This page concerns interpretations of S88 Part 1. Comments are very welcome. This first discussion concerns Storage Tanks.

Is a storage tank a Unit ?

"Remember that a unit performs a major processing activity, and the last time we checked, storing wasn't one of those." So they say over at Batchcontrol.com, and understandably too. Do get their book.

But hold on. Storing a material can be a major part of the life cycle of the material in a vessel. And whilst no 'processing' is happening, it is also essential for the product that all materials were correctly handled in the production process. So maybe a storage vessel IS a unit, one that controls and monitors the storage conditions.

Now there are other things in S88 that seem to suggest that storage vessels should not be defined as units:

"Units are presumed to operate on only one batch at a time. " 
But your storage tank can contain lots of batches! Our milk tank gets filled and part used and then refilled over the day. How can it be a unit?
Well, you have to rethink your batches, which we will come back to

But first lets look another way, from the physical model side. Units are made of Equipment modules and Control modules. So are storage tanks. The task of programming their controls is equivalent. They run phases, contain material etc 

What about the other criteria?

 "A unit frequently contains or operates on a complete batch of material"
"A unit may contain or operate on only a portion of a batch. "
"A unit does not operate on more than one batch at a time"
All of these can apply to a storage tank. Subject to a different view of a batch, for example:
"The batch in a storage tank is all the material that was put into it  from empty to empty."
So if the milk tank starts clean and empty, then over a day is filled from 3 tankers, then used and emptied then that batch comprises the 3 tankers milk.

So, if we accept a Storage Tank as a Unit then what sort of recipes can it run?

A Receive Store and Deliver Recipe. Yes, an entire recipe to look after the tank. (See the latest milkshake model soon for an example)
A CIP Recipe, just like the one that runs in the Mix vessel.

And what does it this mean in regard to batches?

There are probably many more than you first expected. And they can have complex relationships, One to Many, Many to One and even part to part. That's fine. S88 does not say much about it, but as you can see above that this does not mean S88 is wrong. It is just much more flexible than saying that a Storage tank is not a unit.

By the way, in some of the ControlDraw sample models you will find Storage Tanks defined as Common Resources. But in fact, these Common Resources behave exactly like units, they are virtually the same.