I am working for a client who has an evaporator operating under vacuum at 70mmHg which suffers from regular bumpovers. The liquor feed is a mixture consisting of nitric acid and water fed at approx 35C and is fed continuously. During operation, the evaporator has no liquid outlet flow and the function of the evaporator is to concentrate the feed by vaporising essentially water and nitric acid.

Could someone explain to me, why bumpovers would occur in such a system? 

Bumpover is not a word in my vocabulary, but intuitively, I think it must be what I know as carryover, or priming.  If so, I have the following questions.

What kind of heat source drives the evaporator(Pressure Reducing Valves), how is it controlled, and how is it drained or trapped, if it is steam.  And, if it is steam, what is the temperature and pressure?

I assume you are saying 70mmhg absolute rather than 70mm of vacuum below atmospheric.  It can be ambiguous.

Where does the evaporated vapor go??  How is that process controlled??  Are there any flow variations in the vapor outlet.  If it is a condenser, what type condenser is it??  If the vapor outlet is varying, the fluctuation in flow, and hence evaporator vessel pressure could be causing a change in the boiling rate of the nitric acid/water solution.

You really haven't given much information, so I am just stabbing in the dark.  Give us some more information to work with.

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