Here's a question, when it was opened after not being opened for 10 years were any residues found on the plates? Either product residues on the product side, or minerals, or other residues on the heating / cooling sides?
If no (I would be very surprised!) then I would follow what your maintenance team says. However, typically this isn't the case....in various milk plants I've been in it is either as frequent as every 3 months up to a year. It really depends on what you run through it, how long you run it at a time, and how it is cleaned more than anything else.
The residue on the milk side was tremendous after 10 years of build up. Our inspector said he had never seen one so full of deposits. The plates also had a very unpleasant smell of burnt milk or cheese. Oddly enough my aerobic plate counts never went over 30cfu (<10cfu to 10cfu was the normal range).
I learned that the bacterial count is not a good indicator of cleanliness in the plates.
The plate pack had leaked for years but they just ran with it since it was not enough to override pasteurization.
At the second opening (10 weeks) there was discoloration on the product side indicating a protein buildup and a slight oily film in the corners. I manually washed the plates to remove any residue.
We had a variable drive installed to boost the cleaning cycle. Called in our chemical rep to test and adjust the concentrations of caustic and acid (both boosted from 1% to 1.5%)
I wanted to boost the chemical concentration even more 1x a week for a few months (we always did a double strength caustic wash and acid rinse on the farm side during bulk tank wash), but the chemical rep said the caustic wash would not be more effective above 1.5%. Is this true?
We also did a can test to time the volume of water pumped thru at each cycle, and are at our maximum.
One month later we opened it a third time to address the broken gasket we noted only a very slight discoloration on the product side (this is the beginnings of a biofilm to my eyes). Whether I didn't get the previous proteins off by manually cleaning on the previous round, the chemicals are not doing a sufficient job, or we need a larger pump on the HTST, is yet to be known.
35 minutes caustic and acid cycles each @ 72C (161.6F)
Long story, short... I want to open it quarterly for visual confirmation of the cleaning. I'm sure I will loose to maintenance wanting somewhere in the 1-5 year range. Trying to come to a compromise.
Thank you very much for your input.