Why do you have to use metal corrugated expansion joints for steam pipes?
Steam lines are not ordinary hot water pipes. The temperature goes to 400℃, and the pressure is above 4.0MPa. The pipeline switches back and forth between cold and hot states every day, and the displacement is always tens of millimeters. Under this working condition, if you make do with a rubber compensator or an ordinary sleeve compensator, it will leak in less than a few heat cycles.
The rubber compensator can't bear the high temperature, and the sealing packing of the sleeve compensator is easy to wear. Only the metal corrugated expansion energy saving depends on the elastic deformation of the bellows to absorb the displacement, and at the same time bear the pressure and temperature of steam. It's not a matter of preference, it's the laws of physics that force you to choose this way.
Visible and tangible core parts: bellows, guide tube, tie rod nut
In the steam metal corrugated expansion joint, there are only three things that really work.
BellowsIs the core of the core. It has to withstand internal pressure and repeatedly expand and contract at high temperatures. Generally, 304 or 316L stainless steel is selected as the material. No matter how high the temperature is, heat-resistant alloys such as Inconel 625 have to be used. The thicker the wall thickness is not the better-too thick, the stiffness is large, and the compensation amount is not enough; It's too thin and doesn't withstand enough pressure. The balance here depends on the calculation of fatigue life.
guide tubeIs an easy to overlook character. The flow rate of the medium in the steam pipe is high. If the steam directly washes the inner wall of the bellows, the bellows will soon be eroded and thinned. The deflector is equivalent to adding a layer of armor to the bellows to guide the high-speed airflow away. When selecting the model, you must confirm the material and wall thickness of the guide tube. Don't save money on this detail.
tie rod nutThe function is to control the displacement range of the bellows. When a steam pipe is started, if the bellows is overstretched or compressed, the fatigue life will drop dramatically. The tie rod is not designed to withstand internal pressure thrust, but to limit the displacement to the design range. When the nut is not adjusted in place during installation, it is equivalent to buriing a thunder in the expansion joint.
Three structures, how to draw the applicable boundary?
Universal type, external pressure single type axial type, high temperature axial type.
Universal corrugated expansion jointIt is suitable for low pressure and low temperature steam lines, such as saturated steam below 0.6 MPa. It has simple structure and cheap price, but the bellows directly contacts the medium, which is not suitable for high pressure and high temperature working conditions.
External pressure single axial expansion jointWhen the bellows is placed in an external pressure environment, the medium does not directly contact the inner wall of the bellows, but is pushed by external pressure. This structure has better pressure resistance and much greater displacement capacity than the general-purpose model. This is the first choice for medium pressure steam pipeline (1.6MPa ~4.0MPa).
High temperature axial expansion jointSpecially deals with high temperature steam (above 350℃). It usually uses multi-layer bellows with thermal insulation lining, or selects high-temperature alloy materials. This is what is used on the main steam pipeline of the power station. Don't replace it with the general-purpose type. As soon as the temperature rises, the fatigue life of the bellows will drop by a cliff.
Do not only look at the compensation amount in the selection parameters
Many engineers choose steam metal corrugated expansion joints, and when they open their mouths, they ask "how much is the compensation". Of course, the compensation amount is important, but if you only focus on the compensation amount selection, something will happen sooner or later.
The pressure rating determines the wall thickness and number of layers of the bellows. Temperature determines material selection. Fatigue life determines the design stress level of the bellows. These parameters are linked-if you want the expansion joint to compensate 10mm more, you may have to lower the pressure level by one gear; If you want to increase the fatigue life to 10,000 times, you may have to increase the wave number or change the material.
Guide bracket spacing is the parameter that is most easily overlooked. For the corrugated expansion joint to work properly, it must rely on the pipe guide bracket to ensure that the displacement is concentrated on the corrugated pipe. If the spacing of the brackets is not set according to the calculation book, the bellows will be subjected to additional bending stress, and its life will be greatly reduced. Remember: The ability of expansion joints to compensate is based on the correct pipe arrangement.
The easiest pits to step on at the installation site
I met a customer two days ago, and the expansion joint leaked within two months after it was installed. When I removed it, I saw that the guide tube was installed in the reverse direction-the medium directly washed the bellows and wore through a hole in the bellows. There are arrows on the guide tubes, pointing to the flow direction of the medium, which can be avoided by taking a second look during installation.
Not adjusting the tie rod nut is the second common mistake. The tie rod nut is locked when it leaves the factory, which is used to fix the bellows against transportation deformation. After installation, the position of the nut must be adjusted according to the requirements of the drawing. The nut should be loose and the nut should be tight. Many people forget this, and as a result, the expansion joint doesn't work at all, and the pipeline stress is all held in the elbow and equipment interface.
Cold tight pre-displacement this pit is larger. Steam pipeline design generally requires cold tightness, that is, pre-stretching or pre-compression for a certain distance in the cold state, so that the expansion joint is in an equilibrium position when working in the hot state. The construction team does not do cold tightening according to the drawings, or the direction of cold tightening is reversed. In the hot state, the displacement of the bellows directly exceeds the limit, and the fatigue life drops from 10,000 times to 1,000 times. You calculate this account, in order to save hours of construction time, losing the money of an expansion joint, is it worth it?
Looking at Life Short Board from Failure Cases
There are three kinds of failure of steam corrugated expansion joint: corrugated pipe corrosion, water hammer impact and weld cracking.
The problem of corrosion is mostly the quality of steam. The steam contains chloride ions or acidic substances, which can cause pitting or stress corrosion cracking on the surface of stainless steel bellows. There are two preventive ideas: one is to improve the material grade of stainless steel, and the other is to do a good job in pipeline passivation. If your steam system is frequently out of service and condensate drains poorly, the risk of corrosion increases exponentially.
Water hammer is a unique hidden danger of steam pipeline. The drainage of the pipeline is not thorough, and the high-temperature steam condenses instantly when it meets cold water, resulting in a huge impact force, and the bellows is directly deflated. This is not an expansion joint quality problem, but a system design problem. However, when selecting the type, you can choose the structure with reinforcing ring, and the water hammer resistance will be better.
Weld cracking is the most irritating because it can be avoided completely by inspection. The connecting weld between the bellows and the end pipe and the longitudinal weld of the guide tube are all weak links. The manufacturer must do 100% non-destructive testing, and it is recommended to make random inspections after entering the site. Cracks tend to start as tiny weld defects and slowly grow under alternating stress. When I saw the crack visible to the naked eye, it had actually leaked for a long time.
Therefore, the service life of steam metal corrugated expansion joint, seven points in type selection, three points in installation and maintenance. It is normal to use it for ten or eight years if you choose it right and install it. It is common for you to choose the wrong choice and pretend to be smashed. It is common for you to change it half a year. It is much more cost-effective to spend more time figuring out the first five things than to fix the pipes afterwards.