Steel plate metal expansion joint, did you really choose the right one?
Two days ago, a buddy who was doing a power plant project spat with me, saying that the expansion joint they purchased began to leak in less than half a year. When they were removed, the bellows were worn out. Only after asking did he know that when he bought it, he just looked at the price and didn't know the working conditions at all. Alas, I can run into this kind of thing dozens of times a year. Today, I'll break up the metal expansion joint of steel plate and crush it to make it clear. After reading it, you can choose it again, and you can step on 80% less pits.
What is a Plate Steel Metal Expansion Joint? Don't get confused by the name
To put it bluntly, it is a stretchable metal bellows installed in the middle of the pipe to absorb the displacement caused by thermal expansion and contraction, vibration or installation deviation. And the ones in our siteUniversal corrugated expansion joint、High-temperature axial expansion joint is essentially a kind of thing, but it emphasizes using steel plate as material, and the wall thickness is more solid than that of ordinary bellows.
Steel plate metal expansion joints are generally used in high temperature, high pressure and large diameter occasions, such as the main steam pipeline of power stations and the kiln head flue gas pipeline of cement industry. Pay attention to the structure: there are several layers of stainless steel corrugations on the outside, and there is often a guide tube inside. Regarding the function of the guide tube, we specifically mentioned it in the FAQ of our site, which mainly prevents high-speed media from directly washing the bellows and prolonging its life. When selecting, if there is no guide tube and the flow medium carries particles, then wait for leakage.
Focus on these 3 parameters when selecting models to spend less money
Don't come up and ask, "How much is this expansion joint per meter?" Experts ask three numbers first:Nominal pressure, design temperature, compensation amount。 None can be missing.
The first is the nominal pressure (PN).Is it 0.6MPa or 2.5MPa in your pipeline? By one grade, the wall thickness and the number of layers of the bellows may double. For example, those used in the power station industryCorrugated expansion joint for power station industryIn some high-pressure occasions, double-layer or even three-layer corrugations should be selected, and single-layer ones can't bear it. If the pressure is chosen low, it will be a matter of time before the bellows burst.
The second is the design temperature.This stuff is critical, but not so easy to calculate. The temperature resistance of steel plate expansion joint mainly depends on the material: 304 stainless steel can carry about 600℃, and 316L is slightly higher but not more than 700℃. If the temperature exceeds 700°C, you have to considerHigh temperature axial type expansion joint or thermal insulation lining inside. Two years ago, there was a cement factory customer with a pipeline temperature of 850℃, and an ordinary 304 expansion joint was selected. As a result, the bellows was directly peeled and oxidized, and it was wasted in two months.
The third is the amount of compensation.The compensation amount includes axial displacement, lateral displacement and angular displacement. Many novices only give a number, such as "to compensate 100mm", but don't say whether it is axial or horizontal. The compensation ability of steel plate metal expansion joints in different directions is very different: it can generally reach tens of millimeters in the axial direction, and it may only be a few millimeters in the transverse direction. If you want to compensate both directions at the same time, you have to chooseCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointThese structures with tie rods and hinges. The function of the tie rod is to bear the pressure and thrust, so as to prevent the bellows from being blown to death-how to adjust this thing? Our FAQ specifically says that before installation, you must loosen the transport bolt, but don't screw the tie rod nut to death.
Several details of the easiest rollover at the installation site
When the equipment arrives at the site, installation is the real big test. The most rollover cases I've seen are three:
- Install the direction reverse.There is generally an arrow on the expansion joint pointing to the direction of media flow, and that arrow is not randomly drawn-especially the model with a guide tube, with the small mouth of the guide tube facing downstream. Install backwards, the medium directly impacts the root of the bellows, and it can flush and leak in a few months.
