Three core parameters that must be clarified before bidding: pressure, temperature and displacement
What are you most afraid of when bidding for metal expansion joints? I am afraid that it says "general type" on your drawing, and it will collapse as soon as it is installed on the spot. After more than ten years in this business, I have seen too many people throw bids because they don't understand the parameters thoroughly-in the end, either the bellows burst or the pipe pulled off. Therefore, before writing the bidding documents, first kill these three numbers:Design pressure(MPa),Design temperature（℃）、displacement amount(axial/transverse/angular).
Steam pipeline, pressure 1.6MPa, temperature 350℃, displacement requires axial compensation of 80mm. If you take aUniversal corrugated expansion jointRemove the sleeve, the upper temperature limit is only 250℃, and the bellows material is directly softened, which can't last a heating season. In the case of pressure, don't forget to consider the water hammer effect-leave at least 20% margin when selecting the model. The displacement amount should not only look at the value on the drawing, but also include the installation error and cold tightening pre-stretching, otherwise the actual running difference can be twice as different.
What about that? The technical documents must explicitly require the supplier to provideBellows stiffness calculation book(In the FAQ of this site, there is "Stiffness and Calculation Formula of Bellows" for reference). With these two things, you can judge whether the value he reported was calculated or slapped on the forehead.
The 5 easiest pits to step on when selecting models-don't confuse general-purpose models with high-temperature models
The general-purpose type and the high-temperature type are almost identical in appearance, but the material of the corrugated pipe is different by several grades. Generally, SUS304 is used for general purpose, and the upper temperature resistance limit is 450℃ (it is recommended not to exceed 400℃ for actual long-term operation); High-temperature models use SUS316L or Incoloy 800, or even addguide tubeDo insulation. And guess what? Some small factories will sell the general-purpose brush-layer high-temperature resistant paint as a high-temperature type-the paint will be exposed as soon as it burns.
Pressure levels are piled up. Some people think that "you can afford it anyway, so it's always right to choose the highest pressure level". Wrong! The higher the pressure level, the thicker the bellows, the greater the stiffness, and the displacement compensation ability decreases instead. Turn back the amount of axial compensation you need, it can't be given, and the scene has to be hard pulled. If the tie rod nut is twisted askew, the pipe will crack correctly.
The media wasn't clear. Acid and alkaline gases? Contains particulate matter With corrosive? You just write "steam" and you're done, but the steam may be mixed with condensate and oxygen. You have to choose at this timePTFE-lined hoseOrPTFE compensatorInstead of ordinary metal expansion joints.
Ignore installation space. Some equipment rooms are tightly piped, you pickedStraight pipe pressure balanced expansion jointAs a result, the length was over and it couldn't fit in. Why not switch toCompound hinge transverse expansion jointOrExternal pressure single axial expansion jointThe structure is more compact.
Blindly believe in the "big pull rod". The function of the tie rod is to bear the internal pressure thrust, but many people use the tie rod as a fixed bracket. The tie rod nut should be loosened after installation (see the installation section below for details). If it is not loosened, it is equivalent to turning the expansion joint into a rigid joint.
When writing technical requirements, how do you accurately describe them? — — Just apply the product classification of your site directly
Writing bidding technical requirements don't make your own words, directly copy the product name of this site, which is accurate and hassle-free. Something like:
- Desulfurization flue of power plant- -writingDesulfurization flue gas baffle door+Non-metallic expansion joints (fabric fiber expansion joints)
- High temperature pipeline (> 600℃) -writeHigh temperature axial expansion joint(MATERIAL SUS310S OR Incoloy 800)
- Kiln tail of cement industry- -writingMetal Corrugated Expansion Joints in Cement Industry
- Vacuum Pipe-WriteSpecial hose for vacuum
- Direct buried pipeline-writeDirect buried (fully buried) type expansion joint(External protective sleeve required)
Put the working condition parameters into a set, and the supplier will know at a glance that you know what you do, so you don't dare to fool them casually. In addition, the FAQ mentionedexpansion joint guide tubeThe function is to protect the bellows from being washed by high-speed fluid. If it is a gas-solid two-phase flow, the guide tube must be written in the requirements, and the material must be wear-resistant.
