Why is steam pipe inseparable from metal expansion joint? -Thermal displacement and stress are no joke
When the steam pipe starts running, the temperature is always two or three hundred degrees Celsius. Think about it, a carbon steel pipe tens of meters long can stretch out tens of millimeters by thermal expansion and cold contraction. Can the pipeline carry it on its own? Can't handle it. If the top is hard, the pipe frame is pushed, the flange leaks, and the weld cracks, which are all common accidents. The core value of the metal expansion joint steam pipe is here: it absorbs heat displacement and dissipates stress in the bellows, rather than making the pipe and equipment compete.
Two days ago, I met a customer, a 380℃ superheated steam pipe, without an expansion joint. As a result, an elbow collapsed directly, and the maintenance was stopped for three days. Do you say it's worth it? So don't feel bad about the equipment money. Choosing the right special parts under the steam working condition of the metal expansion joint is much more cost-effective than repairing the leak afterwards.
The first step of type selection: pressure, temperature and displacement, three numbers determine life and death
Design pressure. Steam pipelines are divided into low pressure (≤1.6MPa), medium pressure (1.6~4.0MPa) and high pressure (≥4.0MPa). The higher the pressure, the wall thickness and the number of layers of the bellows have to be adjusted accordingly. Second number: design temperature. Ordinary carbon steel bellows will end when they are carried to 350℃. No matter how high they are, they have to use stainless steel (such as 304 and 316L) or even heat-resistant alloys. The third number: displacement-axial, transverse, angular, which direction to move and how much, must be calculated clearly.
A DN300 steam pipe, 50 meters long, with a temperature rise of 250 °C and an axial thermal elongation of about 150 mm. You choose a general-purpose corrugated expansion joint, the rated compensation amount is only 100mm, and it will crack fatigue within two years of installation. What to do? Either change the high-temperature axial expansion joint, or connect two in series. The slightest difference is a thousand miles away-if you choose the wrong model, all the rest will be useless.
General-purpose type, high-temperature type, external pressure type... Which type should the steam working condition be?
There are many types of expansion joints on the market, but the commonly used types of steam pipelines are:
- Universal corrugated expansion joint: Cheap, simple structure, suitable for low pressure, low temperature (≤300℃) small diameter steam pipeline. The disadvantage is that the stability is poor, and the bellows is prone to instability.
- High temperature axial expansion jointSpecially designed for high-temperature steam, the bellows is usually made of Incoloy 800 or high-temperature alloy, with a guide tube inside to avoid high-temperature medium directly washing the bellows. It is suitable for occasions with medium and high pressure and temperature exceeding 350 ℃.
- External pressure single axial expansion joint: The bellows is on the outside, and the medium takes the inner straight pipe. The advantages are that there is no problem of "column instability", large compensation amount, and it is suitable for high-pressure and large-diameter steam pipeline. The disadvantage is that the length is on the long side and it takes up space.
- Straight pipe pressure balanced expansion joint: Used in situations where the blind plate force needs to be balanced, such as when a valve or blind plate end is installed on a steam pipe. It counteracts pressure thrust by balancing bellows and avoids strain on pipes and equipment.
You see, there is no universal model. When selecting the type, you should choose the most matching one from the above categories against the working condition parameters. Don't be superstitious about "universal packages conquer the world". That thing is a ticking time bomb in high temperature and high pressure steam.
Installation That Thing: Do you remove the tie rod or not? Where is the deflector? Should pre-stretching be done or not?
It was hard to choose the right model, but it was still finished if it was installed wrong. There are three common pits:
Pit 1: The transport tie rod is put into operation without dismantling.When the expansion joint leaves the factory for the convenience of transportation and installation, a tie rod or screw will be temporarily installed. Many people forget to dismantle it and directly pass steam. As a result, the bellows is locked by the tie rod and can't be expanded or contracted, and the thermal stress is all transmitted to the pipeline. Correct practice: After installation is in place, the transport rod must be loosened or removed. See the product description for details-some tie rods are permanent structures (such as hinge type), and some are temporary fixings, so don't get confused. Regarding the tie rod adjustment, you can check the question and answer "How to adjust the tie rod nut of the expansion joint" on this site.
Pit 2: Install the guide tube in the reverse direction.The role of the guide tube is to direct the steam flow direction and protect the bellows from high-speed media scour. The direction of the arrow of the deflector must point to the direction of the media flow. Installing it backwards, the guide tube becomes a throttle element, and the bellows is washed out more severely. Remember that the guide tube is opened toward the media and closed toward the media.
Pit 3: Pre-stretching was not done or done wrong.For pipes installed at low temperatures and operated at high temperatures, pre-stretching is recommended if the installation temperature is lower than the design temperature. For example, when you install the pipe in summer, the ambient temperature is 30℃ and the operating temperature of the pipe is 300℃. Calculate the stretching amount according to the formula, and pre-stretch the bellows to the specified length with a tie rod. After this operation, the stretching amount is slowly compressed back to zero position, and the life can be prolonged by more than 30%. But note: Pre-stretch is only for axial type, hinge type or transverse type is not suitable.
Leaks in a few years? Practical Operation Method of Full Life Extension from Failure Cases
There was a steam line in a chemical plant. The metal expansion joint started leaking steam in less than two years. When removed, the outer surface of the bellows is covered with corrosion pits, and the inner wall has scour grooves. Cause analysis: Steam carries water, and condensed water accumulates at the low point of bellows, causing pitting corrosion. Solution: Install a drain valve under the expansion joint, or require the manufacturer to install a drain port during model selection. In addition, steam pipeline starts and stops frequently, and thermal fatigue accelerates. It is recommended to choose multi-layer thin-walled bellows, which is more fatigue resistant than single-layer thick-walled bellows.
For another example, a power plant used an external pressure single axial expansion joint, but the fixed bracket was set too far during installation, which caused the expansion joint to bear excessive additional bending moment, and the bellows became unstable and deflated. So remember: fixing brackets (primary fixing brackets or secondary fixing brackets) must be set on both sides of each expansion joint, and the distance is determined according to the maximum allowable span given by the manufacturer. Beyond this distance, even immortals can't save them.
Regular inspections. Check the bellows surface every six months for cracks, corrosion and deformation. Just use visual and penetration testing. If you find that the guide tube falls off or is seriously worn, replace it in time, and don't drag it until steam leaks.
To put it bluntly, the metal expansion joint steam system is a systematic project, and you can't be lazy in every link of selection, installation and maintenance. If you treat it as precision equipment, it will hold you steadily for ten or eight years. If you fool it, it will show you it in a minute.