Hello Clay,
What type of evaporator do you have on bottom of surge drum? Is it a shell and tube or something else as cant tell from pictures.I was thinking on two lines,one is product you are cooling have a good flow rate (bye the way what is it )?
or or is it freezing up on tubes causing poor heat transfer.Also on fridge side is evaporator logged with oil or a valve say in wet return line not opening properly.
I had ajob recently where a air cooler with a surge drum on ammonia would not pump down at all from a high level because air cooler was oil logged .The surge drum was iced up on level control column etc,
but due to no circulation of refrigerant no cooling or heat transfer took place so level would not lower in vessel.This unit had a leaking liquid line solonoid as well but isolating valve was ok.
Your hot gas low load valve could also have been leaking not helping matters.
Other questions
if the level is ok is commressor loading up and how fast?
has this system been operating ok in the past and is this a new problem ?
has any recent work been carried out which could have some link to your problems
has compressor had any oil carry over or oil return issues?
has product temps been gradually deteriorating?