What are the hard levers defined by the national standard JB/T 12235-2015?
Let's start with standards. The national standard of non-metallic compensator is JB/T 12235-2015. This thing is not a decoration, but it directly blocks the mandatory indicators of material, structure and performance. For example, the fabric fiber used in the loop belt must meet the temperature resistance grade, the thickness of PTFE layer has a lower limit, and the flatness of the sealing surface of the flange must also meet the standard. Compared with the standard of metal expansion joint, the difference is obvious: metal parts mainly look at the fatigue life and pressure resistance of bellows, while non-metal compensators pay more attention to the overall sealing, corrosion resistance and displacement compensation ability of the ring belt. You look through JB/T 12235-2015, which clearly lists the pressure and displacement test methods of airtightness test, as well as the duration and temperature of aging test. Don't underestimate these figures. Even if the difference is 5℃, the scene may leak in half a year. We have actually measured a batch of bands with nominal temperature resistance of 200℃, but actually cracked at 180℃. Later, after checking, it was found that the silicone cloth used was unqualified.
Material Determines Life: Material Selection Threshold for Fabric Fiber, Rubber and PTFE Layer
In the quality requirements of non-metallic compensators, materials are the biggest problem. Common fabric fibers are glass fiber, ceramic fiber, aramid, etc. -glass fiber is cheap but afraid of alkali, ceramic fiber is high temperature resistant but brittle, and aramid is strong but expensive. The rubber layer is usually made of fluororubber or silicone rubber. The temperature resistance and corrosion resistance indexes can't be looked at the data table alone, but the actual measurement: How long can fluororubber last in acid gas at 200℃? Does silicone rubber harden at low temperatures? Polytetrafluoroethylene layer (PTFE) is the key to anti-sticking and corrosion resistance, but if the thickness is less than 0.5mm, it is easy to be sucked and broken under negative pressure. The threshold of material selection is one sentence: working temperature, medium composition and pressure range, the three lines must be met at the same time. For example, desulfurization flue gas, although the temperature is not high (80-150℃), but contains sulfuric acid droplets, the rubber layer has to be acid-resistant, and the fabric fiber has to be surface-treated to prevent penetration. We have a customer who uses a rectangular non-metallic expansion joint in a cement plant. The medium has high dust content and the life of the ring belt is less than half a year. Later, he changed the thickened ceramic fiber + PTFE composite layer, and now he has not changed it for almost two years.
Influence of Structural Design on Compensation Amount: Design Points of Rectangular, Circular and Custom Shapes
Structural design is not a pat on the head. Taking the rectangular non-metallic expansion joint as an example, the displacement amount, the guide tube and the heat insulation layer restrain each other. The four angles of the rectangular ring belt are stressed in concentration, so it is necessary to add circular arc transition at the corner when designing, otherwise it is easy to tear if the deformation is large. The function of the guide tube is to guide the medium and reduce the direct scouring of the circular belt-install a non-metallic compensator behind the flue gas baffle door, and the length of the guide tube must be at least 30% longer than the diameter of the pipe, otherwise the airflow will directly circle the belt. The insulation layer depends on the medium temperature: when it exceeds 300℃, the thickness of the insulation layer cannot be less than 100mm, and a multi-layer structure must be used. The circular non-metallic expansion joint matched with the circular baffle door is designed to focus on the balance of axial compensation and radial deviation. Customizing the shape is more troublesome. Two days ago, a customer asked for an oval shape, so he had to recalculate the belt deployment area and flange stiffness. Remember, the core of structural design is to make the displacement stress evenly distributed, and don't let the local part carry too much.
Factory inspection: air tightness, pressure resistance and displacement compensation ability must be missed
The factory report can't just look at the word qualified. In the quality requirements of non-metallic compensators, air tightness test, withstand voltage test and displacement compensation ability test must be solid. Air tightness usually uses 0.1-0.3MPa compressed air, and the pressure is kept for 5 minutes to see the leakage rate. Soapy water leakage detection is the most earth but the most reliable. The pressure test shall be 1.5 times of the design pressure, and the ring belt shall not have bulge or leakage. The displacement compensation ability test is most easily ignored: the manufacturer said that it can compensate 100mm, but the actual site can only reach 70mm, and the displacement limiter is not adjusted properly. Before the rubber compensator leaves the factory, a three-axis displacement test will be done, and the X/Y/Z directions will be fully pulled to see whether the ring belt has wrinkles or local deformation. By the way, the weld of the guide tube also has to be detected. In one project, the ring belt was cut because the welding slag of the guide tube was not cleaned, and it lost hundreds of thousands. The factory report you get must at least list the measured values of test pressure, holding time and displacement. Don't just give a qualified conclusion.
Installation & Maintenance: What Details Are Prone to Rollover?
Installation rollovers are more common than selection rollovers. The bolt tightening sequence is not correct, resulting in flange deformation and air leakage. Tie rod nut adjustment-As mentioned in the previous question and answer, how to adjust the tie rod nut of the expansion joint? First, loosen the nut to allow the ring belt to stretch freely, and then pre-press or pre-pull according to the design displacement. The arrow of the medium flow direction must be up. If the arrow direction is wrong, the guide tube will directly block the flow, and the back pressure of the ring belt will increase. Two days ago, in a power plant project, the baffle door and the non-metallic expansion joint were installed backwards, and the arrow pointed opposite to the medium flow direction. As a result, the ring belt was deformed when the flue gas was vented for the first time. There is also thermal insulation layer filling: in some sites, in order to save money, ordinary rock wool is used instead of aluminum silicate wool. When the temperature is high, the rock wool shrinks, and the ring belt is exposed and burned through. In terms of maintenance, regularly check whether there are cracks on the surface of the ring belt and whether the flange bolts are loose. Don't screw the locking nut of the tie rod to death, and leave a displacement margin. We have a customer who does an air tightness re-inspection once a year when the machine is shut down, and brushes the welds and ring tape joints with soapy water, which can basically find problems in advance.
How to choose the right product according to the working conditions?
Flue gas, dust removal, desulfurization and high-temperature pipelines have different working conditions, and non-metallic compensators, metal compensators and rubber compensators have their own choices. Flue gas (such as boiler outlet): high temperature (200-400℃), dusty, preferably non-metallic expansion joint (fabric fiber type), can absorb multi-directional displacement, corrosion resistance. Dust removal pipeline: large negative pressure, temperature fluctuation, non-metallic compensator or rubber compensator will work, but rubber compensator has poor negative pressure resistance, so skeleton must be added. Desulfurization system: acidic, low temperature, liquid droplets, PTFE compensator or rubber PTFE compensator is more resistant to corrosion, don't use metal compensator, it will be corroded by acid. High temperature pipeline (> 500℃): metal corrugated expansion joint must be installed, and non-metal can not bear it. The formula is: low temperature multi-directional selection of non-metals, high temperature and high pressure selection of metals, and tetrafluorine or rubber as corrosive medium. Our product line on the station is very complete: non-metallic expansion joints, rubber compensators, PTFE compensators, flue gas baffle doors... Don't just look at the price when selecting the model, but calculate the life cost. For example, a desulfurization pipeline can be replaced once a year with an ordinary non-metallic compensator, and can be used for three years with a rubber PTFE compensator. The latter is 30% more expensive but saves two shutdowns for maintenance.