Let's tell the truth first. Two days ago, a purchaser from a chemical plant complained to me, saying that their hot water circulation pipeline used the expansion joint for less than half a year, and suddenly it cracked, and the scene was sprayed in a mess. I asked him if he heard a loud noise, if the pipe was shaking violently, and he said yes, and when the valve was closed, it was like someone had smashed a hammer in the pipe. Okay, solved the case- -water hammer.
The water hammer is called "invisible killer" in the industry. It doesn't take its time like corrosion, wear and tear, it is an instant crit. Think about it, the high-speed flowing liquid is suddenly stopped by a valve or pump, and the kinetic energy is instantly converted into a pressure wave, which bounces back and forth in the pipeline. The peak pressure can reach several times or even ten times the normal working pressure. If the pipe can't hold it, just blow it up.
How does water hammer happen? Understand this "invisible killer" first
A long water pipe, the water is running at a speed of 3 meters per second, and suddenly you shut the valve sharply-the water can't stop. Its forward pressure is blocked by the valve, and the pressure wave begins to propagate upward at the speed of sound from the valve. When it hits the elbow, tee and blind plate, it will be reflected back and form an oscillation. How violent is the peak of this pressure wave? The measured data show that the peak water hammer value of DN300 steel pipe can soar to more than 15kg/cm² under the normal working pressure of 3kg/cm². It's weird that the pipe doesn't crack.
What about that? There are two ways to solve the problem from the root: one is to control the operation, such as slowly closing the valve and installing a slow-closing check valve; The second is to rely on equipment to carry it hard-the expansion joint is the protagonist here.
Expansion Energy Saving Holds Water Hammer? The principle is actually very simple
The core component of the expansion joint is the bellows. Whether it is metal or non-metal, it essentially uses elastic deformation to absorb the displacement and vibration generated by the pipe. When the pressure wave rushes over, the bellows will compress or stretch like a spring, converting kinetic energy into deformation energy, thus reducing the peak pressure. The principle sounds simple, but whether you can hold it depends on the selection and design.
For example, the product of this siteUniversal corrugated expansion jointThe design is mainly to absorb the axial displacement of thermal expansion and contraction, and the wall thickness and wave pitch of its bellows are determined according to the thermal displacement conditions. The pressure wave of a water hammer is a high-frequency shock, which is completely different from the slowly changing load of thermal displacement. You take the universal model to carry the water hammer, which is like taking an umbrella to block a bullet-not that it is completely useless, but it is easy to break through.
Type selection door: Not any expansion joint can prevent hammer
Then what should I use? The key lies in three parameters: pressure grade, stiffness and fatigue life.
- Pressure rating: Do not select according to the working pressure, at least 1.5 times the peak pressure of the water hammer. For example, the working pressure is 4kg/cm², and the peak value of water hammer is calculated as 12kg/cm², so the nominal pressure of the expansion joint has to start with PN16.
- stiffness: The smaller the stiffness, the stronger the ability to absorb shocks. But the stiffness is too low and it is easy to become unstable, so we have to find a balance. Generally recommendedExternal pressure single axial expansion jointIts bellows is subjected to external pressure, has good stability, and the stiffness can be made relatively low, which is suitable for absorbing shocks.
- Fatigue life: Water hammer is a cyclic load, with few times but huge amplitude. The ordinary thermal displacement design may only require a fatigue life of 1000 times, but it is recommended to do more than 10000 times for water hammer resistance. This site'sStraight pipe pressure balanced expansion jointAndCompound hinge transverse expansion jointThere are redundancies in fatigue design, which can cope with such shocks.
In addition, the design of the guide tube cannot be ignored. The guide tube can guide the medium smoothly through the bellows, avoid vortex impact, and at the same time, it can also prevent high-pressure waves from directly impacting the root of the bellows. In the product information of this site,Corrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryAll of them have been strengthened on the diversion tube, so you can see the specific structure diagram.
If it is a corrosive medium or high temperature working condition, you have to consider the material. Such as usingPTFE-lined hoseOrPTFE compensatorThey are corrosion resistant, but their pressure-bearing capacity is relatively weak, so they need to be installed with a limit tie rod. How to adjust the limit pull rod? You can refer to "How to Adjust the Tie Rod Nut of Expansion Joint" we wrote before. Simply put, loosen the nut after installation, let the bellows expand and contract freely, and tighten it to the set pre-stretching amount.
