A manhole is installed in storage tanks, pressure vessels, drums, columns and serves as a passage and vent for personnel during installation, repair, cleaning and maintenance.
Manholes should be at least 24 inches diameter to be practical from a safety viewpoint. This has been found to be the minimum size for a man with an airpack and tools to gain safe internal access. When a manhole opening is made in a vessel or shell; the shell becomes weaker due to the amount of metal removed.
Therefore a reinforcing pad is provided by welding around the opening.
Manholes come in all shapes and sizes, but this article will briefly describe only the 2 most common in process piping.
Manhole with cover for vertical applications
This type is mounted on the side(s) of drums, pressure vessels, storage tanks or columns. Closing or opening the manhole is done by means of a blind flange connected to a swivel device. If all bolts from the flange connection blind and manhole flange removed the blind flens can be simply separated from the manhole with the swivel device.
The swivle device in pipeline terms is called a Davit. A Davit (especially for manholes) is a mechanically designed hinged guide and support system that facilitates the opening and closing of large and heavy blind flanges. This system can be designed for horizontal and vertical applications. In principle, a Davit can also be applied to other flanges such as the cover of a shell and tube heat exchanger etc.
A blind flange 24 inch e.g. class 150 is about 200kg, so no additional lifting equipment is needed, with the use of a Davit.
Manhole with cover for horizontal applications
This type is mounted on the top of drums, pressure vessels or columns. Closing or opening of the manhole is done in the same way as described for the manhole for vertical application.
Davit for Manhole
Example of Manholes in Distillation Columns
Manholes in a column with trays are normally provided at the top, below the lower tray, at the supply tray and at any other tray around which there are removable internal parts. Passage at intermediate points should be placed so that the maximum distance between two passages does not exceed 15 trays.
There are obvious and logical reasons for installing manholes in distillation columns. There must be limited access to inspect, clean and repair the column from time to time. This can be important, especially when liquids are distilled that can corrode, clog, erode or cause damage in the column.
Some rules of thumb..
This short example has hopefully given an impression of the need for manholes.
Please also read the safety article on entering of Confined spaces