[ayan_dg]

What is the purpose og Gauge Hatch provided in tanks ?

[djack77494]

Despite the wonders of modern instrumentation, there are times when you really need to physically "see or feel" a process variable. A gauge hatch on the roof of an atmospheric tank provides you with the means of doing so. They often have a quick opening hatch so you can conveniently open the hatch, drop your measurement device (often a calibrated "stick") into the tank, and quicklu obtain a physical measurement of the tank's level. These hatches could be used for other purposes as well.

[Kyan]

Say that Hatch (thief hatch) is quite useful when you have a obligation for sampling from the top of oil tank (believe me this is rule in Russia) and when you are also obliged to provide blanketing with inert gas. Stillpipe perforated within tank, with perforation below usual oil level, that asks for oil shipment.
Thief hatch is the quickest way to do this sampling and applicable stillpipe would reduce oil contamination during the sampling.