XPost: sci.electronics.equipment   
   From: presence@MUNGEpanix.com   
      
   In sci.electronics.equipment Ralph Mowery  wrote:   
   > In article , presence@MUNGEpanix.com   
   > says...   
   >>   
   >> OK, so if a PLC croaks, the operators can switch to the redundant one. If a   
   >> probe of whatever you have between it and the current loop or whatever it   
   >> was fails, it's 100% manual control? How do you share the reading from one   
   >> problem? Not trying to pole holes in theory here, just actually curious.   
   >>   
   >> One thing annoying about the probes I deal with is it seems no two are   
   >> alike. Never been able to swap one for another, even with short leads, even   
   >> with three lead compensation and get the same behavior on the same   
   >> controller.   
   >>   
   >>   
   >   
   > Here is sort of how it works.  One PLC quits.  The second PLC takes over   
   > automatically or can be switched manually at any time.   
   >   
   >   
   > A TC feeding the PLCs fails.  An alarm is sounded as the PLC thinks the   
   > process has gone out of limits.  The operator is at a computer (was   
   > running a graphics program on Win XP when I left) ,  He calls up the   
   > control screen and puts the control in manual and sets the signal to the   
   > valve to where it was before the device fails.  There is a secondary   
   > temperature TC.  The operator looks back in the history of the read outs   
   > and sees that the primary control TC was showing 300.2 deg C when the   
   > lab said the process was on target.  At the same time the secondary TC   
   > was showing 301.6 deg C.  So the operator now has the control in manual   
   > and adjusts the output of the contoler to try and keep the process at   
   > 301.6 deg C showing on the readiout now.   
   >   
   > When the bad TC is replaced and the process is checked out by the lab,   
   > The new setting may come back as 301.1 deg C.  So that will be the new   
   > target.   
   >   
   > The vessels are large enough , around 10 feet tall and 5 feet in   
   > diameter so the temperature change takes a long time.   
   >   
   > The process is making polyester material.  We put in a powder that looks   
   > like flour and a liquid Glycol.  It is heated to about 300 deg C.  There   
   > are 5 vessels in the process and it is continious.  The powder and   
   > liquid are put in the first vessel and at the bottom is a pipe that   
   > conveys it to the next one.  The process is repeated and small ammouts   
   > of other chemicals are added at each stage.  It is extruded after the   
   > last vessel to what looks like string.   
   > It takes about 15 hours for the material to make it from start to   
   > finish.  We make about 3000 pounds to 10,000 pounds of material each   
   > hour depending on the size of the process line.   
   >   
   > About 2 years ago the plant that had around 3000 people 20 years ago   
   > went out of business.  Mostly because of other countries makeing the   
   > material much cheaper.   
      
   Interesting.   
      
   I met an engineer who worked at Eastman decades ago. He had good stories   
   about the complexity of starting a new production line of chemicals or   
   plastics whatever they were making at the time. The amusing part was none   
   of the people that designed the new systems could never get the first   
   batches to work at full scale. They'd have an old timer operator figure it   
   out for them after they all gave up. This process could take days.   
      
   The final "say" on the machines I deal with are the colors of test strips   
   that run through the entire process.   
      
   --- SoupGate-Win32 v1.05   
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