Metal expansion joint screw: Don't underestimate this screw, wrong installation is an accident
Two days ago, I met a customer, and I called in a hurry to say that the on-site expansion joint was leaking, and I suspected the quality of the bellows. As a result, as soon as our engineers went over, they found that it was not the bellows at all-the nut on the tie rod was loose twice, and with the vibration of the pipe, the screw collapsed directly. The bellows were fine, and the whole pipeline was out for three days. You say it was wrong or not?
After more than ten years in this business, I have seen too many people stare at bellows and ignore screws. I always think that what big things can happen to so few small things? Tsk, if something happens, it's a big accident. Today, let's make it clear: How many kinds of screws are there on metal expansion joints? How to install, adjust and check?
Tie rods, nuts, screws…How many kinds of screws are there on metal expansion joints? What are they for separately?
A metal expansion joint looks uncomplicated in structure, but in fact, screws alone can be divided into three categories:transport screw、Tie rod assembly (including nut)、fixing bolt。 Each type of work is completely different, and it can be mixed or installed wrongly, which can affect the compensation amount at least and directly scrap the equipment at worst.
transport screw— Temporary workers, to put it bluntly. When the manufacturer leaves the factory, in order to prevent the bellows from expanding and contracting at will during transportation, several screws are used to lock the flanges at both ends. When we get to the site, these screws must be removed. Not dismantling? The expansion joint loses its compensation function, and the force of thermal expansion and contraction of the pipe is all suppressed on the bellows, which cracks in minutes.Remember: the transport screw is not a permanent piece and must be removed after installation.
tie rod assembly(Including tie rod and nuts at both ends) -This is the core force member. Based on this site'sLarge tie rod expansion jointFor example, the role of the tie rod is to constrain the bellows to not exceed the design value in the axial displacement, and at the same time bear the pressure thrust of the pipe. How tight are the nuts? Not the tighter the better, nor the looser the better. More on that later.
fixing bolt— — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — This thing seems simple, but the wrong material choice, annealing at high temperature and rust in corrosive media still cause problems.
How to adjust the expansion joint tie rod nut? Are those transport screws disassembled or not during installation?
Let's start with the transport screw. Article 13 of FAQ on this site clearly answers:Must be removed after installation.However, many on-site workers try to save trouble, or don't know it at all, so they directly ignite and test run with the transport screw. As a result, as soon as the pipeline heats up, the expansion joint can't move, and the bellows is forcibly cracked. I have at least three of these cases on hand.
How to adjust the tie rod nut? You need to know the design principle. Tie rods are generally divided into two groups: one group isPre-tensioned bolt(or positioning bolt), which is used to give the bellows a pre-stretch amount in the cold state to compensate for thermal expansion; The other group islimit nutTo prevent excessive displacement of the bellows. Adjustment steps:
- Step 1: After removing the transport screw, loosen the limit nut to leave the bellows in a free state.
- Step 2: According to the design cold tightening value (usually indicated on the installation drawing), tighten the pre-tensioned bolts with a torque wrench to the specified torque.
- Step 3: Adjust the limit nut to leave a design gap (generally 5-10mm) with the end of the tie rod, and it cannot be pushed to death.
Core principle: the nut cannot be locked, and the margin for the normal expansion and contraction of the expansion joint must be allowed.Otherwise, it becomes a rigid connection and completely loses its compensation function.
What happens when the screws are tightened too tightly? So what if it's too loose? Real lessons from the field
Screws tightened too tightly-especially tie rod nuts, can limit the bellows from displacing properly. For example, one used by a power plantCorrugated expansion joint for power station industryThe installer was afraid of leakage, so he screwed all nuts to death. As a result, after the pipeline heated up, the bellows couldn't be extended, and the force was all transmitted to the equipment at both ends, which made a valve bracket crooked. After inspection, the nut threads were bitten to death, and they couldn't turn at all.
The screw is too loose-the most typical consequence is that the nut vibrates and falls off. Of a chemical plantMetal hoseAt the connection, because the tie rod nut did not add an anti-loosening washer, two nuts fell after three months of operation, the pipe sag directly, and the flange surface was misaligned, resulting in leakage. Fortunately, it was discovered in time and didn't cause a bigger accident.
How tight is that?The torque value is determined according to the bolt diameter and material. Generally, when 8.8 grade high strength bolts are used, the recommended torque of M16 is between 200-250 N·m.If you don't have a torque wrench, from experience: screw until the bolt elongation reaches about 0.2% of the original length, and then use the wrench to return half to one turn. But honestly, it's better to follow the instructions.
Choosing screws depends on the material and working conditions-what to use for high temperature? What to use for corrosive media? Compare the expansion joint products of this station one by one
If the screw material is selected wrong, it is equal to a time bomb. Simply divide into three types of working conditions:
High Temperature Conditions:Such asHigh temperature axial expansion jointThe medium temperature exceeds 400 °C. Ordinary carbon steel bolts (such as 35#steel) will creep and drop in strength at high temperature. Heat resistant alloy steel such as 25Cr2MoVA or GH4169 must be used. The nut material should be one level softer than the bolt to avoid high temperature bite.
Corrosive media:For example, for desulfurization systemsDesulfurization flue gas baffle doorAcid fog and chloride ions are serious in the environment. Carbon steel screws rust and break in a few months. Use 304 or 316L stainless steel bolts at this time. But note: Stainless steel bolts are prone to stress corrosion at high temperature, so if there is high temperature + corrosion at the same time, you have to use duplex stainless steel (such as 2205) or Hastelloy.
Ordinary water vapor/air duct:normalUniversal corrugated expansion jointIt is enough to use galvanized bolts or blackened 8.8 grade high strength bolts. Rust prevention should be considered outdoors, and hot-dip galvanizing or Dacromet treatment is recommended.
Among the products of this site,PTFE-lined hoseAndPTFE compensatorBecause it is lined with fluoroplastic, the flange bolts must be made of stainless steel, otherwise rust contaminates the medium. And the torque can't be too large to prevent the fluoroplastic lining from being fractured.
Don't just focus on bellows in daily inspection. Loose screws are hidden time bombs
Many enterprises only list "inspect cracks and corrosion of corrugated pipes" on their inspection lists, and don't mention screws. This is a big no-no. The bellows is broken and you can at least see it at a glance, but the loose screws are hidden inside.
Recommended inspection frequency:At least once a month, spot check 10% of the bolts with a torque wrench. Focus on the tie rod nut and the fixing bolts near the vibration source (such as pump outlet and fan inlet and outlet).
Use a marker to draw an alignment line on the bolts and flanges and see if the alignment line is misaligned next time. Misalignment means that the bolts are loose. In addition, listen to the sound: if there is a metal impact sound when the pipeline is running, 80% of it is that the tie rod nut is loose and the tie rod is swinging in the guide tube.
There used to be a cement factoryMetal Corrugated Expansion Joints in Cement IndustryThat is, the inspector found that the alignment line on the tie rod nut ran 5mm, and timely tightening avoided parking. Think about it, a screw saves hundreds of thousands of shutdown losses, is it cost-effective?
Although the metal expansion joint screws are small, each one bears the heavy responsibility of pipeline safety. Wrong installation, wrong adjustment, wrong choice, and it is you who pays the bill in the end. If you are unsure about the screw parameters of specific products, please refer to the product page of this site or consult technicians directly. Don't wait for an accident to slap your thigh.