Double the number of bolts, but the leak is worse?
Two days ago, I met a customer who said that all the expansion joint flange bolts in their factory were replaced with double the number, and the leakage became more serious. And guess what? The preloading force was not well controlled, and the bolts interfered with each other, which crushed the gasket instead. Many people think that the more bolts, the safer it is, but in fact, it is completely reversed. The number of metal expansion joint bolts is not determined by a pat of the head. It is directly linked to the nominal pressure, caliber size, gasket type, and even medium temperature. Make a mistake, from leakage to warped flange and broken bolt.
None of the three core parameters can be missing
To decide how many bolts to match a set of expansion joints, first look at three hard indicators:Nominal pressure (PN), nominal diameter (DN), effective width of gasket。 Take the national standard GB/T 9115 as an example. The number of bolt holes and diameter of flat welded flanges with necks of different pressure grades are clearly recommended. For example, PN16 and DN200 universal corrugated expansion joints are equipped with 8 M20 bolts as standard; If it is the large-diameter thick-walled expansion joint of PN25 and DN800, it will jump directly to 24 M27.
But that's just the base threshold. When actually installing, you still need to look at the structure of the expansion joint itself-if you have a guide tube, you should consider the installation space more. The guide tube will occupy a circle of position on the inner side of the flange, and the diameter of the center circle of the bolt hole may have to be fine-tuned. The flange surface of the high-temperature axial expansion joint is wider because the bolt spacing has to be recalculated to counteract the deformation caused by thermal expansion. To put it bluntly, standards are only the starting point, and on-site working conditions are the end.
Different products, bolt configuration is very different
There are so many products on our station, and the bolt configuration is the same as the model. Take the high-temperature axial expansion joint as an example. The working temperature is always five or six hundred degrees Celsius, and ordinary carbon steel bolts can't bear it, so you have to replace heat-resistant alloy steel, such as 25Cr2MoVA. At the same time, the flange will expand at high temperature, and the original calculated number of bolts may not be enough-it has to be recalculated according to the bolt load in the hot state. The number generally does not increase, but the specifications may be upgraded.
The desulfurization flue gas baffle door is more special. The medium is corrosive, so the number of bolts should not only be enough, but also anti-loosening gaskets, otherwise it will be loose after a few vibrations. In addition, the flange of the baffle door is usually made of thick-walled structure, and the number of bolt holes is filled according to the maximum. However, some customers require to reduce the number of bolts for the convenience of maintenance-at this time, the seal specific pressure has to be re-checked, and the risk is not small. If the local specific pressure of the flange surface is not enough, the air leaks directly.
Another example is the compound hinge transverse expansion joint, which itself has a hinge structure, and the force direction of the flange is complicated. The bolt not only has to bear axial force, but also has to resist shear. The bolts should be arranged away from the hinge support position, and the number of holes may be fewer than standard, but the gauge must be larger. The corrugated expansion joint used in the power station industry has been running under high temperature and high pressure for a long time, and the material, quantity and tightening torque have to be recalculated, which is far from enough to look at the standard.
The quantity is correct. If you don't tighten it, it will be useless
This problem is most common at the installation site-workers hit with wind cannons, thinking that the greater the torque, the better. As a result, the bolts either break or the uneven preload force causes the flange to warp. Correct approach: Tighten symmetrically in three steps, the first step is preloaded to 30% of the target torque, the second step to 70%, the third step to 100%, and finally check again in sequence. The adjustment of the expansion joint tie rod nut is similar to the logic. Refer to the question and answer in our station.
In addition, the elastic elongation of the bolt is more reliable than the torque value. The torque value is greatly affected by the friction coefficient of thread. With oil and without oil, the same torque preloading force can be 30% different. If conditions permit, measure with an ultrasonic bolt elongator, and directly see how much the bolt is elongated. That is the real preloading force. Especially for high-pressure and large-diameter expansion joints, don't save this step.
How to check the quantity of bolts when purchasing? Three tricks to do it
First, look at the Faran standard number. Check the number of bolt holes under the corresponding pressure and temperature level, such as HG/T 20592 and GB/T 9115, which are clearly written above. Second, compare the product drawings. Don't just look at the marks on the final assembly drawing, but check the hole diameter and the center circle diameter in the flange detail. Sometimes because the final assembly drawing is simplified, a few holes are missed, and the actual flange drawing is accurate. Third, special working conditions put forward additional requirements. For example, the corrugated expansion joint used in the power station industry may need to run at high temperature for a long time, and the bolt material must be 25Cr2MoVA high-temperature alloy steel. At this time, the quantity remains unchanged but the specification may be upgraded.
There are more than 30 kinds of products on our station, each of which has a corresponding technical parameter table. It is most reliable to ask the manufacturer directly for the calculation book. Don't guess for yourself, and don't listen to field workers say "I used to do this"-data and calculations are the bottom line.