Find out how it is different from other expansion joints
Pipe compensator has many names-general corrugated expansion joint, straight pipe pressure balance expansion joint, double hinge transverse expansion joint... To put it bluntly,Straight pipe metal bellows expansion jointIt is the most common axial type, with no tie rod or hinge, and the simplest structure. But don't underestimate it just because it is simple-it is most afraid of lateral displacement. If you use it in a pipe system with elbows or pipes leading to complexities, wait to blow up.
Then how do you tell? See if it has a tie rod, hinge or balance ring on it. Straight pipe type is a bellows plus two end connections, with a guide tube at most. It can only absorb axial displacement and cannot withstand lateral or angular offsets. If you have to use it as a panacea, such as on a steam main pipe that needs to compensate for lateral displacement at the same time, it is only a matter of time before the bellows becomes laterally unstable. Two days ago, I met a customer, who chose the straight pipe type instead of the double hinge type for cheap. As a result, the pipe vibration directly twisted the bellows into a twist-tsk, the rework cost is three times more expensive than buying the right model.
If you don't look at these points in your selection, it will be a ticking time bomb
Pressure, temperature, displacement, fatigue life-none of these four numbers can be less. Let's talk about pressure first: don't just look at the design pressure, but also the hydraulic test pressure. In one case, a chemical plant selected a general-purpose corrugated expansion joint of PN10, and the pressure was tested to 1.5 times the design pressure (PN16). As a result, the bellows directly bulged. Why? The withstand pressure value given by the manufacturer is for the normal working temperature, which is normal temperature during pressure test. However, the wall thickness of the bellows is calculated according to the design pressure, and overpressure is finished. Therefore, you must confirm with the manufacturer when selecting the model:Whether the allowable pressure of the expansion joint of the straight tube metal bellows includes the test pressure margin.
The temperature thing is more subtle. When it exceeds 400℃, ordinary 304 stainless steel begins to creep, and the fatigue life drops from a cliff. At this time, either add insulation or change to heat-resistant alloys-such as Incoloy 800H or SUS310S. What about media corrosiveness? If it contains chloride ions, ordinary 304 will stress corrosion crack in less than two months. In the working conditions behind the cement flue gas pipeline and desulfurization flue gas baffle door, the Cl-concentration is always thousands of ppm, so 316L or even duplex steel has to be used.
The amount of displacement and fatigue life are twin brothers. In straight pipe design, it is generally calculated according to 1000 fatigue cycles, but in actual working conditions, it may start and stop once a day, and it will be scrapped in three years. What about that? When selecting the model, tell the manufacturer the actual cycle times, let them calculate the meridional stress of the bellows, and control it within one time of the yield limit. Or directly onExternal pressure single type axial type expansion joint, it is more resistant to instability than straight pipe type, the same displacement can carry more times.
There's also a pit-the medium flow rate. High-speed gas-solid two-phase flow (such as the flue of a power station) can erode the guide tube. The guide tube was worn out, and the high-speed medium directly impacted the inner wall of the bellows. The speed of scrapping was tsk tsk. Therefore, when selecting the model, you must confirm the material and thickness of the guide tube with the manufacturer, which is at least two gears thicker than the bellows body.
The most common mistake to make during installation, have you fallen for it?
The tie rod nut is not loosened。 Many manufacturers will pre-tighten the tie rod to prevent transportation deformation after leaving the factory, and some masters will start to take over after welding it directly. Guess what? The bellows can't be moved at all, which is equivalent to being installed for nothing. The correct method is: before installation, loosen the tie rod nut to the position marked by the factory (usually two turns back), and then adjust it to the position corresponding to the design compensation amount after all the pipes are welded and fixed. How exactly to adjust? Flip through the question and answer on "How to adjust the tie rod nut of the expansion joint" on our station, and it is thorough.
Another fatal mistake —Direction of guide tube。 The arrow on the expansion joint is the flow direction of the medium. If the guide tube is installed backwards, it will hinder the flow or even be washed away. The function of the guide tube is to guide the medium smoothly through the inner wall of the bellows to prevent vortex erosion. Installed backwards, the cone of the guide tube faces the incoming flow, which is equivalent to setting an obstacle for the medium. The pressure loss increases at least, and the guide tube falls off and breaks the bellows at worst. So be sure to check the direction of the arrow and the direction of the pipe flow before installation.
Straight tube expansion joints cannot be used for deflection installations. Some on-site pipelines don't match, so the expansion joint is twisted and installed. As a result, the bellows generates additional bending stress, and the life is directly cut in half. It really doesn't match, so you should add short sections and short sections. Don't save that little effort.
Something went wrong after several years of using it? Eighty percent of these three points are ignored
One isDust accumulation on outer wall of bellowsEspecially when used in cement industry or flue gas pipeline of power station. Dust accumulation leads to local overheating, dust becomes an insulation layer, the temperature difference between the inside and outside of the bellows is large, and the superposition of thermal stress accelerates corrosion. Cleanup? Have to shut down, shutdown losses five figures a day. Therefore, when selecting the model, you should consider the design of anti-dust accumulation, such as installing a dust cover, or choosingHigh temperature axial type expansion joint.
Second,The number of cycles is exceeded. The design is calculated according to 1000 times of fatigue, and it actually starts and stops once a day, and it will be over in three years. However, many customers don't record the actual number of starts and stops, and wait until the bellows leaks before they remember to check it. It is recommended to install a counter or count it at least once a year.
Third,The deflector is not regularly checked for wear. After the guide tube is worn out, the high-speed medium directly impacts the inner wall of the bellows-that scrap speed, tsk tsk. The air-swept coal grinding pipeline in the cement industry rushes through the guide tube in half a year, and it is much cheaper to change the guide tube once than the bellows. Therefore, don't just look at the surface of the bellows during inspection, but extend an endoscope to see the wear of the guide tube.
If you buy it right and use it right, your money will not be wasted
Straight pipe metal bellows expansion joint itself is inexpensive, but the labor and shutdown loss of a change is much greater. Don't be superstitious about "universal type" when selecting the type, find out the working conditions-pressure, temperature, displacement, medium characteristics, and one of the four parameters can't be vague. If it doesn't work, go onStraight pipe pressure balanced expansion joint orExternal pressure single type axial expansion joint, although expensive, but high margin of safety. After installation, make records: the initial position of the tie rod nut, the direction of the guide tube, and the arrow of the medium flow direction, and take photos for archiving. Regular inspection, focusing on dust accumulation, deflector tube wear and loose tie rod. Done in place, this thing will last for ten or eight years no problem. If you still have questions, turn through the two questions and answers on our station "How to adjust the tie rod nut of the expansion joint" and "Specific function of the guide tube", and they will be thorough.