28 replies to this topic

[katmar]

@curious_cat - I thought that measuring the pressure close to the reboilers would have an advantage in keeping the flashing zone stable even if it required a long cable, but then realized it wasn't much better and I edited the comment out. 

 

I like your idea of moving the reboiler condensate outlet valves to a lower elevation.  The only question I would have would be the interaction between the reboiler controllers and the outlet pressure controller proposed by DB Shah. But I think that with a bit of careful tuning (or self-tuning from today's high tech controllers) it would work very well.  The reward of totally eliminating the flashing would make the bit of extra work on the control side worthwhile in my opinion. 

 

Maybe with the valves at the lower position as you suggest it would be good to move the pressure control point back to near the reboilers so that the reboiler condensate valves always "see" a steady back pressure.  This should make the reboiler control more stable.  We could spend a few fun and rewarding hours if we could get us all around a white board where we could debate this in real-time!

[curious_cat]

@katmar

 

Indeed! It's been an interesting discussion! 

 

One point I wonder is whether the active PIC at the deaerator inlet is truly needed? Point being that we don't need to keep the line pressure at a strict particular set point but we have a fairly wide band where the P can float.  All we want the pressure to do is not go below the 3 barg or so point at which it may flash.

 

Can a passive orifice or a similar device serve the purpose? That would also prevent those controllers fighting each other. Or maybe even a simple mechanical back pressure valve? 

 

Not sure. But would love your opinion. 

[DB Shah]

@Bobby Strain-Thanks for raising the valid point. We some time rush to conclusion.

 

@Katmar -"You are driving a bus..." and ends with "What is the bus driver's name?" Perfectly said. And indeed it is fun and rewarding to discuss across the board.

 

@curious cat-

 

Yes we can bring the reboiler control valve at lower grade to avoid flashing in the control valve.

 

Passive orifice or a similar device can surly work. NICE POINT.

RO is a fixed restriction element which will work within a specific range of flow, instead I will prefer a local pressure gauge followed by a globe valve. Once in a while (when reboiler load change is too high) shift operator can operate the globe valve if required to adjust back pressure.

 

However this valve should be nearest to deaerator location, can be at ground level (down stream line to be sized for flashed flow). Bringing this valve near to reboiler will again have 1600 mt line subjected to 2 phase.

[katmar]

The decision on whether to use a controller with an automatic valve, or a fixed restriction orifice, or even a manually operated regulating valve will depend on the expected variability in the reboiler duties.  I do not have any practical experience with reboilers where the heat flow is controlled by regulating the condensate out flow (post #11), but I suspect that such an arrangement would work best with a steady back pressure on the control valves.  For this reason I believe it would be worth installing the automatic controller, and controlling the pressure just downstream of the reboiler outlet condensate control valves.  The pressure control valve would still be at the inlet to the deaerator.  The 1600 m condensate line can be reduced from 10" to 6" (or maybe even smaller now that we have curious_cat's method to totally eliminate flashing) and this saving should more than cover the cost of the PIC.