1. What is a negative pressure resistant non-metallic compensator? What is the difference with ordinary non-metallic compensator?
"Can your non-metallic expansion joint (fabric fiber expansion joint) be used on negative pressure pipelines?" To put it bluntly, ordinary non-metallic compensators mainly rely on fabric and rubber layers to bear pressure, and the band tightening is not a big problem under internal pressure conditions. However, once the system is vacuumed, the external atmospheric pressure is pushed inward, and if the ring belt does not strengthen the structure, it will directly deflate in-from deformation to failure at least to tearing and leaking at worst. The negative pressure-resistant non-metallic compensator is based on the ordinary non-metallic expansion joint, and adds a built-in support skeleton (such as stainless steel wire mesh ring, reinforcing ring or internal support frame), so that the ring belt can keep its shape under negative pressure conditions. To put it bluntly, it just adds an "anti-collapse liner" to the "soft shell".
2. The easiest pits to step on in the selection of models under negative pressure conditions
Two days ago, I met a customer who was doing desulfurization flue, and I had to use the standard non-metallic expansion joint in the negative pressure section behind the outlet of the induced draft fan. I told him, "What is the negative pressure of your flue? He said-5kPa. I said that the ultimate negative pressure of ordinary non-metallic ring belts is-2kPa, and the reinforcing ribs inside the lower ring belts of-5kPa are not enough, so they have to be replaced in a month." He still didn't believe it, but the ring belts were concave after less than three weeks of use, and the guide tubes were worn out. Therefore, when choosing a non-metallic compensator resistant to negative pressure, the first parameter is to design the negative pressure value-not to pat your head, but to calculate. According to JB/T 12235-2015 standard, the pressure bearing capacity of non-metallic expansion joints under negative pressure conditions depends on the number of belt layers, skeleton materials and spacing. Generally, multi-layer ring belt + stainless steel wire mesh can be used within-3kpa, and steel inner brace ring or combined frame can be added above-5kpa. How deep do you think this pit is?
3. Doorways in structural design: skeleton, guide tube and seal
There are three key differences between negative pressure resistant non-metallic compensators and ordinary models.
First, the skeletal form. There are two common types: one is a built-in stainless steel spiral wire ring, which has good flexibility but poor pressure uniformity; The other is a segmented steel support ring, which is sufficiently rigid but has a limited bending radius. In the series of "Non-metallic Expansion Joints (Fabric Fiber Expansion Joints)" of our site, the negative pressure-resistant version usually adopts the composite structure of stainless steel wire ring + reinforced lining ring.
Second, the guide tube. Under negative pressure, the guide tube is not only a guide, it must block the external pressure to be transmitted to the ring belt. The end of the guide tube should extend to the inside of the loop belt at least 50mm, otherwise the edge of the loop belt will be sucked into the gap by negative pressure. Install backwards? That's over, just scrap it.
Third, the sealed form. The connection between the ring belt and the flange is prone to air leakage, and the air leakage is more serious under negative pressure. Generally, double press strip + negative pressure sealing gasket is used, and some customers also require silicone gel. Don't feel bad about that glue, it's more expensive to dismantle it if it leaks.
4. Which scenarios must use negative pressure-resistant non-metallic compensators?
Turn over the product line of our station, and there are a lot of them related to negative pressure.
- Flue downstream of desulfurization flue gas baffle door: the outlet of desulfurization tower is usually slightly negative pressure, which can be held by ordinary non-metallic expansion joints, but the negative pressure in the rear section of induced draft fan may reach above-6kPa, so it must be negative pressure resistant.
- Vacuum special hose supporting system: Although there is "vacuum special hose" in our station, the hoses at the elbows of some large-diameter vacuum pipes (such as air-cooled island system) are difficult to arrange, so they have to be replaced by negative pressure-resistant non-metallic compensators.
- Dust collection pipeline matched with metal corrugated expansion joint in cement industry: negative pressure dust collection pipeline has large diameter, low temperature, and low cost of non-metal, but the negative pressure fluctuates high and low, so the skeleton design is redundant.
- Denitrification system of power station: There is negative pressure fluctuation in the flue at the inlet and outlet of SCR reactor. Although our "corrugated expansion joint for power station industry" has many metals, some non-metallic transition sections also have to withstand negative pressure.
5. Those humble but fatal things in installation and operation and maintenance
Install negative pressure resistant non-metallic compensator, the first note: the direction of the guide tube cannot be reversed. The direction of the arrow is the flow direction of the medium, and the medium in the negative pressure pipeline flows from high pressure to low pressure, but the direction of the pressure difference between the inside and outside of the ring belt is opposite to the conventional, so the direction of the opening of the guide tube should be confirmed with the manufacturer, so don't install it backwards. Second, don't screw the tie rod nut to death. In our question and answer, we talked about the adjustment method of the tie rod nut of the expansion joint-the preloading force of the negative pressure-resistant type is about 20% greater than that of the ordinary type, because it is necessary to resist the trend of external pressure flattening the ring belt, but a compensation margin must be left after installation, otherwise the ring belt has no room to extend and collapse directly when the temperature rises. Third, check the inside of the circle band regularly. Under negative pressure conditions, dust is easy to adhere to the inner wall, and more dust accumulates to increase the weight of the ring belt, which may lead to sag deformation. Quarterly endoscopy is recommended. Don't wait until the air leaks to find out, it's too late.
6. How to match with other compensators? Don't choose the wrong type and waste money
"In the same position, use metal corrugated expansion joint or non-metal?" If the negative pressure does not exceed-10kPa and the temperature is lower than 400℃, the negative pressure-resistant non-metal compensator is cost-effective-the price is 1/3 of that of metal, and the installation is light. However, if the temperature exceeds 400℃ or the medium contains particles and is seriously eroded, it has to be made of metal, such as "high-temperature axial expansion joint" or "general-purpose corrugated expansion joint". In addition, some working conditions need to absorb large displacement and resist negative pressure at the same time. You can consider "compound hinge transverse expansion joint" with non-metallic section, but it has to be customized. In short, don't use ordinary non-metals to deal with negative pressure, and don't be superstitious about the universality of metals-although the "rubber compensator" and "rubber PTFE compensator" on our stand have good elasticity, their negative pressure resistance is limited by the hardness of rubber, and they are generally only suitable for slight negative pressure within-2kPa. It is more useful to send the working condition data (pressure, temperature, displacement, medium) to the technology before selecting the model than to read a hundred manuals. Do you think this is the case?