Find out first: What exactly is the metal balance expansion joint balancing?
To put it bluntly, an ordinary expansion joint generates a huge thrust when it is subjected to internal pressure-to what extent is this force? For example, DN500, 1.6MPa steam pipeline, the thrust can easily exceed 20 tons. The traditional practice is to pile reinforced concrete for the main fixed point, and several expansion joints can be bought at the cost of civil construction alone. The metal balance expansion joint (such as straight pipe pressure balance type and curved pipe pressure balance type) is designed with a set of bellows inside, so that the thrust can be offset by itself. The principle is simple: the working bellows and the balancing bellows have equal areas and the same pressure, and the generated axial forces are in opposite directions and cancel each other out. In this way, the pipe support saves a lot of money, and the fixed point only bears the self-weight and friction of the pipe.
What is the difference between ordinary corrugated expansion joint and metal balanced expansion joint? The former is "I bear the thrust, you reinforce the bracket", and the latter is "I digest the thrust myself, you just get a bracket". Use the wrong occasion? Two days ago, I met a customer who took a general-purpose corrugated expansion joint to top the high-pressure and large displacement. As a result, the fixed pier collapsed directly, and the whole pipeline was forced to shut down for emergency repair. Remember: ordinary expansion joints are suitable for scenarios with low pressure and reliable fixing points; Metal balance expansion joint is specialized in treating the difficult and complicated disease of "no place to fix it".
Which of the three common types on the market should you choose?
Straight pipe pressure balanced expansion joint: Suitable for straight pipeline, both ends are welded directly to the pipeline, and the internal working bellows and balanced bellows are arranged coaxially. The structure is compact, the axial compensation is large, and the pressure range of 0.25~2.5MPa can be covered. The main steam pipeline and heating network trunk line of power station are used the most.
Curved tube pressure balance expansion joint: Exclusive for pipes with elbow layout. For example, the main steam pipe from the boiler outlet to the turbine can be used to get angular displacement and axial compensation at the turning, saving two ordinary expansion joints and a fixed point. We have a cement factory customer who uses this for rotary kiln waste heat pipeline, and the support cost has been reduced by 40%.
Compound straight pipe bypass pressure balanced expansion joint: A lifesaver in space-constrained situations. Pipes stick to the wall, and pipes are dense in the pipe gallery-ordinary expansion joints can't fit in, so use this. It puts the balance bellows into the bypass pipe, and the axial length is shortened by more than 30% compared with the straight pipe type. Its shadow is often seen in refinery pipe galleries.
Don't use the universal corrugated expansion joint to deal with high pressure and large displacement, that's not the same thing. Before selecting the type, draw the pipeline direction diagram, mark the number of elbows and space size clearly, and then find the manufacturer to confirm the type.
Don't just look at the caliber when selecting a model. These three parameters are fixed
Nominal pressure (PN): Don't just look at the design pressure, but include the hydraulic test pressure and impact pressure. For the working condition of steam pipeline PN16, the actual selection should at least the balanced expansion joint of PN25, and the safety factor should be sufficient.
Axial compensation amount (mm): Calculated according to the pipe thermal elongation formula Δ L = α · L · Δ T. α is 0.012 mm/ (m·℃), L is the distance between two fixed points, and Δ T is the working temperature minus the installation temperature. Don't forget pre-stretching: 50% of the compensation amount is pulled to the working state during installation, which can effectively utilize the bellows life.
Fatigue life (number of cycles): This parameter is easy to ignore. Thermal power plants start and stop once a day, 365 times a year, and the design life of 15 years requires 5,475 times. Selection should leave a margin, usually according to 10,000 designs. The rigidity of the bellows determines the thrust-I forgot to count this, and the top of the bracket deforms in minutes after installation. The greater the stiffness, the harder the bellows, and the greater the reaction force when absorbing displacement. Although the internal pressure thrust of the balanced expansion joint is cancelled, the elastic reaction force generated by the rigidity of the bellows is still there, and the bracket must be able to hold this force.
By the way, the direction of the arrow of the expansion joint refers to the direction of the medium flow, and installing the guide tube backwards will damage the bellows. The role of the deflector is clearly written in the FAQ: to protect the bellows from high-speed media. The arrow is reversed, the bell mouth of the guide tube is facing away from the medium, and the airflow directly hits the root of the bellows, which will be perforated in one year.
The easiest pits to step on at the installation site
Do you want to remove the tie rod nut?I have answered this question no less than 50 times. Transportation tie rods must be removed-those thick screws are used to prevent the bellows from deforming during transportation. Without removing, the expansion joint cannot expand and contract properly. Don't move the adjusting screw for pre-stretching. That is the position calculated by the manufacturer according to the compensation amount. If it moves, it may exceed the limit or not be in place. For specific adjustments, refer to the Q&A "How to adjust the tie rod nut of the expansion joint".
The bell mouth of the guide tube should be directed toward the medium。 Have you ever encountered a case where the arrow is put backwards? I've seen it, and the whole pipe is shaking. That time, it was the steam pipeline of a chemical plant. The bellows was not broken, but the vibration of the pipeline reached the amplitude of 2mm, and the instrument pipe joint was all loose. When disassembled, the direction of the guide tube is reversed, and the airflow washes the inner wall of the bellows to generate vortex. Install it upside down, and the vibration disappears immediately.
Pre-stretching/pre-compression: When the ambient temperature is lower than the pipeline installation temperature, it should be pre-stretched; When above installation temperature, pre-compress. Before operation, look at the installation compensation value on the product nameplate, adjust it with the screw, and lock the nut after it is in place. Alternating diagonally when screwing to prevent the bellows from deflecting.
The end face of the flange connecting the expansion joint and the pipe should be parallel, and the deviation should not exceed 1mm. Plug it with a gauge, and if it exceeds the standard, the stainless steel spacer levels it. Don't use asbestos rubber pad, it will age and leak under high temperature.
How long does the lifespan last?
There is no fixed age for this thing. The steam pipeline can run for 10 years, and the high-temperature flue gas pipeline with dust in the cement industry may have to be replaced in 3 years. Key factors: temperature (creep acceleration over 400℃), medium corrosiveness (chloride ions are bellows killers, 304L stainless steel has limited chlorine resistance, so 316L or Incoloy825 must be used), number of cycles (short life of pipelines with frequent start-and-stop).
The key is not to wait for it to break, but to regularly check whether there are cracks on the bellows surface and whether the tie rod is loose. Quarterly visual inspection and annual penetration test (PT). If you find corrosion pits or micro-cracks on the surface of the bellows, change them quickly. Don't compare with non-metallic expansion joints-non-metallic expansion joints (fabric fiber expansion joints) are suitable for low-temperature and corrosive flue gases, and their pressure and temperature resistance are one gear worse. They are suitable for flue desulfurization systems, but for steam pipelines.
Although the metal balance expansion joint saves the fixing point, the bracket still needs to be limited-to prevent the bellows from being twisted by excessive lateral displacement of the pipe. The spacing of limit brackets should be according to the specification, so don't save this money.