What is the difference between anti-corrosion non-metallic compensator and ordinary non-metallic expansion joint?
To put it bluntly, ordinary non-metallic expansion joints (also called fabric fiber expansion joints) mainly carry high temperature and displacement, but when they encounter corrosive media such as strong acid and alkali, wet chlorine gas and desulfurization slurry, they will be wasted in a few days. The anti-corrosion version is specially treated in structure-either lined with polytetrafluoroethylene (PTFE), corrosion-resistant rubber (such as fluororubber and EPDM) instead of ordinary silicone, or directly lined with PTFE hose-like composite structure. This thing is not simply "non-metal", but a multi-layer composite compensator. We call itAnti-corrosion non-metallic compensatorIn essence, it is to add a layer of "body armor" to ordinary non-metallic expansion joints.
Why do you have to use this thing in desulfurization, denitrification and chemical flue gas pipelines?
Look at the flue gas at the outlet of the desulfurization tower of the power plant. Although the temperature drops to about 80℃, the humidity is close to saturation. It is full of sulfuric acid and sulfurous acid condensate, and the pH value is as low as 2. When ordinary metal corrugated expansion joints (such as corrugated expansion joints used in power station industry) encounter this working condition, pitting and stress corrosion cracking are common phenomena. The non-metallic compensator itself is not conductive and is not afraid of electrochemical corrosion. With the addition of PTFE layer partition, it can hold it.
Two days ago, I met a chemical plant customer. The rubber compensator for their sulfuric acid pipeline bulged in less than half a year. After replacing it with PTFE compensator, it has not been touched for three years now. If you think about it, if you change it once every six months, the shutdown loss alone will be enough to buy several compensators. Is that the truth? What about that? Select anti-corrosion non-metallic compensator in one step.
Materials should be matched according to the working conditions, don't blind matching
The outer layer is silicone glass fiber cloth (temperature-resistant and aging-resistant), the middle layer is polytetrafluoroethylene film or integrated PTFE-lined (anti-penetration), and the inner layer is formed with corrosion-resistant rubber (such as fluororubber) or simply with PTFE-lined metal hose. If the pipeline medium contains particles and dust, wear-resistant layer must be added. If the temperature exceeds 200℃, you have to consider non-metallic expansion joints (fabric fiber expansion joints) combined with ceramic fiber mats, but the corrosion resistance will decrease-which requires a trade-off between temperature resistance and corrosion resistance.
Flue gas desulfurization pipeline, with a temperature of 80-120℃ and a large amount of condensed acid, is much more reliable to choose rubber PTFE compensator than pure rubber compensator. But if you insist on using regular rubber, it will crack in three months, and it will be too late to cry.
There are three pits that are easiest to fall into when selecting a model
First: Just look at temperature and not at pressure
The pressure bearing capacity of non-metallic compensators is generally weaker than that of metals. Low-pressure pipelines (below 0.1MPa) can be used casually. If the air pressure is slightly higher (such as above 0.3MPa), steel wire rings or split frames have to be added. Before, there was a customer whose flue gas pipeline pressure was 0.25MPa, and he chose an ordinary non-metallic expansion joint without frame, which exploded directly after one month's operation. Miserable or not?
Second: Ignore the guide tube
If there is no guide tube in the flue gas pipe, the high-speed airflow directly washes the fabric layer, and the hole will be worn out in two months. The function of the expansion joint guide tube is to protect the corrugated body, especially for anti-corrosion conditions-the guide tube itself should also be treated with anti-corrosion treatment (such as lining with PTFE). Some manufacturers cut corners, and the carbon steel used in the guide tube is not anti-corrosion. As a result, the guide tube rotted first, and then the corrosive medium directly eroded the fabric layer, and the whole was scrapped.
Third: Calculate the displacement too full
The allowable compressive displacement of non-metallic compensators is usually larger than the tensile displacement, so the amount of pre-tension must be allowed during installation, otherwise it will crack when it is operated. How to calculate? According to the formula in the national standard JB/T 12235-2015 for non-metallic expansion joints, don't rely on your feeling. Especially for pipes with large thermal expansion and cold contraction, the amount of pre-stretching is not enough, and the compensator is directly pulled into a "halter shirt".
There are several aspects of installation and routine maintenance that are especially easy to overlook
How to adjust the tie rod nut?
The expansion joint tie rod nut is locked in the transportation state and must be loosened after being installed in place to allow the compensator to expand and contract freely. Don't forget to disassemble the transport screw (does the screw of the expansion joint need to be disassembled? Of course, if you don't disassemble it, it is not installed). We have encountered it several times after sales. After installing and running for a few days, the customer found that the compensator was bulging. After checking, the transportation screw was still locked. You say it was wrong or not?
Alignment: Non-metallic compensators cannot withstand torque
The pipe flanges must be centered parallel, hard screwed on diagonally, and torn apart as soon as they expand and shrink. Especially for large-diameter rectangular non-metallic expansion joints, the flange is crooked by 2mm, and the fabric layer has been twisted when the bolts are tightened, and the service life is directly cut in half.
Inspection: If there is exudation, bulging or local hardening on the surface of the fabric, it has to be changed
According to JB/T 12235-2015, the normal service life is 5-8 years, but if the medium contains strong oxidants (such as chlorine), the life will be cut by half. During regular inspection, take a flashlight to shine on the inner wall to see if there are any white crystals or yellow-brown leakage spots. Once found, change it quickly, and don't feel bad about the thousands of dollars-if the pipeline system is shut down for maintenance due to compensator leakage, the loss will start at least tens of thousands.
Choosing an anti-corrosion non-metallic compensator depends not only on the material collocation, but also on the structural design, and more importantly on the installation details. If one of the three is missing, it will be for nothing.