The correct installation method of large diameter metal expansion joint? The 3 easiest pits for construction teams to step on
Two days ago, I met a customer. A DN1200 steam pipe in the factory cracked less than a month after the general corrugated expansion joint was installed. When I went to the scene, the installation team screwed the tie rod nut tightly, and the guide tube arrow was installed backwards. Do you think the money was spent unjustly? Large-diameter metal expansion joint, with a diameter of 800mm, has a completely different temper than a small diameter-its own weight is measured in tons, its stiffness doubles, and the thrust of the pipe system is often tens of tons. If it is not served in place, it will leak at least and the bracket will collapse at worst. Today, let's break it up and talk about the three easiest pits for the construction team to step on on this road, and the real correct installation method.
Why Large Diameter Expansion Joints Are More "Delicate" Than Small Diameter
Small-diameter expansion joints (such as DN100 or below) have light weight and low stiffness, so the installation deviation of a few millimeters is not a big problem, and the bellows can "digest" by itself. But what about large diameters? Take the large-diameter thick-walled expansion joint as an example. The bellows section alone may weigh several hundred kilograms. Coupled with the self-weight of the medium, the vertical load of the whole assembly on the pipeline soars. More critically, the larger the diameter, the axial stiffness of the bellows rises nonlinearly (stiffness increases approximately 4 times for every 1-fold increase in diameter), which means that the additional stress generated on the larger diameter may be more than 10 times that of the smaller diameter for the same installation deviation.
In addition, in structures such as external pressure single axial expansion joint, because the bellows is subjected to external pressure under working condition, the requirements for installation coaxiality are more stringent than those of internal pressure type-if the deflection exceeds 1°, the side wall of the bellows may appear wrinkled. To put it bluntly, it is not that large-diameter expansion joints can't be installed, but that they can't stand the "almost" mentality.
5 parameters that must be checked before installation
When the equipment arrives, don't rush to hoist it. First, check these five parameters with the design drawings and product nameplates one by one, and one less item may lay mines.
- Model and displacement:Confirm is the universal type, external pressure single type or double hinge transverse type? Does the design compensation amount meet the actual working conditions? For example, the pre-tension value of directly buried (fully buried) expansion joint has strict regulations, and the difference of 10mm can't be done.
- Pressure Class:The operating pressure of the pipeline must match the nominal pressure of the expansion joint, with special attention being paid to the possible instantaneous overpressure of the steam pipeline.
- Direction of guide tube:The purpose of this thing is to direct the flow of media and protect the bellows from washing. The arrow must point to the direction of the media flow (from upstream to downstream). In the case of the customer, the arrow was installed backwards, and the medium directly washed the root of the bellows, which wore out in two months.
- Tie Rod Status:Is the transport protection tie rod removed? Is the working tie rod nut in the factory preload position? Many construction teams tried to save trouble and removed the nut of the working tie rod. As a result, the expansion joint drooped and deformed under gravity.
Key steps in the installation process
Let's start with pre-stretching. In the correct installation method of large-diameter metal expansion joint, pre-stretching is a step that many masters are prone to make mistakes. Simply put, in order to compensate for the thermal elongation of the pipeline from the cold state to the hot state, the expansion joint needs to be "pulled apart" by a certain distance during installation. The specific stretch amount is given by the design and is generally equal to half the amount of thermal displacement. During operation, use two hoists to pull symmetrically, and measure the wave pitch of the bellows while pulling to ensure uniform deformation. After pulling, install the temporary limit bolt, hold your strength and then go to the butt welding.
Should the tie rod nut be removed or not? According to the situation. For external pressure single axial expansion joint, the working tie rod is a load-bearing part, and the nut must not be removed-removing the bellows will be excessively elongated under the action of internal pressure. For the models with tie rods such as the transverse type of compound hinge and the pressure balance type of curved tube, the angle and limit of the nut have been adjusted when it leaves the factory, and it is only necessary to check whether it is loose on the spot, and it is strictly prohibited to adjust it without authorization.
