Steam Condensate Pot High Level

[Ahmedmabrouk]

Dear Chemical/Process Engineers,

I need your assistance. I have MP steam reboiler with the typical control scheme. FV on the steam Line and LV on the Condensate Pot. We have a problem that the Level in this Pot is always 100 %. I reviewed the Design documents and found that the design pressure inside the reboiler ( PI on equalizing Line) is much bigger than current operating ( 10 barg vs 3.4 current).

As a result the FV is always working with low opening (9%).

Is the above problem is normal? What could be the reason for this high level? What should I check in Design to tackle this problem? Does it affect the reboiler performance?

 

[don1980]

With the reboiler steam-side operating at a pressure that is lower than expected (3.4 rather than 10 barg), this pressure (3.4 barg) is apparently too low to maintain a constant level in the condensate pot. That is, there's not enough dP across the pot's level control valve to consistently push the condensate from the pot to the condensate collection system.

Why is the reboiler operating at a much lower pressure than expected? You're either running the system at a much lower rated than the design rate, or there's too much over-design in the reboiler. It it's the latter, the designer probably used an excessively high fouling factor, or an excessively low U value, or perhaps he just purposely designed the reboiler for an excessively high over-design (net effect is that the reboiler has too much area).

The dynamics of the system are going to follow the governing equation Q=UAdT. If the Q is much lower than the design Q, then the system compensates by operating at a lower dT. The reboiler steam-side pressure falls, causing the dT to fall, until an equilibrium is established that satisfies Q=UAdT. The steam side pressure seeks out whatever value is necessary to satisfy this equation - you're not directly controlling reboiler chest pressure.

This isn't a good situation for long-term operation, but it's not necessarily a show-stopper either. This distillation system may be able to operate sufficiently (achieve the necessary purity specs) despite the poor design. Whether or not it can be tolerated long-term, depends on the system's need for precise process control. What's happening now is that the pot is full and this liquid level is likely into the reboiler itself. When this liquid covers some of the tube, it effectively eliminates that surface area - rendering it ineffective heat-transfer area. If the condensate keep rising inside the reboiler, because there's not enough pressure for it to flow into the condensate collection system, then eventually the reboiler's heat-transfer area will be lower than that necessary for maintain the demanded Q. The control system will automatically respond by opening the steam valve further, to get more dT so the Q can be satisfied. This dynamic continues, until eventually the reboiler steam side pressure gets high enough to cause flow out of the condensate pot.

You can run long-term like this as long as the control system (steam valve and condensate level valve) are tuned sufficiently. This is not the way we normally design reboiler steam systems, but that doesn't mean that this system won't work. The system is simply operating on a different set of dynamic conditions, as compared to how the designer intended it to operate.