1. Why should metal expansion joints be specially used for vertical pipes? — — Triangular puzzle of dead weight, axial displacement and lateral oscillation
In the selection of expansion joints on horizontal pipes, you can basically make no big mistakes by turning them according to the samples. But once the pipes go up — like the downpipe of a power station boiler, the preheater outlet of a cement kiln — it's a whole different story. Vertical pipe metal expansion joints face three interrelated problems simultaneously: pipe dead weight, axial thermal displacement, and lateral oscillation caused by wind load or earthquake.
Self-respect is an old dilemma. The weight of the horizontal pipe is borne by the supports and hangers, and the expansion joint is only responsible for absorbing displacement. But what about vertical pipes? Dead weight will press directly against the expansion joints, especially bottom-mounted ones, and if the structure is not strong enough, the bellows may be squashed. What's more troublesome is that vertical pipes are often very long, with thermal expansion of tens of millimeters and huge axial displacement. At the same time, because the pipe is vertical, when the wind blows or the equipment vibrates, it will also swing laterally-the double hinge transverse expansion joint is specially designed for this kind of scenario.
Can you use a common general-purpose corrugated expansion joint? The answer is premised. Although the universal expansion joint can absorb axial and angular displacements, it has no supporting structure. On the vertical pipeline, if it is carried hard only by the bellows, its own weight and pressure thrust will make it fail quickly. Therefore, vertical pipes must be designed with special products with tie rods, hinges or pressure balance.
2. Which products can be used in vertical pipelines? — — Applicable boundary of straight pipe pressure balance type, external pressure single axial type and compound hinge transverse type
According to our product system, there are three main types of metal expansion joints commonly used on vertical pipelines:
- Straight pipe pressure balanced expansion joint: The biggest advantage of this thing is that it doesn't produce pressure thrust. Internally, the thrust generated by the medium pressure is counteracted by a balanced bellows, so there is no need to set a main fixing bracket on the pipe during installation. Especially suitable for situations where valves or equipment are immediately adjacent to vertical pipelines, such as the main steam pipeline of power stations. Note, however, that it can only absorb axial displacement and has almost zero lateral capacity.
- External pressure single axial expansion joint: The structure is simpler, the bellows bears pressure on the outside, and the inside is protected by a guide tube. This expansion energy saving withstands large axial displacement, and because of the external pressure design, the bellows has better stability, which is suitable for high temperature and high pressure vertical pipelines. It is often used on preheater outlet pipelines in the cement industry.
- Compound hinge transverse expansion joint: Consists of two bellows and a set of hinges that can only absorb lateral displacement, not axial displacement. This is great-for vertical pipes, if axial displacement has been resolved by other means (such as pipe natural compensation), but lateral oscillation cannot be avoided, then it is the best choice. Double hinge expansion joints are commonly used on air-cooled island vacuum pipelines, and the reason is the same.
None of these three products is absolutely good, and the key depends on the on-site working conditions. Experienced engineers will calculate the pipeline stress analysis first, then determine the displacement type and magnitude, and finally select the type.
3. When selecting the model, not only look at the pressure and temperature-the guide tube, the tie rod and the self-weight support must not leak
Many people only look at the medium, pressure, temperature and caliber when selecting models. Take these parameters to check the sample and place an order. If you do this on a vertical pipeline, something will happen nine times out of ten.
The first is the deflector. In a vertical pipe, the medium flow direction is usually top-down or bottom-up. If the medium has a high flow rate and contains particles (such as pneumatic conveying pipelines in the cement industry), and there is no guide tube, the bellows will quickly be worn out. The direction of the deflector must coincide with the direction of the medium flow, which will also be mentioned later when installing.
Followed by the pull rod. The function of the tie rod is to restrain the pressure thrust generated by the bellows when it is subjected to internal pressure, and at the same time bear the self-weight of the pipe. For straight pipe pressure balance type expansion joints, the tie rod is part of the structure and cannot be dismantled at will. Another detail is the adjustment of the tie rod nut-the nut is usually in a pre-tensioned state when it leaves the factory, and it needs to be adjusted to the working position according to the drawings after installation, otherwise it will affect the displacement ability of the expansion joint.
