Two days ago, I met an engineer who was doing heating network design. He asked me with the drawings, "What is the unit of angular displacement of the expansion joint? Why do some of the drawings write ° and some of them write mm?" I saw that on his expansion joint selection sample, the angular displacement column was marked with "±5°", while the other company gave it with "±12mm". If you compare these two directly, something will go wrong. Today, break this parameter apart and crumble it into pieces to explain clearly.
What exactly is angular displacement? First distinguish the difference between it and lateral displacement and axial displacement
What the expansion joint does in the pipeline, to put it bluntly, is to absorb displacement. There are three kinds of displacement: axial displacement is the expansion and contraction along the axis of the pipe, transverse displacement is the misalignment perpendicular to the axis, and angular displacement-is the relative rotation between the flanges at both ends of the expansion joint. You can think of it as the pipe "folded at the waist", and the size of this folded corner is the angular displacement.
Understanding this is critical. Many people confuse angular displacement with lateral displacement, which is actually two different things. The lateral displacement is translation and the angular displacement is rotation. For example, a piece of equipment is lifted up by 20mm because of thermal expansion. If the pipe connecting it turns a corner, the expansion joint may actually bear an angular displacement of several degrees, instead of a simple 20mm lateral displacement. You just report a "lateral displacement of 20mm" to the manufacturer, and people really can't choose a model for you.
Is the unit of angular displacement only degrees (°)? Why do some drawings say millimeters?
The standard unit of angular displacement is degrees (°), which is nothing to contend with. But why do some drawings say millimeters? Because in many manufacturers' samples, angular displacement is expressed by "equivalent lateral displacement". What do you mean? It is to convert this angular displacement into the arc displacement at one end of the bellows.
The arc length is equal to the radius times the radian. If the radius of rotation of the bellows is 500mm and the angular displacement is 2°, the corresponding arc length is 500×2× π/180≈17.5mm. But there is a big pit here: different expansion joint structures have different radii of gyration at all. The same 2° angular displacement may be several times different in millimeters between a single hinge expansion joint and a large double hinge transverse expansion joint.
So when you see mm written on the drawing, don't rush to convert it. You have to find out where this mm comes from, whether it is the equivalent lateral displacement calculated by the manufacturer according to its own product structure, or it refers to the swing arc length of a certain point of the bellows. If you take this mm directly as an angular displacement to select the model, then you can't install it on the spot.
Generation of angular displacement from bellows structure: Why are single hinges different from double hinges
Angular displacement is not produced for nothing, it has to be guided by hinge structure. Single hinge expansion joint, that is, a hinge is installed at both ends of the bellows, which can only rotate in one plane, and its ability to absorb angular displacement is very limited. The compound hinge transverse expansion joint has two sets of hinges and a section of pipe in the middle. The bellows can undergo more complicated deformation and absorb large angular displacement and transverse displacement.
For example, the double hinge expansion joint used on the vacuum pipeline of the air-cooled island has a more specific structure. The two hinges cooperate with the bellows to deal with the thermal expansion of large-area horizontal pipelines. You see, for the same angular displacement, the magnitude that a single hinge and a double hinge can absorb is completely different. Therefore, when selecting a model, don't just look at an "angular displacement of 2°", but you have to confirm what structure it is. If you give the manufacturer an angular displacement of 4 °, the double hinge can be easily handled, but if you take the single hinge to carry it, the bellows will become unstable sooner or later.
How to read angular displacement values? Common unit misunderstandings and conversion traps in engineering type selection
Get a copy of the parameter table of the expansion joint. The angular displacement column says "±6°", and you can read it directly as "plus or minus 6 degrees". However, the pipeline layout drawings issued by some design institutes will mark the angular displacement as "6mm/m", which is easy to be confused. 6mm/m is actually the slope, which when converted into an angle is arctan (0.006) ≈ 0.34°. Don't think of it as a simple 6mm displacement.
Think of the unit of angular displacement "°" on the sample as radians. If anyone thinks of 5° as 5 rad, the selected expansion energy saving is twisted into a twist. Remember, the default unit of angular displacement in engineering is degrees, not radians, or millimeters.
Then how to check whether the tonnage given by the manufacturer is correct? Let me tell you a way: you first calculate the total angular displacement required for the pipeline, then look at the allowable angular displacement of each bellows on the manufacturer's sample, and then multiply it by the number of sets of hinges. If it is a single hinge, a bellows carries an angular displacement; If it is a duplex hinge, each set of hinges shares a portion. Don't expect one bellows to eat all the angles.
Type selection practical combat: the cooperation relationship between angular displacement and tie rod and hinge structure, and how to raise capital to manufacturers
The tie rod and hinge are the "seat belt" of the expansion joint. The tie rod is responsible for bearing the internal pressure thrust and preventing the bellows from being pushed open by the pressure; The hinge is responsible for constraining the direction of displacement, so that the angular displacement goes honestly along the designed plane. Without tie rods and hinges, a bellows alone can't absorb angular displacement alone.
Therefore, when selecting and raising capital, you need to provide the manufacturer with these things: pipe diameter, design pressure, design temperature, medium, and the size and direction of angular displacement. The angular displacement must be clearly written as "degrees" and explain which plane it is rotating. It is best to attach a simplified diagram. Don't just say "need to absorb angular displacement". The manufacturer is not a god and can't guess the direction of your pipeline.
The universal corrugated expansion joint itself does not absorb angular displacement, so it can only be carried by the flexibility of the bellows itself, and its life will drop suddenly if it is carried too much. If you want to absorb angular displacement, choose a compound hinge transverse expansion joint or a special hinge expansion joint. If you want to save that little money, it will be more troublesome to replace the corrugated pipe if it cracks in the future.
What is the unit of angular displacement of the expansion joint? The standard answer is one word: degrees. However, understanding the structural logic behind it, the conversion trap and the cooperation relationship with the hinge tie rod will prevent the selection from overturning. Next time I see mm written on the sample, ask more "how to convert it", which is much more reliable than stuffy selection.