropriate gap." The word "appropriate" sounds mysterious.
In practice, the heat insulation cotton cannot be stuffed above the crest and trough of the corrugation. If the insulation cotton is filled with the trough for good looks, once the ripples are compressed, the batting is squeezed into the bottom of the trough, and the wave peak will directly press against the inside of the insulation layer, generating a reaction force-which is equivalent to adding a rigid limit to the expansion joint. For high-temperature axial expansion joints, heat dissipation itself is also a part of design considerations. It is the industry practice to expose the corrugated part or adopt a detachable insulation sleeve. The inside of the guide tube is the medium flow channel. If the insulation layer is wrapped around the outside of the guide tube, the medium heat will be held in the wall of the tube, causing temperature difference stress. The position of the pull rod and the nut should also allow an adjustment margin. Some projects use cable ties to tie the tie rod and the insulation layer together, and when the nut is to be adjusted, it is found that it can't turn at all.
Inventory of Common Construction Errors
The first, insulating cotton stuffed into the corrugated valley. This is the most common mistake. Insulation cotton itself is a flexible material, so there is no problem in the short term. When the system runs for a period of time, the bellows expands and contracts repeatedly, the cotton batting is compacted and agglomerated, and finally the trough is blocked. Second, lock the pull rod. The tie rod of the expansion joint was originally designed to restrain the internal pressure thrust, not to let you lock the displacement. In order to fix the insulation layer, some construction workers wrap the wire directly on the tie rod, and even screw the tie rod nut to death. When the bellows needs to be stretched, the tie rod becomes a rigid support, and the displacement is all transferred to the pipe elbow and bracket. Third, ignore the heat dissipation requirements of high-temperature axial expansion joints. High Temperature Axial Expansion Joint Under high temperature conditions, the bellows material relies on natural heat dissipation to keep the metal temperature within the allowable range. If the insulation layer is made too thick or too dense, the temperature of the bellows will increase, and the creep speed of the material will be doubled. To put it bluntly, bellows are not afraid of heat, but they are afraid that the temperature is high and they can't disperse.
How to check and accept after construction?
After the metal expansion joint insulation construction is completed, the acceptance should be viewed in three positions.
First, look at the displacement markers. Regular manufacturers have displacement indication signs (red arrows or scale rulers) when the expansion joints leave the factory. After the construction of the insulation layer, this mark must be exposed, visible and readable. If the mark is covered by insulation cotton, it means that the insulation layer is out of range of motion, and it should be reworked and redone. Second, look at the way the insulation layer is fixed. It is recommended that the expansion segment adopt a detachable thermal insulation structure, which is fixed with stainless steel clamps or Velcro, and it is strictly prohibited to bind with iron wire. Third, look at the temperature monitoring point settings after operation. After the high-temperature pipeline insulation is completed, the surface temperature of the insulation layer generally does not exceed 50℃. In the corrugated section of the expansion joint, if the local surface temperature is abnormally high, it means that there are voids inside the insulation layer; If the thermal insulation of the expansion joint body is too tight, the surface temperature will be low, and this kind of "over-protection" should be vigilant. On the acceptance form, record these data on the record. Whether the effect is good or not, we will see after three months of operation.
In the final analysis, the insulation construction of metal expansion joints is a matter of one sentence: the place where it should be packed wraps the heat, and the place where it should be exposed leaves enough activity. Don't think of bellows as ordinary pipes, and more than half of the problem is solved.