How to determine the compensator temperature range? Understand these points before choosing a model
Two days ago, I met a customer, and I asked: Can your high-temperature axial expansion joint be used in 600℃ steam pipeline? I asked him back: What's the steady-state temperature? Is there an instantaneous overtemperature? He said he didn't know, but it was 600 on the nameplate anyway. And the result? After checking the process parameters, the actual operating temperature fluctuation range is 450~620℃, and there is a cold start once a day. In this working condition, the design temperature of 600℃ can't bear it at all-the fatigue life of bellows drops directly from a cliff. So, the compensator temperature range is determined? You can't just look at the nameplate.
1. Why is the temperature range the first factor in selection?
The high-temperature axial expansion joint used in a power plant has a design temperature of 600℃, but it leaked after half a year of operation. After checking, the actual temperature fluctuation of the medium exceeded the upper limit. It's not that the design temperature is not enough, but that the designer didn't take the temperature cycle into account. Compensators are not universal parts, and temperature directly determines three things: the bellows material (304 or 316L or Inconel 625), the structural thickness (every 0.5mm increase in thickness may double the fatigue life), and the overall fatigue life. The permissible stress of austenitic stainless steel decreases by about 30% ~40% for every temperature increase of 100℃. If you think about it, choosing the wrong temperature is equivalent to planting a ticking time bomb in the system.
2. Temperature limits of different materials
Metal hoses and corrugated expansion joints mainly rely on stainless steel to support the surface. The maximum temperature resistance of 304 is about 450℃. If this number is exceeded, oxidation and peeling will begin; 316L can carry it to about 500℃, but pay attention to chloride ion corrosion; The Inconel 625 can withstand more than 800℃, but the price is five or six times that of the 304. Non-metallic expansion joints (fabric fiber expansion joints) rely on coatings and thermal insulation layers, and usually have temperature resistance in the range of 300~600℃, but it depends on the bonding process of fiber substrate and silica gel/PTFE coating. Don't think about the rubber compensator. When it exceeds 120℃, it starts to age and become sticky. The rubber PTFE compensator can barely last to 150℃, but its life is not long. What about PTFE-lined hoses and PTFE compensators? PTFE temperature resistance is-200~260℃, and highly toxic gas is decomposed when it exceeds 260℃, so extra caution should be taken when used in high-temperature flue gas containing acid-condensation corrosion when the temperature is low, and material collapse when the temperature is high.
Three hard indicators for determining the temperature range
Don't think that knowing the maximum temperature of the medium is done. The first is the operating temperature — the temperature of the medium during normal operation, such as the steady-state temperature of a steam pipe. The second is the design temperature-it must contain a safety margin, usually 10~20℃, but don't forget the temperature difference impact during cold start-up. The third is the temperature cycle frequency-frequent heating and cooling will make the fatigue life drop off by a cliff. I have seen a project that uses a general-purpose corrugated expansion joint. The working condition is to start and stop once a day, and the temperature difference is 400℃. As a result, it cracks after 6000 cycles. The standard design life can be 10,000 times. Temperature difference impact is the killer, especially when the wall thickness of the pipe is large, the thermal stress generated by the temperature difference between the inner and outer walls may directly tear the bellows.
Fourth, what to do under special working conditions?
Compensators behind flue gas baffle doors often face acid-containing dew point corrosion. The compensator matched with the desulfurization flue gas baffle door should be temperature resistant (the flue gas temperature is usually 120~160℃) and corrosion resistant (dilute sulfuric acid, chloride ion). At this time, PTFE compensator or PTFE-lined hose is more reliable-the PTFE layer isolates the corrosive medium, and the metal layer provides strength. But don't forget, if the flue gas temperature exceeds 260℃, PTFE will be wasted, and you have to use non-metallic expansion joints (fabric fiber expansion joints) plus acid-resistant coating instead. In addition, the metal corrugated expansion joint used in the cement industry (metal corrugated expansion joint in the cement industry) faces the problem of high temperature dust wear. The temperature itself may not be high (400~600℃), but the dust particles scour the wall thickness of the bellows, and the wear rate is positively correlated with the temperature-the higher the temperature, the softer the material and the faster the wear. Therefore, it is necessary to leave a margin for the wall thickness or add a guide tube when selecting the type.
5. Selection misunderstanding: Don't regard "instantaneous temperature" as "continuous temperature"
Two days ago, I met a customer who said that the rotary compensator should be used in 800℃ steam pipeline. I asked him to adjust the process curve. In fact, it was just instantaneous overtemperature during driving, and the steady state was only 550℃. This kind of working condition does not need to go to Inconel at all, 316L plus guide tube is enough. Under instantaneous overtemperature, although the bellows can be tolerated for a short time, frequent instantaneous high temperature will lead to accelerated intergranular corrosion of materials. Therefore, the temperature range should be clearly confirmed with the process engineer: is it continuous or intermittent? Is there emergency overtemperature protection? Don't ask me how I know-there was a project before that I didn't ask clearly, so I chose a high-temperature axial expansion joint of Inconel 625. As a result, the actual temperature at the site was less than 500℃, which cost three times more.
6. Finally, give a self-examination list
Before the next selection, go through these four items yourself:
① Media type (steam, flue gas, corrosive gas?)
② Maximum/minimum operating temperature (note that it is not the design temperature, but the actual long-term operating temperature)
③ Temperature change rate (℃/min) -this data is not provided by many manufacturers, but it affects the fatigue life calculation
④ Expected number of cycles (including cold start and load fluctuation)
Take these four items to ask the manufacturer, and the quotation given by the other party will have reference value. If you are still worried, ask us directly for the temperature test method in the national standard JB/T 12235-2015 for non-metallic expansion joints, and you can compare it yourself. Compensator temperature range determined? If you understand these four points, you will basically not step on a pit.