ISA88 Batch Control discussion: Part 1, number 6: What's in a process cell

More on ISA88 terms: What goes into a process cell? Dave, I don’t know how to start on this without putting my foot in it, so here it is with my foot planted firmly in the middle. I recognize that you are trying to get terms straight, but I couldn’t disagree more with the interpretation you are putting on the process cell. By limiting it to the equipment that is needed for a single control recipe, you are complicating life immeasurably.

Let the control recipe use what it needs

If it fits one control recipe ,that implies that it doesn’t fit all of the control recipes that might use that equipment. I guess that explains why you were talking in a recent meeting about sharing equipment between process cells. If the next control recipe that comes down the pike uses equipment that is not a permanent part of the process cell, you have to go through complications to make the required equipment available for that next control recipe that needs the equipment. Why not put all of the equipment that can be needed by likely control recipes that might be run and let the control recipe simply use the equipment it needs.

The process cell, in my view, should be determined largely by the operational aspects of the process and the span of control of the people who are responsible for operating it. The boundaries may also be influenced by the span of control of the automation system that drives it even though in today’s world that should not be a big problem.

Likewise, considering the meaning of the term “Train” to the degree that it influences the process is a waste of time. The term “Train” came from the era before real automation and was nothing more that a convenient way to refer to the general layout of equipment that tended to be used together. An example might be clearer.

Example: Plant with four reactors

In a plant I know, we had four reactors. Because two of the reactors were operated by one crew and the other two were operated by another crew we had two very similar process cells. Though relatively close together, the equipment was separate enough that neither process cell used equipment in the other process cell. Within the process cell the A reactor had a primary feed tank that was pretty well hard piped to the A reactor much like the B reactor feed tank. Based on that we could talk about the A train or the B train because those were the limiting equipment that had to be used together by virtue of the way the things were piped up. However – and a very big HOWEVER, a recipe running on the A reactor would probably use the A feed tank but there were a lot of other tanks and other equipment that could be used with both the A and the B train. Calling those things trains did not limit anything, it was simply a way to refer to the central equipment related directly to the A or the B reactor. Trains are left over from the early days of characterizing process equipment – way before we settled on the process cell as a super important boundary that really means something.

Likewise, the term “Path” was not intended as a limiting definition, it was intended to be the definition of the equipment that was used in the execution on a single control recipe. It is a way to define (AFTER THE FACT) the – lets call it what it is – path a specific control recipe ended up using in its trip through the process cell.

All equipment available to make products

Now – why do we disagree? The process cell contains all of the equipment we choose to make available to the people and automation systems that they can use to make products. The more flexible and complete the equipment is, the more capability the people and automation systems have in the use of the equipment. The boundaries are really there because a process cell can’t be infinite and the capabilities of the people and automation systems need to be adequate but can’t be infinite either. If it has enough equipment, then a broad array of recipes can run on the process cell simultaneously without a problem and terms like train and path are only useful as an easy way to refer to things that might be of interest. A process cell can run any number of recipes at the same time and might even run a recipe that makes up a solution that is used by several other recipes. Why hobble something that flexible.

Lynn W. Craig

Read prior posts in this discussion series. Add your comments below.

5. ISA88 Batch Control discussion: Part 1, number 5, multiple trains, paths - more diagrams

4. ISA88 Batch Control discussion: Part 1, number 4, look at the scope of operator control, advises Dennis Brandl

3. ISA88 Batch Control discussion: Part 1, number 3, better as 1, says Paul Nowicki

2. ISA88 Batch Control discussion: Part 1, number 2, either approach, says Lynn Craig;

1. ISA88 Batch Control discussion: Part 1, number 1, questions from Dave Chappell (includes diagrams)