Two days ago, the equipment supervisor of a thermal power plant called me, saying that the non-metallic expansion joint at the outlet of their induced draft fan was leaking, and the noise at the scene was so loud that the air volume was obviously not enough. He asked me if I wanted to replace it with a new one directly. I asked him not to rush to change it, and checked it in order, and finally solved the problem for less than 500 yuan. I have seen too many similar cases. Today, I will write the investigation ideas clearly. If you encounter them, just follow them.
Air leakage is no trivial matter: energy consumption and corrosion will come to your door
Let's talk about the harm first, otherwise you don't take it seriously. The most direct impact of non-metallic expansion joint leakage is the increase of fan power consumption. If a part of the sealed flow rate of the flue gas system is missed, the fan has to increase the power to make up. A simple comparative data: for every 1% increase in air leakage rate, fan power consumption increases by about 2% to 3%. It is normal for the air duct matched by a 300,000-kilowatt unit to consume hundreds of kWh of electricity an hour. The extra electricity bill in one year is enough to replace several expansion joints.
Even worse is corrosion. When the flue gas leaks out, along with the condensed water, sulfur dioxide and chloride ions are all attached to the surrounding pipelines and equipment surfaces. We have seen a case where the pipe bracket was next to the air leak point. In less than half a year, the corrosion layer of the bracket peeled off like fish scales. If you really have to wait until the equipment is thoroughly corroded before repairing it, the cost is not to be stopped by another expansion energy saving.
Find the leakage point first: Look at it with a flashlight, and you will know it with the smoke method
There is no need to guess the location of the air leak, it is just those few places.
First, the skin surface. The skin of non-metallic expansion joint is a fabric fiber layer, which is washed and aged in high-temperature smoke for a long time, and is most prone to tiny cracks and even holes. Use a flashlight to stick to the skin slowly, and see that the light transmitting point is the leakage point. When the crack is fine and can't be seen, take a piece of paper and rub it on the surface of the skin. The place that is sucked is the air leakage point.
The second place, the flange seal. Where the flange surface is connected with the steel pipe, the sealing gasket is aged and deformed or not compacted during installation, so the gap will come out. This is not easy to find by looking outside. Take a smoke pipe and walk along the flange to see where the smoke drifts.
The third place, the beading. The bead is used to press the skin tightly on the flange frame. When the bolts on the bead are loose or the bead itself is deformed, the skin will come out of the groove, and the air leakage is not small. None of these locations are difficult to check. It takes ten minutes.
Emergency treatment: Patches and bolts have their own specifics
Find the leakage point, and decide whether to fill or replace it depending on the situation.
Small cracks or pinholes on the skin can be patched with high-temperature repair tape or silicone cloth, which can last for a while. Note that the surface is cleaned before repairing, and the dust and oil stains are not wiped off. The tape is also a white sticker, and it will fall off after a few days. This method is suitable for temporary repair, and it is no problem to last until the planned shutdown for maintenance, but it is not a long-term solution.
If there is an air leak caused by a loose bolt, don't screw it when you come up. Let's first see if there are any signs of displacement on either side of the flange-thermal expansion will pull the bolts loose, especially in the thermal power plant case I mentioned earlier. The normal practice is to loosen the bolt, adjust the natural position of the expansion joint, and then tighten it symmetrically according to the standard torque. Direct hard screwing may deform the pressing strip, or tear the skin folds, which is really the worse the more you repair it.
Radical cure plan: change when you should, don't make up for it blindly
What circumstances don't make up? The skin is aging and hardening in a large area, the surface cracks are densely packed, or the flange and bead have been deformed. At this time, it is not something that can be solved by repair. It's safer to replace it directly with a new one.
Rectangular non-metallic expansion jointIt is suitable for rectangular air ducts, such as the inlet and outlet of induced draft fan and the inlet and outlet of desulfurization tower;Non-metallic expansion joint (fabric fiber expansion joint)It covers circular and special-shaped pipes, and the fabric fiber skin itself is the core sealing layer and displacement compensation layer.
Don't just look at the interface size when selecting a model. The three parameters of medium temperature, working pressure and displacement amount determine the number of layers and material structure of the skin. For example, if the flue gas temperature is 200 °C or less, a conventional combination of silica gel cloth + glass fiber is sufficient; If the smoke temperature exceeds 350℃, fluororubber or ceramic fiber layer should be considered. The pressure is simple. Generally, the pressure of flue gas duct is not high, but it should be noted that the skin is easily deflated under negative pressure conditions, so a supporting skeleton has to be added.
Don't forget matching inspection: lax baffle door will aggravate air leakage
Expansion joint air leakage is sometimes its own problem, but half the probability is related to supporting equipment. Two days ago, I met a customer. The expansion joint was replaced with a new one or leaked. After checking it, the problem was in the upstream and downstream baffle doors.
The desulfurization flue gas baffle door or ordinary flue gas baffle door is not closed tightly, which causes the system pressure fluctuation during operation, and the skin of the expansion joint is repeatedly agitated, and fatigue cracks appear. Double-seal structures such as circular baffle doors (double seals) and double-seal single-axis circular baffle doors, although the sealing grade is high, the wear of the sealing surface must be checked regularly. It is best to open and close the manual plug-in type insulation door or the electric plug-in type insulation door once when the machine is shut down for maintenance to see if there is any sticking. The whole air duct is a system, and the expansion joint is just one of the links. It protects the duct, and you have to protect it.
Routine Maintenance: Spend Your Effort at Usual Times
At the end of the day, non-metallic expansion joints shouldn't wait until there is an air leak to attract attention. During daily inspection, take a flashlight to see if there are any scratches or cuts on the surface of the skin, especially pay attention to whether there are any places scratched by the insulation iron sheet. The torque of the flange bolt is checked every six months. Don't be too troubled. The standard requires that the torque value be as much as it is. Don't rely on the feel. The fixing bolt of the guide tube is also easy to loosen. After falling off, the high-temperature smoke directly washes the inner layer of the skin, and the wear speed is more than ten times the normal.
According to JB/T 12235-2015 standard, in the inspection items of non-metallic expansion joints, the skin condition, flange connection and the integrity of the guide tube are listed. If you do maintenance in the front, it is very common for an expansion joint with passing quality to last three or five years, and the air leakage problem has nothing to do with you. If we wait for the air leak to deal with it, then the money for repairing it is almost enough to buy a new one.
So, what should I do if I encounter air leakage in non-metallic expansion joints? First, reassure yourself and go through it in the above order: find the leakage point, assess the severity, deal with it urgently, change it when you need to, check the supporting equipment, and then adjust the daily maintenance strategy. In most cases, there is no need to go to war at all.