# What Can Be Changed and When?

I think we should take a quick break before applying the Andersen's Power Estimation method in our METSIM model, and we should talk a bit about what you can and should do.

Using METSIM in an advanced manner is a bit like making the transition from being a child to being an adult - you can do anything, and are supposed to apply wisdom when choosing what to do, and you can't "run to mummy" when it goes wrong.  Some are better at this than others, and all of us have our moments.

Using METSIM is supposed to help you think about and understand your process and operation better.  The more it spits me out for doing something stupid the more I like it (when I realise what I did, and why it was so wrong, and then get it right).  I imagine surfing is like that.  If you got it wrong, then lets finish it right now, in a painful way, and you can think about it and try again.

METSIM is not a chain saw.  You can't really do any worse than lose what you did since your last save (to a different filename).

Obviously we cannot change something which is going to be subsequently changed by something else (well we can, but there is no point).  And we should never try and set something that has to have a degree of freedom to solve the mass balance (for example you can only control the flow of some streams from some unit operations).

So thinking about what to change and when to change it to achieve your requirements is "an art" - or is really just common sense which is accumulated by experience and then becomes trivial when explained.

Everything in METSIM is understandable.  The timing sequence is obvious, the structure is clear to see, and if you want to you can work with it because you understand it - but equally you can just jam a screw driver through a data array and twist it around and hope it will all be OK.

We are going to change the calculated value for the KW in a crusher model using our own function to modify the METSIM function.  We need to do this after METSIM has done its calculation for the crusher and set the KW value, and before METSIM has gone on to calculate the next unit operation.

That is what CtA is for. It is "After" the unit operation calculations, but it is executed within the unit operation (and therefore before the next unit operation).