After doing pipeline compensation for so many years, I have encountered countless pits. But what hurts Party A the most is that the effective length of the non-metallic compensator is wrong-two or three centimeters away, the whole pipeline is scrapped and redone, and hundreds of thousands are wasted. You might be thinking: isn't it just the length? What can I do with a few centimeters more and a few centimeters less?
Yeah, yeah. Today, let's break out this key parameter and spread out the pits we stepped on.
What exactly is the effective length? -Don't confuse it with installation length
"What is the total length of this non-metallic compensator?” Hey, the total length is the installed length, which is different from the effective length.
The effective length of the non-metallic compensator refers to the actual length of the elastic body that can produce axial displacement compensation-that is, the distance from the root of the flange at one end to the root of the flange at the other end, removing the length of rigid structures such as metal flanges, presses and guide tubes at both ends. Simply put: the length of the fabric or rubber layer that you can pinch with your hands is the effective length.
A certain type of non-metallic expansion joint (fabric fiber expansion joint) has a total length of 400mm, but the flanges on both sides account for 50mm each, and there are two layers of thermal insulation cotton interlayer in the middle. The effective length actually participating in deformation may only be about 250mm. When installing, do you calculate the compensation amount according to the total length of 400mm? Just wait for an accident.
Two days ago, I met a customer, took a tape measure to measure the total length of the non-metallic compensator, and filled it in directly in the pipe diagram. As a result, the thermal displacement of the pipeline exceeded the limit of the compensator after the temperature was raised, and the surrounding brackets were pulled and deformed. After asking, I found out that he used the installation length as the effective length.
How does the effective length affect the amount of compensation and life? — Numbers Talk
The compensation ability of non-metallic compensator depends on the effective length. Here's a simple relationship:
Axial compensation ≈ effective length × allowable strain rate of material
For example, the allowable strain rate of a rubber compensator is 30%, the effective length is 300mm, and the theoretical maximum axial compensation amount is 90mm. If the actual thermal displacement requires 100mm, then this compensator is deadlifted when installed-it will definitely crack within a few months.
More troublesome is the lateral displacement. The effective length is shorter, the lateral angular displacement stiffness rises sharply, and the pipeline thrust soars. I have seen a case: a rectangular non-metallic expansion joint with a short effective length was used in the flue gas pipe. After turning on, the expansion joint bulged, the fabric layer was torn, and the site was shut down urgently. Once removed, the effective length is 35mm less than the design value, which is equivalent to compressing the elastomer to the limit state-who will scrap it if you don't scrap it?
In terms of life, for every 10% reduction in effective length, the fatigue life of the fabric fiber layer may decrease by more than 40% under the same displacement. This is not scaring people, this is measured data.
Three factors influencing the effective length: working condition, structure and material
1. Working condition parameters have the final say
Temperature, pressure, medium, amount of displacement, these four items directly determine the effective length you need. High-temperature flue gas (such as the flue behind the baffle door of desulfurization flue gas), when the temperature is above 350℃, the non-metallic layer will accelerate the aging, so sufficient margin must be left, and the effective length should be 20% ~30% more than the theoretical value. Under high pressure conditions (over 0.1MPa), too large effective length will lead to unstable bulge, which will be shortened. Therefore, there is no universal formula, and it must be calculated item by item.
2. Structural design determines whether the effective length can be fulfilled
The same non-metallic expansion joint, with and without the guide, the effective length algorithm is completely different. The guide tube will occupy the internal space, and the actual deformable length should be deducted from the overlapping area of the guide tube. There is also a double-layer insulation structure-some customers insist on thickening the insulation layer. As a result, the strip occupies the activity space of the elastomer, the effective length shrinks by half, and the compensation amount is completely insufficient.
3. The material itself is the ceiling
The allowable strain rates of rubber compensators (such as EPDM) and PTFE compensators differ significantly. The former can achieve 30% ~50%, while the latter is generally only 10% ~15%. So the same effective length, the rubber can absorb much more displacement. When selecting materials, don't just look at temperature resistance and corrosion resistance, and the strain rate must be clearly confirmed.
How to correctly determine the effective length when selecting a model? — — Practical experience
After talking about the theory for a long time, we got dry goods. How do we calculate the effective length?
Get the pipeline stress analysis report, or at least know the maximum axial displacement, lateral displacement, angular displacement. These are the input conditions.
Determine the material allowable strain rate for the non-metallic compensator. Ask the manufacturer for the actual measurement report, and don't believe the number of "maximum compensation" in the brochure-that is an ideal working condition, generally with a safety factor.
Inferred the required effective length with formula. For example, if the axial displacement is 100mm, the allowable strain rate of the material is 25%, and the effective length should be at least 400mm. Note: This is the minimum value, and a safety margin of 15% ~30% must be added. Why? Because there are deviations in the actual installation, fluctuations in the medium temperature will also affect.
Add the effective length to the structural dimensions such as flange, insulation layer and pressing bar to obtain the final installation length. Many people make the reverse of this step-first set the installation length and then deduct the effective length, but the effective length is not enough. The correct way is to determine the effective length first, and then match the total length.
Check instability risk. The effective length is too long and may bulge under internal pressure. Experience: The effective length of non-metallic compensators generally does not exceed 1.5 times its nominal diameter. For example, the effective length of DN800 pipeline is best controlled within 1200mm. Beyond this value, you have to add a guide ring or a segmented design.
Common Myth and Guide to Avoiding Pits
Myth 1: Thinking that the longer the non-metallic compensator, the better
Some people think that longer is more compensatory. Not right. Increased length leads to decreased overall stiffness, easy instability, and skyrocketing costs. The reasonable range is backwards according to the displacement amount, not the longer the better.
Myth 2: Copy the length of the old model directly
The effective length of products of different working conditions and different manufacturers may be much different. You think you can fit the same size if you change the supplier? The effective length may be 20mm shorter, and your pipe displacement is just stuck at this critical point-wait for the pipe to burst.
Myth 3: Ignoring the installation of pre-compression/pre-stretching
Many non-metallic compensators are free length when they leave the factory, and need to be pre-stretched or pre-compressed to accommodate hot and cold conditions when installed. This amount of pre-adjustment changes the actual effective length. If you mount according to the theoretical length on the drawing, the effective length in actual work has changed. What to do? Ask the manufacturer to indicate "effective length after pre-stretching/pre-compression" on the invoice and review it on site.
Myth 4: Does not distinguish between axial and lateral effective length
Some non-metallic compensators are angular or hinged, and the effective length behaves differently in different directions. For example, the effective length of the transverse expansion joint of the compound hinge is defined for the transverse displacement, which is different from the axial direction. When selecting a model, check the product information in detail.
The parameter of effective length of non-metallic compensator may be a row of numbers on the selection table, but it involves pipeline safety, equipment life and project duration. Don't slap your head to decide, calculate what should be done, ask what should be asked.