What exactly is the pressure of the flue expansion joint? Don't be fooled by the numbers
After more than ten years in this business, the most common problem I encounter is that customers ask as soon as they open their mouths, "How much pressure does your expansion energy saving bear?" It seems that the larger the pressure number, the more awesome things will be. To put it bluntly, this is not the case at all when the flue expansion joint is under pressure-it is more like a "tailor-made shoe" where the fit matters, not the bigger the better.
The pressure in the flue system, usually referred to as the design pressure, is in kPa or MPa. But this number is not determined by patting the head. It is related to the full pressure of the fan, the resistance of the pipeline and the flow rate of the flue gas. Even if you buy a metal corrugated expansion joint that can carry 10MPa, the result is to use it in an atmospheric flue with only 5kPa, that is, to kill a chicken with a knife-money is wasted, and it is easy to limit the displacement of the pipeline because of too much stiffness, which will cause problems.
The nominal "nominal pressure" of many manufacturers is measured at normal temperature and static conditions. However, high-temperature smoke runs in the flue, and some of them also carry corrosive media. As soon as the temperature rises, the allowable stress of the material plummets.
Metal corrugated expansion joint vs non-metal expansion joint, who can carry it better?
It's a classic multiple choice question. Metal corrugated expansion joints (such as corrugated expansion joints and high-temperature axial expansion joints used in power station industry) can indeed withstand higher pressure-stainless steel corrugated pipes are pressure-resistant and temperature-resistant, and their structural strength is there. However, the problem is that the flue is often large section, low pressure, too heavy metal parts, high cost, and limited ability to compensate for displacement.
Although the non-metallic expansion joint (fabric fiber expansion joint) has weaker pressure resistance, usually not more than 0.1MPa, it has several unique skills: it can absorb multi-dimensional displacement, corrosion resistance, light weight and easy installation. Especially when used in desulfurization flue, cement kiln tail and other low temperature and low pressure but corrosive occasions, non-metal is the correct solution. If you have to use metal, it will be corroded and perforated by acidic smoke, so you might as well save some money for non-metal.
High pressure, high temperature, small diameter, choose metal corrugated expansion joint; For low pressure, large displacement and environmental corrosion, choose non-metallic expansion joint (rectangular non-metallic expansion joint). There is no absolute good or bad, only fit or not.
The cost of wrong pressure rating: a real lesson from a thermal power plant
Two years ago, a Northern Thermal Power Plant asked me for help. The expansion joint used in the outlet flue of their boiler induced draft fan has a design pressure of only 10kPa. As a result, it was cheap when purchasing, and a batch of general-purpose corrugated expansion joints with a nominal value of 0.25MPa were bought. You might think, 0.25MPa is already 250kPa, more than enough? What's the problem?
They didn't think about the seals at the flange joints. Because the stiffness of the expansion joint is too large, the pipeline thrust crushes the flange gasket during thermal expansion, and the air leakage is serious. The current of the induced draft fan soars, and the energy consumption directly increases by 8%. After removing it, the bellows itself is fine, but the bolts are loose and the guide tube is misaligned. Finally, replace it with a suitable non-metallic expansion joint, select the pressure level according to the actual working conditions, and add another setFlue gas baffle doorDo quarantine, and all the problems are solved. Do you think this wrongful money is worth it?
Don't just look at pressure: temperature, medium, and displacement are the chain
When many engineers select models, they only see the numbers on the pressure gauge, but forget the two "hidden killers" of temperature and displacement. For example, if the flue expansion joint is under pressure at normal temperature, but the actual operating temperature reaches 400℃, the yield strength of stainless steel will decrease by more than half. At this time, the bellows may be plastically deformed before it reaches the pressure limit.
Media is also key. The wet flue gas in the desulfurization flue contains sulfide, which isMetal hoseAnd bellows are corrosive. Even if you are 304 stainless steel, you can't bear long-term washing. Therefore, the use of polytetrafluoroethylene linedPTFE-lined hose, or simply put on non-metallic expansion joints, don't compete with corrosion.
Let's talk about displacement. The flue expands under heat, resulting in axial, transverse and even angular displacements. If the expansion joint can only absorb axial displacement, but you install it where lateral compensation is needed, no higher pressure level can handle twisting. That's when we have to considerCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion joint
Structural design is used to resolve the stress.A wrong bolt is screwed during installation, and the design pressure is wasted
This is the most injustice I have ever seen. Obviously, the selection is correct, but the installation went wrong. The screw of the expansion joint is locked during transportation to protect the bellows. Once installed in place, the screw must be loosened or removed to allow the expansion joint to expand and contract freely. As a result, some workers didn't understand, so they directly screwed the bolts and put them into operation. The force of thermal expansion and contraction had nowhere to go, so the pipe support was crooked and the bellows cracked. What do you say this has to do with the pressure on the flue expansion joint? It matters a lot-no matter how high the design pressure is, if you use it as a rigid pipe, the pressure is bullshit.
And installation directions. Many expansion joints (such asThe direction of the arrow of the expansion joint refers toFlow direction) is marked with an arrow indicating the direction of the media flow. If it is installed backwards, the guide tube will be washed over by the smoke, and the bellows will be directly exposed to the scour, shortening the life by more than half. So be sure to read the instructions when installing, don't be self-righteous.
In addition,Correct installation method of expansion joint of large tie rodIt is necessary to ensure that the nuts at both ends of the pull rod can be freely adjusted, and the pull rod cannot be used as a fixed support. Twist a wrong bolt and the entire pressure system is destroyed.
Practical Selection: How to Find a Balance Between Stress, Life and Cost
Having said all that, how do you choose? I generally suggest three steps:
- Step 1: Clarify the working condition parameters。 Pressure is just one of them, list the temperature (maximum/minimum), media composition, pipe routing, connection mode, space size. Especially the amount of displacement-axial, transverse and angular displacement, select the corresponding type. For example, for large displacementCompound straight pipe bypass pressure balanced expansion jointFor small displacementUniversal corrugated expansion joint。
- Step 2: Calculate the safety factor。 Don't be superstitious about the nominal "maximum pressure" of the manufacturer. It is enough to select the model according to 1.25 times the actual working pressure. For high temperature working conditions, we must look at the allowable stress of the material at the corresponding temperature. If you don't understand, ask the manufacturer for a calculation book, from a regular manufacturerStiffness and Calculation Formula of BellowsAll can be provided.
- Step 3: Coordinate Life Cost。 If the pressure grade is selected high, it is not only expensive, but also rigid, which may affect the pipeline stress distribution. If you choose low, the consequences of leakage and rupture will be more serious. Where are the trade-offs? For example, the non-metallic expansion joint of the flue of a thermal power plant, even in low-pressure conditions, can be replaced once every three to five years, and the cost is more cost-effective than the one-time high-pressure metal parts. After allService life of expansion jointAnd the replacement cycle should be taken into account.
The higher the pressure the flue expansion joint bears is not the better, and you can save money if you choose the right one. Don't be fooled by the numbers, understand the working conditions thoroughly, and find a reliable supplier (such as ours with a full range of products), which is better than anything else. And guess what? Many customers have changed their selection ideas according to what I said, and they can save hundreds of thousands of spare parts fees a year. Tsk, that's the power of professionalism.