Every time a customer asks "Which metal expansion joint is better?" With the parameters of the pipeline, I have to ask first: Does your pipeline run steam or smoke? What's the temperature? How stressful? Have you calculated the compensation amount? I don't mean to make things difficult for people, but I can't recommend them without asking them clearly. The expansion joint is never the more expensive the better, but the more matched the better. Today, I will explain the five questions that must be understood before choosing a model.
First distinguish the type: what kind of expansion joint are you buying
Metal expansion joints are a large category, and there are many types below. Someone took the data of the flue gas pipeline to buy a general-purpose corrugated expansion joint, and it leaked within two months after installing it. When they turned back, they scolded the product. In fact, the selection was wrong.
According to the structure and working principle, there are several common categories:Corrugated expansion joint(Including a general-purpose corrugated expansion joint, a high-temperature axial expansion joint, a straight tube pressure balance expansion joint, a curved tube pressure balance expansion joint, etc.),Sleeve type pipe expansion joint、Rotary compensator。 The bellows absorbs the displacement by corrugation deformation, the sleeve slides by the inner and outer tubes, and the rotary compensator absorbs the displacement of the pipeline by rotation, each of which has its own application scenarios.
In large-diameter low-pressure scenes such as flue gas pipes and air ducts, non-metallic expansion joints or rectangular expansion joints are usually used; Steam pipelines and chemical pipelines use metal corrugated expansion joints; Direct buried (fully buried) type expansion joint for buried pipeline; Dual hinge expansion joint for vacuum special hose or air cooled island vacuum pipe for vacuum pipe. If the working conditions are wrong, no matter how expensive it is, it will be useless. Just like you can't wear leather shoes to run a marathon.
Three hard indicators: compensation amount, pressure and temperature. None of them will work
These three parameters are the selected foundation. The amount of compensation is not enough, and the bellows is pulled to the limit and directly torn; When the pressure exceeds, the bellows bulges or even bursts; The upper temperature limit leaves no margin, the creep of the material accelerates, and the life of the material decreases by a cliff.
For example, the hot air duct of a power station boiler has a temperature of 350 °C, a pressure of 5 kPa and a lateral displacement of 40 mm. If you take a general-purpose expansion joint with normal temperature and pressure to install it, the wall thickness of the bellows can't withstand high-temperature oxidation at all, and intergranular corrosion cracks will appear in less than a few months. This working condition must be at least a high-temperature axial expansion joint, or a special corrugated expansion joint for power station industry, and the material must be temperature-resistant alloy.
Media corrosiveness is often a "hidden killer". For corrosive media such as desulfurization flue gas and chemical tail gas, the material of corrugated pipe must be upgraded to 316L or even higher, and lined with PTFE if necessary. Otherwise, in less than two years, the bellows will be corroded and perforated. A cement factory customer chose 304 material for cheap at first. As a result, the flue gas with excessive SO₂ content directly corroded the bellows, and the loss of shutdown maintenance was far greater than the saved material cost.
Look at the structure selection: how to go the pipeline, you can follow the selection
There are many structural types of expansion joints, but in fact, they correspond to the displacement form of the pipeline. The selection logic is very simple: in which direction the pipeline moves, you choose the expansion joint that absorbs the displacement in which direction.
- Mainly axial displacementThe pipeline is straight, short and has large thermal expansion-choose axial type, such as external pressure single axial expansion joint, which has good external pressure structure stability and is suitable for high pressure working conditions.
- Lateral displacement is mainly, Z-shaped pipe section or L-shaped pipe section-choose transverse type, such as compound hinge transverse expansion joint, and two hinge groups cooperate to absorb transverse displacement.
- Multiple directional displacement need to be absorbed simultaneouslyBut the thrust cannot be too large-choose the universal hinge type or the double hinge type. Single hinges can only absorb angular displacement in one plane, while universal hinges can absorb spatial angular displacement.
- Afraid that the pipeline pressure thrust is too great and the equipment will be damaged-choose the pressure balanced type, such as the straight pipe pressure balanced expansion joint or the curved pipe pressure balanced expansion joint. The blind plate force generated by the internal pressure of the bellows is balanced by the internal structure, and no additional thrust is applied to the equipment.
- Space is constrained to fit large corrugated expansion joints— — Consider the rotary compensator or sleeve-type pipe expansion joint, which is small in size and large in compensation.
Don't take a universal corrugated expansion joint and try to solve all the problems. The general-purpose type is cheap, but it has certain compensation ability in three directions, and the compensation amount in each direction is not large. When the working conditions are complicated, it is hard to use it, just like taking a Swiss Army knife to cut down trees. It is capable, but it is extremely inefficient and easy to collapse.
The material and manufacturing process determine how long it lasts
In the same working condition, some people use expansion joints for five years, while others leak after half a year. What's the difference? Material and craftsmanship.
Let's talk about the material first. Although the main materials of 304 and 316L are austenitic stainless steel, their molybdenum content is different, and their chloride ion corrosion resistance is a grade poor. Desulfurization, seawater, chemical industry, 316L is the bottom line. If the chloride ion in the medium is high, it has to be upgraded. When the temperature exceeds 800℃, ordinary stainless steel won't work, so high-temperature resistant alloys have to be used, and the manufacturing process is completely different.
The wall thickness of bellows is ignored by many people. The thinner the wall thickness, the better the flexibility, the greater the compensation amount, but the smaller the pressure resistance and corrosion resistance margin. The thicker the wall thickness, the stronger the pressure bearing capacity, but the greater the stiffness, the weaker the compensation capacity. There is a balance point here, which has to be calculated according to actual working conditions. Tell you to pat your head to fix the wall thickness of the manufacturer, as soon as possible black.
Fatigue life is another hard indicator. Corrugated pipe absorbs displacement by corrugated deformation. Every time it is deformed, the root of corrugated pipe is subjected to alternating stress. How many cycles can be withstood in the design life, the manufacturer must do a fatigue test and issue a test report. Under normal circumstances, the fatigue life required by the standard is more than 1000 times, and good ones can achieve thousands of times. If you don't ask this data clearly, you will find the manufacturer when the bellows is cracked, and they will send you off when they say "the working conditions have changed".
Don't just focus on quotes when picking manufacturers. These questions must be asked
Do you really dare to use the one with the low quote? The expansion joint is a "air bag" in the pipeline system, and the pressure, temperature and displacement are all pressed on it. Once it fails, the whole system has to stop. Compared with the loss of shutdown, what is hundreds of dollars more expensive?
Just ask three questions.
First, are there any cases of similar working conditions? The working conditions of power plants, steel plants and chemical plants have their own tempers. Manufacturers who have done similar projects have their experience points directly. Ask him to send you a few cases and ask about the use effect by the way.
Second, can you produce a calculation book? The regular manufacturer will calculate the selection of expansion joints according to the pipe system parameters provided by you, and produce a calculation book to explain the verification results of compensation amount, pressure and temperature. If you can't come up with a calculation book, you basically rely on experience to make a decision at your own risk.
Third, how fast is the after-sales response? Problems with expansion joints are often emergency shutdown. Can the manufacturer respond within 24 hours, arrive within 48 hours, or at least give an emergency treatment plan. Bargains without after-sales guarantee are "cheap to buy and expensive to use" in the end.
So which metal expansion joint is better? The answer is clear: if it is suitable for working conditions, the right material is used, the manufacturing process is excellent, and the manufacturer is reliable, that's good. There is no shortcut to model selection. Only by confirming the parameters one by one and asking the questions one by one can we avoid detours and spend less money.