Find out first: What exactly is the "PTFE expansion joint" you asked about
To put it bluntly, the PTFE expansion joint is two layers of skin-a metal bellows on the outside and a layer of polytetrafluoroethylene (PTFE) on the inside. These two things are put together for a clear purpose: metal bellows to carry pressure and displacement, and PTFE lining to deal with strong corrosive media. Like the one in our stationPTFE-lined hose, andPTFE compensatorThey are similar in structure.
Usually, most of the working conditions we encounter are positive pressure (internal pressure), the medium pushes outward, and the bellows and lining are comfortable. Once it is changed to negative pressure (vacuum), the tube sucks hard, and the thin PTFE lining hangs-it is afraid of being deflated and falling off. So don't ask "Do PTFE expansion joints have negative pressure?", You have to find out what kind of structure and level you are using first.
What are the tests faced by PTFE lining and bellows under negative pressure
Let's start with bellows. The metal bellows itself is not so sensitive to negative pressure, especially the multi-layer thin-walled structure. The external pressure stability is worse than that of the internal pressure, but as long as it is not high vacuum or large diameter, conventional design can deal with it. The real "soft spot" is the lining.
Soft. When the temperature is high (for example, over 150℃), it is softer. Under the action of negative pressure, the lining is easily sucked in, causing bulging, collapse, and even coming out of the flange connection. There was once a case in a chemical plant, where a standard PTFE expansion joint was used on a vacuum pipeline of-0.05MPa. Just two months after driving, the lining was sucked into "corrugated paper" and blocked the pipeline-this is a typical design mistake.
So, to judge whether it can be used or not, the key depends on two parameters:Vacuum magnitudeAndOperating temperature。 The cooler the temperature, the better the lining rigidity; The shallower the vacuum, the less risk.
Standard PTFE expansion energy saving can not be used in negative pressure system? The key is to see two parameters
Standard PTFE expansion joints are generally not recommended for use in negative pressure conditions。 Unless both conditions are met: the vacuum is not less than-0.02 MPa (gauge pressure) and the operating temperature is not more than 80 °C. In this case, the lining still barely holds up. But if you really want to meet a working condition of-0.05MPa and 120℃, the standard type is a time bomb.
Why? Because the standard PTFE lining is only loosely lined on the inner wall of the bellows, it is pressed at both ends by the flange flange. As soon as the negative pressure sucks, there is no support in the middle, and the lining hangs. If you go to those manufacturers that do vacuum, they will add a layer of stainless steel wire reinforced mesh on the outside of the lining, or simply useSpecial hose for vacuumThat monolithic molded construction.
Don't be cheap, you won't save enough money to fix the plumbing once.
If you really want to put on negative pressure, which structure should you choose? Vacuum special hoses and reinforced schemes
In the negative pressure scenario, there are two safe choices:
- Special hose for vacuum: This kind of product has a special introduction in our station. It is not simply lined with PTFE, but PTFE is made into a corrugated shape, and the outside is covered with stainless steel woven mesh. The overall pressure bearing capacity is strong, and the negative pressure can be-0.095MPa. The disadvantage is that it is expensive and not as flexible as ordinary metal hoses.
- Reinforced PTFE-lined expansion joint: On the basis of the standard type, stainless steel support rings or coil springs are added inside the lining to prevent the lining from being deflated. The applicable vacuum degree is within-0.05MPa, and the temperature can reach 150℃. The cost is lower than the vacuum special hose, but it needs to be customized by the manufacturer.
In addition, there is a trick: Install the expansion joint outsideExternal pressure single axial expansion jointStructure, so that negative pressure is applied to the shell, and the lining is not stressed-but this thing is large in size, and it is generally only used in power stations and large chemical pipelines.
Don't step on pits: Three common mistakes made by customers and suggestions for model selection
I have been in this business for more than ten years, and I have watched many Party A stumble on PTFE expansion joints. There are three most common questions:
Mistake 1: Use the standard type as the vacuum type.As mentioned earlier, these two structures are completely different. Write "PTFE expansion joint" on the purchase list, without indicating the vacuum degree, and it will be wasted as soon as the negative pressure is pumped at the site.
Mistake 2: Ignore the effect of temperature on the liner.Many people only ask "how much negative pressure can it withstand", and no one asks about the temperature. The same-0.05MPa can be used at room temperature, but there will be problems at 120℃. It is common sense that the mechanical strength of PTFE decreases sharply with the increase of temperature, but some people always ignore it.
Mistake 3: Improper handling of the flange sealing surface.Negative pressure pipes have extremely high sealing requirements. If the flange of PTFE lining is not compacted, or there are scratches on the flange surface, the negative pressure will not be kept as soon as the air leaks in. To make matters worse, once there is an air leak, a local positive pressure will form on the back side of the liner, which directly bulges the liner out.
Type selection suggestions? Simple and crude:
-vacuum degree80℃ → Honestly put on vacuum special hose
-Vacuum degree-0.02~ -0.05MPa, temperature ≤100℃ → Reinforced lining PTFE expansion joint
-Vacuum> -0.1MPa → Don't consider PTFE, replace metal bellows with PTFE coating
Summary: Three things that must be confirmed before placing an order in a negative pressure scenario
After talking so much, back to the original question: "Do PTFE expansion joints have negative pressure?" – the answer is yes, but depends on the case. Before placing an order, please take these three to line with the manufacturer:
- Have you given all the working condition parameters?Degree of vacuum (absolute pressure or gauge pressure?), operating temperature, media composition, do you start and stop frequently? Without a parameter, the manufacturer can't match it for you.
- Did you choose the right structure?Is it standard, reinforced or vacuum-specific? Don't think about saving money with the standard model, and the consequences are endless.
- Does the manufacturer have the ability to test negative pressure?This one is the easiest to overlook. Many small factories only do positive pressure airtight, and there is no negative pressure test equipment. What do you want them to guarantee? Our products will do negative pressure holding test before leaving the factory to ensure that there is no leakage at-0.095MPa for 24 hours.
Well, all the words are on this. If you are still unsure, send the parameters directly to the manufacturer and let them come up with a selection plan. Remember one sentence: negative pressure is not a child's play, and PTFE expansion joints are not all-purpose glues.