Let's be clear: What is DLD standard metal expansion joint? Not all bellows are called DLD standard
Many customers ask with the drawings, and they say "I want DLD standard metal expansion joint". However, DLD is not a national mandatory standard, let alone a unique brand. It is more like a tacit name in the industry for a specific working condition-large diameter, low stiffness and high durability. To put it bluntly, it is mainly aimed at those occasions where the pipeline diameter is large, the pressure is not high, but the displacement requirements are strict, and it has to run stably for a long time.
For example, you can often see this thing when you look at the circulating water pipeline of the power station and the cold air pipeline of the cement plant. This site'sLarge diameter thick wall expansion joint、High temperature axial expansion joint、Straight pipe pressure balanced expansion jointIn fact, they are typical representatives of DLD standard system.
Then how to tell if an expansion joint meets DLD standards? Look at three core points:Is the number of bellows layers reasonable(generally multilayer thin wall),Is the guide tube complete、Is the port structure able to withstand local stress。 Don't just look at the nameplate, you have to dismantle the structure. Some manufacturers dare to raise the price by printing "DLD" on the nameplate. As a result, there is only one layer of bellows and no deflector, which will crack in less than half a year.
Type selection is mine clearance: temperature, pressure, displacement, a data wrong has to be reworked
I met a customer two days ago. It is not said that the DLD standard is that the thicker the thickness, the better. As a result, the compensation amount is not enough when installed, and the pipe bulges directly. When choosing DLD standard metal expansion joint, the most taboo thing is to pat your head.
Normal temperature is generally 304, over 400℃ get 316L or Inconel, even higher, such as above 600℃, you have to consider lining with heat-resistant layer, which is likely to use usHigh temperature axial expansion jointStructure of. Look at the pressure again: DLD standard medium pressure is generally between 0.25-2.5MPa. If it exceeds 2.5MPa, you have to use compound hinge or straight pipe pressure balance type, because these structures can share the internal pressure thrust. The displacement is more critical: axial, transverse and angular, each displacement has to be calculated separately. If you are buyingUniversal corrugated expansion joint, which mainly absorbs axial displacement; But if it is a large tie rod expansion joint, it is mainly used to get the lateral displacement. If you choose the wrong model, you can make abnormal noise or crack the weld. Therefore, when selecting the model, take the working condition table directly, and we compare the parameters one by one.
Installation is not screwing: pull rod, guide tube, cold tightening, one step wrong step wrong step
Many masters think that the expansion joint is an elastic joint. If you go to the middle of the flange, the bolts will be tightened. In fact, the DLD standard metal expansion joint has so many installation details that it gives you a headache.
Let's start with tie rod nut adjustment-this pit has the most people stepping on it. The tie rod is to limit the bellows. Before installation, the nut should be screwed to the pre-stretched position (see the cold tightening amount marked in the product), and then loosened after installation. Many sites were screwed to death directly, resulting in the corrugated pipe not being able to expand and contract freely, and it cracked after a few days. About this, this siteHow to adjust the tie rod nut of expansion jointThat article makes it very clear.
Looking at the deflector again, many people ask what this thing is for? To put it bluntly, it is to direct the medium to prevent high-speed fluid from washing the bellows. There is an article in the station that specifically talks aboutSpecific Function of Expansion Joint Guide Tube: If it is installed in the opposite direction-the arrow must point in the same direction as the medium flow-then the guide tube becomes a jamming piece, and the bellows will quickly be scrapped.
For pipes with a temperature higher than 300℃, cold tightening must be carried out according to the calculated value during installation, otherwise the expansion joint will exceed the limit in hot state. If you are not sure, remember a formula: welding the pipe first, and then installing the expansion joint; Loosen the tie rod first, then test the pressure.
Maintenance is not so mysterious, but life depends on these three tricks
The design life of DLD standard metal expansion joint is usually between 1000-3000 cycles, but how long it can actually be used has much to do with the state after installation.
Regularly check the outside of the bellows for cracks or corrosion pits. Visual inspection will do, every three months. Tip 2: Look to see if the tie rod and nut are loose. If the pipe vibrates heavily, the locking nut of the tie rod will slowly back, and if it is not treated, it will cause the bellows to bear additional stress. The third trick: pay attention to whether there is fluid accumulation in the expansion joint. Especially when horizontally installed, if water accumulates inside the guide tube, long-term rust will rot through from the inside. There was a customer of a cement factory before, and the deflector was worn after two years of use, just because the drainage hole was not opened. Therefore, you must leave a good drainage hole when installing, or choose a structure with drainage function.
If you've already noticed a minor leak, don't rush to change it-first determine whether it is at the trough or the crest. Trough leakage is usually stress fatigue. You can try to repair the leak temporarily with epoxy resin, but don't drag it and replace it as soon as possible. If the peak leaks, there is a high probability that it is corrosion and perforation, and you can replace it with a new one without saving it.
What's so good about it? Speak with cases
To be honest, DLD standard metal expansion joint is not a black technology, but it is better than reliability and versatility. For example, the steam pipeline of a coal-fired power plant, which used to use ordinary corrugated expansion joints, exploded three times a year. Then it was usStraight pipe pressure balanced expansion joint(Design that meets the DLD standard for large diameter and high requirements), and has been running continuously for three years without any problems.
For example, the rotary kiln exhaust gas pipeline in the cement industry has a large temperature fluctuation and a lot of dust, so ordinary nonmetallic compensators can't hold it for two months. Replace withMetal rectangular expansion jointWith ceramic fiber lining, the service life is directly pulled to more than 18 months. These cases are not bragging. The core reason is that the DLD standard puts sufficient safety margin in structural design-the wall thickness is added by 0.5mm, the wave pitch control is stricter, and the welding process passes the fatigue test. Of course, the price is more expensive, but if you calculate the general ledger, you will lose hundreds of thousands at a time when you shut down, and you will spend three to five thousand more to buy peace of mind, which is much more cost-effective.