Even high-quality 316L stainless steel manholes can lead to accidents-whether due to stress corrosion or improper operation-if maintenance is neglected.
1. Prevention of "Pitting Corrosion" and "Intergranular Corrosion"
Although stainless steel is rust-resistant, in environments containing high concentrations of chloride ions (such as certain chemical cleaning solutions), the flange faces of manholes are susceptible to developing needle-point "pitting corrosion."
Recommendation: Perform periodic passivation treatments. Additionally, after cleaning, rinse the manhole with deionized water and dry it thoroughly to prevent process media from accumulating for prolonged periods within the sealing gaps.
2. The Scientific Approach to Bolt Tightening Sequences
When closing a pressurized manhole, strictly avoid tightening the bolts in a sequential, circular pattern.
Correct Procedure: Employ the "cross-pattern method," applying the preload force gradually over 2 to 3 rounds. This ensures uniform pressure distribution across the gasket, thereby preventing the flange from warping due to excessive localized stress.
3. Safety Warnings
Before entering any vessel equipped with a manhole, one must strictly adhere to the "Ventilate First, Test Next, Enter Last" safety protocol for confined space entry. For automated pressure systems, it is highly recommended that the manhole be equipped with a safety interlock device-specifically, a mechanism that mechanically locks the manhole handle until the internal pressure of the vessel has been completely relieved.
