Re: Handling of ramps and DVs in DMC sub-controllers

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Posted by Venky Venkataraman on March 04, 2003 at 14:18:44:

In Reply to: Handling of ramps and DVs in DMC sub-controllers posted by Keith Landells on February 20, 2003 at 18:54:18:

: I'd like to see if others agree with some of our thoughts on :

: 1. Ramps in a sub-controller. Even though a ramp is assigned to a selection of subs, like any CV, if the ramp is imbalanced the whole controller switches off. It is then possible to re-commission the subs not involved with the ramp. This seems inconsistent to me.

: 2. It is not possible to assign DVs to a sub-set of the sub-controllers. Hence, if we make a DV critical it switches the whole controller off, even though it may only affect 1 of the subs within the controller. It would be nice to assign the DV within Build to 1 or more subs, and use this to switch off parts of the controller if the critical DV is bad

: Have we missed something ? I'd like to understand if there are issues we've missed.

1. Yes you are right about the fact that sub-controller containing the ramp can be turned OFF and the rest of the controller commissioned. The only reason why that can be an advantage is during normal operations, if the valve (flow) that controls the ramp is sticky or has gone bad, we can turn OFF the sub and keep the rest of the plant ON DMC. Sometimes there is no telling when they will fix the valve.

For that very reason you mentioned, commissioning the ramp – sub controllers first seems to make commissioning go a whole lot smoother IMHO.

2. IMHO - DV’s should also be assigned to sub-controllers similar to MV’s. This is one of the issues that was brought up in the users group. Quote below is comment from Aspen on this issue from WUG voting form.

Each MV is required to be in a subcontroller, and only in one subcontroller. It does not make sense to force every FF to belong to one and only one subcontroller. Changing this is difficult because it changes some fundamental assumptions about the implementation of subcontrollers.

You might already know the work around, but nevertheless thought that I might add it. The temporary work around is to not make the DV critical, but to look at the INDSTA of the DV and then write a CCF logic to turn OFF the sub-controller (in the DCS, just to ensure the MV’s shed) when INDSTA = -1 or -2. INDSTA for a FF is –1 when it is bad or operator has turned it OFF and –2 when the engineer switch is turned OFF.

Venky

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Comments:
: : I'd like to see if others agree with some of our thoughts on :

: : 1. Ramps in a sub-controller. Even though a ramp is assigned to a selection of subs, like any CV, if the ramp is imbalanced the whole controller switches off. It is then possible to re-commission the subs not involved with the ramp. This seems inconsistent to me.

: : 2. It is not possible to assign DVs to a sub-set of the sub-controllers. Hence, if we make a DV critical it switches the whole controller off, even though it may only affect 1 of the subs within the controller. It would be nice to assign the DV within Build to 1 or more subs, and use this to switch off parts of the controller if the critical DV is bad

: : Have we missed something ? I'd like to understand if there are issues we've missed.

: 1. Yes you are right about the fact that sub-controller containing the ramp can be turned OFF and the rest of the controller commissioned. The only reason why that can be an advantage is during normal operations, if the valve (flow) that controls the ramp is sticky or has gone bad, we can turn OFF the sub and keep the rest of the plant ON DMC. Sometimes there is no telling when they will fix the valve.

: For that very reason you mentioned, commissioning the ramp – sub controllers first seems to make commissioning go a whole lot smoother IMHO.

: 2. IMHO - DV’s should also be assigned to sub-controllers similar to MV’s. This is one of the issues that was brought up in the users group. Quote below is comment from Aspen on this issue from WUG voting form.

: Each MV is required to be in a subcontroller, and only in one subcontroller. It does not make sense to force every FF to belong to one and only one subcontroller. Changing this is difficult because it changes some fundamental assumptions about the implementation of subcontrollers.

: You might already know the work around, but nevertheless thought that I might add it. The temporary work around is to not make the DV critical, but to look at the INDSTA of the DV and then write a CCF logic to turn OFF the sub-controller (in the DCS, just to ensure the MV’s shed) when INDSTA = -1 or -2. INDSTA for a FF is –1 when it is bad or operator has turned it OFF and –2 when the engineer switch is turned OFF.

: Venky

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