Find out what you want to measure first: 4 core sizes
Two days ago, I met a customer. On the phone, I said anxiously that the expansion joint was leaking and I needed to replace it with a new one. I asked him what the diameter of the pipe was, and he was stunned for a long time and said, "About one meter". Tsk, this "probably" is in trouble. How to measure metal expansion joints? It's really not a matter of just taking a tape measure and measuring it casually.
Before measuring, figure out what you are going to measure. There are only four core sizes:
- Diameter (DN)— — The inner diameter of the pipe, which is the first factor in the selection. The common ones are DN50, DN100, DN200, etc., according to the nominal diameter.
- Installation length— — The actual length between the end faces at both ends of the expansion joint. This directly determines whether you can fit it.
- displacement amount-Amount of axial compression, stretching, and lateral offset. This is the core parameter of the expansion joint, which will be described in detail later.
- Ripple parameters— — Wave number, wave height, wave pitch, inner and outer diameter of bellows. These determine the stiffness and compensation capacity of the expansion joint.
How to choose a measuring tool: Don't measure the inner diameter of the corrugation with a tape measure
Many on-site masters are used to walking the world with a tape measure. Tape measure the installation length is fine, but measure the corrugated inner diameter? The error can reach more than 5mm. The bellows is a thin-walled piece, and once the tape measure is pulled, the tension of the ruler itself can make the reading too large.
Install length and diameter, either with a tape measure or laser rangefinder; The corrugated inner diameter must be with vernier caliper or special inner diameter gauge; Vernier caliper for wave height and wave distance; If you need a precise corrugation profile, use a dedicated template or projector.
For 90% of on-site measurements, a vernier caliper plus a tape measure is enough. Don't be superstitious about laser rangefinders. The dust in the pipeline well is large and the light is poor, so the laser is not good to use. For what occasions do you use lasers? Large-diameter pipelines, above DN500, can't be reached by a tape measure, so it is most reliable to measure the installation length with a laser rangefinder.
Teach you step by step: standardized operation process
How to measure the metal expansion joint to be standard? Give you a set of operating procedures:
The first step is to measure the path.If the expansion joint is installed, measure the inside diameter of the flange or the inside diameter of the pipe directly. If it is removed, measure the inside diameter of the bellows. Note that measure in three directions and take the minimum value. Why? Bellows ellipticity is very common, and you can't fit it when you take the maximum value.
The second step is to measure the installation length.The distance between the end faces of the flanges at both ends is measured at 4 points evenly along the circumference, and the average value is taken. If the difference between the four points exceeds 5mm, it means that the expansion joint is crooked or the flange surface is uneven. At this time, it has to be corrected before measuring.
The third step is to measure the corrugation parameters.Use a vernier caliper to measure wave height (the vertical distance from crest to trough) and wave pitch (the distance between two adjacent crests). Count the wave numbers. There is a common mistake here-someone included the end edge wave, and the result was that the number of waves was 2 more, and the selection was directly wrong.
Step 4, record the nameplate information.If the nameplate is still there, write down the model, nominal diameter, design pressure, design temperature, and displacement on it. This information is much more accurate than you can measure yourself.
Common Errors Demonstration
Once I went to the scene, I saw a master put a tape measure on the ripples to measure the length. Ripples have peaks and troughs, and he measures the length of the arc between the peaks, which is several centimeters longer than the actual installation length. This kind of error is particularly hidden. After testing, I took it to correct the model. If I didn't match it, I thought it was the manufacturer who did it wrong.
The most difficult thing to measure is actually the displacement
The previous ones are easy to handle, but the real test of kung fu is the measurement of displacement. Why? Because the displacement is divided into cold state and hot state, the data of the two states are completely different.
How to measure cold pre-stretch?Before the expansion joint is installed, it is generally necessary to pre-stretch or pre-compress. Universal corrugated expansion joints and axial expansion joints usually leave the factory with transport rods to hold the bellows in a predetermined position. When installing, what you want to measure is the change in the amount of displacement from the natural state to the installed state. Specific method: After installation, adjust the tie rod nut, and measure the distance change of the flanges at both ends of the bellows with a vernier caliper. Stop when you pull to the design value, and the nut locks.
How to estimate hot operation data?This really can't be measured directly-the pipe is so hot that you can't touch it. How do you measure it? It can only be calculated. The thermal expansion coefficient of the pipe material is multiplied by the length of the pipe and multiplied by the temperature difference, and the theoretical thermal displacement is calculated. For example, a 20-meter-long carbon steel pipe, with a temperature difference of 200℃, has a thermal expansion of about 20×0.012×200=48 mm. This means that the expansion joint has to absorb at least 48mm of axial displacement.
What about that? The reliable way is to look at the running traces. After the expansion joint is installed, if there are strains or scratches on the bellows surface, or the tie rod bolts are bent, it means that the actual displacement exceeds the design value. Conversely, if there is dust accumulation on the bellows surface but not locally, the boundary of that clean area is often the actual displacement range. Earth methods are sometimes more accurate than instruments.
Don't hurry to place an order after testing: How to convert data into model
After measuring for a long time, the data is all written in the notebook, and then what? Just take the size and buy it? Wrong. The expansion joint is not a standard part, not that you can buy it in stock by quoting a DN200 with a length of 500mm. It is a non-standard part designed according to working conditions.
After you get the data, you have to do three things:
First, check the displacement amount.Multiply the measured displacement by the safety factor (usually 1.2~1.5), and report it to the manufacturer. For example, if you calculate that the thermal displacement of the pipeline is 48mm, then you have to report the compensation requirement of about 60mm. Why? The actual working condition is more complicated than the calculated working condition, with vibration, installation error and occasional working condition fluctuation.
Second, check the medium and temperature.The diameter and length are just the shell. What really determines the material and number of layers of the bellows is the medium, pressure and temperature. Stainless steel bellows for high-temperature steam pipelines, PTFE-lined hoses may be used for corrosion-resistant media, and flue gas pipelines should consider wear resistance, so guide tubes must be added. It is also the expansion joint of DN200, room temperature air and 500℃ flue gas, and the material and thickness of the corrugated pipe used are different from one thousand miles.
Third, choose the right structural form.General corrugated expansion joints are used for straight pipe sections, compound hinge transverse expansion joints are used for transverse displacement, and straight pipe pressure balanced expansion joints may be used for large displacement and high pressure. If the structure is chosen wrong, no matter how accurate the measurement is, it will be useless.
There is one of the worst things here-the expansion joint is installed cold during measurement, but you have to tell the manufacturer the displacement direction under hot working conditions when selecting the model. Is axial displacement compression or tension? Is there a lateral offset? If this information is not provided, the manufacturer can only do it according to the most unfavorable working conditions, and the products made are big and expensive, so you can't complain yet.
In the final analysis, how to measure the metal expansion joint is the measurement of the size and the understanding of the working conditions. Dimensional measurement is a basic skill, and what really opens the gap is whether the field data can be converted into accurate selection parameters. Only after measuring the right amount and asking about the working conditions can you buy the right expansion joint.