Find out first: What does the DN700 skin non-metallic expansion joint look like?
The caliber of DN700 is not small, and it is more used in smoke ducts, dust collector inlet and outlet, desulfurization towers and other places. When many purchasers look at the "DN700 skin non-metallic expansion joint" marked on the drawing, they think that just like the metal expansion joint, they can report the diameter directly. In fact, the size system of non-metallic expansion joints (that is, we often call fabric fiber expansion joints) is different from that of metal-it has not only the nominal diameter, but also the key data of skin width, frame thickness and guide tube length. Two days ago, I met a customer and asked for a price with a picture of DN700. As a result, only a diameter was given. I said you should measure the total length between the two flanges first, and the other party was stunned: Huh? Isn't this thing a fixed length? I tell you, the axial length of non-metallic expansion joints is often calculated according to the design compensation amount, and there is no such thing as a "standard value".
Therefore, before talking about the size, we have to first understand the structure of the non-metallic expansion joint of DN700 skin: metal frames (or flanges) at both ends, flexible skin (multi-layer fabric + rubber/fluororubber composite layer) in the middle, and sometimes a guide tube (to prevent airflow erosion) inside. The "dimensions" you asked include at least: nominal diameter DN700, total length L, skin width B, flange bolt hole center circle diameter D1, number of bolt holes and hole diameter. If one is missing, you can't quote.
The core dimensions of DN700 expansion joint: diameter, length, skin width, one must not be less
① Nominal diameter DN700: to put it bluntly, it is the inner diameter of the pipeline, 700mm. But be warned: the flange inner diameter of the skin non-metallic expansion joint usually matches the inner diameter of the pipe, but the outer diameter of the frame will be larger by a circle. The diameter of the center circle of the flange bolt hole can refer to GB/T 9119 or HG/T 20592 standard. The diameter of the center circle of PN6 or PN10 flange of DN700 is about 860mm (specifically, depending on the pressure level). However, non-metallic expansion joints are commonly used as angle steel frames or channel steel frames, and the position of bolt holes is not necessarily strictly according to the standard flange, because the frame itself needs to be matched with the matching flange on the pipeline or equipment.
② Total length L: This number is the easiest to ignore. The total length of the non-metallic expansion joint depends on the design compensation amount. For example, the axial compensation required by the flue is 50mm, and the width of the skin part (that is, the effective working width) should be at least 50mm. Coupled with the width of the frame plates at both ends, the total length is usually between 300 and 600mm. With the caliber of DN700, the total length of small compensation amount may be 400mm, and the total length of large compensation amount can reach 600mm or even longer. If the "size" you asked only gives the diameter but not the total length, then the one you got may not fit at all.
③ Skin width B: This is the actual width of the flexible section, which directly determines the compensation ability. The skin width is generally equal to the total length minus the thickness of the frame platens on both sides. For example: the total length is 500mm, the frame plates are 50mm each, and the skin width is 400mm. This 400mm is the "elastic segment" that can really absorb thermal displacement.
④ Frame thickness and material: Q235 carbon steel or weather-resistant steel is commonly used for non-metallic expansion joint frames. The frame channel steel of DN700 is generally selected from 10# ~14#, and the thickness depends on the pressure. If it is a desulfurization flue gas system, you have to consider anti-corrosion, and the frame can be lined with rubber or stainless steel.
There is also a small detail-the length of the deflector. If there is high-temperature dusty air flow in the pipeline, a guide tube is usually added to protect the skin. The length of the guide tube is generally 50~100mm shorter than the skin width, and the outer diameter of the guide tube of DN700 is slightly less than 700mm (usually 5~10mm smaller) to avoid interference with the pipeline.
How to choose the size? Don't just look at the DN number, the working condition is father
To be honest, many customers ask "DN700 skin non-metallic expansion joint size", and the real subtext is "Give me a general size table so that I can copy it". But there really is no uniform size chart for non-metal expansion joints. Because the same DN700, used in hot air ducts and used in cold air ducts, may be twice as different in size.
The flue gas pipeline at the kiln tail of a cement plant has a medium temperature of 350℃ and a pipeline length of 30 meters. The calculated axial thermal elongation is about 120mm. If a non-metallic expansion joint is selected, the total length should be at least 500mm (skin width 400mm, leaving some safety margin). At the entrance of the desulfurization tower of another power plant, the medium temperature is 80℃, the compensation amount is only 30mm, and the total length of 300mm is enough. You said the same DN700, can the size be the same?
So how do you size it? The core is three steps: ① calculate the compensation amount (axial, transverse and angular); ② Select skin materials (silicone rubber, fluororubber, polytetrafluoroethylene, etc., different materials have different temperature and pressure resistance, and the rolling width is also different); ③ After determining the width of the frame and skin, reverse the total length. If the site space is limited and the total length has an upper limit, it can only be compensated by increasing the number of skin layers or replacing the skin material with a large compensation amount.
In addition, pressure is also a factor affecting the size. Non-metallic expansion joints are usually used for low pressure (≤0.1MPa), but if it is a positive pressure pipe, the skin needs a thicker interlayer or an added wire mesh reinforcement, which will slightly compress the effective compensation width of the skin-to put it bluntly, the total length of the expansion joint with high pressure will be slightly longer than that of the low pressure with the same compensation amount.
Guide to common "pits" and avoidance of non-metallic expansion joints with DN700 skin
Use non-metallic expansion joints as metal bellows. The size of metal bellows is standardized, such as the general-purpose corrugated expansion joint of DN700, which is usually 300mm or 400mm in length, and has national standard reference. The size of non-metallic expansion joints is customized. If you take the idea of metal expansion joints to set them, there is a high probability of buying the wrong one.
Ignore the pairing dimensions of the frame flanges. The pipe flange of DN700 may be PN6, PN10 of HG/T 20592, or it may be American standard or Japanese standard. Frame bolt holes for non-metallic expansion joints must correspond one-to-one with the customer's pipe flange. Don't think up a standard yourself.
Only the space size was measured and the installation clearance was not considered. For example, the distance between the on-site pipeline interfaces is 600mm, and you bought an expansion joint with a total length of 620mm, but it can't fit in. Both ends of the frame of the non-metallic expansion joint have an overlap length of welding or bolting, and an adjustment margin of 5~10mm is required during actual installation.
The skin width is too extreme. Some designs calculate the skin width to just equal the compensation amount in order to save cost. However, the skin will creep at high temperature, and the compensation ability will decrease after long-term operation. It is recommended to leave at least 20% ~30% margin. For example, calculate the compensation amount of 100mm, the skin width should be 130mm, and the total length should be increased accordingly.
Don't only quote DN700 when ordering, but give the total length, flange specifications, medium temperature, pressure and compensation amount on the drawings to the manufacturer. This is a professional practice. You save communication time and avoid the hassle of returning goods when they arrive.
(Oh yes, if you need more specific size reference, you can flip through the product page of "Non-metallic Expansion Joints (Fabric Fiber Expansion Joints)" on this site, which has the recommended size range for typical working conditions. Although it is not a panacea, it can at least make you have a spectrum in your heart.)