Do you really understand the specification sheet of compensation amount of metal expansion joint? Practical guide to model selection
Two days ago, a customer came to me with pipeline drawings and asked, "How much is the compensation for your expansion joint? Give me a specification sheet". However, the amount of compensation is really not something that can be concluded by looking at a table. How many millimeters of thermal expansion and contraction of the pipe, how much lateral displacement, and where the interface direction is-these data are incorrect, and there will be problems when you buy it back and install it. Today, let's break it up and talk about the compensation amount. By the way, I will take you to find out the specification table of the pile of products on this site (general type, high-temperature axial type, straight pipe pressure balance type, double hinge transverse type...).
First, we should understand the three basic dimensions of the compensation amount: axial, transverse and angular displacement
The axial direction is easy to understand, that is, the tube is elongated and shortened; The lateral direction is the left and right swing of the vertical pipeline; The angular displacement is the rotation of the pipe end. In most cases, all three will happen at the same time, but the design of expansion joints is not a panacea-for example, if you choose a general-purpose corrugated expansion joint, it has a large axial compensation, but a weak lateral and angular displacement. In turn, the lateral expansion joint of the compound hinge is dedicated to lateral displacement, and the axial direction is almost immobile. Therefore, the string of numbers in the specification sheet has to be looked at according to your pipeline layout diagram, and don't just look at the largest number.
You think you'll be all right by getting a metal expansion joint compensation amount specification sheet? Tsk, that's naive. The compensation of the expansion joint of the same DN300 can be twice different under different fatigue life. The line "Axial compensation ± 50mm" in the table is often followed by a line of small words: "Design fatigue times 1000 times". Your pipeline starts and stops once a day, 365 times a year, and it will exceed its lifespan in less than three years. If it is replaced with a straight pipe pressure balance expansion joint, the compensation amount may be doubled, and the number of fatigue times can also be mentioned as 3000 times. Don't forget to ask about this "hidden condition" when selecting a model.
How to quickly find what you want from the specification sheet?
For example, look at the "Universal Corrugated Expansion Joint" parameters on our website: usually mark the nominal diameter DN, working pressure, temperature, axial compensation X, transverse compensation Y. However, note that the compensation amounts in many tables are the design values under a single fatigue life. For example, a certain specification states that the axial compensation amount is 100mm, the working pressure is 1.6MPa, and the temperature is 350℃-these values are measured under certain conditions. If you use it on a steam pipeline with a temperature of 500℃ and a pressure of 2.5MPa, the compensation amount will have to be discounted. Therefore, when looking at the watch, you can't just look at the numbers, but you have to find out the test conditions.
What about that? Teach you a local method: List the working condition parameters, compare them with the "allowable compensation amount" column in the specification table, and multiply them by a safety factor. To be safe, the lateral displacement should not exceed 80% of the table mark, and the angular displacement should not exceed 90%. This is the blood and tear experience of many old fitters, which is more reliable than theoretical calculations.
Let's talk about special working conditions
For high-temperature axial expansion joints used in steam pipelines in power plants, the compensation amount should be calculated by considering the high-temperature creep of materials-the displacement difference that can be swallowed by the same wave number at 300℃ and 500℃ is large. This high-temperature axial expansion joint in the station is specially designed for the power station industry. The wave number is two more layers, and the material is upgraded to heat-resistant stainless steel. The "high-temperature correction coefficient" will be additionally marked in the compensation scale. As for the metal corrugated expansion joint in cement industry, because of the large dust and large temperature fluctuation, the compensation amount design will leave 30% margin compared with conventional products.
There is also a large tie rod expansion joint, and its tie rod is not a decoration-as soon as the tie rod angle changes, the compensation direction will change accordingly, and the line "angular displacement ± 5 °" in the specification table will be discounted if you install it crooked. Two days ago, a brother who was doing a desulfurization project chose a general-purpose type to save trouble. As a result, the pipeline vibration tore the bellows and replaced it with a double hinge horizontal type with hinges.
And guess what? Compound hinge transverse expansion joint can absorb transverse displacement and a small amount of axial displacement simultaneously, and is suitable for pipe system with limited space. However, the "lateral compensation amount ±30mm" written on the specification sheet refers to two-way swing. If you only install it in one direction, the actual compensation amount will shrink by half. The first thing a knowledgeable person does when they get a table is the "allowable deflection angle" rather than the displacement value.
Don't trust some "universal specification sheets" on the Internet
The compensation amount is linked to the pipe support, medium pressure and ambient temperature, and even the pre-stretching/pre-compression amount during installation will affect the actual compensation value. For example, if you buy an external pressure single axial expansion joint, it is more suitable for high-pressure steam pipelines than ordinary axial type, but it has to be pre-stretched according to the instructions during installation-this data specification table is generally not written, so you have to ask the manufacturer for construction guidance. All kinds of products in this site (large-diameter thick-walled expansion joints, curved tube pressure balance type, direct burial type, etc.) have drawings and technical parameters, but it is recommended that you throw the working condition parameters over, and let's check the compensation amount one by one, which is much worry-free than turning the table yourself.
The specification table of compensation amount of metal expansion joint is an engineer's mirror, which can show the numbers, but can't show the scene. The direction of the pipe, the stiffness of the support, and the corrosiveness of the medium, which is not a variable? Next time someone throws you a table for you to pick the model directly, you can shoot this article on his desk: talk about the pipeline diagram first, and then talk about the compensation amount.