What is the metal expansion joint of pump connection exactly? First distinguish the difference between it and metal hose and compensator
Many people ask as soon as they come up, "Is the metal expansion joint connected to the pump a metal hose?" Tsk, these two are really not the same thing. The metal hose is mainly used to absorb vibration and flexible displacement, such as the high-frequency vibration at the pump outlet, and relies on the hose to remove the force; The metal expansion joint of the pump is essentially a kind of corrugated expansion joint, which is used on the inlet and outlet pipelines of the pump to compensate the axial displacement and transverse displacement caused by thermal expansion and contraction, and by the way, absorb part of the vibration of the pump. As for compensator, that is a general term, and expansion joint is a kind of compensator, just like the relationship between "car" and "car". If you have to use the metal expansion joint connected to the pump as a metal hose and let it bend around, it will definitely be scrapped early.
More in detail, the metal expansion joint of the pump generally has a guide tube. Why? Because the flow rate of the medium from the pump is high, the guide tube can straighten out the flow field and prevent the high-speed fluid from directly washing the ripples. After the expansion joint without the deflector is used in the pump, it is only a matter of time before the bellows will be punched through.
Don't look at the eye in the selection: medium, pressure, temperature and displacement, four parameters determine life and death
The matter of model selection is really not just quoting the caliber and length. A while ago, a customer brought an old expansion joint, saying that the metal expansion joint of the pump connection leaked after half a year. When I looked at the parameter table, the medium was desulfurization slurry, the pressure was 0.6MPa, and the temperature was not higher than 80℃, but the displacement was only given 5mm in the axial direction. There is a problem-the desulfurization slurry contains chloride ions and solid particles, and the bellows material has to be 316L or even 254SMO, which he quoted as 304. Moreover, the layout of the inlet and outlet pipelines of the pump is complicated, and the actual displacement is 8mm in the axial direction and 3mm in the transverse direction, but the 5mm he gave is simply not enough.
- Medium: Corrosive media determine the material of corrugated pipe, so don't use 304 to dominate the world.
- pressure: The pressure level determines the number and thickness of bellows layers, and the multi-layer thin-walled structure is used for high pressure, which has better fatigue resistance.
- Temperature: Material creep and fatigue life attenuation should be considered in high temperature environment, and heat insulation layer should be added if the temperature is too high.
- displacement amount: Axial, transverse and angular displacements should be given. Don't only give one direction, otherwise the selected products can't be installed at all.
Think about it, these four parameters are like the four major pieces of finding an object. If one is less, you will be blind. If you choose the wrong model, no matter how good the brand is, it will be useless.
Analysis of common failure scenarios: What are the causes of cracking of bellows, loosening of tie rod and falling off of guide tube
The most common failures of the metal expansion joint of the pump connection are cracking of the bellows, loosening of the tie rod/nut, and falling off of the guide tube.
Cracking of bellows is mostly caused by fatigue. The pulsating pressure of the pump, the vibration of the pipeline, and the frequent start-and-stop will make the bellows bear alternating stresses. If the displacement is small during the selection, and the bellows is hard broken in place after installation, it is equivalent to working with injuries every day, and it is strange that it does not crack. Another, if the medium contains chloride ions, 304 bellows will have stress corrosion cracking, and the cracks are like tree branches, which is scary to look at.
Many people ignore the loose lever. If the metal expansion joint of the pump connection has a tie rod or a limit rod, it is used to restrain the bellows to absorb the displacement in the specified direction, not to be used as a hanger. When installing, the tie rod nut was not adjusted according to the instructions. After running for a period of time, the nut withdrew, and the bellows would become unstable, bulging and twisting all came. Judging whether the tie rod should be disassembled depends on the working conditions-if the expansion joint only compensates for axial displacement, the middle tie rod nut can't be locked, so a displacement margin should be left; If the pipe only vibrates without thermal displacement, the tie rod should be locked instead.
When the guide tube falls off, nine times out of ten are the welding quality or the flow rate too high. The spot welding of the guide tube is not firm, and the high-speed medium impacts it, and it is shocked off in hundreds of hours. After dropping, the bellows directly faces the medium to scour, the wall thickness is thinned, and then the perforation leaks. Therefore, the guide tube of the metal expansion joint of the pump must be integrated molded or fully welded, so don't save that little money.
