What causes small floating head bolts to break?
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For many years, our company has been responsible for the maintenance of heat exchangers in oil refining and chemical processing plants. Each time, about a dozen of the small floating head bolts (35CrMoA) in the liquefied gas cooler L-405 of the catalytic unit crack. Most are located at the ribbed blank areas. Steel No. 35 also broke! I seek advice from experienced experts!
Your liquid hydrocarbon cooler should have the gas-phase and liquid hydrocarbon flowing in the shell side, while the circulating water flows in the tube side, right? Moreover, the liquid hydrocarbon cooler should be placed before the desulfurization process; it is advisable to have the hydrogen sulfide content in the liquid hydrocarbons tested. Additionally, check the water content in those liquids. If the hydrogen sulfide level exceeds 30PPM, it can lead to wet hydrogen sulfide stress corrosion. The manifestation of hydrogen sulfide stress corrosion is cracks, which first appear at certain surface defects or areas with localized stress concentration. After the induction period, the cracks grow rapidly under the tensile stress of the bolts, causing the cross-section capable of withstanding the tensile force of the bolts to shrink rapidly, ultimately leading to fracture.
It is recommended to conduct hardness testing on the bolts as well as fracture metallographic failure analysis. It is preliminarily determined that stress corrosion is the cause; stress concentration combined with a H2S-containing environment led to this situation
Thank you, Floor 2. You’re right; the crude oil at our factory contains sulfur, and the coolers are located before the desulfurization process.
Another factor is the bolt working stress level. If the bolt stress is high, it becomes more sensitive to hydrogen; conversely, if the stress is low, sensitivity is lower. One factor contributing to high bolt stress is the tightening method; the use of a torque wrench generates torsional stress, and due to the uncertainty associated with friction, the tightening force often becomes excessive, exceeding 70% of the bolt material’s yield limit. There is indeed a reason why it is recommended to use tensioners to preload bolts of foreign heat exchange equipment (1 inch and larger)! This post was last edited by Lao Jiu on 2008-12-6 at 16:32.]
The guys on the second and fifth floors are really professional; I won’t comment on LZ’s questions – just use them as a reference for learning! Thank you~
A very typical type of failure: a wet hydrogen sulfide environment, local high stresses – it’s extremely difficult to deal with. Improvement measures: Reduce the hydrogen sulfide content (which implies changes to the process); if that doesn’t work, the only option is to produce more spare parts for these bolts. When removing the floating head cover for inspection, pay attention to checking the small floating head bolts. Recommended method: Perform PT testing on the bolts after cleaning them.
For production, this is a pretty serious problem. It is highly significant for sections to carry out discussions on this type of questions, as it has a clear impact on guiding production. More of such issues should be addressed.
It breaks at the area without a cutting tool, as that spot is already a stress concentration point during bolt processing; moreover, the condenser causes significant vibrations in the heat exchanger tubes, which leads to cracking
I learned it! ! ! Regarding the factor of bolt tightening mentioned on the 5th floor, we manufacture hydraulic wrenches, so we have considerable experience in this area as well!
As a newcomer who is not familiar with the rules, upon seeing this issue I reminded the design and manufacturing teams that for the shell side floating head bolts where H2S-induced stress corrosion might occur, 35CrMoA bolts should not be used; instead, 40Cr, 40MnB, or SA 193 B7 can be chosen. Our company used to encounter such problems frequently, but they have not occurred in the past seven or eight years.