- Cold pull didn't do it.The difference between the installation temperature and the operating temperature of the pipeline is tens or even hundreds of degrees, and the expansion joint should be pre-stretched or pre-compressed by a certain amount when it is cold. For example, the design temperature is 300℃, the installation temperature is 20℃, and the axial compensation amount is 60mm. When it is cold, it should be stretched by about 30mm. This amount is not calculated correctly, and it either can't be pulled or pushed over when it runs.
- The bolt is screwed.Some installers think that the tighter the bolt, the better, and screw the tie rod nut of the expansion joint to the end. As a result, the expansion joint can't move at all, and the force generated by the thermal expansion of the pipe is all transmitted to the fixed bracket, and the bracket will break for you. The correct method is: remove the transport bolts after installation, and keep a certain clearance after the tie rod nut is adjusted in place to allow the bellows to deform freely.
Regarding the installation of the expansion joint of the large tie rod, there is also a special question and answer on our site. You can search directly, which is more detailed than I said.
Steel plate metal expansion joint vs rubber compensator vs non-metal expansion joint, who is more built?
Look at the temperature, the medium, and the pressure.
rubber compensator(For example, ourrubber compensatorAndRubber PTFE compensator) The advantages are cheap, easy to install and good vibration reduction effect. However, the temperature resistance generally does not exceed 120℃, and the pressure resistance is not high. It will age and crack in one or two years. It is suitable for normal temperature and low pressure ventilation ducts or water pump inlet and outlet.
Non-metallic expansion joint(i.e.Fabric fiber expansion jointWe haveRectangular non-metallic expansion joint) The temperature resistance can be very high-silica gel cloth plus ceramic fiber can carry above 1000℃, and there is almost no elastic reaction force, which is especially suitable for large smoke ducts. However, the disadvantage is that the pressure bearing capacity is weak, so it can only be used in micro-positive pressure or negative pressure systems, and it is easy to wear, so the life of 2-3 years is considered good.
WhileSteel plate metal expansion jointIt's the tough guy in the middle: wide temperature resistance range (-200℃ ~800℃), pressure can reach tens of MPa, and long life-it is designed properly and will be used for more than ten or even twenty years. The cost is expensive, heavy and high installation requirements. To put it bluntly, if you are working in the main steam pipeline of the power station, the chemical high-pressure pipeline and the tail of the cement kiln, you have no choice but to put on steel plate metal. If it is an ordinary air conditioner water pipe, it is enough to use a rubber compensator, so why spend more money?
It leaked when you used it? Check these 5 reasons first
Don't rush to scold the manufacturer if you leak it. Let's see if it is the following problem:
- Corrosion perforations.The medium contains chloride ions or acidic gas? Stainless steel is most afraid of chloride ion stress corrosion cracking. Did you use 304 for your steel plate metal expansion joint? Try 316L or duplex steel.
- Fatigue cracks.Too many cycles or too high a vibration frequency. Check whether the design only considers thermal displacement, but not mechanical vibration. If yes, add aMetal hoseIt would be much better to do the vibration damping transition.
- The guide tube fell off.If the welding quality of the guide tube is not good, it will fall off and directly jam the bellows during operation, and it is only a matter of time before it wears out. There is a schematic diagram in the article on the diversion tube in our FAQ, which can be understood at a glance.
- Excessive mounting deviation.The pipeline is uncentric and the support settles, resulting in the expansion joint bearing additional displacement that is not in design, and the local stress exceeds the limit. In this case, the pipe stress is re-checked.
- Water in the insulation layer.If the exterior of the expansion joint is covered with heat insulation, the insulation layer leaks or condensed water accumulates, and the exterior surface will also corrode after long-term immersion. Don't ignore.
Oh yeah, there's another trivia: the expansion joint and the compensator are actually the same thing, but they are called differently. We have a special explanation in our FAQ, so don't be fooled by sales again.
Steel plate metal expansion joint this thing, three points depends on the product, seven points depends on the selection and installation. If you get the parameters right and keep an eye on the construction, it can run safely for at least ten years.