Understand the structure before drawing: Don't draw bellows as springs
Every time I see someone drawing a bellows as a circle of coil springs, I know this picture is waste. The core of a metal expansion joint is a bellows, but a bellows is not a spring-it is a corrugated cylinder formed by hydraulic pressure or rolling from thin-walled stainless steel plates, with parallel annular folds between waves, not spiral rising threads. You use the technique of painting springs to paint bellows, but the workers can't understand it, and the mold master wants to swear even more.
Set the model before drawing. The universal corrugated expansion joint is a single wave group plus end tube, which has a simple structure. The main view is drawn as several symmetrical semicircular arcs connected in a row. What about the double hinge transverse expansion joint? There are hinge plates, tie rods and pins in the middle, and the bellows are two sections, connected by an intermediate pipe in the middle. The painting methods of these two products are completely different, and even the weld positions are different. The product type is not determined, and the labels at the back are all castles in the air.
Also, don't forget to draw the deflector. The guide tube is a layer of thin-walled cylinders installed inside the bellows, and the function is to allow the medium to directly flush the guide tube instead of the bellows. It is drawn in the cross-sectional view with a two-dot dashed line, distinguished from the thick solid line of the bellows body. This detail was missed, and the processed product was installed on the pipeline, and the bellows was washed through in three months.
Drawing standards and legends: what is specified in GB/T 12777
The national standard GB/T 12777 "General Technical Conditions for Expansion Joints of Metal Bellows" has clear requirements for the drawing method of expansion joint drawings, so don't invent and create them yourself. The legends specified in the standard include: bellows deployment diagram, cross-sectional view, weld symbol, medium flow direction arrow, guide tube representation method, simplified drawing method of tie rod connecting plate, etc.
Take the guide tube as an example. In the cross-sectional view, the double-dot dash line represents the guide tube, the thin solid line represents the end tube, and the peak of the bellows is outlined by a thick solid line. The tie rod connecting plate can be simplified as a thick solid line plus a center line in the assembly drawing, but the location and number of bolt holes must be drawn in the component drawing. The weld symbol is more particular-butt weld, fillet weld and plug weld are marked in different ways, and the expansion joint with high pressure grade should be marked with the weld detection ratio and standard code.
It is a trivial matter to be beaten back when reviewing the drawing, and it is tens of thousands of pieces of waste if it is processed wrong. Two years ago, a customer drew a drawing of the expansion joint himself to a small factory for processing, and the guide tube was drawn with a thin solid line. As a result, the worker welded the guide tube to the outside of the bellows according to the thin solid line position. After installing the pressure test, the bellows will bulge and be scrapped directly.
Step-by-step drawing: from main view to partial enlargement
The first step is to draw the center line and the interface size.Draw the horizontal center line first, and then set the outer diameter and length of the flange or connector at both ends. What is the diameter DN, and what is the diameter of the center circle of the flange bolt hole, drawn according to the actual interface size. Draw the center line with thin dots, and both ends should be 3-5mm beyond the contour line.
In the second step, draw the middle diameter and wave height of the bellows.The middle diameter of the bellows is the average diameter of the crest and trough, and the wave height is the radial distance from the middle diameter to the crest or trough. The wave pitch must be uniform-a single wave pitch deviation exceeds ±1.5mm, and the fatigue life is directly discounted by 30%. When drawing, draw the peak outline with thick solid lines, and the trough with thin solid lines. After drawing a set of waves, match the end tube.
In the third step, draw the end tube and the guide tube.End tubes are straight sections of a bellows welded at both ends to connect pipes or flanges. The guide tube is drawn inside the bellows, the left end tube is welded and fixed, and the right end tube is left with a gap, so that the guide tube can expand and contract freely with the temperature change. Note that the direction of the notch of the guide tube should be consistent with the flow direction of the medium. Wherever the medium enters, the notch will face-this detail directly determines whether the guide tube protects the bellows or accelerates the wear of the bellows.
The fourth step is to mark the key dimensions.Diameter DN, Wave Number, Total Length L, Design Pressure PN, Bellows Wall Thickness, Wave Height, Wave Pitch, End Tube Wall Thickness and Length, Flange Standard No. The wall thickness of the bellows is the thickness of the plate before forming, not the measured wall thickness after forming-it can't be done if the mold is wrong, and the stiffness of the bellows is wrong.
