It is also to compensate for the displacement of the pipeline. On the smoke duct of the power plant and the dust removal pipeline of the steel plant, why did the old master name the non-metallic skin expansion joint? The metal expansion joint has high pressure resistance, and it can encounter large cross-section, low pressure and high temperature dusty smoke, so it can't really play. Non-metallic skin expansion energy-saving absorbs multi-directional displacement, especially lateral and angular displacement. The metal parts are straightforward, but the skin is as flexible as a joint. Coupled with its heat insulation, corrosion resistance, vibration and noise reduction, the cost performance is directly full under the working conditions of desulfurization and dust removal.
Take apart to see the structure: one layer less may have an accident
The non-metallic skin expansion joint looks like a cloth bag with an iron frame, but in fact, it pays attention to a lot. Typical rectangular products, such as the rectangular non-metallic expansion joint of this site, are roughly in this order from the inside to the outside:
- guide tube: Innermost layer, diverting medium, reducing scour. Without it, the airflow patted directly on the insulation cotton, and it took less than two months to hollow out.
- insulation layer: Aluminum silicate wool and the like, blocking high temperatures. In some working conditions, the temperature reaches 600℃, and this layer is not thick enough, so the surface of the skin is hot and the enameled wire is burnt.
- Skin (non-metallic fabric layer): A flexible layer that truly takes on sealing and displacement. Fluorine rubber, silicone gel, PTFE composite fabric, one layer laminated, both temperature and corrosion resistance.
- frame: The inner and outer frames press the skin tightly and connect with the pipe flange. The stiffness of the frame is not enough, the skin will wrinkle when the bolts are tightened, and the seal will be completely destroyed.
Some people want to be cheap and save the deflector. What is the result? The smoke wear the insulation layer, and the inner layer of the skin directly contacts the high-temperature particles, which will leak in half a year. These four things are standard, and whatever layer is missing is working overtime for the maintenance class.
Not just rectangles: How to choose between circle and compound
When many people hear about non-metal skin expansion joints, they think of the rectangular flue thing. In fact, circular pipes and fan inlets and outlets can still be used. The circular structure bears uniform force and is suitable for small and medium-diameter pipelines; Rectangular is mostly used in large sections such as flue gas desulfurization and power station boiler air duct. The circular baffle door (double seal) of this station is equipped with rectangular non-metallic expansion joint, which is a common collocation in desulfurization system.
There is also a composite type, which is a combination of non-metal + metal, or a multi-layer belt insulation layer. For example, high-temperature axial expansion joint has large axial stiffness, but does not need to be so flexible; At this time, you may not be able to support the pressure with pure non-metal, and you can't compensate the radial direction with pure metal, so you have to use a composite design scheme. Select type do not die type, calculate the displacement and thrust first, and then determine the structure.
Pits for Installation and Maintenance: Details Determine Life
I met a customer two days ago, and the non-metallic skin expansion joint tore after three months of installation. When I removed it, I saw that the pre-stretching was not done-the skin was loose when it was installed in the cold state, and it was directly pulled out when it was heated. Remember that non-metallic expansion joints usually have a pre-offset when they leave the factory, and the tie rod must be adjusted to the ambient temperature during installation. And guess what? Many people directly remove the pull rod and throw it aside, and the displacement makes the skin hard to carry it. Can it not break?
There are also guide brackets. The expansion joint can only absorb the displacement in its own direction, and the pipeline is deviated, which is all constrained by the guide bracket. If you are lazy to widen the distance between the guide frames, the lateral force will all eat on the skin, and the life span will be cut in half. As for the skin replacement cycle, there is no fixed number, depending on the working conditions. If the flue gas has high sulfur content and many particles, it shall be inspected once a year; Clean hot air, it will be fine for three or five years. Every time you stop, check the surface for cracks, bulges and scratches, which is stronger than anything else.
Three Parameters Most Easily Overlooked in Type Selection
If you choose a non-metallic skin expansion joint, anyone will see the displacement marked on the drawing. What is really easy to roll over is the temperature and pressure. The temperature does not depend on the temperature of the medium, but on the temperature of the outer surface of the skin-the design of the insulation layer is unreasonable, and the outer temperature still exceeds the limit. The same is true for pressure. Non-metallic expansion joints are generally suitable for low pressure, and if they exceed 0.1MPa, they have to be carefully selected.
The third is media corrosivity. The corrosion of the skin and frame by the SO₂, NO, and hydrochloric acid mist in the flue gas is completely different from the same magnitude. Fluororubber is acid and alkali resistant, silica gel is high temperature resistant but poor wear resistant, and polytetrafluoroethylene is resistant to almost everything but expensive. You list the media components and let the manufacturer choose composite materials. Don't just say "high temperature resistance".
JB/T 12235-2015"Non-Metallic Expansion Joints". There are hard levers for materials, design and inspection in this standard. When selecting, take it one by one, especially the fatigue life and sealing test, which can save a lot of trouble later.
When not to use it? The boundary conditions should be cleared
Non-metallic skin expansion joints are not a panacea. The pressure exceeds 0.1MPa, the diameter is large, and the medium is still steam. The skin is bulging like a balloon. At this time, honestly choose metal corrugated expansion joints, such as corrugated expansion joints and straight pipe pressure balance expansion joints used in power station industry, which have an order of magnitude higher pressure.
The vacuum condition is also suspended. The non-metallic skin itself is a flexible fabric, which will deflate when the external pressure is large, and the internal guide tube can't maintain its shape. If you really want to use it on the air-cooled island vacuum pipeline, use the vacuum special hose or the double hinge expansion joint of the air-cooled island vacuum pipeline, which is the professional counterpart.
In the final analysis, the advantages of non-metallic skin expansion joints are "soft" and "resistant", not "strong" and "pressure". You have figured out its temper, and the selection is a matter of looking at the working conditions.