I met a customer two days ago, and as soon as I came up, I asked, "Which is better, metal expansion joint or non-metal expansion joint?" I asked him what kind of medium was going in the tube, and he was stunned and said, "It's just normal steam." That's the problem-directly comparing the good with the bad regardless of the working conditions, like asking "which is better, a hammer or a screwdriver"-you first have to know whether to nail or screw.
First, don't worry about which one is better, it depends on what is going in your pipe
The expansion joint has one core task: absorbing the displacement caused by thermal expansion and contraction of the pipeline, and by the way, absorbing shock and noise. However, different media, different temperature and pressure, and different selection logic.
Steam pipeline, the temperature is always above 300℃, the pressure starts at 1.6MPa, and the non-metallic fabric fiber layer can't bear it at all; On the other hand, the flue gas pipeline contains sulfur and acid, and the metal bellows will corrode in a few days, while the non-metal is corrosion-resistant.
Therefore, the first step is not to compare good or bad, but to find out the four parameters of medium, temperature, pressure and displacement. Otherwise, if you choose the wrong choice, you can leak at least and explode the pipe at worst.
Second, metal expansion joint: a tough guy resistant to pressure and temperature, but it also has weaknesses
Metal expansion joints (such as oursUniversal corrugated expansion joint、A high-temperature axial expansion joint,Corrugated expansion joint for power station industry) by deformation of stainless steel bellows to compensate for displacement. The advantages are obvious:
-High withstand voltage: Conventional energy to 2.5MPa, special design (e.g.Large diameter thick wall expansion joint,directly buried expansion joint) Can carry higher.
-High temperature resistance: With the guide tube and the heat insulation layer, the high-temperature axial type can reach above 800 ℃.
-Long life: Designed properly, fatigue times can reach tens of thousands.
But the weakness is also eye-catching:
First,Not corrosion resistant。 When it comes to wet chlorine and concentrated sulfuric acid, stainless steel is useless, and it has to be lined with PTFE (such as oursPTFE-lined hose).
Second,Unable to withstand large lateral displacement。 The single-wave compensation amount is generally 10-20mm, which requires multi-wave or double hinges (such asDuplex hinge transverse type expansion joint) to make up for it.
Third,High installation accuracy requirements。 How to adjust the tie rod nut and where to point the direction of the guide tube are all particular, and it is wasteful to install it wrongly.
3. Non-metallic expansion joint: flexible master, but don't let it touch high pressure
Non-metallic expansion jointNon-metallic expansion joints (fabric fiber expansion joints), anda rubber compensator,Rectangular non-metallic expansion joint) Flexible deformation by glass fiber cloth, silicone cloth, fluororubber and other composite layers. Where are the advantages?
One isLarge amount of compensationIt can be eaten in axial, transverse and angular directions, especially suitable for large displacement flue gas pipelines.
Second,Corrosion resistanceThe acid and alkali exhaust gas can't bite directly.
Third,Vibration and sound insulationThe fan outlet uses a rubber compensator, and the noise can be reduced by several decibels.
But don't let it touch high pressure and high temperature:
-Pressure exceeds 0.1 MPa, 0.25 MPa is the red line.
-When the temperature exceeds 300°C (silicone cloth limit), ordinary rubber carbonizes at 200°C.
So the flue gas desulfurization system (such asDesulfurization flue gas baffle door) love to use non-metal, and don't even think about the main steam pipeline.
4. Comparison table of working conditions: Who should be chosen for what scene, you can understand at a glance
Don't just look at the principle, go directly to actual combat:
Scenario A: Main steam pipeline, pressure 1.0MPa, temperature 300℃
- Metal expansion joints (e.g.External pressure single axial type expansion joint orStraight tube pressure balance type expansion joint) → suitable
- Non-metallic expansion joints → Don't kill you, internal pressure can tear the fibers
Scenario B: Boiler flue, temperature 150℃, containing SO₂, large displacement
- Metal expansion joints → half year perforated unless lined with PTFE or super stainless steel
- Non-metallic expansion joint (Rectangular non-metallic expansion joints orNon-metallic expansion joint (fabric fiber) → positive solution, corrosion resistant and absorbing multi-dimensional displacement
Scenario C: Air-cooled island vacuum pipeline, negative pressure condition, with large displacement
- Metal expansion joint (Double hinge expansion joint for air-cooled island vacuum pipeline) → preferred, because vacuum sealing requires high, non-metal is prone to air leakage
Scenario D: Fan inlet and outlet, vibration is large, vibration reduction is needed
- Rubber compensator (a type of non-metal) → works best, but metal transmits vibration when it hits hard
You see that? There is no absolute good or bad, only appropriate or inappropriate. Which is better, metal expansion joint or non-metal expansion joint? We have to talk about the work conditions.
5. A key overlooked point: Installation and maintenance costs vary greatly
A lot of people just keep an eye on the purchase price and end up spending twice as much on installation and maintenance.
Metal expansion joint: Carefully align during installation, otherwise the bellows will deflect and break in advance; Also check regularly whether the guide tube falls off and whether the tie rod nut is loose. Change a set of high-temperature axial type, and the labor cost alone will cost thousands.
Non-metallic expansion joint: The installation is easy, and the flange can be clipped, but the maintenance frequency is high-the fabric fiber ages in about two years, and the rubber compensator cracks in three or five years. However, it is quick to change, and it takes half an hour to finish, so the cost is low.
Calculate the general ledger: Assuming that the life span is 5 years, the metal may invest 8,000 yuan at a time, with zero maintenance fee; The non-metal investment is 3,000 yuan, and the skin flower is 1,500 yuan once in the middle, totaling 4,500 yuan. Who's more cost-effective? Look at the budget cycle.
6. A practical suggestion for procurement: Don't just look at the price, but look at the comprehensive life span
Many purchases dropped the price, and as a result, the wall thickness of the "cheap metal expansion joint" bellows bought was not enough, and it was fatigued and cracked in half a year; Or the "low-price non-metallic expansion joint" uses not PTFE layer, and acid corrosion perforates in three months. Saved a small amount of money and lost the overhaul discontinuation cost.
Correct way to do this:
1. Give the working condition parameters (medium, temperature, pressure, displacement) to the technical party and let them recommend the model. For example, for steamUniversal corrugated expansion jointStillHigh temperature axial typeIt depends on whether the temperature exceeds 400℃.
2. Require fatigue test report and material certificate, and don't trust verbal promises.
3. Calculate the overall cost: purchase price + installation fee + expected maintenance fee + shutdown loss. Usually metal expansion joint life is as long as 10-15 years, non-metal 5-8 years, but non-metal repair is convenient.
Finally, let me tell you the truth: Which is better, metal expansion joint or non-metal expansion joint? There is no standard answer. But if you have high-temperature, high-pressure steam or oil going through your pipe, close your eyes and choose a metal (such asUniversal corrugated expansion joint orCorrugated expansion joints for power station industries); If it is flue gas, dust, low-pressure and high-temperature exhaust gas, select non-metal (Non-metallic expansion joints (fabric fibers) orrubber compensator) is more cost effective. Don't let the salespeople lead you, it is king to master the core parameters yourself.