Question

Process storage tanks are typically incapable of withstanding much pressure or vacuum. Standard tanks are designed for a maximum of 0.1-psi vacuum and 0.2 psi pressure. That is, they can withstand only a slight vacuum of 0.1 psi below atmospheric pressure and a slight excess pressure of 0.2 psi above atmospheric pressure, so that if atmospheric pressure is assumed to be 14.7 psia, then the tanks are rated for a pressure range of 14.6 to 14.9 psia. While this may seem like a very small range, it is usually sufficient for normal operations.

The tank has an overflow line to prevent overfilling; any overflow drains into a collection vessel often called a “catch-pot”. During a routine filling operation, the operator was told to stop filling when liquid began to overflow into the catch-pot. When this occurred the operator stopped the input flow and the tank was subsequently sucked in due to vacuum. How did this occur?

Answer 1

The input flow is closed and over flow line is open. the liquid is draining into collection vessel ( catch pot). Because of gravity the liquid continue to flow in catchpot untill and unless gravitational pull is balance by vaccume created in vessel. This is similar to the phenomenon of siphoning of water from drum without using any kind of pump by the action of gravity. hence when out flow is open more liquid drain from storage tank by the action of gravity , and creating vacuum in storage tank. hence it is not recommended to do this practice in the industries.