What exactly is expansion joint metal compensation for?
To put it bluntly, the pipeline will expand and contract under high temperature and pressure. Without it, it will be a matter of time before the weld collapses and the flange leaks. Metal compensators (that is, metal expansion joints) absorb displacement by bellows deformation, while also withstanding medium pressure and corrosion. Look at the product line of this site, from general-purpose corrugated expansion joint to high-temperature axial type and direct burial type. The principle is the same but the details vary widely-if you choose the wrong one, you will have endless maintenance.
The first step of type selection: find out the working condition parameters
Temperature, pressure, media corrosivity, displacement type (axial, transverse, angular displacement) -these four numbers are set to screen products. For example, high temperature and high pressure of steam pipeline, priority is given to high-temperature axial expansion joint or external pressure single axial type; If the desulfurization flue gas contains sulfide, it is necessary to use the desulfurization flue gas baffle door with non-metal expansion joint or lined with PTFE metal hose. Don't copy the model number as soon as you come up, calculate the compensation amount first. Did you know? Some customers take other people's drawings and place orders directly. As a result, once installed on the spot, the compensation amount is 30% different, and the pressure test collapses directly-that's no joke.
The corrosiveness of media is often overlooked. Ordinary stainless steel 304L can withstand most working conditions, but when encountering environments containing chloride ions or strong acids, polytetrafluoroethylene compensator or PTFE-lined hose must be considered. What about the temperature? The upper temperature resistance limit of 304L is about 800°C. If it exceeds the temperature, it needs to be superalloy or lined with heat insulation. Don't expect general-purpose products to take over the world, that's unrealistic.
How to build the product matrix?
More than 30 types of products on this site are not for viewing. Corrugated expansion joints for power station industry and metal corrugated expansion joints for cement industry-industry-specific models have been optimized in structure. For example, large-diameter thick-walled expansion joint is suitable for large-diameter and high-pressure working conditions, and compound hinge transverse expansion joint solves the transverse displacement with limited space. There are also double-sealed single-axis circular baffle doors used in conjunction, and the smoke and air duct system is a set of combination punches.
In the outlet flue of the rotary kiln of the cement plant, the temperature fluctuates frequently from normal temperature to 850°C, with an axial displacement of 150mm and a transverse displacement of 30mm. In this working condition, the universal corrugated expansion joint can't bear it at all-the corrugated pipe will soon be fatigued and cracked. We recommend a combination of metal corrugated expansion joints plus non-metal expansion joints for cement industry, the former absorbing axial displacement, and the latter bearing lateral and large angular displacement. The next factory is cheap to use the general type, and it will crack in half a year. The maintenance cost is enough to buy three sets of special products.
How to choose a scene with limited space? For example, the transverse displacement at the corner of the pipeline is large but the axial compensation requirement is small, so the transverse expansion joint of compound hinge is the positive solution. Another example is the buried pipeline. The directly buried expansion joint has its own waterproof and anti-corrosion structure, so there is no need to make additional wells directly buried in the soil. Rotary compensator is another routine-suitable for the thermal displacement of long-distance straight pipe sections, with simple structure and long service life.
Record of installation pit stepping
A few days ago, a customer called and said that the directly buried expansion joint was stuck during pressure test. I asked him to take a picture and see-hey, the tie rod nut wasn't removed at all! The function of the expansion joint tie rod is to limit the position and prevent the bellows from over-stretching during transportation and installation. After installation, it must be adjusted or removed according to the design. That guy thought it would be reinforced by keeping it, but the compensator was destroyed.
Direction of guide tube arrow. The direction of the arrow points to the flow direction of the medium, and the guide tube can not prevent erosion if it is installed backwards. Especially the high-speed airflow containing particles can wear the bellows through in a few days. How to judge? Look at the arrow, don't take it for granted.
Check the bellows surface for bumps and scratches before installation. Near the weld is the weak zone, and a small crack may propagate under high cycle fatigue. In addition, the large-diameter thick-walled expansion joint has a large self-weight, so the pipeline support point should be calculated well, so as not to let the expansion joint bear extra bending moment.
Lifespan Management Don't wait for something to happen
Bellows stiffness, fatigue life and material are hooked directly. The upper temperature resistance limit of stainless steel 304L is about 800°C. If it exceeds the temperature, it needs to be superalloy or lined with heat insulation. Rubber compensators and PTFE compensators are suitable for corrosive media but have low temperature resistance, with the former generally not exceeding 120°C and the latter within 200°C. Regularly inspect the bellows surface for cracks, corrosion pits, especially near welds. The service life of expansion joints is mentioned in the Q&A of this site, and most of them are designed to be more than 1000 full displacement cycles, but the actual working conditions will be discounted-in cases where the temperature fluctuates frequently and the vibration is large, it is recommended to check it every six months.
Those who think "put it on and leave it alone" will have to pay tuition sooner or later. How to check? Look at the appearance, measure the wall thickness, and do non-destructive testing. If the local thinning is found to exceed 20%, it is time to consider replacement. In addition, medium fouling can also affect the free deformation of the bellows, resulting in local stress concentration-this situation is particularly prominent in flue gas systems.
Cases speak
The temperature of the outlet flue of the rotary kiln of a cement plant fluctuates frequently from normal temperature to 850°C. The combination of metal corrugated expansion joint + non-metal expansion joint in cement industry was selected, with axial compensation of 150mm and horizontal compensation of 30mm, and no leakage for three years. However, the next factory chart is cheap and uses the general type, which will crack in half a year-the selection is not about the price, but about the matching degree of working conditions. There is also a case of a power plant: the pressure of the main steam pipeline is 10MPa, the temperature is 540°C, and the high-temperature axial expansion joint is used, with an external pressure single structure, and the operation is zero for five years. In the other pipeline, because the angular displacement was ignored during the selection, the bellows was twisted and failed, and the loss exceeded one million after three days of shutdown.
Therefore, the metal compensation of the expansion joint looks like a bellows and flange, but the tunnel inside is deep. From working condition analysis to product matching, every step should not be fooled. If you are not sure, send the parameters, and we will compare them with the database, and put the risk first-it is much better than remediation after the fact.