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Re: DA operating level

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Posted by David Anton, Sr Power Engineer on April 14, 2004 at 15:05:57:

In Reply to: DA operating level posted by JM on February 27, 2004 at 10:38:16:

: Our department is having a discussion.
: Looking for others thoughts as to how critical Deaerator storage tank level (Spray type DA) is.

: Here's the Theories:
: 1) Water level needs to be high enough to maintain proper NPSH to feedwaterpumps, but not above the scrubbing column. Essentially storage capacity. On/off level control ok if maintained within those ranges

: 2)Critical because that's where someone put the line on the tank some time ago

: 3) Steady operating level is critical to heat transfer and flash steam venting.

: Anyone have any thoughts or other ideas?
There are a number of good issues here. Here are my thoughts based on many years of experience with DA units.
1 - You can't change the height, some other engineer already decided that matter for you and you probably wish it was 5 or 10 feet higher. So determine the minimum level that you can safely operate your pump at. Let's say that this is 30% fill level for your DA storage tank. Now define the maximum level that you can have in the storage section (let's use 80% for this discussion). This is especially critical for large DA units with trays. Large units should also have a high level dump valve for protection. Allow for sufficient disengagement space. This is critical in case of a loss of steam pressure which will result in a surge in the storage section.
Thus you have determined the control range for your level control. (Much like you would with a boiler).
2 - Design your flow control into the DA like a boiler 3 point level control. You don't want on/off or fixed point level control. Use your boiler steam flow as the basis with DA level as your trim. You want slow gradual changes in flow to the DA unit. WHY? Because the DA unit is essentailly a stripping column and anyone who has operated a stripping column knows you want to maintain an equilibrium and all changes should be gradual. So let the DA level be set at 60 - 70% (not an actual recommendation just for this discussion). This allows the level to vary from 30% to 80% with the SP schewed toward the high end. I like to run a DA unit at 66% to 75% of the span. 30% + .7(80% - 30%) = 65%
3 - Tightly control your DA pressure. This is much easier when you make gradual changes in water flow. Once again just like a stripping column you want the unit to operate at constant pressure. While most of the deaeration is achieved by heating the water to its boiling point and increasing the surface area of the water (spray atomization / trays / etc) the final removal is achieved by steam scrubbing. This is one reason why it is important to maintain a steady steam pressure. The scrubbed gases are removed by venting a small fraction of the supplied steam. Typically about 0.75% to 1.00% of the supplied steam must be vented. This completes the mechanical method of deaeration and the removal efficiency is typically very good. The water should now have an oxygen concentration of less than 0.005 cc/liter or about 5 ppb. The rest will be removed chemically, sulfite is the preferred method for boilers operating below 1000 psig.

Keep in mind that that the DA water is stored energy. If your DA is operating at 15 psig (H = 219 Btu/lb) and because of poor controls (level and steam pressure) the pressure drops to 5 psig (H = 196 Btu/lb)about 2.4% of the water will boil. In some cases, engineers will wrongly determine that there is non-sufficient head and thus increase the DA level SP causing other problems such as DA flooding and more instances of pump cavitation. THe DA is a process vessel so run it like a process and you will not be sorry.

Hopes this helps and respond back if you have any questions.

FYI - I am a PE and do consulting in the following areas.
Incoming raw water treatment Clarifiers, sand filters, and biotreatment processes
DI Water Systems Ion exchange, softeners, reverse osmosis
Deaerators Maintenance and optimization of these units.
Cooling Tower & Boiler Water Treatment programs
Water Metrics and Conservation
Energy Conservation
Air compressors and dryer

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