What exactly are metal expansion joints used for? -"Flexible Joints" of Piping Systems
Have you ever seen dozens of meters of steam pipes "twist" themselves in the sun on a hot day? It doesn't matter if you haven't seen it in person, but you have to know that when the temperature changes, metal pipes can stretch or shorten. A 100-meter-long carbon steel pipe has about 12 centimeters more for every 100℃ increase in temperature. Where are these 12 centimeters going? If you carry it hard, the pipes, valves and brackets will all be pulled out. At this time, the metal expansion joint is the "flexible joint"-it absorbs displacement, relieves stress, and allows the whole pipe network to live safely in thermal expansion and contraction.
To put it bluntly, the core functions of metal expansion joints are three:Absorbing axial expansion and contraction, compensating lateral displacement, reducing vibration and noise。 Power stations, cement, chemical industry, metallurgy, desulfurization…which high-temperature and high-pressure industry can live without it? Two days ago, a buddy who did a thermal power project spat with me, saying that they had used it in their factory for ten yearsCorrugated expansion joint for power station industrySuddenly it leaked. When I removed it, I saw that the ripples had been fatigued and cracked-the choice was wrong when I installed it. The design life was only five years, and it lasted for ten years, so it was a fortune.
Products are divided into categories: general-purpose type, high-temperature axial type, pressure balance type... Don't choose the wrong one
"Bring me an expansion joint, a DN400." I said you wait, expansion joints are not socks and can be worn regardless of size. If you choose the wrong type, it will be a ticking time bomb. Let's take a look at the common categories on the market:
- Universal corrugated expansion joint: Tiger oil contestants, ordinary temperature and pressure are not high, such as room temperature water pipes and low pressure air pipes, so it's fine to use it.
- High temperature axial expansion joint: Specialized in serving high-temperature media, such as steam pipeline and flue gas pipeline. Inside with insulation and deflector, the bellows will not be directly grilled.
- Pressure balanced expansion joint: IncludesStraight pipe pressure balanced expansion jointAndCurved tube pressure balance expansion joint。 These guys have their own internal pressure balance structure, the thrust digests itself, and won't transfer force to the fixed bracket. Suitable for large diameter, high pressure lines.
- Compound hinge transverse expansion joint、Compound straight pipe bypass pressure balanced expansion joint: Specialized in absorbing lateral displacements, such as L-pipe turns.
- Sleeve type pipe expansion joint: Old school but reliable, by sleeve sliding compensation, suitable for low pressure and large displacement occasions.
- Non-metallic expansion joint(fabric fiber expansion joint) andrubber compensator: It is cheap and corrosion-resistant, but the pressure and temperature resistance are limited, and the desulfurization flue and fan import and export are more used.
- And something likeSpecial hose for vacuum、PTFE-lined hoseThese special products, corrosive media or high vacuum working conditions, just choose them.
Before choosing, first look at the on-site medium temperature, pressure, pipeline direction and displacement. Don't buy a general-purpose steam pipe at 400℃ cheaply, which will leak in less than three months.
Selection does not look at these 3 parameters, install it and wait for problems
The three most critical things in model selection, I said one by one:
First, the displacement amount.Which direction will your pipe go? Axial, lateral or angular displacement? How many millimeters? When the metal expansion joint is designed, the number of corrugations is calculated according to the specific displacement combination. You said "about 5 centimeters"-sorry, no approximation, you have to get the pipe stress analysis report, or at least figure out the thermal elongation.
Second, the pressure rating.Nominal pressure PN6, PN10, PN16, PN25... Don't just look at the pipe diameter. In the same expansion joint of DN500, the wall thickness, wave height and layer number of PN6 and PN25 are completely different. If the pressure level is selected low, the bellows will bulge and burst directly. Refer to this site'sMetal expansion joint weight tableAndMetal hose pressure standardThe structural differences corresponding to different pressure levels can be understood.
Third, medium and temperature.Is the medium water, steam, flue gas or corrosive gas? What's the temperature? The fatigue life of stainless steel at high temperature will drop by a cliff. For example, 304 stainless steel begins to oxidize and peel above 550℃, so it has to be replaced with heat-resistant alloy. Some operating conditions require the addition of corrugated pipesguide tube(The specific function of the guide tube can be seen in the FAQ of this site), protecting the bellows from being washed by high-speed media.
