Re: [foxboro] GDEV functionality

  • From: "tjvandew@xxxxxxxxx" <tjvandew@xxxxxxxxx>
  • To: foxboro@xxxxxxxxxxxxx
  • Date: Wed, 27 Sep 2006 21:02:17 -0400

Like Corey, we have converted all versions of PID,E,X,XE to PIDA's and 
are gradually converting from PLB, MTR, and VLV blocks to GDEV's.  We 
have modified the default GDEV faceplate into a generic format for each 
specific function.  One for Motors/Pumps, a different one for on/off 
block valves, and another for 3-way valves that only have two possible 
flow paths.
        My only complaint about the GDEV is associated with the motor 
start/stop functionality.  We have a lot of Hand/Auto switches on pumps 
and motors in the field and in many cases we only have one DCS COUT used 
to both start and stop a motor and one CIN for RUN Status.  Using ladder 
logic we pulsed the start and sealed around it with the RUN Status 
contact to hold the COUT until we pulsed a stop or the RUN status 
failed.  We wish the GDEV could pulse the start using the pulse option 
and then use the feedback from the run status, (.DEVLM2) to latch in 
.COUT_1 just like a seal in , (AUX), contact would do in wiring in the 
field.  If .DEVLM2 went false while the request to run, (.MANDSR or 
.AUTDSR), was still true then the GDEV would issue a Mismatch alarm and 
drop .COUT_1 and the .MANDSR or .AUTDSR request so the MTR could be 
started again without first having to pulse a stop and then pulse a 
start to restart. The MTR block has a function that is close to this but 
it doesn't work with the PULSE option.  I'd write a CAR but my past 
experience tells me that an enhancement to a block function is even less 
likely to get done than fixing a bug, so why bother.
        Brad, when you are talking about a valve with two solenoids and only 
two limit switches how do you tell if the valve is half open and how do 
you hold it in that position?  Do you wait until neither limit is made 
and then turn both the COUT's off?  We have a call for "Fail Last" 
valves that utilize two solenoids and two COUT's but have never used a 
block valve in a three position mode.
Cheers,
Tom VandeWater
Dow Corning Corp.

brad.s.wilson@xxxxxxxxxxxxxx wrote:
> Yes, I use GDEVs extensively and am converting the numerous VLV & MTR
> blocks that uninformed engineers have built over the years.  There are
> numerous advantages to GDEV, not least of which is STAIND for bitmapping to
> display elements.  However, I really wish there was 3-state capability.  As
> I stated earlier, many of our block valves have closed, open & half-open,
> and there's no way to accomplish this with a single block.  We use a CALC
> coupled with 2 CINs for valve limits & 2 COUTs for solenoids.  I suppose if
> all the valves were wired differently, I could use MCIN & MCOUT, but that's
> not the case ... aaargh.
> 
> Brad Wilson
> Process Control Engineer
> ExxonMobil Chemical Co
> Edison Synthetics Plant
> 732-321-6115
> 732-321-6177 fax
> Brad.S.Wilson@xxxxxxxxxxxxxx
> 
>  
>  
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