First, it can't be changed-which signals tell you that the metal expansion joint should move?
After more than ten years of pipeline engineering, what I am most afraid of is not that the design drawings are deviated, but that the stress has been exceeded, and the site is still hard to carry it. You may ask: What does stress excess look like? Say a few real signals.
The first one,Fracture or deformation of pipe support。 Last year, in a chemical plant project, the sliding bracket on the steam pipeline was directly skewed by 20mm. The inspection found that the expansion joint selection was too small, and the thermal displacement couldn't stand at all. The second one,Frequent leakage of flange interface— If this thing has to tighten the bolts every month, don't hesitate, it is definitely not the gasket problem, it is the expansion joint that doesn't absorb the displacement. Third,Cracks or obvious depressions appear on the bellows surface(especially the general-purpose corrugated expansion joint), which is already an early warning of fatigue failure.
Vibration anomaly。 The pipe buzzed when it ran, and a dozen vibrometers exceeded the amplitude by several times. Don't just stare at the pipe rack at this time, the expansion joint may have become a "vibration amplifier". Remember, the decision-making window period of metal expansion joint modification is only a few months. When the bellows cracks and the medium leaks, the shutdown loss is not as simple as replacing the parts.
Second, transformation is not a simple replacement-type selection and matching is a technical job (how to choose general type, high-temperature axial type and pressure balance type)
The old expansion joint is broken. Wouldn't it be over to buy one of the same size and install it? Alas, this is the most pit line of thought. Piping conditions may have changed long ago – the temperature has increased? Large pressure fluctuations? Or is the media changed to be corrosive? If you don't change the selection, it is equivalent to changing a new trash can to fill the same garbage, and then it will break after changing it.
Then how to choose? Watch three scenes.
Scenario 1: Conventional steam and hot water pipelines with temperature below 400℃ and axial displacement as the main factor— UseUniversal corrugated expansion jointIt has low cost and convenient installation. However, note that the general type is not resistant to torsion, and the guide bracket should be added if the pipeline has lateral displacement.
Scenario 2: High-temperature flue gas or hot air ducts with temperatures exceeding 600℃ or even higher— — At this time, ordinary bellows can't hold up, so we have toHigh temperature axial expansion joint。 Its bellows material is usually Inconel 625 or a similar superalloy, which has an order of magnitude higher thermal fatigue life than 304. Two years ago, a waste incineration power plant project, after changing the high-temperature axial type, the maintenance cycle was extended from 6 months to 2 years.
Scenario 3: The internal pressure of the pipeline is high and the space is limited, so the blind plate force cannot be withstood by the support-Must be usedStraight pipe pressure balanced expansion jointOrCurved tube pressure balance type expansion joint. This kind of product has a balanced bellows inside, which can offset the internal pressure thrust and eliminate heavy fixed brackets. Oh, by the way, andCompound hinge transverse expansion jointAndCompound straight pipe bypass pressure balanced expansion jointSpecialized in dealing with large-angle lateral displacement or three-dimensional displacement. Each column of parameters on the selection table corresponds to a working condition, so don't pat your head.
Third, don't just look at the bellows-the guide tube, tie rod, baffle door and other accessories must also be evaluated simultaneously
When transforming metal expansion joints, the most taboo is to only stare at bellows. You get a new bellows, but the deflectors are worn out, the tie rod nuts are rusty, and the baffle doors are not closed tightly — and then the whole system is still leaking and vibrating.
guide tubeMany people don't understand its role: it protects bellows from direct scour by high-speed media, especially particle-containing flue gas or steam pipes. If the length of the guide tube is not enough or the material is wrong (for example, 304 or 316 should be used, but Q235 is used as a result), it will be perforated in half a year. WeSpecific Function of Expansion Joint Guide TubeAs written in the article, it can also reduce flow resistance and turbulence-induced vibration.