Installation and configuration: wrong location, wasted effect
If you choose the right product, it will be useless if you can't install it. Under water hammer conditions, there are several iron laws for the installation of expansion joints:
- The fixing bracket must be strong enough: The expansion joint itself does not bear the blind plate force, and the blind plate force should be carried by the fixed bracket. Under the impact of water hammer, if the stiffness of the fixed bracket is not enough, the whole pipe will be pushed away, and the expansion joint will directly exceed the displacement limit and tear. The main fixing point should be set near the valve, elbow and reducing diameter.
- The spacing between guide brackets should be calculated well: In order to prevent the pipe from swinging laterally under the action of pressure waves, the spacing of the guide brackets shall not exceed 80% of the allowable span of the pipe. Especially for horizontal pipe sections, if the spacing is too large, the pipe will swing up like a fish's tail and twist the expansion joint.
- Pre-stretch/pre-compression should be accurate: If there is a large difference between the installation ambient temperature and the working temperature, it is necessary to pre-stretch or pre-compress according to the calculated value. For specific operation, please refer to the steps in "Correct Installation Method of Large Tie Rod Expansion Joint". The core is to loosen the nut → pull or press to the predetermined size → lock the nut.
- Don't reverse the direction of the expansion joint: Many products have arrows on them, which is where the medium flows. If the direction of the arrow is wrong, the position of the guide tube will be reversed, and the high-pressure wave will directly hit the inside of the corrugation, which is easy to tear.
Two days ago, another customer asked me that they had installed the pipelinerubber compensatorIs it more resistant to water hammer than metal? The rubber compensator does have a good vibration damping effect, but its pressure resistance is not as good as the metal bellows, and it ages quickly. If your water hammer has high pressure and high frequency, it is recommended to use itMetal expansion jointRubber is suitable for low pressure and small amplitude applications, such as water pump inlet and outlet.
Comparison of real cases: How big is the gap between putting it right and putting it wrong
Last year, there was a steam condensate recovery system in a paper mill, with a design pressure of 6kg/cm² and an operating temperature of 95℃. They originally used a certain brand of general-purpose corrugated expansion joint, which took about 8 months. After an emergency pump stop, a DN250 expansion joint on the main pipeline exploded, and the corrugated pipe directly tore open a 30cm hole. The scene was filled with water vapor, almost injuring people.
The peak water hammer is at least 20kg/cm², while the nominal pressure of the expansion joint they use is only PN10, and the fatigue life design is only 500 times. Moreover, the spacing between the guide brackets is too large during installation, and the expansion joint is twisted by transverse sliding of the pipeline during impact, and the stress at the root of the bellows is concentrated, which leads to direct brittle fracture.
Later we changed the one we recommendedExternal pressure single axial expansion jointWith a nominal pressure of PN25 and a nominal fatigue life of 12,000 times, a fixed bracket is added on the outlet side of the valve, and the spacing between the guide brackets is reduced from 6 meters to 3.5 meters. I haven't had any accidents for more than two years now. Is that the truth? Choosing the right model and installing the right position doesn't cost much, but it saves a lot of worry.
Another case is the steam turbine bypass pipeline of a power plant, high temperature and high pressure steam, and the risk of water hammer is also high. They usedStraight pipe pressure balanced expansion jointThis structure has its own pressure balance cavity, which can effectively offset the blind plate force. At the same time, the bellows adopts multi-layer thin-wall design, which has good flexibility and obvious effect of absorbing shock waves. After using it for three years, when I opened it and inspected it, I couldn't even find cracks on the surface of the bellows, and there was only slight oxidation on the top. Therefore, don't save that little money, spend more time on the selection, and the maintenance cost can save a lot.
Finally, water hammer protection can not be solved by expansion joints alone, but also by integrating pipe system design, operation process and bracket configuration. However, as the last line of defense, the expansion joint can save your life if you choose it right, but if you choose it wrong, it will be a hidden danger. If there are any specific project conditions, feel free to ask. Let's calculate the parameters together and don't let the pipeline crack again.