What exactly does a pressure rating mean: the difference between nominal pressure, design pressure and test pressure
Many engineering purchases get the drawings of metal expansion joints, and at first glance they look at the PN number on the nameplate. PN2.5, PN4.0, thinking that if this number is enough, everything will be fine. And the result? Within two months of installation, the bellows cracked, the media leaked, and the scene couldn't be cleared.
Let's figure out three concepts first. The nominal pressure is the nominal pressure value given at the time of design. It is only a nominal value and does not represent the limit that can actually be withstood. The design pressure is the highest pressure that may occur in the operation of the pipeline system, which is the number that should be really benchmarked when selecting the type. The test pressure is the value of pressure applied during water pressure or air pressure test before leaving the factory, usually from 1.25 times to 1.5 times of the design pressure. The PN number on the nameplate is neither the design pressure nor the test pressure, it is just the code name of the pressure rating. Think of PN2.5 as a working condition that can run 2.5MPa, and it is a matter of time before problems go wrong.
Who determines the pressure capacity of bellows: wall thickness, wave height, wave pitch, number of layers, material grade
For the metal expansion joint of the same DN300, Manufacturer A quoted 2.5MPa, Manufacturer B quoted 1.6MPa, with a price difference of 30%. Is the expensive one tricking you? Not necessarily. The pressure capacity of bellows is not determined by a single number of nameplates at all. It is determined by a set of parameters together.
The thicker the wall thickness, the higher the pressure, but the stiffness is also large, so the compensation amount is small. The higher the wave height, the better the flexibility, but the pressure bearing capacity decreases. The smaller the wave distance, the denser the corrugation, the better the pressure bearing, but the difficulty of forming is greater. The multi-layer bellows with many layers can improve the pressure resistance while maintaining flexibility, and the material grade directly determines the allowable stress-the strength of SUS304 and SUS316L at high temperature is completely two levels. There's another one that's easy to overlook: the fatigue life of the bellows. The higher the pressure, the lower the cycle life. The same expansion joint can run 20,000 times at 1.0MPa, and there may be 8,000 times left when it comes to 1.6MPa. When selecting a model, just staring at the pressure level on the nameplate is equivalent to leaving your life to luck.
How to distribute common pressure levels: from 0.25MPa to 4.0MPa, or even higher
Common metal expansion joint pressure ratings in engineering range from 0.25MPa to 4.0MPa. General-purpose corrugated expansion joint is generally used on low-pressure pipelines. 0.25MPa to 1.6MPa is its comfort zone. The environment is mild, the displacement requirements are not high, the price is cheap, and it is okay to run water and walk away with superheated steam. External pressure single axial type expansion joint is suitable for 1.6MPa to 2.5MPa pipe section. Its bellows bears external pressure and has better stability, which is not as easy to be unstable by the column as internal pressure. Straight pipe pressure balance type expansion saves energy to more than 2.5MPa or even 4.0MPa, because it is the opposite compensation of two sections of bellows, which internally digests the blind plate force generated by the medium pressure, and the main fixed bracket does not have to carry so much thrust. High-pressure steam pipelines in power stations, metallurgy and chemical industries basically have this structure.
What about above 4.0MPa? Large-diameter thick-walled expansion joints do this, with a wall thickness of 8mm or even thicker. The material is Inconel 625 or Hastelloy, and the pressure resistance can be lifted up. But the price is ten for a top ordinary expansion joint, and you can do what you can.
How to choose under high temperature and high pressure conditions: Special considerations in power stations, cement and desulfurization flue gas pipelines
In the main steam pipeline of power station, the kiln tail flue gas pipeline of cement plant and the original flue gas pipeline of desulfurization system, pressure is only one of the parameters, and temperature and medium corrosiveness are the real killers. The yield strength of materials drops sharply at high temperature-the allowable stress of SUS304 is about 130MPa at room temperature, and it drops to over 40MPa at 600℃. For the same pressure level, as soon as the temperature is high, the actual allowable pressure has to be discounted.
Desulfurization flue gas pipeline is more headache, the medium contains sulfur and chlorine, and the acid dew point corrosion can't be avoided. When many projects are selected, the pressure level is calculated clearly, but as a result, this crop is forgotten, and the bellows is corroded and perforated. We also have to consider the fatigue life-the flue gas pipeline starts and stops frequently, and the number of pressure cycles is large, so the fatigue life of the bellows directly reduces the actual allowable pressure. When selecting, it is better to increase the design pressure by 30% than to make up the boundary of working conditions. The safety margin is not a waste, but an insurance purchase.
The easiest pits to step on when selecting a model
After running for more than ten years, there are only three pits that I see most. First, the nameplate pressure is sufficient but the actual working conditions are not satisfied. Customers often ask with the expansion joint of PN1.6, saying that the running pressure of my pipeline is only 1.2MPa, why is it still leaking? Looking at the parameter table, the medium temperature is 350℃. At 350℃, the actual allowable working pressure of the 1.6MPa expansion joint may be less than 1.0MPa. Second, ignore the hydraulic test pressure. Some purchases are bluffed by the supplier's sentence "test pressure 2.4MPa", thinking that it is good goods if the voltage is stabilized for 24 hours and it doesn't explode. However, the hydraulic test is to test the strength and sealing performance, not for you to use as working pressure. The high test pressure can only show that this expansion joint is qualified before leaving the factory, which is different from whether it can run stably in the system for a long time. Third, use the blasting pressure as the working pressure. The burst pressure is the pressure at the moment the bellows breaks, usually 3 to 5 times the design pressure. Some customers use the blasting pressure report to prove that their expansion energy saves high pressure-that is not to prove that it is pressure-resistant, but to prove that it has been wasted.
Summary: A reliable list of stress level confirmations
To put it bluntly, choosing metal expansion joints is not to pick the size of numbers, but to check a complete set of working condition parameters. Here's a list, take it and follow it directly:
- Confirm that the design pressure of the pipeline system, not the working pressure, is the highest possible pressure-including water hammer, steam hammer, instantaneous overpressure caused by valve malfunction.
- Confirm the design temperature, and hand this temperature and design pressure to the manufacturer, so that the manufacturer can check the wall thickness and the number of layers of the bellows according to the actual material allowable stress.
- Tell the manufacturer that the media composition, especially the media containing chloride ion, sulfide, acid, base and salt, should give sufficient corrosion margin.
- Clarify fatigue life requirements – how many cycles do you want the expansion joint to cycle at maximum displacement? 5,000 or 20,000? This number directly affects bellows thickness and wave height design.
- The manufacturer is required to provide a hydraulic test report, but it is not used as a working condition, but to confirm that the strength is qualified.
- Finally, allow at least 10%-20% of the design pressure margin, and more margin is recommended for high temperature and high pressure conditions.
After checking this list, the metal expansion joint pressure level will not be selected wrong again. If you choose the wrong expansion joint, you can shut down the machine for maintenance or burst the pipeline. That price is much more expensive than spending 20% more budget.