1. National standard system of metal expansion joints: who has the final say between GB/T and JB/T?
Two days ago, I met a purchaser for a power station project. When I came up, I asked, "Does your metal expansion joint meet the national standard?" I said yes, and he immediately asked, "Which is the mandatory standard, GB/T 12777 or JB/T 10617?" This question is a good question, but many people actually confuse one thing- -GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Corrugated Pipe" is a product standard, while JB/T 10617 "Expansion Joints of Metal Corrugated Pipe" is more an industry standard for design, manufacture and inspection.The two are not a substitute relationship, but complementary. GB/T 12777 specifies the terms, classification, size series, technical requirements, test methods, etc. of expansion joints, which belong to the basic general standard; JB/T 10617 is more detailed, such as more specific clauses on bellows raw materials, forming process and welding requirements. Which one has the final say, you ask? In actual selection,The national standard is the bottom line, and the industry standard is the implementation rules.If someone takes "conforming to the national standard" as a universal shield, you have to ask: "Is it GB/T 12777 or JB/T 10617 specifically? Has the fatigue life test been done?"-Alas, many small factories can't even produce the type test report.
Second, working condition determination standard: which specification should power station, cement and flue gas desulfurization be compared with?
National specification for metal expansion joints under different working conditions? The emphasis is completely different. For example:Corrugated expansion joint for power station industryIt mainly takes high-temperature steam or hot water, with high pressure and large pipe diameter. At this time, you have to stare at the Technical Regulations on the Design of Steam and Water Pipelines in Thermal Power Plants DL/T 5054, which clearly requires that the fatigue life of the expansion joint is not less than 1000 times, and there are strict regulations on the thickness of the guide tube and the wall thickness of the nozzle. We sold a batch to a thermal power plant, and the other party directly took the terms of DL/T 5054 to accept it one by one. If the wave pitch deviation of the bellows exceeds 0.5mm, it will be returned.
Look at the cement industry again. The expansion joint on the cement production line is full of dust and high-temperature gas in the medium, which is not corrosive but abrasive.Metal Corrugated Expansion Joints in Cement IndustryIt is usually carried out according to JB/T 10617, but the key is to thicken the guide tube and choose wear-resistant lining. The national standard does not mandate the material of the guide tube, but it can be actually used. The 304 guide tube will be worn out in three months, so it has to be replaced with 316L or wear-resistant coating.
Flue gas desulfurization (FGD) systems are even more complex. The flue gas after wet desulfurization contains corrosive media such as sulfuric acid and sulfurous acid, and the temperature is about 80℃ but the humidity is extremely high. At this timeDesulfurization flue gas baffle doorAnd matching expansion joints, non-metallic expansion joints or metal expansion joints lined with F4 should be used. The standard should refer to the industry specifications such as Technical Conditions for Non-metallic Expansion Joints for Flue Gas Desulfurization Engineering. I have seen a case: a chemical plant installed a general-purpose corrugated expansion joint on a desulfurization pipeline for a cheap picture. As a result, it was perforated and leaked in three months, and millions were lost when it was stopped for maintenance. So, don't take the universal standard set for special working conditions, death.
3. Key clauses that cannot be avoided in model selection-pressure, temperature, displacement and fatigue life
Selecting a metal expansion joint is essentially to find a balance among four parameters:Design pressure, design temperature, compensated displacement, fatigue life.GB/T 12777 provides a lot of formulas, but in actual purchasing, you only have to focus on three points:
First,Fatigue life。 According to the standard, the fatigue times of expansion joints used in steam pipe network should not be less than 1000 times (calculated according to the maximum displacement). If it is a pressure pipeline, some design institutes require 3000 times or even 7,000 times. What do you think? Look directly at the manufacturer's type test report, which will indicate the test conditions and results. Don't trust verbal promises.
Second,displacement direction。 Is the pipe axially telescopic or laterally offset? Or angular displacement? Different displacement types correspond to different structures:Universal corrugated expansion jointMainly absorbs axial displacement;The double hinge transverse expansion joint is suitable for absorbing transverse displacement;Curved pipe pressure balance expansion joint is used for large diameter pipeline and needs to balance blind plate force. You've chosen the wrong structure, and no matter how good the standard is, it's useless.
