Wednesday, 5 May 2010

ISA106: Procedural Automation for Continuous Process Operations

A new standard is being proposed, called ISA106: Procedural Automation for Continuous Process Operations
The WBF website provides a description of the forthcoming effort.

In the very early days of S88 Part 1, in October 1995 I presented a paper at the EBF in Dublin titled “Extending SP88 into continuous and semi-continuous processes.”

One point I made was that the Recipe operations for a continuous process are distributed in Space rather than time. So the sequence of process operations to make stuff happens continuously in a stream of equipment.

The essence of this is exactly the same as defined in part 1 for batch, so a continuous operation still carries out a process operation - A major processing activity that usually results in a chemical or physical change in the material being processed .

And in a continuous plant the ‘sequence’ is achieved continuously as the material passes through the equipment, whereas a batch plant carries out the sequence of actions batch by batch and may carry out many of the sequence of operations in the same equipment.

Since then I have further developed my understanding this to encompass how to make continuous processes (or discrete ones) exactly the same as batch..

The essence here is that the procedures needed to make a quantity of stuff or things as from the operation viewpoint can be identical for both batch and continuous.

This is predicated on the idea that the procedural operation of plants, whether batch, continuous or discrete can be the same. And that these procedures do not need to work real time – or at least in the time domain needed to control equipment.

Unfortunately (from an s88 purist’s point of view like mine) the procedural aspects of part 1 have been taken by many to prescribe a means of controlling equipment.

Now the concept of combining sequential control with state orientated control is a very powerful one, capable of controlling highly complex processes. But in my view these equipment sequences are not the same as the procedural level that puts together product independent equipment control to make stuff.

And I have never agreed that the procedural level as described in the original Part 1 was intended for equipment control. It was intended to provide the means to map equipment control to the operational procedures needed to make stuff. And no more.

But practise has not been that, and what we now have is a lot of systems which use recipe managers to perform equipment control - and I am convinced that this has been detrimental to many projects.

Now, referring to the Dow/ABB State based Control architecture I know it very well and I have in fact worked with Dow and Yahya to develop ControlDraw to support it even better than it used to.

But I do not believe that it is really S88 based, apart from the division of units into equipment modules (which by the way are not recipe aware) and control modules. The entire procedural level in the examples of this approach is missing, yes there are sequences to start up and shut down the equipment, but they are not in my opinion even phases as they do not perform process actions. For example, a sequence to start a distillation column is not a phase because it does not perform a process action, it just sets the column into the state where it can perform the distil process.

There is much more to say, for example about equipment requirements and routing etc. But I do ask that the ISA106 group consider what I am saying about the procedural level being an operational level rather than a control level.