Why do refinery flues have to use special expansion joints?
The flue of the refinery is not an ordinary pipeline-the temperature is often above 400℃, and the flue gas is sandwiched with sulfide and catalyst dust, and there is positive or negative pressure. Ordinary rubber compensator goes up, and it burns in a few months; The universal corrugated expansion joint can withstand high temperature but cannot withstand corrosion, and the weld leaks when it cracks. Three mountains are pressing down: high temperature, corrosion and dust. The wrong choice is not to dig a pit for the maintenance team, but to lay mines directly for the whole factory.
What about that? Expansion joints designed for flue operating conditions must be used. This expansion joint has targeted design in structure, materials and accessories-such as deflectors, baffle doors, and acid-resistant coatings. Not just any compensator can top it.
Metallic or non-metallic? You can't slap your head when choosing materials
Are metal expansion joints more durable? It depends on the location. In the flue gas system of an oil refinery, the high temperature and high pressure section (such as the outlet of a waste heat boiler) is usedMetal rectangular expansion jointOrHigh temperature axial type expansion joint, that's right. The metal can withstand high internal pressure, and the bellows structure compensates for displacement, no problem.
However, there are plenty of places in flue systems that have variable cross-sections and large displacements-such as transitioning from circular flues to rectangular flues, or needing to absorb displacements in multiple directions. At this timeNon-metallic expansion joints (fabric fiber expansion joints)Is the main force. It does not rely on corrugation deformation, but on the softness of the fabric layer to absorb displacement, and can easily cope with multi-directional displacement of tens of millimeters.
Don't be superstitious about "metal must last longer". In the smoke environment with corrosive media, metal bellows are easily corroded by acid dew points, while non-metallic fabric layers are more resistant to corrosion-as long as the right material is selected. The lining layer of the non-metallic expansion joint can be made of fluoroplastics such as PTFE and FEP, which are acid-and alkali-resistant. My suggestion is: use metal for the high temperature and high pressure section, and use non-metal for the large displacement section with variable cross section, and use both together.
Deflectors and baffle doors-the "bodyguards" of expansion joints
The flue gas velocity in the flue is high, and the catalyst dust is hard as sandpaper. If the bellows or fabric layer is directly exposed to the airflow, erosion wear can compromise life in half. That's when you needguide tube-It is installed inside the expansion joint to divert the high-speed flue gas away, so that the medium does not directly flush onto the compensation element.
There is a common question and answer in our station that specifically talks about the function of the guide tube. The core is one sentence: the flue expansion joint without the guide tube is equivalent to letting the bellows streak naked.
Another key accessory isFlue gas baffle door。 If you have to change the expansion joint during maintenance, you can't stop the whole factory, right? So the flue system has to be fitted with baffle doors-includingDesulfurization flue gas baffle door、Electric plug-in insulation door。 When the baffle is closed and the flue gas passage is cut off during maintenance, the expansion joint can be safely replaced. Not fitted with flapper doors? Then replacing an expansion joint would have to stop the whole catalytic cracking unit, losing millions a day. Do you think this money should be spent?
Installing That Thing: Tie Rod, Screw, Pre-Stretch
Buying the expansion joint back is only the first step, and installing it wrong is equal to buying it for nothing. Say a few common pits.
First, how to adjust the tie rod nut?expansion joint tie rodThe function is to limit the excessive deformation of the bellows during transportation and installation, and is not used to withstand the thrust of the pipe. After installation in place, the tie rod nut should be loosened to the designed compensation position and cannot be screwed to death. Some customers use the tie rod as a fixed bracket. As a result, once the thermal expansion comes, the compensation amount is all locked, and the bellows is directly flattened.
Second, should the screw be disassembled? Depends. If it is an axial expansion joint, the transport screw must be removed or loosened after welding, otherwise it will not expand or contract properly. However, if it is hinge type or tie rod type, the screw is a structural part and cannot be disassembled. How exactly to operate? There are directional arrows and installation instructions on the product. Before installation, read the direction of the arrow-the arrow points to the flow direction of the medium, and the thermal expansion direction will be reversed if the arrow is installed backwards.
Third, pre-stretch. For pipes that need to absorb cold shrinkage, the expansion joint should be pre-stretched during installation to compensate for the cold clearance. However, the flue is generally hot expansion, so in most cases, it is not necessary to pre-stretch, but to reserve the compression amount. Confused, something happens as soon as you get lucky.
Case: Lessons Leaked in Half a Year
The flue expansion joint leaked after less than half a year of use. When I disassembled it, I saw that the weld of the guide tube was cracked, and the fabric layer was corroded by the acid dew point. What is the root cause? Two mistakes were made in the selection.
First, ordinary non-metallic expansion joints were chosen without acid-proof coating. The refinery burns high-sulfur crude oil, and the sulfur dioxide content in the flue gas is high. When it encounters water vapor, it condenses into sulfuric acid, and the pH value is as low as 1. Ordinary fabric layers can't bear it at all.
Second, flue drainage is not smooth, condensed acid accumulates at the bottom of the expansion joint, and long-term immersion plus acid etching accelerates failure.
How was it solved later? Replace withNon-metallic expansion joint with PTFE liningThe inner lining layer is polytetrafluoroethylene, resistant to any strong acid, and is equipped withDesulfurization flue gas baffle doorSo as to facilitate regular discharge of the condensate. With the optimized design of the guide tube, this expansion joint has not been changed for three years. The cost is a little higher, but the loss of labor and shutdown has long exceeded the material price difference.
Five steps of model selection, each step is money
In the selection of flue expansion joints in oil refineries, none of the five details can be missed. Recognize the working conditions of high-temperature corrosion dust, metal and non-metal perform their respective duties, the guide tube and baffle door must be matched, and the installation should be according to the regulations. The case lessons should be used as mirrors. If you choose the right one, there will be no accident for three or five years; If you choose the wrong one, you will be reworked in half a year. Don't use the overhaul workshop as a testing ground – one stop loss is enough to buy dozens of expansion joints.