First, find out what kind of metal expansion joint you are facing
The name metal expansion joint is actually a general term. What you have in your hand may beUniversal corrugated expansion joint、High temperature axial expansion joint, could also beDirect burial type、External pressure single type axial type, even large-diameter thick-walled expansion joints. Different structures correspond to different working conditions: high-temperature axial type for high-temperature steam pipeline, direct burial type for buried pipeline, and occasions where transverse displacement needs to be absorbedCompound hinge transverse expansion joint。 Get the wrong model, and it will be a ticking time bomb.
Before selecting, look at the pipeline medium temperature, pressure and compensation direction-is it axial, transverse or angular displacement? For example, the power station industry usesCorrugated expansion joint for power station industryHigh temperature resistance and fatigue resistance are usually required, while the cement industryMetal Corrugated Expansion Joints in Cement IndustryTo deal with dust and abrasion. Each product has a clear application scenario, so don't mix it.
2. Four things before installation: arrow, guide tube, pull rod and bracket
Many people's first reaction when they get the expansion joint is to weld it to the pipe. Stop it. The first step is to look at the direction of the arrow on the expansion joint cylinder. That arrow represents the flow direction of the medium. If the direction is installed in the opposite direction, the guide tube will become a throttle plate, which will increase the pressure loss at least and cause the bellows to flush and fail at least —The direction of the arrow of the expansion joint refers toMedia flow, this common sense rollover case I have seen no less than ten times.
The second step is to check whether the guide tube is in good condition. The role of the deflector is to protect the bellows from being directly washed by high-speed media, especially fluids with particles (such as the pipe behind the smoke baffle door). Once the guide tube falls off, the bellows life plummets. Step 3, confirm the position of the tie rod nut. Adjustment nuts on tie rods are usually in a pre-tensioned or pre-compressed state when they leave the factory to compensate for installation errors. Once installed in place, these nuts should be loosened or adjusted according to the design requirements-how exactly? refer toHow to adjust the tie rod nut of expansion jointThe core principle of that question and answer is: let the expansion joint be in the design pre-displacement in the cold state, and only in the hot state can it work normally.
Step 4, check the pipe support. The expansion joint itself does not bear the weight of the pipe, so it is necessary to set up a fixing bracket and a guide bracket. The lack of guide brackets can cause the expansion joint to twist, which is the most overlooked mistake in installation. And guess what? Some construction sites did not install guide brackets, and three months later, the bellows was directly twisted into a twist.
3. Hard rules of upper pipe welding: torque and misalignment are not allowed
Before welding the expansion joint to the pipe, make sure that both ends of the pipe have been adjusted to the same axis. Forcibly using expansion joints to compensate for pipeline installation deviations (such as non-parallel flanges and uneven gaps) is equivalent to keeping the bellows in a torsional state after welding. The biggest fear of metal bellows is torsional loads-it is good at tensile compression, but its ability to resist torsion is almost zero.
The correct thing to do is to position the pipe with a jack or a reversed chain so that the flanges or welded joints at both ends of the expansion joint are naturally aligned. After welding, remember to review the straightness of the expansion joint. If the deviation exceeds 1% of the nominal diameter of the pipe, it should be reworked. Also, if you useDouble-sealed single-axis circular baffle doorOrElectric plug-in insulation doorIn this type of sealed structure, welding heat may damage the internal seal and require cooling measures. Alas, I've seen cases where seals melt directly after welding on site, and the rework cost is enough to buy three new expansion joints.
4. Debugging stage: Is the tie rod disassembled or not?
It was the most asked question on the scene.Do you need to remove the screw of the expansion joint？ First of all, be clear: the tie rod (or screw) plays a protective role during transportation and installation, preventing the bellows from excessively elongating or compressing. Once installed in place, whether to remove it depends on the expansion joint type.
For general-purpose corrugated expansion joints, if the design allows for axial displacement, then the locking nut on the tie rod needs to be loosened to allow the bellows to expand and contract freely. But if the tie rod is used aslimit tie rodDesign (such as double hinge transverse expansion joint), it cannot be removed at will, but the nut position should be adjusted according to calculation. How to judge? Look at the product nameplate or drawing: If the value of "pre-deformation" is marked, it is usually necessary to adjust the nut to the specified scale in the cold state. For specific operation, first loosen the nut on one side, and then symmetrically adjust the other side to avoid force on one side. After adjustment, check the bellows for skew. To put it bluntly, whether the pull rod is removed or not depends on whether it plays a "bodyguard" or a "traffic police"-the bodyguard can be withdrawn, but the traffic police have to keep the command.
5. Fatal misunderstanding in operation: the insulation layer cannot wrap the bellows
In order to keep heat at many sites, the expansion joints are wrapped tightly with rock wool. This is a big no-no. The inside of the bellows is a cavity structure, which needs heat dissipation. If the insulation layer is directly applied to the bellows surface, local superheating zones will be formed and material fatigue will be accelerated. The correct way is: the insulation layer should only be made on the pipes at both ends of the expansion joint, and the expansion joint itself should be exposed, or a detachable insulation sleeve should be used.
In addition, for high-temperature axial expansion joints and corrugated expansion joints used in power station industry, we should also pay attention to whether there is condensed water accumulation inside the bellows-when starting at low temperature, if the medium is steam, the condensed water may form a water hammer and impact the bellows instantly. The solution is to set the trap in the low position. Is that the truth? Just like when a person wears a cotton-padded jacket, he can't wrap his head around him, so he must leave the heat dissipation port.
6. The maintenance cycle is very simple: look at the displacement and fatigue times
Metal expansion joints are limited life components.Service life of expansion jointDepends on the actual number of cycles rather than simply the time. For example, an expansion joint with a design life of 1,000 full displacements can be used for about three years if it starts and stops once a day; But if the pipe vibrates frequently (like connecting the pump outlet), it can fail in a few months.
During maintenance, focus on checking whether there are cracks at the crest of the bellows and corrosion pits on the surface. ForLarge diameter thick wall expansion jointAnddirectly buried expansion joint, due to being buried in the soil or difficult to disassemble, it is recommended to reserve inspection holes or use models with leak detection during installation. If you useRotary compensatorOrSleeve type pipe expansion jointThe maintenance method is different-this type of product relies on sliding seals and needs regular grease injection. In short, different structures have their own rules. Don't apply the same maintenance method to all expansion joints.
Two days ago, I met a customer who installed a general-purpose expansion joint on the steam main pipe, without a guide bracket, and the insulation layer was still wrapped tightly. Three months later, the bellows cracked, and I came to ask "How to use the metal expansion joint?"-I can only say that the wrong installation is equal to white installation, and one step of type selection, installation and debugging can't be saved.