Find out first: Which national standards and industry standards manage high-temperature metal expansion joints? Don't just stare at one number
As soon as many people come up, they ask, "What is the latest standard of national specifications for high-temperature metal expansion joints?" When they open their mouths, they are GB/T 12777. That's right, GB/T 12777-2019 "General Technical Conditions for Expansion Joints of Metal Bellows" is indeed the core national standard, but if you think that just looking at it is enough, there is a high probability that you will be planted.
The design pressure of high-temperature pipelines used in power stations and cement industries is always several MPa, and the temperature rushes above 600℃. Is GB/T 12777 alone enough at this time? Not enough. You still have to read JB/T 12235-2015 Technical Specifications for Non-Metallic Expansion Joints? No, that's non-metallic. There are also JB/T 10617-2006 Bellows Expansion Joints for Power Stations, and the fatigue life algorithm in ASME B31.3 Process Pipeline Specification, which is tacitly adopted in the industry. To put it bluntly, GB/T 12777 is a "general chassis" for high-temperature metal expansion joints, but according to specific working conditions, you have to superimpose industry standards, design specifications and even material standards (such as GB/T 3280 stainless steel plate and GB/T 14976 fluid tube) layer by layer.
Two days ago, I met a customer who made cement waste heat power generation, and reported the type directly with GB/T 12777. As a result, the bellows cracked after three months of installation. Why? The fatigue life test in the standard is calculated according to normal temperature. His flue gas temperature is 550℃, and the material creep is not considered at all. So, don't be superstitious about a number, first find out the working conditions, and then check the number to find the standard group.
How strict is the standard card under high temperature conditions? Material, structure, fatigue life, every pit has to be filled
Compared with the old version, the biggest change of GB/T 12777-2019 is the tightening of the assessment indicators for high-temperature working conditions. How strict? Three hard bars:
- Materials: The upper limit of the service temperature of austenitic stainless steel in the old standard is not so dead. The new standard directly says that heat-resistant steel or high-temperature alloy, such as Incoloy 800H and GH3030, must be used if it exceeds 425℃. Someone take 304 hard to carry 600℃? Sorry, the allowable stress gauge in the standard at high temperatures has blocked the road.
- Structure: The deflector is no longer "optional". For high-temperature axial expansion joints, the standard explicitly requires that the guide tube be be set, and it should be designed as "labyrinth type" or "ring plate type", in order to reduce the direct erosion and heat radiation of high-temperature medium to the bellows. And guess what? In order to save money, some manufacturers steal the thickness of the guide tube, or simply don't do it. As a result, local overheating cracks appear on the inside of the bellows-which is too common in the accident of metal corrugated expansion joints in the cement industry.
- Fatigue life: This is the main event. The new standard introduces a "temperature correction factor", and the fatigue life curve at high temperatures should be discounted. For example, at 400°C, the number of allowable cycles should be multiplied by 0.7; It may only be 0.4 at 500℃. In the past, bellows that could carry 10,000 times according to normal temperature may actually hang up 3,000 times at high temperature. Many design institutes now require manufacturers to provide high-temperature fatigue test reports instead of just giving a factory certificate.
Tsk, if you don't fill these pits, you will end up changing pipes, shutting down and losing money on the spot. Standards are not decorations, they are life-saving talismans.
Select the model according to the standard: Take our "high-temperature axial expansion joint" as a matter, how to match the parameters
Suppose you have a high-temperature steam pipe, DN300, with a design pressure of 1.6MPa, a temperature of 520℃ and an axial compensation requirement of 80mm. How to choose the products in the station? It is correct to report the model "high-temperature axial expansion joint" directly, but the parameters must be repeated according to the new standard.
First of all, don't want to use 304 for bellows material, you have to use 316L or higher, and the wall thickness should be more than 1.5mm according to GB/T 12777-2019. Then there must be a deflector, and the material should match the medium-304 is enough here, the thickness should not be less than 3mm, and the direction arrow must point to the flow direction of the medium (don't reverse it when installing, otherwise the deflector will become a "jacket board").
How to verify fatigue life? The standard requires that the number of safe cycles at high temperature is not less than 1000 times (actual adjustment according to the design documents). In the scheme we make for customers, we will provide a high-temperature fatigue calculation book based on the new standard, including stress analysis and creep check. The product you receive will have a nameplate attached to it.
Let's talk about the pull rod. High-temperature axial expansion joints usually have a limiting tie rod, and the standard requires that the tie rod nut must be loosened to the working length after installation. Someone asked, "How do you adjust the tie rod nut of the expansion joint?" The new standard makes it clear: the tie rod nut should be adjusted to the position of the compensator pre-tension amount, and the screw cannot withstand additional bending moment. Alas, this detail is directly ignored by many installation teams. If it is screwed to death and used as a fixed bracket, it is equivalent to letting the bellows hold its strength, which is weird if it is not bad.
Installation and acceptance don't step on lightning: the direction of the guide tube and whether the tie rod nut should be removed or not are all written in the standard
Regarding the installation, GB/T 12777-2019 is not written in detail, but you have to read the supporting construction code JB/T 5290-2016 "Regulations for Installation, Commissioning and Acceptance of Expansion Joints of Metal Bellows". Two of the easiest things to do:
- Direction of guide tube arrow: An arrow will be marked on the expansion joint to indicate the flow direction of the medium. Don't think that you are smart to install it in reverse. When the guide tube is designed, the damping structure is made according to one-way flow. Reverse installation will cause the medium to impact the bottom of the guide tube, resulting in eddy vibration, and the life of the bellows will be directly discounted. Two days ago, a power plant customer reported that the deflector was noisy, and it was checked that the direction was reversed. Solution? Remove the re-welding, and the construction period is three days longer.
- Tie rod nut should not be removed: When many projects were accepted, it was found that the adjustment nut on the tie rod had not been removed. The standard reads in black and white: Temporary tie rods (or adjustment nuts) for transportation and installation must be removed after installation is in place and the pipe is secured. If it is a limit rod that does not need to be removed, the nut should also be loosened to the designed position. You can look at the role of the expansion joint tie rod. It is mainly to prevent the bellows from over-stretching, not as a rigid fixture. Screwing the nut is equivalent to turning the expansion joint into a rigid section, and the thermal stress of the pipe is all transmitted to the equipment-this is the reason why the inlet and outlet pipe of the customer's compressor cracked.
The latest standard of national specifications for high-temperature metal expansion joints is not a sheet of paper, but a scale that runs through the whole process of selection, design, manufacturing, installation and acceptance. Only by overcoming these obstacles can the system run steadily for ten or eight years. Otherwise, tsk, wait for the rework.