Third,Pressure thrust。 On high-pressure pipelines, the bellows expansion joint will produce a huge blind plate force (the force is equal to the pressure multiplied by the effective area of the bellows). If this force is not balanced by the tie rod or the main fixing bracket, the pipe and equipment will fly directly out. SoLarge tie rod expansion joint、Pressure balanced expansion jointThese have special fatigue stress calculation chapters in the national standards.
4. Specification details neglected in installation and inspection: guide tube, tie rod, arrow direction
In terms of installation specifications, GB/T 12777 is actually clearly written, but the on-site construction team often ignores three details:
Direction of guide tube。 What is the direction of the arrow on the expansion joint? The answer isDirection of media flow。 The role of the guide tube is to guide the medium smoothly through the bellows, preventing vortices and erosion. If it is installed backwards, the guide tube will become a "baffle", and the airflow will impact the bellows, ranging from abnormal noise to tearing. There is a case of returning goods in our warehouse, that is, the cement factory installed the guide tube backwards, and the edge of the guide tube cracked after half a year.
How to adjust the tie rod nut?When expansion joints leave the factory, tie rod nuts are typically pre-tightened to limit accidental deformation during transport. However, after installation in place, if the pipe is installed cold, the nut must be loosened, otherwise the expansion joint will not be able to expand and contract freely. Many sites remove the tie rod directly-that's wrong! The function of the tie rod is to bear the blind plate force. If the tie rod is removed, the bellows may be pushed into a "balloon" by pressure. Correct practice: Adjust the nut clearance according to the drawing requirements, and allow the amount of pre-tension or pre-compression when cold.
Weld flaw detection。 According to the national standard, the circumferential weld between the bellows and the connecting pipe and the longitudinal weld of the bellows itself need to be inspected by radiation or ultrasonic according to different pressure levels. But how many small factories do you know that skip this step directly? We have seen a project in which the weld seam was not detected, and it leaked after half a year of operation. Therefore, when purchasing, it is best to ask forWeld flaw detection report, at least the reports of filming (RT) or magnetic particle (MT).
V. Metal vs. non-metallic expansion joint: What is the difference between national standards? Don't mix things up
Many people think that expansion joints are the same, and metal and non-metal are different materials. Wrong.The national standard for non-metallic expansion joint is JB/T 12235-2015While the metal expansion joint is GB/T 12777 and JB/T 10617, the two standard systems are completely different.
Non-metallic expansion joints (e.g.Rectangular non-metallic expansion joint、Fabric fiber expansion joint) Mainly depends on the skin materials (silicone cloth, fluororubber, polytetrafluoroethylene, etc.) to bear pressure and seal, and the bellows skeleton generally plays a supporting role. Its advantages are large compensation, corrosion resistance, good vibration isolation, but low pressure resistance (usually no more than 0.1MPa), and fast skin aging. The metal expansion joint is just the opposite: it has high pressure and temperature resistance, but the displacement compensation is relatively small and the rigidity is large.
When selecting, do not mix: flue gas pipeline, dust pipeline, low pressure and large displacement working conditions, preference is non-metal; High pressure, high temperature, metal sealing is preferred. If you install the metal expansion joint on the low-temperature wet flue gas pipeline after desulfurization, wait for it to be corroded by sulfuric acid; In turn, if a non-metallic expansion joint is attached to a steam pipe, the skin will burn through directly-this is common sense, but there are still purchasers who make this mistake.
6. Avoiding procurement pits: How to use normative clauses to judge whether products are compliant
When purchasing metal expansion joints, don't just look at the quotation and delivery date. Teach you to recruit quick judgment:
First, check the number of bellows layers and materials.GB/T 12777 recommends the number of bellows layers with different pressure grades. For example, above 2MPa, multi-layer bellows (such as 3 or 4 layers) are usually used to improve pressure resistance and flexibility. Some manufacturers take single-layer bellows as multi-layer, and their service life is several times worse.
Second, check the size and weight.We have aMetal expansion joint weight tableYou can compare it. If the wall thickness, wave height, wave distance and standard value reported by the manufacturer deviate too much, there is a high probability of cutting corners. For example, the general-purpose corrugated expansion joint has a nominal diameter of DN500 and a standard weight of about XX kg. If the weight of the quoted manufacturer is 20% less, it can basically be determined that the wall thickness has been thinned.
National Code for Metal Expansion Joints? It is not placed there to watch, but is actually used to ensure safety. If you buy the wrong expansion joint, you may cause enough parking losses to buy dozens of qualified products.