Where does the deflector arrow go? Bottom line: the arrow points in the direction of the media flow, and the small mouth end (reduced diameter end) of the guide tube must be directed downstream. This point is clearly explained in the question and answer of "The Specific Function of Expansion Joint Guide Tube", with the purpose of forming a gradually expanding channel and reducing resistance. If installed backwards, the medium will form a vortex that washes through the bellows, and the noise and vibration will immediately increase.
Configuration principles of pipe supports and fixed piers
The large-diameter expansion joint weighs too much, and it can't be supported by the pipe itself. The main fixed pier (or pipe rack) must be set to withstand the blind plate force of the expansion joint. How to calculate? For example, for DN1000 and 1.6MPa pipelines, the blind plate force is about 1.6MPa multiplied by the cross-sectional area (about 0.785m²), which is calculated to be 125 tons. If the design bearing capacity of the fixed pier is only 80 tons, the pier will directly crack as soon as the expansion joint is put into operation.
For straight pipe pressure balanced expansion joints, because they can balance the blind plate force by themselves, the fixed pier can be simplified, but it can't be eliminated-at least a guide bracket is needed to prevent pipe twisting. However, the sleeve-type pipe expansion joint needs to be provided with thrust brackets on both sides, otherwise the sleeve will be pushed out.
What happens to "unsteady standing"? Two years ago, there was a power plant, and the metal corrugated expansion joint in the cement industry was installed on the air-cooled island pipeline. Because the support foundation was not anticorrosive, it was corroded and unstable one year later, and the whole expansion joint was smashed down with a 5-meter pipeline. Therefore, the support of large diameter expansion joint should not only calculate the strength, but also the fatigue life and foundation settlement.
Commissioning and Acceptance Criteria
Brush the weld and bellows surface with soapy water, charge to 1.5 times the working pressure, and hold the pressure for 10 minutes without bubbles before it is qualified. Next, measure the displacement: set marking points at both ends of the expansion joint, measure the wave distance with a vernier caliper, and confirm that the actual stretching amount is within ±5% of the design value. Finally record and archive: write down the model number, installation date, pre-stretch amount, measured displacement value and operator's signature-this paper will be a life-saving straw in future maintenance.
The acceptance criteria of rubber compensators and PTFE compensators are different from those of metal expansion joints, which are sensitive to temperature and usually do not make high-pressure airtight, but low-pressure water pressure. Don't get mixed up.
Review of real accidents
Case 1:A chemical plant uses a general-purpose corrugated expansion joint to compensate the steam pipeline. In order to save trouble, the installation team regards the transportation tie rod as a working tie rod and directly welds it to the pipeline. After the pipeline is put into production, the expansion joint cannot be expanded and contracted, and the bellows is forcibly stretched and cracked. Solution? Cut off the heavy welding and lose heavily.
Case 2:The flue gas pipeline of a cement production line adopts external pressure single axial expansion joint, and the guide tube arrow is installed backward during construction. After 3 months of operation, the inner wall of the bellows was washed out with obvious pits by dust, and it was forced to stop for replacement. Later, Party A checked the installation records and found that the direction of the arrow was clearly marked on the construction drawings, but the installer didn't look at it.
Case 3:A power station uses a double hinge transverse expansion joint, and the support foundation is designed according to ordinary pipeline, ignoring the blind plate force. When the fixed pier is put into operation, the position of the fixed pier is displaced by 40mm, and the adjacent weld seam collapses. Fortunately, the inspectors found it in time and caused no casualties. Afterwards, the anchor bolts were made up and the foundation was reinforced, and the construction period was delayed for 45 days.
Alas, to put it bluntly, these things are not technical problems, but questions of responsibility. The correct installation method of large diameter metal expansion joint? In fact, there are three core points: check according to the drawing, construct according to the regulations, and accept according to the standards. Don't be lazy, don't take it for granted, pretend to test one, and be steady.
If the site conditions are complex (such as high temperature, high pressure, large diameter, with corrosive media), it is recommended to directly find the manufacturer's technical support. You are thinking about whether to remove the tie rod and how much to pre-stretch it yourself. It is better to ask clearly by phone. After all, an expansion joint can range from several thousand to hundreds of thousands. Who will carry it if it is wasted?