Partial enlargement draws the cross-sectional shape of the waveform separately, and marks the wave height, wave pitch, wave peak radius, wave trough radius and wall thickness. This part is for the design of molding molds. If you don't draw them clearly, the mold master can only rely on guessing.
Differences in painting methods under different working conditions: What are the particulars of high temperature, large diameter and direct burial
High temperature axial type expansion joint should draw heat insulation layer and high temperature resistant alloy bellows. The insulation layer is painted on the outer surface of the bellows, distinguished by section lines, and the name and thickness of the insulation material should be marked. Painting with ordinary 304 stainless steel as high temperature resistant material? 304 fails above 600℃ at high temperature, and Inconel 625 or GH4145 alloy is clearly written on the drawing.
Large diameter thick wall expansion joint has less wave number but large wave height. For expansion joints above DN2000, the wave height may be above 200mm, and the wave pitch may be increased accordingly. Draw this diagram to indicate the position and size of the reinforcing ring. The reinforcing ring is welded on the outside of the trough to prevent the columnar instability of the bellows under the internal pressure. If the reinforcing ring is not marked, the bellows will bulge as soon as the design pressure comes up.
Direct buried (fully buried) type expansion joint should draw the outer shield and end seal. The outer protective pipe is a steel pipe sleeved outside the bellows, and the end seal is a sealing structure at both ends of the outer protective pipe to prevent groundwater from seeping into the annular space between the bellows and the outer sleeve. When embedded on site, the end seal should be waterproofed. If you paint the directly buried expansion joint without painting the outer protection pipe and end seal, there is no way to construct it on site-do you ask the construction team to add it on site? Nine times out of ten what they add out is not waterproof.
The external pressure single type axial expansion joint shall be marked separately with the external pressure cylinder. The external pressure cylinder is a pressure-bearing shell sleeved outside the bellows. The medium pressure acts on the outer wall of the cylinder, and the bellows bears external pressure instead of internal pressure. The bellows of this structure will not produce columnar instability, but attention should be paid to the wall thickness and material of the external pressure cylinder, after all, it is the real pressure-bearing part.
Labeling pitfalls and common mistakes: arrow direction, tie rod nut, weld symbol
Media flow direction arrow drawn backwards?When the guide tube is installed backwards, it will directly flush the bellows. It can't be seen in the picture, but it will explode on the spot-the medium rushes to the fixed end of the guide tube, and the bellows is completely exposed to the medium scour, and the perforation can't run after three months. After drawing the arrow, check it by yourself. The flow direction should be consistent with the direction of the notch of the guide tube.
The adjustment margin of the tie rod nut is not standard, so it can't be pre-stretched during installation.Adjustment nuts are installed on the tie rods of the double hinge transverse type expansion joint and the straight tube pressure balance type expansion joint. What is the amount of pre-stretching, where is the nut position when it leaves the factory, it is not indicated on the drawing, and the installation team does not know which way to screw it when it gets it. The bolt is overscrewed, and the bellows is directly pulled and cracked; If it is not screwed in place, the compensation amount is not enough and the pipeline deformation will still be damaged.
The weld mark is missing or incorrectly marked, and the pressure test leaks directly.The circumferential weld between the bellows and the end pipe is a pressure-bearing weld, which generally requires full welding penetration, and 100% radiographic inspection or 100% ultrasonic inspection is carried out after welding. The flaw detection ratio and qualification grade are not marked on the drawings, and the welder welds according to ordinary fillet welds. During pressure test, water seeps at the welds, and the whole equipment is scrapped.
Another one that is easy to overlook —Nonmetallic expansion joints Do not draw with the legend of metal expansion joints.The cross-sectional drawing method of non-metallic expansion joint (fabric fiber expansion joint) is different from that of metal. The fabric fiber layer is represented by an oblique cross-sectional line, the metal mesh reinforcement layer is represented by a thin solid grid, and the layers of skin and thermal insulation cotton should be marked separately. People who are used to drawing metal expansion joints draw non-metal drawings, and it is easiest to draw the fabric layer into steel plates with thick solid lines-the materials are all wrong, how can the workshop cut the materials?
After all, there is not so much metaphysics about drawing metal expansion joints, but you have to put your heart into it.