You asked these three parameters can be found in the selection data? Yes, but many accidents are caused by "thinking" and "default"-you think the pressure is not high, but in fact the system has an instantaneous water hammer, and the peak pressure doubles. Don't ask me how I know.
5 details of the easiest rollover at the installation site (with commissioning method)
If you choose the right product, if you don't install it according to the rules, it will still be useless. The most outrageous case I've ever seen: an installation teamHigh temperature axial expansion jointThe transportation screw on the pipeline forgot to be disassembled, and the expansion joint directly exploded as soon as the pipeline passed through steam-the transportation screw is to fix the bellows to prevent transportation deformation, and it must be loosened or disassembled after installation. What to do?
- Detail 1: Transport screw/tie rod must be adjusted.Under normal circumstances,Expansion joint tie rod nutRequires adjustment to specified length according to design drawings, ensuring pre-stretch or pre-compression in place. If it isLarge tie rod expansion joint, the adjustment bolt of the tie rod also needs to be symmetrically tightened to prevent deflection. How exactly to adjust? See the question and answer on this site "How to adjust the tie rod nut of the expansion joint".
- Detail 2: The flow direction arrow cannot be mistaken.The expansion joint housing has an arrow printed on it, which is the direction of the medium flow. Install it backwards, the deflector rides, and the bellows flush intensifies. EspeciallyFlue gas baffle doorAndDesulfurization flue gas baffle doorThis kind of directional products, the seal failure and leakage exceed the standard when installed backwards.
- Detail 3: Brackets must be arranged according to the drawings.Both ends of the expansion joint should be provided with fixed brackets and guide brackets. Many on-site diagrams save trouble, and one guide frame is less installed. As a result, the lateral offset of the pipeline twists the expansion joint into a twist.
- Detail 4: You can't force matching during installation.If the pipe is too misaligned, don't use the expansion joint to forcibly straighten it. The corrugated pipe is twisted, and the weld is cracked. The pipe bracket should be adjusted so that it is naturally centered.
- Detail 5: Bellows cannot touch welding slag.During welding, the splashed welding slag falls on the bellows, which is a small hole corrosion point. Be sure to protect it with an asbestos cloth or metal cover. After the installation is completed, do a hydraulic pressure test or air pressure test to confirm that there is no leakage before commissioning.
Daily inspection and fault judgment: when to replace and when to repair
Expansion joints are not fitted to last a lifetime. according toService life of expansion jointQ&A, the general design life is 10~15 years, but the actual life is greatly affected by the working conditions. What do you see for daily inspections?
A look at the corrugated appearance.Are there obvious corrosion pits, cracks, bulges? Illuminate with a visual flashlight, focusing on crests and troughs. If there is an "orange peel"-shaped oxide peel on the surface, it means that the temperature exceeds the standard.
Second, check the leakage traces.Especially flange joints and bellows end welds. If you shine with a flashlight at night, there will be dripping water or vapor mist at the leakage point. Use ultrasonic leak detector if possible.
Three measurements of displacement.See if the expansion joint tie rod is loose? Has the bellows been compressed to the limit? If the bellows has died, it means that the compensation amount is not enough, and the pipe may have been stuck. Stop the machine for inspection quickly.
So when should I change? If there are penetrating cracks, serious corrosion and thinning, or the bellows has become unstable and deformed, replace it directly and don't repair it. It won't last long even if it is fixed. However, if it is a minor problem, such as aging and leaking of the flange sealing gasket, or loosening of the tie rod nut, then replace the gasket and relock it accordingly. Other situations, such asRotary compensatorThe seal ring is worn and the seal can be replaced.
A clinker conveying air duct of a cement plant,Metal Corrugated Expansion Joints in Cement IndustryAfter eight years of use, the corrugated surface had only slight pitting, but the deflector was worn out. They only changed the deflector, and the bellows continued to be used. They ran for another two or three years without any problem-the key is to have professional judgment, don't "change it" as soon as they come up, and don't try to save trouble and "make do". Do it regularlyMetal expansion joint knowledge trainingLet the inspectors know what is normal and what is abnormal, and you can spend a lot less money.