The Light Green Machine Institute

27 Jan 16: Mailbag

Hi Jim,
Just re-read your column on Ladder diagrams.  In my opinion there is still a place for ladder logic.  Not so much for ladder diagrams unless you have some hardwired logic.  The design of the programming should be done using Boolean logic diagrams and SAMA diagrams.  The logic diagram then gets converted into the ladder logic when programmed into the PLC or DCS and the SAMA into function block.  Ladder logic is still a good way to think about discrete equipment control.  Today's PLCs and DCSs generally have a couple of different programming languages and they can be generally used interchangeably, so depending on the part of the process you are dealing with, one can use the language that best suits that application.  I programmed the reel nip control on a paper machine and used a combination of ladder, function block, and structured text.  The problem we often run into is that maintenance people are not familiar with the newer languages and techniques and companies don't want to change because of this.  So upgrading the maintenance peoples skills is an important part of the process. 
Alan W. Martin, P.Eng.
Vancouver, B.C

I noticed that most of the Think Light ideas seem to avoid our electrical friends. If a pump can be plastic why not a motor? I realize a rotor and stator are the heaviest parts but why not the
stator casing and junction box? Every bit helps.

On another note, I've been working on a few projects recently that involve repurposing older machines (1980's and 1990's). One of the
items I've noticed is that we (collective) tend to overdesign for "what if" and "maybe" reasons and we could be doing this for no reason at all. It's not uncommon to see a machine or complex that was designed for 500 t/d bombing along at 800 t/d a few years later. Why is this? Well, there are a couple of reasons one of which are the design standards we all work to. They are too conservative for one thing and there is a tendency to add a safety margin on top of an
already conservative calculation that uses these standards. The result is a 20% design margin to start with plus another 20% built into the standards. If you apply this to a pump circuit, the pump, piping and controls are all oversized for real world use and cost more to install and maintain.

We need to rethink our standards and how we implement them.


Bryan Creagan
Montreal, PQ
Hi Jim
Impellers made of fiber-reinforced ceramic; ceramic powder-filled epoxy resin, or polysulfone thermoplastic resist hot solution corrosion and erosion.
All are castable, with specific gravity respectively 40%, 25 % and 15% that of metal impellers.  Surely the power required to spin them decreases too?
Dave Bennett
Light Green Machine Institute

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with "LGMI Frontiers" in the subject line. 

As always, your comments will be appreciated.
Think light!


Brian Brogdon, Ph.D.
Executive Director




Jim Thompson

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or Jim Thompson!

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