Systems generally are limited either by overhead condensing
capability or tower capacity. If the overhead system can control
pressure within the operating margin, it isn’t a limit. A quick
review of control parameter ranges (for example, valve position)
in the overhead system will rapidly identify if you have an
overhead system limit. In general, most towers will eventually
flood as pressure drops. However this isn’t always true, so be
careful. In the unusual case where flooding isn’t a concern, you
can safely reduce pressure. In the more common situation where
tower flooding becomes more of a problem at lower pressure, you
need to know how to keep out of flood. This requires monitoring
pressure drop across the tower’s limiting section, so you can
operate as close as possible to tower flood (and at the lowest
and cheapest pressure). Direct measurement of pressure drop is
most common, but the flow of the vapor  or liquid by internal
restrictions also can provide such data. Are you controlling
energy consumptio