Insufficient expansion joint displacement? Don't be in a hurry to change, start by checking these four steps
I met a customer two days ago, and I called in a hurry to say that the displacement of the expansion joint was not enough, and the pipeline was almost deformed, so I wanted to replace it with a new one directly. I asked him to take a few photos and send them to see-but after looking at the photos, I found that it was not the displacement at all. I've seen something like this more than once or twice.
What should I do if the expansion joint displacement is not enough?Don't think about tearing it down and replacing it when you don't hear enough. If you waste money, you can't afford to delay the construction period. Check according to the following four steps, and 80% of the problems can be solved by themselves.
Step 1: Confirm is it true that the displacement is not enough, or is the pipeline thrust making trouble?
Many so-called "insufficient displacement" are actually "crushed to death" by the pipeline thrust. How to distinguish?
Look at the phenomenon:When the displacement is insufficient, the bellows should be expanded but stuck; Excessive thrust will cause bellows to bulge, twist and even crack. You have to take out the design drawings first, find the rated axial/lateral/angular displacement compensation amount, and then measure the field thermal displacement value with a caliper or displacement sensor. If the measured value does not exceed the rated value, but the expansion joint has deformed and failed, it can basically be concluded that it is a problem of stiffness or pressure-regardless of the displacement itself.
Inspect the guide tube and pipe holder:If the guide tube is deformed or stuck by the medium, it will hinder the normal movement of the bellows; Pipe frame offset also forcibly transfers displacement to the expansion joint. In the question and answer of this site, we specifically talked about "the specific function of the expansion joint guide tube". The guide tube mainly protects the inner wall of the bellows, but once it is stuck, it becomes an accomplice. In addition, check whether the fixed support is loose-if the thrust that the pipe should be bearing by the fixed support is thrown to the expansion joint, the expansion joint will not be able to carry it even with 100% margin.
First exclude "false is not enough", and then judge whether you really choose small.
Step 2: Selection stepped on a pit?
Wrong selection is the most common cause of insufficient displacement, and it is often the most injustice. I've seen general-purpose corrugated expansion joints to absorb large angular displacements-that's not the job at all.
Common Error One:The system needs to absorb lateral displacement or angular displacement, so a single axial expansion joint is selected. Single type axial type can only absorb axial expansion and contraction, and forcibly twist it, and the bellows will fail fatigue soon. Correct practice: If it is multi-directional displacement, you should choose compound hinge transverse expansion joint or compound straight pipe bypass pressure balance expansion joint. The latter has the advantage that it can absorb both axial and lateral displacements, and the internal pressure thrust is balanced out and does not act on the pipe support.
Common error two:Use the universal corrugated expansion joint to cope with the high temperature steam pipe of power station. The temperature of high-temperature steam pipe is always five or six hundred degrees, and the material of general-purpose bellows is not temperature-resistant enough, so the bellows will soften and deform in advance. In this case, high-temperature axial expansion joints must be used-this category is specially available in our station, and the temperature resistance grade and compensation amount are designed according to the working conditions of the power station.
Common mistake three (save money instead):In the air duct of cement industry, the temperature is not high but the dust is large. It is costly and easy to wear with metal corrugated expansion joints, so it is more cost-effective to replace them with non-metal expansion joints (fabric fiber expansion joints). Non-metallic expansion joints are corrosion resistant, absorb multidimensional displacements, and are inexpensive. The national standard JB/T 12235-2015 also has clear specifications for it.
Therefore, first turn over the decision to pat your head when selecting the model at that time, and don't let the expansion joint take the blame.
Step 3: The pit dug in the installation link
There is a problem with installation, which is more concealed than the selection. And guess what? Most of the so-called displacement is not enough, in fact, it is not installed in place.
Tie rod nut not adjusted in place:The tie rod nut of the expansion joint is for pre-tightening, not for transport locking. Specifically, we have a question and answer in our station "How to adjust the tie rod nut of the expansion joint"-simply put: after the installation, the tie rod nut should be screwed to the specified position to release the initial displacement capacity of the bellows. If it is not adjusted, the bellows is equivalent to being pressed all the time, and the natural compensation amount will be discounted.
The pre-stretch is done backwards:Some pipes need to be pre-stretched (for example, steam pipes are pre-stretched for a section when installed in cold state, and just reset when hot state), but workers have gone in the opposite direction-what should have been stretched becomes compression. After the system heated up, the expansion joint was directly pushed to death. Of course, the displacement was not enough. This phenomenon is common on power station steam pipes.
Failure of pipe fixed support:The function of the fixed support is to guide the thermal displacement of the pipe to the expansion joint. Once the support is loosened or fallen off, the pipe will be turbulent, and the displacement that should have been absorbed by the fixed section is all superimposed on the expansion joint. Check for any loose welds or bolts on the holding bearings-don't underestimate this step, it can solve a lot of problems.
Step 4: Classification of Remedial Protocols – Mild, Moderate, Severe
To be honest, after the first three steps of troubleshooting, most problems can be found at the root. But what if the selection is really small or the working conditions change? According to the degree of excess tolerance, there are three situations:
Slightly out of tolerance (actual displacement exceeds rating by less than 10%):Adjust the tie rod nut to redistribute the displacement, or add a limiting device (such as adding a limiting plate next to the expansion joint) to limit the out-of-range displacement so that the expansion joint only works within the safe range. This operation is simple and low cost.
Moderate deficiency (more than 10% ~30%):Change! But you don't have to change the full pipeline. If space allows, switch to a general-purpose corrugated expansion joint of the same caliber but with more wavenumbers-the more wavenumbers, the more compensation. Or simply change it to a double straight pipe bypass pressure balancing expansion joint, which doubles the compensation amount by combining two bellows and can also balance the pressure. Note that the thrust and support load of the pipeline should be recalculated before replacement.
Serious insufficiency (more than 30%):Don't expect to change the same type of expansion joint, there is no room for design. The entire pipeline needs to be recalculated and the combined compensation scheme is used instead. For example, use a rotary compensator (which can absorb large angular displacement by rotation, suitable for long straight pipes) with a sleeve-type pipe expansion joint (absorbing axial displacement), or change single-point compensation to multi-point segmented compensation. Our station has two mature products, rotary compensator and sleeve-type pipe expansion joint, and the technical scheme can be customized for customers.
What should I do if the expansion joint displacement is not enough?The core is "diagnose first and prescribe medication later". Don't change it for a new one at every turn, maybe it won't work if you change it. Follow the above four steps, 90% of the problems can be solved on the spot, and the money saved is enough to buy several sets of equipment.