Preface: Choosing the wrong expansion joint can cost you tens of thousands more
Two days ago, a friend from a power plant complained to me, saying that the expansion joint on their flue cracked after just one year of use, and it would have to be stopped for two days to replace it. I asked him the parameters of the working condition-what is the temperature? How stressful? He said he had never tested it and bought it by feeling. Tsk, isn't this a pitfall?
In fact, the structure of the metal expansion joint in the flue of the power plant is not complicated, but the probability of problems in the later period is particularly high due to wrong selection and sloppy installation. Today, I will break up this selection and installation process into four steps. If you follow it, you can save at least tens of thousands of dollars of wrongful money.
Step 1: Find out your flue conditions first-temperature and pressure are hard indicators
Don't rush through product samples, the first step is always to find out the actual working conditions in your flue.
Maximum operating temperature, maximum operating pressure, medium composition。 Temperature determines what material the bellows is made of-ordinary 304 stainless steel can only carry to about 650℃, and it has to be used if it is highHigh temperature axial expansion jointOr Inconel alloy. What about the stress? The flue is generally negative pressure or slightly positive pressure, but if it is close to the outlet of the induced draft fan, the pressure may exceed 0.1MPa, so you have to chooseCorrugated expansion joint for power station industryIts pressure-bearing layer has been reinforced.
In addition, don't forget the sulfide and chloride ions in the medium-the flue gas after desulfurization is extremely corrosive. At this time, ordinary metal bellows can't bear it, so you have to consider itPTFE-lined hoseOr dedicatedDesulfurization flue gas baffle doorWith corrosion resistant expansion joints.
Tips:Don't just look at the parameters on the design drawings. In actual operation, the temperature may be 30~50℃ higher than the design value. It is best to find the operation class to confirm the historical highest record. Surely you don't want to leak as soon as you put it on, do you?
Step 2: Choose the right structure type – rectangle or circle? Single or Duplex?
After figuring out the working conditions, the next step is to look at the shape and displacement mode of the flue.
If your flue is square-plant tail flues are mostly rectangular, choose honestlyMetal rectangular expansion joint。 A circular flue (such as a fan inlet pipe) is usedUniversal corrugated expansion jointOrCorrugated expansion joint for power station industryThese two are most commonly used in circular pipes.
Next look at displacement. If the pipe expands and contracts thermally only in a straight line, the menu axial type will work, such asExternal pressure single axial expansion jointOrStraight pipe pressure balanced expansion joint。 But if the pipeline needs to absorb lateral displacement at the same time (such as a corner or a misalignment of the equipment interface), then it needs to be a duplex structure-Compound hinge transverse expansion jointOrCurved tube pressure balance expansion jointYou can handle it all.
Then when to use the pressure balance type? When there are equipment at both ends of the pipe (such as valves and fans), you don't want the thrust of the expansion joint to destroy the equipment, so you need the pressure balance type. For example: the outlet of the induced draft fan is installedStraight pipe pressure balanced expansion jointThe fan itself does not have to bear additional thrust.
Note:Don't mix round and square expansion joints. In order to make it cheap, some people put a round one hard on a rectangular flue, but as a result, it couldn't be sealed and the air leakage rate soared. How much more does that cost for electricity?
Step 3: Understand the key accessories-how to match the deflector, tie rod and baffle door?
The expansion joint body is selected, but the accessories are not paired, so it is also useless. Here are three attachments you remember.
The first one is the deflector tube.The function of the guide tube is to block the high-speed flue gas from directly flushing the bellows. If you askSpecific Function of Expansion Joint Guide TubeTo put it bluntly, it is life-saving-without it, the bellows will wear out in half a year. How to install? The direction of the arrow of the guide tube must be consistent with the flow direction of the medium, and the opposite direction will be a ride. When selecting, tell the manufacturer your flow rate. Generally, it must be added if it exceeds 15m/s.
The second is the pull rod.The role of the tie rod is to withstand pressure thrust and initial mounting force. Especially for large diameter pipes, you don't add a tie rod, and the bellows will be pushed out like a balloon by pressure. How to adjust? Refer to the article in our siteHow to adjust the tie rod nut of expansion jointQ&A-Remember: Loosen the transport screw when installing, and adjust the tie rod nut to the design length after the pipe is in place. Don't screw the nut to death as soon as it is installed, so the expansion joint will lose its compensation effect.
The third is a flapper door.If you need to isolate smoke during overhaul, get a setFlue gas baffle doorOrDouble-sealed single-axis circular baffle door。 Some power plants directly useElectric plug-in insulation doorWith better sealing performance. Note: Should the flapper door be installed upstream or downstream of the expansion joint? It is generally recommended to install it close to the device side for easy separate shutdown.
Tips:If you are usingNon-metallic expansion joint (fabric fiber expansion joint)Andrubber compensatorWith metal expansion joints, the guide tube must be thickened, because non-metallic parts are more afraid of scour.
Step 4: Installation & Maintenance-These 3 Details Determine Lifespan
If you buy the right thing, install the wrong thing equals buying it for nothing. I've seen too many on-site rollovers during installation. For the following three details, you should be more careful when installing them.
Detail 1: Pre-stretched/pre-compressed.Expansion joints are generally at free length when they leave the factory, but the ambient temperature and operating temperature at the time of installation are very different. For example, when the temperature is low in winter, you have to pre-stretch an amount to compensate for the elongation after heating up. How much does it stretch specifically? The manufacturer will write it in the product manual, or refer toUniversal corrugated expansion jointInstallation parameters of. Be careful not to pull over the head, otherwise the bellows will break from fatigue.
Detail 2: Hoisting and supporting.The expansion joint itself cannot bear the weight of the pipe, so independent supports or hangers must be added on both sides. Many people try to save trouble and directly press the weight of the pipe on the expansion joint, and as a result, the bellows is flattened and deformed. Also, remember to remove the shipping set screw when installing-if you askDo you need to remove the screw of the expansion joint, the answer is: it must be dismantled! It is temporarily fixed. If it is not disassembled, the expansion joint cannot be displaced, which is equal to a dead pipe.
Detail 3: Anti-corrosion and periodic inspection.The acidic condensate in the desulfurization flue is the number one killer of bellows. It is recommended to make an anti-corrosion coating on the outside of the expansion joint, and replace it with an anti-corrosion coating if the internal corrosion is seriousPTFE compensatorOrPTFE-lined hose。 Check the bellows surface for cracks, corrosion pits or wear spots by opening the insulation every six months. Once it is found to be replaced in time, don't wait until the air leaks before shutting down, and the loss will be great.
To conclude
The selection and installation of flue metal expansion joints in power plants is not difficult. The key is whether you are willing to spend half a day going through these four steps. From working conditions to structural selection, from accessory matching to installation and maintenance, common pits are avoided in every step. Isn't this wrongful money saved?