First, let's start with the word "balance": what is the balance of this expansion joint?
Let's talk about live phenomena first. Steam line of DN500, 1.6MPa pressure, temperature 300℃. The ordinary corrugated expansion joint is installed, and the fixed bracket is pushed forward. The pipe frame is deformed, the weld cracks, and finally the bellows bulges. What's the problem? Blind board force.
The blind plate force is the axial thrust generated by the medium pressure acting on the effective area of the bellows, which is not small. The blind plate force of DN500 at 1.6MPa is about 32 tons. The common axial expansion joint itself does not bear this force, and it has to be carried hard by the main fixed bracket. The larger the pipe diameter, the higher the pressure, the more exaggerated the bracket, and the civil construction cost soared.
It has an extra balance bellows or balance ring inside, which digests the blind plate force inside the expansion joint and prevents it from being transmitted to the bracket. Equipment outlet, pump outlet, turbine inlet and outlet such sensitive positions, as soon as the pipe system thrust exceeds, the equipment will be displaced. The balanced expansion joint is installed, the external force is locked in the compensator itself, and the pipeline and equipment are relieved.
2. Straight pipe pressure balance type and curved pipe pressure balance type: structural differences, applicable scenarios and key points of type selection
Straight pipe pressure balanced expansion jointAndCurved tube pressure balance type expansion joint. The name is like, but the structure is different.
The straight pipe pressure balanced type consists of two sets of bellows-working bellows and balanced bellows, which are connected by a communication pipe in the middle. After the pressure medium enters, the forces on the bellows at both ends cancel each other. It mainly absorbs axial displacement and is used on straight pipe sections, such as main steam pipe and hot water heating main pipe. When selecting, look at the axial compensation amount and pressure level. If DN600 or above, it is recommended to confirm whether the fatigue life meets the design requirements.
Curved tube pressure balance type is a working bellows plus a balanced bellows, and the thrust is balanced through hinge and tie rod system, mainly absorbing lateral displacement and angular displacement. It is suitable to be arranged at the turning point of pipeline-L-shaped and Z-shaped pipe sections, and it is commonly used at the turning point of smoke duct and hot air duct of power plant boiler.
Don't turn your selection backwards. The straight pipe is hard-mounted on the elbow, and the lateral displacement can't be compensated at all; The curved pipe is installed on a long straight pipeline, and the axial compensation ability is not enough. If the bellows is unstable, it will be replaced.
3. On-site case: how to use steam pipeline, smoke duct and power station pipeline
Let's talk about a case of steam pipes. A DN400, 2.5MPa steam pipeline in a chemical plant was originally designed with three general corrugated expansion joints, and the main fixing bracket was made of reinforced concrete foundation. Results the bracket was offset by 18mm and the flange surface leaked during the test run. After the scheme was changed, one of the expansion joints near the equipment was replaced with a straight pipe pressure balance expansion joint, which reduced the system thrust by 40% and saved the bracket reinforcement cost by more than 200,000 yuan.
As for smoke and air ducts, rectangular ducts are mostly used hereMetal rectangular expansion jointOr curved tube pressure balance type. The flue at the tail of the boiler and the inlet of the desulfurization tower have frequent temperature fluctuations and heavy dust wear. Curved tube pressure balance expansion joint can not only absorb thermal displacement at the turn, but also change the flue support from "carrying thrust" to "only bearing load", and the cross section of the support is smaller by more than one specification.
The corrugated expansion joint in the power station industry is more particular. For steam turbine exhaust pipeline and feed water pump recirculation pipeline, equipment manufacturers have strict restrictions on the force of pipe nozzle. The outlet pipeline of the feed water pump of a power plant, with a pressure of 25MPa, used to use ordinary axial expansion joint, and the vibration of the pump body exceeded the standard. After replacing the straight pipe pressure balance expansion joint, the force on the pump port dropped by 60%. In this kind of occasion, the balanced expansion joint is not only "icing on the cake", but also "it must be".
IV. Installation and operation and maintenance warning: pre-stretching, guide bracket, cold tightening-which details will lead to the instability of bellows if they are not done well?
Expansion excerpt is correct, but the installation is not done correctly. Three details are the easiest to roll over.
First, pre-stretch. For pipes with high design temperature, the bellows should be pre-stretched when cold, leaving a compression margin for hot expansion. Some construction sites find it troublesome to install it directly. After operation, the bellows is compressed to the limit, and the fatigue life decreases by a cliff. The amount of pre-stretching is calculated according to the difference between the installation temperature and the design temperature, so you can't pat your head.
Second, the guide bracket. Although the balanced expansion joint balances the blind plate force, the instability and lateral deformation of the pipe system do not mean that the guide bracket is not needed. The spacing of the guide frame is too large, the pipeline deflects under the action of its own weight, the bellows bears extra bending moment, the deformation is uneven, and the instability is faster than overpressure. The spacing is calculated according to the pipe diameter and wall thickness, don't save.
Third, cold tightness. On high temperature pipelines, cold tightening is an important means to improve the force distribution of supports. In a project, the steam pipeline was not cold tight, and the thrust of the fixed support was 15% higher than the design value in hot state, and cracks appeared in the concrete at the root of the pipe frame. In the later stage, the thrust will return to normal until it is cold and tight. How to calculate the amount of cold tightness? Pre-deformation is done according to half to two-thirds of the thermal elongation of the pipeline, which will be given by the specific numerical design institute, and the construction party can follow it, but many sites are too troublesome to skip it directly. That buried the mine.
In addition, can the transportation protection tie rod be disassembled or not? Many expansion joints come out of the factory with transportation fixtures, which must be loosened after installation on site. It is reported that the operator of a cement plant did not dismantle the tie rod. During the pressure test of the pipeline, the pressure resisted the tie rod, and the bellows did not displace at all. As a result, the weld at the other end of the pipeline was torn. Such low-level mistakes have serious consequences.
5. How to compare with the general corrugated expansion joint and the double hinge transverse expansion joint? When should you not choose balanced style?
The universal corrugated expansion joint has simple structure and cheap price, so it is fine to be used on low-pressure small-diameter pipelines, provided that the main fixing bracket can hold the blind plate force. Cost general ledger: the money saved by expansion will be filled in the civil engineering support with high probability. The pressure is not high, the pipe diameter is not large, and the bracket is surplus, so it is suitable to choose the general type.
The double hinge transverse type expansion joint bears the internal pressure thrust through the hinge, allows large transverse displacement, and is suitable for Z-shaped pipe segments. But its limitation is that it can only absorb lateral and angular displacements, and is almost useless for axial displacements. Moreover, the hinge system itself has friction resistance, and the blind plate force is converted into the load of the hinge pin, which still has certain requirements for the pipe frame.
Then when should you not choose balanced style? For normal temperature pipelines with low pipeline pressure (
In the final analysis, the selection is just one sentence: first calculate the blind board force, then calculate the bracket, and finally look at the budget in your pocket. The pipeline thrust is large, the equipment nozzle is sensitive, and the bracket layout is limited. These three conditions account for more than two. The balanced metal expansion joint is more stable than the ordinary corrugated expansion joint.