Key Points of Bid Document Evaluation: How to Give Technical Score? Don't be fooled by fancy parameters
When evaluating the bid, look at the full article of "high quality", "import" and "high standard", how to screen it? Stay pegged to three hard indicators:
First, fatigue life.The national standard requires bellows to withstand at least 1000 cycles, but good manufacturers can do more than 3000 times.
Second, the stiffness value.The stiffness directly affects the force on the pipe support. If your pipe support design is calculated according to a certain stiffness, as a result, the stiffness of the expansion joint provided by the manufacturer doubles, and the support will be wasted. Therefore, the bidder is required to attach the technical documentsMetal expansion joint weight tableAnd stiffness calculations, weight and stiffness have a positive correlation-too light tends to have insufficient stiffness.
Third, the manufacturing process details.For example, is the bellows hydroformed or mechanically formed? Is welding argon arc welding or laser welding? The weld should be radiographed. If these details are not written clearly, you can't check them later when you accept them.
Arrow direction of expansion joint(FAQ#10)! The direction of the arrow is the direction of the media flow and must be installed in line with the direction of the pipe flow. You can add a line to the technical score: "The expansion joint shell is required to be clearly marked with the flow direction arrow, and the flow direction mark is treated for corrosion resistance before leaving the factory." This one can sieve out many small factories-they often print arrows into stickers and tear them off.
Critical control points in installation and acceptance (should the tie rod be removed or not? Where is the arrow?)
When the goods arrive at the scene, you think everything will be fine? Wrong. Installation is the high incidence of accidents. Here are 5 must-check items, which you can take as an acceptance list:
- Is the tie rod/screw disassembled or not?Articles 12 and 13 of the FAQ make it very clear: the tie rod nut plays a fixing role during transportation and installation to prevent the bellows from being crushed. Once installed in place,The locking nut must be loosenedLet the expansion energy-saving freely expand and contract. But loosening is not tearing it down! Keep the tie rod as a guide to prevent the bellows from becoming unstable. If you see that the nut is still tightly screwed during acceptance, it will be directly judged unqualified.
- Arrow Direction: Must align the flow direction of the line media. When installed backwards, the guide tube will direct the medium to the back of the bellows, accelerating erosion.
- Cold-tight pre-stretching: If the working temperature of the pipeline is lower than the installation temperature, the expansion joint needs to be cold tightened (pre-stretched), and the value shall be provided by the manufacturer. Check whether the cold tightness is within ±10% of the allowable deviation of the drawing at the time of acceptance.
- Stent spacing: Guide brackets and fixing brackets must be provided on both sides of the expansion joint. The root cause of many accidents is that the bracket is not done as required, and the expansion joint is unevenly stressed.
- Sealing face inspection: Especially at the flange connection, see if the gasket model is correct. Ordinary rubber pads cannot be used in high temperature situations, but metal winding pads or flexible graphite pads should be used.
FAQ: These details will not be actively told by manufacturers (life, stiffness, weight table)
"How many years will it take you to expand and save energy?" There is no standard answer to this question, just like asking "how long does a car last?" The life depends on the working conditions-temperature fluctuation frequency, corrosion rate, actual displacement amount. However, manufacturers usually give you a theoretical life (such as 1000 cycles), which is laboratory data, and it is not too much to calculate it on the spot. General experience: steam pipelines are replaced once every 2~3 years, and gas pipelines are replaced once every 5~8 years (refer to the FAQ of this site "Gas bellows are replaced once every few years").
Another, stiffness is not taken seriously by many people. When the stiffness of the expansion joint is greater, the force on the pipeline will be greater, and the metal hose and bellows are prone to fatigue fracture. Therefore, the manufacturer should be clearly required to provide the stiffness curve when bidding. If you find it troublesome, just ask the other party to give the Metal Expansion Joint Weight Table-the same type of product, the weight and stiffness are positively correlated, which is an empirical shortcut.
Some manufacturers will falsely mark the displacement compensation amount in the technical parameters. For example, the nominal axial compensation is 80mm, but the actual test can only reach 60mm. How to prevent it? Do a displacement test during acceptance! Put the dial indicator on the bellows, press and push it to the design displacement value to see whether the bellows has plastic deformation. If a false label is found, return it directly.
The bidding of metal expansion joints, to put it bluntly, is just one sentence: write the parameters accurately, screen the manufacturers through, and make the acceptance solid. If you understand these points thoroughly, you can avoid at least 80% of the pits.