Why should non-metallic compensators keep an eye on flange standards?
To put it bluntly, the flange is the "joint" that connects the pipe to the compensator. Once the standard is selected wrongly or mismatched, air leakage and ash leakage will occur at least, and the stress of the whole pipeline will exceed the standard and the equipment will be broken down at worst. Especially non-metallic compensator (also called fabric fiber expansion joint or non-metallic expansion joint), its loop belt is flexible material, and once there is a problem in the rigid connection of flange, the probability of seal failure is much higher than that of metal compensator. Therefore, understanding the flange standard of non-metallic compensators is not to pretend, but to save your life. Think about it, a smoke and air duct is several hundred meters long, and several expansion joints in the middle absorb the heat displacement. If the flange leaks, the whole system will have to stop.
What are the mainstream flange standards in China? How to choose?
GB/T 9119 (plate flat welding), HG/T 20592~20635 (European system, American system), SH/T 3406 (petrochemical), JB/T 81 (cast iron flange), etc. The most commonly used non-metallic compensators are HG/T 20592 (PN series) and GB/T 9119, because these two standards cover most low-pressure and high-temperature flue ducts. Like the common rectangular non-metallic expansion joints in our factory, many customers require flanges according to PN6 or PN10 of HG/T 20592, and RF (protruding surface) or FF (full plane) for the sealing surface, depending on whether the medium is corrosive. For special working conditions such as desulfurization flue gas baffle doors and air-cooled island vacuum pipes, American standard flanges are sometimes made according to ANSI B16.5-but domestic projects still dominate by national standards and chemical standards. You said that you have to set petrochemical standard SH/T 3406 for a cement plant pipeline? It is not impossible, but the cost is high and the delivery time is long, so it is unnecessary.
What is the difference between the flange structure of non-metallic compensator and ordinary flange?
Its flange face is usually not welded directly to the compensator. The actual structure is: the metal flanges (or angle steel frames) at both ends of the compensator clamp the non-metal ring belt through bolt pressing plates. This leads to the special design of bolt hole spacing, sealing surface width and platen groove size of flange. For example, rectangular non-metallic expansion joints often have a circle of "stops" or grooves on the inner side of the flange to position the sealing gasket and prevent blowing off. Some engineering sites complain that "the flange doesn't match", and nine times out of ten, they don't pay attention to the "non-standard customization" of the flange of the non-metallic compensator-although the connection size is according to the national standard, the thickness and bolt hole distribution may be fine-tuned. Therefore, when purchasing, we must write the flange standard, pressure grade and sealing surface type clearly, and it is best to attach flange drawings. Don't just write a sentence "According to HG/T 20592 PN10", and the supplier will make it according to the standard for you. As a result, the bolt holes on the spot don't match, which will be embarrassing.
Choosing flange criteria is actually a trade-off question
Medium temperature exceeds 300℃? The non-metallic ring belt itself may not be able to bear it. To choose a high-temperature non-metallic expansion joint, the flange material has to be upgraded (for example, Q235B is replaced with 16Mn or 304). The medium contains sulfur and humidity? For example, desulfurization flue of power plant, HG/T 20592 is recommended for flange standard, RF is used for sealing surface, and composite gasket in PTFE or rubber PTFE compensator is used for gasket. Two days ago, there was a customer of a cement factory with a pipe diameter of 2 meters. They wanted to use PN2.5 flange of GB/T 9119 to save cost. However, the non-metallic compensator manufacturer reported that the bolt arrangement of PN2.5 under large diameter was too thin, and the ring belt could not be pressed tightly. Later, it was changed to PN6 to solve the problem. Therefore, the flange standard can't only look at the pipe end, but also the structural stiffness of the compensator. You don't believe it? Flip the national standard of JB/T 12235-2015 "Non-metallic Expansion Joints", which clearly says: when the design pressure is ≤0.1MPa, the spacing between flange bolts is not more than 150mm; When the pressure is> 0.1MPa, the spacing is not greater than 120mm. PN2.5 simply can't meet this requirement under large calibers.
On arrival installation, flange standards may go wrong even if they are aligned
First, the flange sealing surface has rust or burrs that have not been cleaned, so it is installed directly. The non-metallic ring belt is under uneven pressure and will leak in a few months. Second, the bolt tightening sequence and torque are not controlled. First tighten diagonally and then tighten in turn. The torque value refers to the recommended value in the appendix of HG/T 20592 (generally, the torque of M16 bolts is 80~120N·m). Don't rely on brute force. Third, the compensator itself has pre-displacement (such as cold tightening), and the flange clearance should be aligned with the marking line during installation, otherwise the flange stretches excessively in hot state and causes the bolt to break. According to our after-sales service statistics, 70% of air leakage accidents are related to sloppy flange installation, which is really not the poor quality of the compensator itself. Think about it, the ring belt is fine, the bolts are not tightened, and the gasket is offset. Who should carry the pot?
Still want to dig deeper? Look at JB/T 12235-2015
When the design pressure is ≤0.1MPa, the flange bolt spacing is not more than 150mm; When the pressure is> 0.1MPa, the spacing is not greater than 120mm. These data are reflected in the selection table. If you flip through the nameplate of your own equipment and compare it, you will know whether there was a pit in the original selection. In addition, don't forget that the flange of non-metallic expansion joint often bears the function of compressing the ring belt. Its structural form-such as whether it has a stop or not and the depth of the pressing plate groove-is not refined to every article in the standard, so experienced manufacturers will provide flange drawings. After getting the drawings at the time of purchase, check them again against the actual size of the pipe flange, which is more effective than anything else.
In the final analysis, the flange standard of non-metallic compensator is not an isolated technical parameter, but is closely interlocked with the pressure grade, medium working condition, compensator structure and installation accuracy. Understand it, and your plumbing system can run smoothly.