【Q&A Question 228】December 04, 2016

The sequence for opening manholes is from top to bottom; that is, the manhole at the top of the equipment (tower or vessel) should be opened first, followed by the remaining manholes in order from top to bottom. This is to facilitate natural ventilation, prevent the accumulation of combustible gases inside the equipment, allow such gases to escape quickly and avoid explosion accidents, while gradually creating conditions for people to enter the tower (vessel). In some factories, the manholes at the top, middle, and bottom are opened first, from top to bottom. It also facilitates natural ventilation, creating conditions for people to enter the tower. Before opening the manhole at the bottom of the equipment (tower or vessel), it is also necessary to check once again whether the low-point vent valve is indeed open and that all residual material at the bottom of the equipment has been completely removed before the manhole can be opened. This prevents hot residual liquid from remaining at the bottom of the equipment, which could cause burns when the access hatch is opened. The tower (vessel) must be ventilated naturally; only after passing the testing and analysis can a work permit be issued to proceed with work inside the tower (vessel). Sampling, testing, and analysis of towers (vessels) are carried out to prevent suffocation and poisoning due to entry into the tower, as well as explosions and fires that may occur when working with residual oil and gas, thereby ensuring the safety of such work.

Removing the manhole from top to bottom facilitates natural ventilation. Upon entering the container, analyze the oxygen content and flammable gases, and issue an analysis report. And entry is only permitted after obtaining a confined space work permit. In any case, safety must be the starting point

The sequence for opening the manholes on the container during shutdown for maintenance is from top to bottom. The main consideration is ventilation; it was sealed before opening. If one opens it from bottom to top, it’s difficult to access the topmost manhole due to the high concentration of toxic and harmful gases inside. By opening it from the top first, there is no convection inside, so the toxic gases don’t easily escape, which makes it easier to open the manhole. The same principle applies when sealing it.

Answer: The sequence for opening the manholes during maintenance is from top to bottom; that is, the manhole at the top of the equipment should be opened first, followed by the manholes below it, in order from top to bottom. The reason is for safety, to prevent incomplete purging inside the tower or to avoid harm caused by the natural formation of iron sulfide.

The sequence for opening the manholes is from top to bottom; that is, the manhole at the top of the equipment should be opened first, followed by the remaining manholes in sequence from top to bottom. This facilitates natural ventilation, prevents residual combustible gases from remaining inside the equipment, allows such gases to escape quickly, and thus avoids explosion accidents.

During shutdown for maintenance, the manholes should be opened from top to bottom, as starting from the upper part facilitates ventilation of the container and helps to remove any remaining flammable or harmful gases as quickly as possible； At the same time, it is necessary to ensure that the drain or vent lines at the bottom of the container are open, so as to prevent any residual liquid or impurities at the bottom from catching fire or exploding, which could cause harm to people.

From top to bottom, it prevents air from entering the upper part of the tower from below, thus preventing the formation of ferrous sulfide in that upper area and addressing safety issues