Let's talk about the temperature first: it's not just filling in a number
Two days ago, a customer came to me with a purchase order that said "temperature 200℃" and asked me if I could do it. I asked him back: Is this 200℃ the medium temperature or the design temperature? Steam pipes or thermal oil? Is it a continuous 200℃ run or an occasional flush? He was stunned for a moment and said, "This…the manufacturer asked me to fill it in."
And the result? Less than two months after installing the equipment, the bellows leaked. When disassembled, the material has creep cracks. The problem is not the manufacturer, but the "200℃"-he filled in the medium temperature, but the design temperature required for model selection is the highest temperature of the medium plus the safety margin, which is dozens of degrees different. How to write compensator selection temperature? If you really think you can just fill in the numbers, then there are pits behind.
First thing: The three temperatures must be clearly distinguished
Medium temperature, design temperature, and ambient temperature are three things, not the same thing.
Medium temperatureIt is the actual degree of fluid in the pipe, which is easy to understand.Design temperatureIt is the highest temperature that the compensator can bear. When selecting the model, the highest temperature of the medium plus the safety margin is taken-for example, the highest operating temperature of the steam pipeline is 250℃, and the design temperature is at least 280℃ or even 300℃.Ambient temperatureIt is the external environment. Outdoor pipelines are exposed to minus 20℃ in winter and 60℃ in summer, which affects the selection of insulation layer and material toughness.
Many people just write the medium temperature and are done. The design temperature is not written, the ambient temperature is not mentioned, and the manufacturer can only guess. If you guess right, you are lucky. If you guess wrong, whose equipment is scrapped?
The second thing: Metal and non-metal, the way to write temperature is two sets of logic
Metal corrugated expansion joints (such as general corrugated expansion joints and high-temperature axial expansion joints) absorb displacement by elastic deformation of corrugated shell, and temperature directly determines the allowable stress of material. 304/316L can hold up at room temperature, and above 400℃, the allowable stress is visible to the naked eye. At this time, either change the alloy material with higher temperature resistance, or increase the number of layers or wall thickness to compensate for the strength loss. Therefore, when selecting the metal compensator, the temperature should be clearly written as "maximum operating temperature", and whether there is an instantaneous overtemperature condition should be indicated-for example, when the steam pipe is heated, it may instantly rush above the design value, which directly affects the fatigue life.
The temperature resistance of non-metallic expansion joints (fabric fiber expansion joints, rectangular non-metallic expansion joints) depends on the skin material. Silicone coated glass fiber cloth, polytetrafluoroethylene film, ceramic fiber felt, each material has different temperature resistance upper limit: fluororubber can only reach around 200℃, and ceramic fiber can reach above 1000℃. Selecting and filling temperature of non-metallic expansion joint, you can't just write an average value. You have to write clearly the sustained temperature, peak temperature and duration-the non-metallic skin burns through this thing, which is often caused by instantaneous overtemperature, not slow aging. It looks reasonable for you to write "350℃". In actual working conditions, the bypass rushes to 600℃ for half an hour, and the skin burns through directly. Whose do you think this belongs?
Third thing: Temperature and pressure are bound, don't disassemble and write
The same general-purpose corrugated expansion joint can withstand 2.5MPa at room temperature, and may not even be able to withstand 1.0MPa at 400℃. When the temperature is high, the yield strength of the material decreases and the creep rate accelerates. Therefore, the temperature and pressure in the selection parameter table must be paired: "200℃/1.6MPa" and "400℃/0.6MPa"-this is called working condition pair, not two separate numbers. If only one temperature is written without the corresponding pressure, the manufacturer can't calculate the fatigue life and stability at all.
And the temperature cycle frequency. Alternating hot and cold several times a day is completely different from starting and stopping only once a year, and the fatigue life calculation is completely different. Each time the steam pipeline starts and stops is a temperature cycle, and each cycle of the bellows consumes a fatigue life. When writing the temperature, write the working system clearly: continuous operation or intermittent operation, and start and stop several times a day. It's much more important than struggling with those few degrees of error.
Fourth thing: Case is straightforward than theory
Some time ago, a cement factory purchased a non-metallic expansion joint for the flue gas pipeline. The working condition table wrote "temperature 350℃", which looked fine. I asked: Is there an instantaneous high temperature in the kiln tail flue gas? He said that yes, when the kiln is shut down for maintenance, the bypass is opened, and the flue gas temperature can rush to 600℃ for about half an hour.
That's the key message. The material cost difference between the skin selected according to 350℃ continuous and 600℃ instantaneous working conditions is more than double that of the material selected according to 600℃ continuous working conditions. If the communication is not in place, the manufacturer will continue to design according to 600℃, and you will spend 600℃ money; The manufacturer designs according to 350℃, and what you get is 350℃ goods, which will be burned through as soon as the bypass is opened.
Practical suggestion: Write this in the temperature column of the selection single
Don't bother, just fill in these:
- Medium temperature range: xx℃ ~ xx℃
- Design temperature: xx℃
- Instantaneous maximum temperature: xx℃/duration xx minutes
- Ambient temperature: xx℃
- With or without insulation: Yes/No
If you are not sure, ask the manufacturer directly, explain the working conditions thoroughly, and let the other party help you judge. Compensator is a waste of money if you choose it big, and a safety hazard if you choose it small. Only when the temperature parameters are written accurately can the equipment be installed and used for a long time. How to write compensator selection temperature? The answer is four words: write the whole working condition.