Two days ago, I met a customer, and the drawing said "non-metallic expansion joint". When the goods arrived, I was dumbfounded-the rectangular piece installed on the smoke duct was taken to connect the circular opening of the pipe, but the size did not match, and the construction period was delayed for three days. It's not that people's products are not good, it's that they don't know the "direction" at all.
The direction of the non-metallic expansion joint, the first meaning refers to the shape selection. Rectangular non-metallic expansion joints are used on rectangular flue ducts, which are the most common in power plant desulfurization, cement kiln tail and steel plant dust removal systems. Circular ones are used on pipes, matched with metal pipes, and absorb axial and angular displacements. Another easy confusion is the rubber compensator, which has strong low-frequency vibration and is suitable for the inlet and outlet of water pumps and fans. If you install it on the high-temperature flue, it will age in less than two months. If you use it in the wrong place, no matter how expensive the product is, it will be useless.
The core of the selection direction: not the price, but the working condition data
Many people buy non-metallic expansion joints, and when they come up, they ask for the price and size, which is the opposite. The core of the selection direction is four parameters: displacement, temperature, pressure and media corrosivity. These four parameters correspond to the structural design and material selection of the expansion joint. If any one is wrong, it will be a hidden danger when installed.
Standard JB/T 12235-2015 is clearly written-the technical requirements and inspection rules of non-metallic expansion joints are all in it. If you are engaged in design, it is recommended to turn this standard out and have a look, which is more effective than listening to sales fools.
The temperature of the flue gas pipe is 180°C, the pressure is ±3kPa, and the displacement is 20mm in the axial direction. This working condition looks simple, but if you ignore that the medium is desulfurized wet flue gas, which contains sulfite ions, it will be in great trouble. The life of ordinary fabric fibers drops sharply in acid-alkali environment. At this time, non-metallic expansion joints (fabric fiber expansion joints) made of polytetrafluoroethylene (PTFE) should be selected, and their corrosion resistance is completely different from the same level.
Material direction determines life: How to choose three mainstream materials
The material direction of non-metallic expansion joint directly determines its service life and maintenance cycle.
Fabric fibers: The temperature resistance range is the widest, and there are corresponding combination schemes of silicone cloth, ceramic fiber cloth and glass fiber cloth from 200°C to above 1000°C. Suitable for high-temperature flue gas and hot air ducts. The disadvantage is that the withstand pressure is low, and it is generally used in low positive pressure occasions such as fan outlet or flue.
Rubber: Mainly EPDM or neoprene rubber, good weather resistance, high elasticity, good vibration isolation and noise reduction effect, used in room temperature air duct, water pump inlet and outlet, air conditioning system. The rubber compensator carrying low-frequency vibration is a strong point, and the price is affordable. However, the temperature resistance generally does not exceed 120°C, and the medium will accelerate aging if there is oil stain.
Polytetrafluoroethylene (PTFE): Extremely chemically stable, resistant to almost all strong acids and bases. Desulfurization system, wet dust removal, chemical tail gas treatment, these places must use PTFE compensator or rubber PTFE compensator. The upper limit of temperature resistance is lower, but it is the optimal solution for corrosive media scenarios.
In another case, the medium is both high-temperature and corrosive, such as incinerator tail gas, so a composite layer scheme has to be used: the outer layer is high-temperature resistant fabric, and the inner layer is PTFE to isolate the corrosive medium. This kind of non-metallic expansion joint with combined structure can be done by professional manufacturers, but you should know what working conditions you need.
The installation direction is wrong, and everything is wasted
In this step of installation, the most prone to problems is the reverse direction. Which way does the deflector face? Here there is a hard rule: the direction of the guide tube must coincide with the direction of the airflow. The guide tube is used to protect the bellows and fabric layer from the direct erosion of high-speed airflow. If the direction is reversed, it is equivalent to not being installed, and the weld seam will be worn out in less than a year.
So how to determine the direction of non-metallic expansion joint? Look at two signs: one is the airflow direction arrow on the product, and the other is the position of the guide tube. Keep three things in mind:
- Airflow direction sign: The flow direction arrow is marked on the takeover when the product leaves the factory. Install against the direction of airflow in the pipe, with the arrow pointing to the outlet.
- Guide tube orientation: The guide tube and the connector are welded into a whole, and it should be installed along the flow direction of the medium to prevent the medium from directly impacting the corrugation.
- tie rod nut: The tie rod nut is in a locked state when it leaves the factory, this is to prevent distortion in transport. After installation, loosen the nut and adjust it to the specified position according to the design requirements. Note: The tie rod is subjected to internal pressure thrust and cannot be screwed randomly.
The metal hose absorbs small displacement and high-frequency vibration, while the non-metal expansion joint absorbs large displacement and multi-directional deformation. When the two are installed in series, there should be enough straight pipe section transition in the middle, otherwise they interfere with each other and the hose will be pulled out first. When cooperating with smoke baffle door or electric plug-in insulation door, the expansion joint should be installed on the low-pressure side of the valve to avoid the dead angle of airflow, otherwise the wear will be aggravated. The pipeline coming out of the desulfurization flue gas baffle door has large temperature fluctuation and strong corrosion, so it is more reliable to choose non-metallic expansion joints with PTFE lining. This point is easily ignored by many design institutes. As a result, there is always air leakage during installation and commissioning. After checking, it is found that it is a layout problem.
What direction to look at in later maintenance: wear, aging and glue opening
Non-metallic expansion joints are not consumables that are installed and left alone, and regular inspection is a must. There are only three directions to inspect: fabric wear, rubber aging and joint opening.
Fabric wearLook at the surface: the surface of fiberglass cloth or silicone cloth is fluffy and broken, indicating that there is long-term airflow washing or dust friction. This frequency of wear observation is recommended to be checked quarterly. If the wear area exceeds 20%, it's time to consider replacing it. If there is only a local hole, it can be repaired with the same material patch, but ensure that the patch is firmly bonded.
Rubber agingLook at elasticity: Press the rubber surface with your fingers, and the rebound is slow, the surface is sticky or cracked, which means it is aging. Rubber compensator generally has a life span of 3-5 years, and if the environment is harsh (direct ultraviolet radiation and high ozone concentration), it may not work in 2 years.
Joint open glueIt is the most common failure mode of fabric expansion joints. Press the lap hard to see if there is delamination. If a small area is found, open the glue, clean it and re-bond it, and treat the gap between production stoppage. If it is a large area of degumming, it is recommended to replace it as a whole, not hard repair.
How to set the maintenance cycle? Be conservative according to the working conditions:
- Continuously running: It is recommended to shut down for inspection every 6 months.
- Intermittently operated: Inspected once a year.
- High temperature, corrosive occasions: once a quarter, don't be too troublesome.
If the displacement is found to exceed the design value, don't rush to change the expansion joint, and check whether the pipeline support and hanger are stuck. The failure of the support and hanger causes the displacement to concentrate on the expansion joint, which should not be carried by it. Similarly, if the nut is loose and the tie rod does not play a limiting role, it will also stress the fabric layer too much.
At the end of the day, direction is a systemic issue
So you see, the direction of non-metallic expansion joints, from type selection to installation to maintenance, is a whole set of decision-making chains. Choose the wrong shape, and it will leak when installed; Choose the wrong material, corrosion penetration in half a year; Installing in the wrong direction, the guide tube is equal to white installation; Neglect of inspection, the loss of shutdown maintenance is enough to buy dozens of expansion joints.
What is the medium temperature? How corrosive is it? What is the displacement amount? Is the pipe round or rectangular? If you can answer these four questions, the direction will not go wrong.