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Help on Portfolio Question

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Atetos:
Thanks guys !  Me remember sloooooowwwwlyyy.

M1Ark:
Three element control pertains to FW level control scheme.  Basically it is a level dominant system that uses S/G level (or Reactor Level for BWR's), Feedwater Flow and Steam Flow to make up the three elements.  The system uses Steam and Feed Flows as an anticipatory signal to give you finer control of level.  At my previous plant I had the ability to switch to single element and you could instantly see that level control was more of a coarse control (more errratic).  The three element logic scheme controls feedwater flow by adjusting a feed regulating valve or a variable speed main feed pump to adjust level.  Level dominant refers to the logic scheme swapping to level control only if it sees level outside of a specified tolerance.  Below ~25% reactor power my old plant procedurally swapped to single element control.  At my new plant this is done automatically and substitutes feedwater temp and reactor power for feed and steam flow as the anticipatory signals.  I'm not sure what NucEng sent you but I'll send you the student text for the three element control scheme when I get to work.

M1Ark

shayne:
I have seen 3 element and single element used in more applications than just FW (boiler) level control.  Although it seems that most boiler level controls use these terms, any tank that maintains level control by level error, flow in, flow out to control level could have a term of 3 element level control.

Simple terms that help me...
Single Element - Level Setpoint compared to Level Actual (Level error) to control level.

3 Element - Level error, Steam/water out compared to Water/steam in.

M1Ark
I never understood why the DCS couldn't autoswitch at power levels below 25%.  It has all the inputs it needed, and switched to single level for everything else.  (Steam flow errors, feed flow errors, post scram feed water logic, etc.)  Or maybe I'm missing something with the startup level controls since we switched to that train at powers less than 25%.

M1Ark:

--- Quote from: Shayne on Oct 14, 2006, 01:57 ---I never understood why the DCS couldn't autoswitch at power levels below 25%.  It has all the inputs it needed, and switched to single level for everything else.  (Steam flow errors, feed flow errors, post scram feed water logic, etc.)  Or maybe I'm missing something with the startup level controls since we switched to that train at powers less than 25%.

--- End quote ---

The switch for 3 element to 1 element was there prior to DCS upgrade.  It was procedurally driven and the two things changed when you went <25% power.  You no longer used a highly responsive variable speed steam driven feed pump but rather an sluggish 14 second stroke air operated Startup Level Control Valve (15% Valve in PWR speak). This change along with going to single element control made FW control characteristic very different and needed deliberate operator action to go into that mode.

With all of that said... DCS could have automated it.  I've seen DCS upgraded at two different facilities with different philosophies.  One plant tried to keep the same controller layout and fundamentally operated the plant in the same manner as before DCS for human factors reasons.  Another plant went to all digital touchscreen controllers and changed how everything was accomplished (Human factors was over-rated).

Each approach has benefits.

Fermi2:
Simple, they didn't want to pay for it. If they could have at all kept the GMACs operating they'd still be using them. I think PWRs led the way in DCS type controls simply because Level Control errors are SO unforgiving at low powers.

Sequoyah uses a Flow Error Dominant system. Any level deviation has a time delay to let flow mismatch try to correct the error first. Below a certain power it uses bypass Reg valves that control solely on level. These use some sort of digital controller. The Feed Reg Valves use a Foxboro. They transfer by slow opening the Reg Valve until the Bypass valve is closed, then popping the Reg in auto.

I remember transferring from the start up level control valve to Single Element at Fermi. It was QUITE the ride. The old plant actually had THREE modes of control.

1: Start Up Level Control: Where all feed was routed via a smaller line which had a valve controlling on level. Feedpumps were operated in manual.

2: Single Element: Feed was routed via the big feedpipes with the Start Up Level Control Valve Isolated. Feedpump Speed was based on Level.

3: Three Element, Same as number 2 except Feedpump speed is controlled by Flow error vs Level error.

Mike

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