Pits in the installation link: Is pre-stretching done or not? Is the fixed bracket and the guide bracket set right?
Installation is easier to get out of the pit than selection. When the metal expansion joint of the pump is installed, it is particularly easy to neglect the pre-stretching. What is pre-stretching? That is, when the pipeline is installed in the cold state, the expansion joint is pre-stretched by an amount, which is equal to half of the hot displacement, so that the expansion joint can basically return to the "middle position" in the hot state, and the force is the most uniform. If you don't do pre-stretching, it is equivalent to letting the bellows work to one side as soon as it comes up, and the life is directly discounted by 30%.
Then there's the bracket. The expansion joint itself can only absorb displacement, it can't bear the weight of the pipe, and it can't let the pipe run around. Pump inlet and outlet pipes must be equipped with fixed brackets and guide brackets. The fixed bracket fixes a point so that the displacement goes towards the direction of the expansion joint; The guide bracket ensures that the pipe can only move in the axial direction without generating additional bending moment. Many sites are not fixed on one side, and the expansion joint is pressed like a bent old man by the gravity of the pipe. Can it be durable?
In addition, when installing the metal expansion joint of the pump connection, don't use strong pulling and hard pulling to the flange hole. The deviation of the bolt hole should be solved by adjusting the bracket. Take the expansion joint to make up the installation error, and the bellows has long been internally injured.
Practical Application Case of Metal Expansion Joint with Pump in Power Station, Cement, Desulfurization and Other Industries
In the power station industry, the temperature of the outlet pipe of the feed water pump changes greatly and the pressure is high. Generally, an axial expansion joint with a guide tube is used, made of 316L or Inconel 625. We have made a batch of pump-linked metal expansion joints for a power plant. The medium temperature is 160℃, the pressure is 4MPa, and the axial compensation amount is 12mm. They have been running for five years, and there are no cracks on the bellows surface.
The cement industry is rougher, the medium is dusty gas, the temperature is high, and the abrasion is strong. At this time, the metal expansion joint of the pump has to have an extra layer of wear-resistant lining, or simply use non-metal expansion joint with wear-resistant guide tube. Before, a cement factory used an ordinary corrugated expansion joint, which was worn out for three months and replaced with a metal expansion joint with wear-resistant guide tube. It lasted for two years and was still normal.
The desulfurization system is the easiest to "teach to be a human being". The chloride ion content of the slurry is several thousand ppm, and the pH value jumps up and down randomly. Anticorrosion is the first priority when selecting the type. We suggest using high nickel alloy or PTFE-lined scheme, but PTFE-lined scheme is afraid of negative pressure and high temperature. Therefore, the metal expansion joint of the pump in the desulfurization system must be matched with targeted materials, and cannot be dealt with by general products.
Daily inspection and life judgment: When should you change it, don't wait until you miss it before you regret it
Look at the bellows surface regularly. Are there deep corrosion pits? Are there any obvious scratches? The expansion joint often doesn't have a big problem in appearance, but if the corrugation spacing changes, it means that it is being stretched or compressed too much. Take a caliper to measure the wave distance. Compared with the original data, if the change exceeds 10%, you should be vigilant.
Also, listening to the sound is not bad. There is an abnormal "hissing" sound near the metal expansion joint of the pump, which is probably leaked. If you smell a peculiar smell during the inspection, or see medium crystallization in the insulation layer, you can basically judge that the bellows has cracked, so you have to stop and replace it quickly.
There is no absolute "change every few years" for expansion joints, it depends on the number of cycles and stress levels. According to the fatigue life design of GB/T 12777, the safe life of general civil working conditions is 1000 to 5000 full displacement cycles. However, if the pump starts frequently, it will start and stop more than a dozen times a day, and 1000 times will be used up in three months. Therefore, when selecting the model, be sure to tell the manufacturer the start-stop frequency clearly, and let the manufacturer help you check the fatigue life.
In the final analysis, the metal expansion joint of the pump connection can't be used. In most cases, it is not the problem of the product itself, but the selection and installation are not in place. You report the parameters of medium, pressure, temperature and displacement clearly, and then make the pre-stretch and bracket. This thing is much more durable than you think. If you are afraid of saving trouble, the parameters will not be reported completely, and the installation will come casually, no matter how good the expansion joint is, it will not be able to hold it.