Finally, the self-weight supports. If you use an external pressure single axial expansion joint, it has no capacity to bear the self-weight of the pipe, so you must set up another bracket or hanger. Some engineering drawings are too lazy to calculate, and the expansion joint is directly used as a bracket. As a result, the bellows is pressed out of wrinkles, and its life is greatly reduced.
4. Three details that are easiest to overturn at the installation site: Should the tie rod nut be removed? Where does the deflector arrow go? How to arrange the screw adjustment sequence?
The mistakes that installers often make are tears if they say too much. Two months ago, in a power station project, the manufacturer installed a straight pipe pressure balance expansion joint, and put it into production without removing the transportation fixing nut. As a result, the thermal expansion value of the pipe was not enough, and the bellows was cracked. It must be made clear here:
- Do you want to remove the tie rod nut?A: When installing, the transport set nut (usually the lock nut) at the end of the expansion joint must be loosened to allow the tie rod to expand and contract freely. However, the working nut (adjustment nut) of the tie rod itself cannot be removed. It is used to set the amount of pre-tension or pre-compression of the expansion joint. Which ones to dismantle will be marked on the drawings, so don't think of it as a matter of course.
- Where is the deflector arrow pointing?A: The direction of the arrow is the direction of the medium flow. On a vertical pipe if the medium is top-down, the arrow must be down; Conversely upward. Installed backwards, the guide tube not only does not protect the bellows, but will form a vortex inside the bellows, which will aggravate the wear. We had an article before that specifically talked about the role of the deflector, which you can turn over.
- How to arrange the screw adjustment sequence?A: For duplex hinge transverse expansion joints, there are usually multiple screws (or adjustment bolts). When adjusting, it should be gradually tightened or loosened diagonally and evenly. Do not screw one piece in place at once, otherwise it will cause the hinge to jam or the bellows to deflect. To put it bluntly, it's the same as changing tires and screwing them.
In addition, after the expansion joint is installed, remember to check if there are foreign objects stuck inside the bellows. And guess what? Last year, there was a cement factory, and the welding slag of the weld fell into the bellows. When the pipe vibrated, the welding slag grinded a hole in the bellows, and the furnace was shut down for two days for emergency repair.
5. From power stations to cement plants, how does the vertical pipeline expansion joint really perform? — — Comparison of typical working conditions in two industries
Comparing the two industries of power station and cement plant, the selection ideas of vertical pipeline metal expansion joints are very different.
Power station industry: Take the main steam pipeline of the boiler as an example. The medium is high-temperature and high-pressure steam, with a temperature above 560℃ and a pressure of more than ten MPa. The vertical section of the pipe is usually adoptedCorrugated expansion joint for power station industryAmong them, the straight pipe pressure balance type is the most used. Because steam pipes do not allow pressure thrust, otherwise the pipe support design would be very complicated. In addition, the thermal insulation layer of power station pipeline is thick, and the installation space is limited, so the compact structure of straight pipe pressure balance expansion joint is just suitable. In actual operation, this kind of expansion energy-saving stable operation for one overhaul period (3-5 years), the key is to do a good job of pre-stretching, otherwise thermal stress will deform the top of the guide tube.
Cement industry: Taking the preheater outlet pipeline as an example, the medium is dusty high-temperature flue gas with a temperature of 400-500℃, but the dust concentration is high and the flow rate is fast. Commonly used hereMetal Corrugated Expansion Joints in Cement IndustryMost of them are external pressure single axial type or double hinge transverse type with wear-resistant guide tube. Because there are many pipeline elbows in cement plant, the vertical section often has thermal displacement in multiple directions, the transverse type of compound hinge can absorb lateral swing, and the external pressure structure makes it difficult for dust to accumulate at the root of bellows. But there is a pain point: the shutdown time of the cement plant is short, and once the expansion joint is worn out, the replacement work is very painful. Therefore, in the past two years, many cement plants have begun to upgrade, choosing large-diameter thick-walled expansion joints with ceramic fiber barriers, which can double their life.
You see, it is the same vertical pipeline, different industries, and the product selection is very different. People who know the business will ask for the details of working conditions, while people who don't understand will only report the caliber pressure. Therefore, next time you purchase metal expansion joints for vertical pipes, you may wish to ask more questions: Have you considered the guide tube and self-weight support?