What displacement is the metal expansion joint "compensating" for? — — Axial, transverse and angular directions at once
Friends who have done pipeline installation know that the temperature of a steam pipeline several hundred meters long rises from normal temperature to five or six hundred degrees, and the thermal expansion is not a joke. The displacement of the metal expansion joint, to put it bluntly, is to help the pipeline "take a breath".
Axial, transverse, angular. The axial direction is to stretch and contract along the direction of the pipe, pulling and contracting like an accordion. Lateral displacement is a translation perpendicular to the axis of a pipe, such as a lateral misalignment at a bend of a pipe due to thermal expansion. Angular displacement is more naughty, which is the angular deflection of the pipeline in a certain plane, which is common in Z-shaped or L-shaped pipe sections.
How does that count? Simple estimation formula: Δ L = α × L × Δ T. α is the linear expansion coefficient (about 12×10⁻⁶/℃ for carbon steel), L is the tube length between the two fixed brackets, and Δ T is the temperature difference. The total amount of axial displacement is calculated. However, the actual scene is often the superposition of three kinds of displacements. At this time, you have to rely on professional selection software or manufacturers to calculate-don't pat your head, I have seen many cases of accidents.
Which metal expansion joint should I choose under different displacement scenarios? — — From general-purpose hinges to duplex hinges, how to match the products
Model selection is like matching keys, what lock matches what key. I'll list a few common scenarios:
- Large axial displacement of straight pipe section: withUniversal corrugated expansion jointOrExternal pressure single type axial type expansion joint. The external pressure type can put the bellows in the medium flowing to the outside, has high pressure resistance and good stability, and is suitable for high-pressure steam pipes.
- Lateral displacement dominated: ConsiderationDuplex hinge transverse type expansion joint. It "eats" the lateral displacement through two hinge groups, and the compensation angle of a single hinge is limited, so the combination can handle large lateral deviation.
- Need to absorb axial displacement without wanting to generate blind plate force: Then onStraight tube pressure balanced expansion joint orCurved tube pressure balance type expansion joint. This kind of product comes with its own balanced bellows, which internally digests the thrust generated by the medium pressure, and is very friendly to the equipment interface and bracket.
- High temperature operating condition: For example, the main steam pipeline of the power station, the temperature exceeds 500℃, the first choiceHigh temperature axial type expansion joint. The material is heat-resistant alloy, and the guide tube is added to the structure to reduce high-temperature erosion.
- Space is constrained and large compensation is required：The double straight pipe bypass pressure balance expansion joint can be one-to-two, and one expansion joint can be used for two.
Never forcefully plug the universal type in situations where angular displacement is required. Although the general-purpose model can withstand a small number of angles, structural limitations can accelerate fatigue. Please use hinge type or universal hinge type for angular displacement.A double hinge transverse expansion joint is a typical example.
Easy pits in displacement calculation and selection: stiffness, guide tube, tie rod nut, none of which can be missed
The design institute calculated the axial displacement of 30mm and selected the general type. As a result, the expansion joint was directly flattened after the on-site installation and operation. Why? Because the system stiffness is ignored.
The rigidity of a bellows determines its ability to resist deformation. The stiffness is too large and the displacement compensation is not in place; If the stiffness is too small, it will easily become unstable under internal pressure. When selecting the model, let the manufacturer provideThe rigidity and calculation formula of corrugated pipe, combined with the pipeline thrust to check the support strength. Don't figure cheap, stiffness parameters are not transparent all the time pass.
Another pit is a guide tube. Many people think that the deflector is a "baffle". In fact, it is a life-saving part under high temperature and high frequency vibration conditions.Specific Function of Expansion Joint Guide TubeIs to guide the flow of the medium, avoid the high-speed airflow directly rushing the bellows, and reduce the flow resistance at the same time. When selecting, be sure to confirm the material and thickness of the guide tube-at high temperature, the guide tube burns through itself first, and the bellows is directly exposed, so wait for the perforation.
And tie rod nuts. You thought it was done when you tightened it at the factory? Wrong. The screw during transportation is used for pre-tightening protection and must be loosened after installation, otherwise the displacement is "locked". As forHow to adjust the tie rod nut of expansion jointThere is a standard process: first disassemble the transport screw, and then adjust the limit nut according to the designed displacement, leaving a compensation gap. For specific operations, please refer to the Q&A we wrote before.
How to ensure that the expansion joint displacement is not "locked" during installation? — — How to disassemble the screw and pre-stretch it, on-site experience
Two days ago, I met a customer, and I was extremely anxious to call: "The expansion joint is installed, and the pipe will leak when it is hot!" When I went to the past, I saw that the installation screw was not removed, and the bellows was suffocated. Tsk, such low-level mistakes are not uncommon on construction sites.
Do you need to remove the screw of the expansion joint？ Transporting screws and adjusting screws are two different things. The transport screw must be removed after it is installed in place and the pipeline connection is completed; The adjustment screw (nut on the pull rod) should be adjusted according to the design displacement, and cannot be completely screwed or loosened. Simple formula: the transport screw is completely disassembled, and the adjustment nut leaves the seam.
Let's talk about pre-stretching. For pipes that need to absorb thermal expansion, they are often cold-drawn, that is, pre-stretched a distance, when installed. For example, when the pipeline is running, it is extended by 50mm, and when installed, it is cold-pulled by 25mm, so that the expansion joint can "eat" half of the displacement in advance. In this way, the bellows is in a neutral position and the fatigue life is doubled. However, the amount of pre-stretching must be accurately calculated. If you pull too much, the bellows will have internal stress, while if you pull less, it will be dry for nothing. It is recommended to use a tie rod with hydraulic tools for construction, and don't use a crowbar hard-I've seen the bellows not cracked.
Installation directions. There are generally arrows on the expansion joint cylinder to indicate the flow direction of the medium. Don't install it backwards, otherwise the guide tube will not play a protective role. The question of the direction of the arrow, ourThe direction of the arrow of the expansion joint refers toThis Q&A is very clear.
Practical Case: Displacement Compensation Scheme of Steam Pipeline in Power Station-Selection Logic from High Temperature Axial Type to Straight Pipeline Pressure Balance Type
Tell me a practical case. The main steam pipeline of a power plant has a design pressure of 10MPa, a temperature of 540℃, a pipe diameter of DN400 and a distance of 30 meters between two fixed brackets. After calculation, the axial thermal expansion is about 95mm, and there are also transverse displacement of 18mm and angular displacement of 2° due to pipeline turning.
At first they were going to use aHigh temperature axial type expansion joint to fix the axial direction, and use anotherDuplex hinge transverse type expansion joint handles transverse. However, after calculation, the pipe support between the two expansion joints has to bear a huge blind plate force-the pressure is 10MPa, the area is 0.1256m², and the thrust is 1256kN, which can not be carried by ordinary supports.
Select in horizontal straight pipe sectionStraight pipe pressure balance type expansion joint, which comes with its own balance bellows, absorbs the blind plate force internally, and the bracket only needs to bear the self-weight and friction of the pipe. At the turn, useDuplex hinge transverse type expansion joint cooperates with guide bracket. This not only solves the 95mm axial displacement, but also handles the transverse and angular directions, and requires much less civil foundation.
Practice has proved that the equipment is in good condition since it was put into operation for three years, and there is no instability or leakage. Choose the right product, calculate the displacement, and install each nut, and the displacement of the metal expansion joint will be stable.