tie rod(Including the expansion joint of the large tie rod) is to control the displacement direction of the bellows. During the transformation, check whether the tie rod can slide freely and whether the nut is stuck — —How to adjust the tie rod nut of expansion jointThere are standard steps: allow the amount of pre-stretching or pre-compression in the cold state, and then adjust to the working position in the hot state. In addition, many sites forget to remove the transport screw after installation-this thing is the most asked in the FAQ: "Do you need to remove the screw of the expansion joint"— The answer is: it must be demolished! Not letting it be dismantled is equivalent to putting a shackle on the expansion joint, the bellows can't move, and the stress is all transferred to the pipeline.
baffle doorThis piece is easy to ignore. In the flue gas system, the expansion joint andFlue gas baffle door、Desulfurization flue gas baffle doorOften used in series. When modifying the expansion joint, check the tightness of the baffle door by the way- -Difference between electric butterfly valve and baffle doorBecause the baffle door is a louver-type multi-blade structure, the seal grade is higher. If the air leakage rate is found to exceed 1%, it should be consideredDouble-sealed single-axis circular baffle doorOrSingle-axis double-flapper doorThey changed together.
Fourth, installing it is only the first step-don't step on these pits of installation gap, cold tightness and screw removal
Once at the site, the construction team put the newCorrugated expansion joint for power station industryWhen installed, the bellows was directly pulled and cracked during trial operation. Check the cause-the mounting clearance was left wrong. Installation of metal expansion joint after transformation,Cold tightness(pre-tension or pre-compression) must be calculated at the design temperature. For example, the working temperature of steam pipeline is 300℃, and when it is cold, it usually requires 50% ~70% of the pre-stretching compensation amount. Not cold tight? When thermal expansion comes, the bellows is deadlifted and its life is cut in half.
Guide bracket wrong position。 The distance of the guide bracket near the expansion joint is too far, the lateral displacement of the pipe is not constrained, and the bellows is subjected to additional bending stress. Generally, it is recommended that the distance between the first guide bracket and the expansion joint should not exceed 4 times the tube diameter.
After installation, remember to putTransport Screws and Adjustment ScrewsDismantle them all. Many small manufacturers deliberately don't remind them, or the construction team is lazy and doesn't dismantle them. As a result, the screw ejects a pit on the bellows, and it leaks in three months. After removing the screw, check whether the pull rod moves freely and screw the nut to the designated position.
5. How to accept after transformation? — — The displacement, leakage point and vibration data are all indispensable
Acceptance is not to sign the process, but to leave the bottom for the next round of maintenance. Focus on measuring three indicators.
First, the displacement amount。 Use displacement scale or laser rangefinder to record the axial and transverse displacement of the expansion joint in the cold and hot states of the pipeline respectively. Compared with the design value, if the deviation exceeds ±10%, you have to check the reason-it may be that the bracket friction is too great, or the guide bracket is offset.
Second, the leakage point。 Soap water leak detection can only deal with low pressure, and high-pressure pipelines must be inspected by helium or ultrasonic leak detection. Pay special attention to the weld connecting the bellows and the end pipe, and the gap between the guide tube and the end pipe. If the retrofit involvesNon-metallic expansion joint(For example, fabric fiber expansion joint), focus on checking whether the flange sealing surface has wrinkles or deformation.
Third, vibration data。 Measuring points are arranged at the front and rear pipe sections close to the expansion joint, and the velocity or displacement values are collected with a vibration analyzer. Common reasons for vibration exceeding the standard: ① Airflow excitation (unreasonable design of guide tube); ② Mechanical resonance (the natural frequency of the expansion joint coincides with the natural frequency of the pipeline); ③ Improper installation clearance causes bellows to slap. We saw a project, changedLarge diameter thick wall expansion jointAfter the vibration is even greater-later, it was found that the pitch design of the guide tube is too small, so it will be good to change the two-layer guide tube immediately.
Finally, a reminder:Transformation of metal expansion jointNot a one-time sale. After the transformation, make a ledger-record the model (e.g.External pressure single axial expansion jointStilldirectly buried expansion joint), installation date, operating parameters, maintenance records. Next time something goes wrong, it is ten times more efficient to check the ledger than to take it down.