Posted 10 August 2009 - 04:19 PM
I fully endorse Qalander's response and comments. Not only is he taking a firm stance on a potential safety issue, he is also bring out the practical fact that the scope of the quality of the two drains is entirely different due to the type of location and the knowledge of what gravity can cause in any process.
Empirically, one will almost always find that should there exist any solids or heavy components in the tank or reactor, they will most certainly gravitate to the lowest part of the vessel - the drain valve. And conversely, the lighter and cleaner liquids will rise to the top and exit through the overflow.
Experienced field engineers inherently know that it is very unwise to rely on a single, common drain system design to handle both type of liquids. The heavier and probably solids-contaminated drain will require a totally different type of piping and drain design and should not be allowed to jeoparize the effective draining of the overflow liquids. That would be asking for trouble.