Why is the power plant smoke air duct system inseparable from the non-metal compensator? Understand its positioning first
Have you ever seen a smoke duct in a power plant? Those thick pipelines are often high in Baidu, and they are also sandwiched with dust and corrosive smoke. The pipe will expand when heated, and shrink when cooled. If you don't leave it some "activity space", the pipe gallery will wait to crack. At this time, you have to rely on non-metallic compensators-also often called non-metallic expansion joints or fabric fiber expansion joints.
Absorb thermal displacement, reduce vibration, compensate for installation errors. Especially in the scene of large section and low pressure such as smoke duct, the metal expansion joint is prone to fatigue cracking due to stress concentration. Non-metallic bands are laminated with glass fiber, silica gel, polytetrafluoroethylene, etc., which can not only withstand high temperatures (some models can carry above 1000℃), but also are not afraid of moisture, acid and alkali. Two days ago, I met a power plant maintenance squad leader who complained to me: their factory replaced three metal rectangular expansion joints last year, and as a result, the welds were all cracked. Finally, they were replaced with our non-metal expansion joints, which have not moved at all until now.
To put it bluntly, the non-metallic compensator is the "flexible joint" of the smoke duct system. Without it, the whole system is stiff. Then here's the question: Why have 90% of maintenance workers stepped on pits when installing this thing?
Three parameters that must be checked before installation: displacement, medium temperature and flange spacing. If one is missing, it may be scrapped
Don't rush to unpack and hoist. Take the drawings first and check the three numbers against the physical object. The wrong one is the scrap list.
The first one: the amount of displacement.The radial and axial compensation capabilities of non-metallic compensators are specified at the time of design. For example, a certain model allows axial stretching of 50mm and compression of 30mm. If you don't leave enough allowance during installation, as soon as the pipe heats up, the ring belt will burst directly. I have seen a case: the rectangular non-metallic expansion joint of our factory was used at the outlet of the induced draft fan of a power plant, and the design displacement was ±40mm. The installer pressed 20mm hard in order to align the flange, and as a result, the ring belt was torn after less than three days of operation. Miserable or not?
Second: Medium temperature.The temperature resistance rating of the non-metallic loop belt depends on the inner material. Ordinary silicone cloth can only hold 250℃, PTFE compensator can hold 300℃, and high-temperature axial expansion joint can even be used above 600℃. If the actual temperature of the flue gas on site exceeds the design value, the ring belt will age and brittle. How to check? Look at the product nameplate.
Third: flange spacing.Many maintenance workers think they just have to stick the compensator in the middle of the flange and tighten it. Wrong! The free length of the compensator must match the actual flange spacing. If the spacing is greater than the free length, the loop band will be stretched and the life will be discounted; If the spacing is small, the loop belt is in a compressed state, which also causes problems. The standard practice is: measure three places (upper, middle and lower) with a tape measure, take the average value, and adjust the pipe support if the deviation exceeds ±5mm.
"Three Don'ts and Three Don'ts" for the installation of non-metallic compensators
Three Essentials
Twist the bolts symmetrically.The bolts must be pre-tightened step by step from the middle to both sides, alternating diagonally. Screw to death once? The flange surface will warp, and the band will be unevenly stressed. The standard torque value is usually in the instructions, and without writing it, press the M16 bolt and screw it to about 80N·m. You think about it, dozens of bolts, a man's hands sore after one turn and then a second — but it's life-saving work.
The transport tie rod is to be kept.When the non-metallic compensator leaves the factory, in order to prevent the ring belt from deforming during transportation, the flanges at both ends will be fixed with tie rods (that is, screws). Before it is installed in place, the tie rod must not be removed! Some impatient masters unloaded the tie rod as soon as they opened the box, and the compensator stood there softly, and the displacement exceeded the standard when touched. Correct sequence: First hoist the compensator in place, align the flanges, put on the bolts for pre-tightening, and then remove the tie rod. As for how to adjust the tie rod nut, refer to the FAQ we wrote before: The tie rod nut of the expansion joint should be loosened first and then tightened to ensure a free state.
Avoid high-temperature radiation.If there are high temperature pipes or equipment next to the compensator, insulation must be added. Although non-metallic bands are temperature resistant, long-term near-range radiation will accelerate aging. I have seen a compensator near the slag outlet, only 30cm away from the slag pipe, and the surface of the ring belt hardened and cracked after three months. What to do? Add baffles or fill with ceramic fiber cotton.
Three Don't
Don't hoist savagely.The loop part of the non-metallic compensator is soft fabric, and the sling will leave indentations or even cuts if it is strapped directly on the loop. The flange must be covered with a sling or a special hoisting rack. Once, when a project was hoisted, the steel wire rope was directly tied to the loop belt, and two holes were tied out on the spot, which leaked before the pressure was tested. This is no joke.
Don't cut the wrong layer of fabric.In order to give way to the deflector, some maintenance workers pick up the utility knife and open holes in the loop belt. Do you know the structure of the ring belt? The ring belt of non-metallic expansion joint usually has multiple layers such as silicone cloth, glass fiber, polytetrafluoroethylene, etc. If you cut one layer, the whole sealing performance will be wasted. The deflector is fitted to the internal pipe, not for you to dig holes in the loop. The specific function of the guide tube-preventing the medium from directly washing the ring belt and reducing wear-was written in the question and answer. Remember: The ring belt is a seal, not a punch plate.
Do not omit the deflector.In high-pressure or high-speed airflow pipelines, if there is no guide tube, the medium directly washes the ring belt, and it will wear out in a few months. The guide tube is generally made of stainless steel sheet and installed on the inlet side of the compensator. Some installers are too troublesome, saying that "the deflector is in the way" and don't install it directly. Alas, waiting until there is an air leak before removing the flange and reinstalling it is the real obstacle.
How to accept after the installation is completed? Pressure test, displacement inspection, sealing test, no one step can be saved
It's done when you put it on? Acceptance is the key to success or failure.
Step 1: Test the pressure.Pressure tests are carried out with compressed air or water, usually at 1.25 times the design pressure, and held for 10 minutes. Observe the surface of the ring belt for bulging and leakage. If the loop is partially bulged, it means that the internal layers are debonded and must be replaced. This is simple, but someone always jumps over-"Our factory never does pressure test". Tsk, wait for an accident to make up, the price is not this little time.
Step 2: Displacement check.Verify that the compensator is not stretched or compressed in the initial state. How to check? See if there is a uniform gap between the two flanges, and measure whether the position of the tie rod nut is at the zero point marked by the manufacturer. If the tie rod has been screwed to the limit, it means that the compensator is in a stressed state, and the pipe support and hanger must be adjusted or repositioned.
Step 3: Sealability test.Brush with smoke inspection or soapy water on the flange surface and ring band joints, and see if there are bubbles after ventilation. Especially the four corners (rectangular non-metallic expansion joints are most likely to leak corners). If it leaks, check that the bolt torque is even first, and then check that the gasket is displaced. Don't be foolish to think that tightening will solve it.
After acceptance, keep the transportation tie rod well, and it can be used for the next maintenance. In addition, data such as installation date, model, medium temperature, etc. are recorded to facilitate tracking of service life in the future. The life of non-metallic compensators is generally 3~6 years, depending on the working conditions.
Okay, all that has to be said. Next time you go to a power plant to install it, going through these pits in your mind will save at least half the rework time. If you are doing related projects at hand, you might as well open the product page of our station to see the specific specifications of non-metallic expansion joints (fabric fiber expansion joints). If you select the right type, the installation will be stable.