Review tank designs first
I read PLANT ENGINEERING every month. There is always good information. However, as I read the May 2005 Solutions tip titled “Changing hoses and fittings” I was alarmed. Though your “Safety First…” disclaimer does generically cover most concerns with this idea, it does not cover the mechanical design of the tank to which someone might apply this tip.
Tanks are designed with both internal and external forces in mind. So, when considering this tip, make sure your plant engineer has reviewed it in detail. Especially, the tank design for external pressure. When a vacuum pump is connected to a tank as described in the tip, the vacuum pump will create a lower pressure inside the tank than the atmospheric pressure acting on the outside of the tank.
I have witnessed a tank implosion (collapse) due to the principles of this tip. In the case I witnessed, the lower internal tank pressure was caused by a pump removing the liquid from the tank when the tank vent was not opened. As the liquid was removed, no air was allowed to fill the void. This caused a lower tank internal pressure than the outside atmospheric pressure, and the tank imploded.
The cost of replacing a tank will far exceed the time saved by improperly evaluating all potential hazards of any maintenance evolution. Even if the tank fluid is water, I would always recommend the proper Lock Out/Tag Out procedure, instead of this Tip.
Kenneth M. Smith
Plant Mechanical Engineer
Innovia Films, Inc.