Two days ago, I met a customer who asked for a price with the drawing of non-metallic expansion joint. As a result, after on-site inspection, the temperature was 450℃, the pressure was 2.5MPa, and the medium was still flue gas with particles. Fabric fibers for this condition? It's not that it's impossible, it's that you have to replace it if you can't last two maintenance cycles. It's not a joke, it's something that happens every day.
So don't rush to compare prices. There are many manufacturers who specialize in providing metal expansion joints, but there may not be many who can really help you make the right selection. Before choosing a model, understand the following five things.
Are expansion joints and compensators the same thing?
It is one thing. Compensator is the scientific name, and expansion joint is the common name. It refers to the flexible element in the pipe that is responsible for absorbing heat displacement and reducing stress. But if you think that all the expansion joints look the same, then you will step on a pit.
Metal expansion joint (corrugated expansion joint) is made of stainless steel corrugated pipe, which is resistant to high temperature and pressure, and is the most common in power stations, cement and chemical pipelines. Non-metallic expansion joint (fabric fiber expansion joint) is made of flexible materials such as glass fiber and silicone cloth, which is not as resistant to temperature as metal, but has large compensation displacement and no thrust, so it is suitable for large air duct and low pressure scenes, such as desulfurization flue. Rubber compensator mainly focuses on pipelines with low pressure and normal temperature, which need vibration isolation. Rubber PTFE compensator can also be selected for acid-alkali medium environment. Sleeve-type pipe expansion joint absorbs displacement by sliding inner and outer pipes, which is cheap, but has average sealing performance, so it is not used on toxic and flammable media.
Look at the medium, temperature and pressure first, and then decide on the category. What category does that working condition in your hand belong to? I can't figure it out, and the rest is all useless.
Different working conditions, selection logic is very different
The same manufacturer, the product line pulls out a large row, but each product is designed for specific working conditions.
The main steam pipeline of power station has high temperature, high pressure and axial displacement, so it is necessary to use high-temperature axial expansion joint or external pressure single axial expansion joint. In the kiln head and tail of the cement industry, there are a large number of dust particles in the air duct, and the ordinary corrugated pipe will wear out in a few weeks. It is necessary to bring a guide tube, and choose the metal corrugated expansion joint of the cement industry. The flue gas baffle door and expansion joint in the desulfurization system should withstand the corrosion of SO₂ and chloride ions. The desulfurization flue gas baffle door, non-metal or PTFE-lined scheme is more reliable. Vacuum pipeline-such as air-cooled island, the pipe system is complicated and the displacement direction is many, so it is necessary to use double hinge expansion joint of air-cooled island vacuum pipeline. Directly buried pipelines cannot be routinely overhauled, so directly buried (fully buried) expansion joints must be selected, and external anticorrosion and internal structures must be specially designed.
You take a universal corrugated expansion joint to go around the world? It would be weird if there were no problems at the scene. Large-diameter thick-walled expansion joint, compound hinge transverse expansion joint, curved tube pressure balance expansion joint... The name of each product corresponds to a kind of working condition, which is the basic plate of type selection.
Stiffness, deflector, material: Three details that determine life
If the parameters don't match, no matter how good the product is, it will be useless. These three details are the easiest to leak.
Stiffness.The rigidity of the bellows determines how much displacement the expansion energy saving absorbs and how much reaction force it generates. The stiffness is too large, and the force of pipeline and support exceeds the standard; The stiffness is small, the actual displacement is not enough, the bellows is hard-pulled, and the fatigue life plummets. This calculation is the core of model selection, not an estimate by slapping the head.
Deflector tube.When there is particulate dust in the medium, the guide tube is life-saving. Its function is to allow the medium to go away from the liner cylinder without directly washing the bellows corrugation. Without the guide tube, the airflow carries particles on the ripples at high speed, which is like sandblasting, and can wear through in a few weeks. In the smoke duct of cement industry and power plant, the guide tube is not optional, but standard.
Material.Ordinary 304 can do work, 316L can also do it, but the cost is high; In turn, 316L can't bear the strong corrosive environment, so you have to consider the PTFE lining scheme. PTFE-lined hose and PTFE compensator are prepared for strong acid, strong alkali and toxic media. Wrong choice of material, cracking, corrosion, leakage, which is enough for you to drink a pot.
Four Most Common Mistakes at the Installation Site
There is no problem in the design of the expansion joint body, but the result is wrong. This situation accounts for a good proportion at the scene, conservatively speaking, more than 30%.
First, the direction of the arrow is reversed. There is a flow direction arrow on the expansion joint, which is an indication of the flow direction of the medium, which directly affects the orientation of the guide tube. Install it backwards, and the medium directly impacts the bellows, which is equivalent to disassembling the guide tube. Second, the tie rod nut is locked. The tie rod and the nut are used to restrain the displacement. The position adjusted when leaving the factory must be loosened after installation on the spot, so that the expansion energy saving can be freely expanded and contracted. If it is locked, it is a rigid pipe section. What is the compensation? Third, the screw is not disassembled. The transport protection screw is temporary and should be removed after installation. Some people don't dismantle it because it is troublesome. The expansion joint is equal to decoration, and the stress of thermal expansion and contraction of the pipeline is all held in the system. Fourth, forget about pre-stretching. For pipelines installed at low temperature and running at high temperature, the expansion joint should be pre-stretched, otherwise the compensation amount of cold state will not be reserved enough, and the hot state will directly exceed the limit.
Each of these four mistakes can make qualified expansion joints scrapped early. Do you say this is a product issue or an installation issue?
How to judge whether a supplier is really professional
There are three differences between suppliers who specialize in providing metal expansion joints and traders who resel expansion joints.
Look at the product line. Only one generic model is sold, mostly by traders. For manufacturers who really make products, their product lines will cover multiple categories such as metal/non-metal/baffle doors, because different products have to be used for different working conditions. Metal rectangular expansion joints, rectangular non-metallic expansion joints, flue gas baffle doors, circular baffle doors (double seals), plug-in insulation doors... these products are put together, which shows that manufacturers have seen various difficult working conditions on the spot.
Look at the customization ability. Are your pipes of non-standard dimensions, special materials, or multi-directional displacements? Can the supplier come up with a plan according to the working conditions instead of asking you to "make do with a similar model"? This directly determines whether the project can be accepted smoothly.
Look at after-sales. Installation instruction, displacement pre-stretching training, life assessment, these services are fundamental to reducing failure rates. There are three suppliers quoting together, who is carefully checking the parameters you gave, and who just sends a form for you to fill in, it is clear at a glance.
In this matter of model selection, you should do enough effort in the front and save worry for ten years in the back. I hope this article can help you ask the right questions when communicating with suppliers.