Deaerator System Capacity?

[engcentral]

Does any one know if the rated Deaerator system has to exceed the max capacity of the boiler system?

We are trying to determine if the Deaerator system has to be revamped to accomodate a new and larger boiler that will replace an existing one. A couple points regarding safety (Deaerator capacity < Boiler capacity = theoretical meltdown? if boilers went full blast for a long time) and code was brought up. Thanks in advance.

 

[HVMC]

First, one must consider the amount of water one wishes to deaerate. Included in this amount must be the worst case makeup conditions including temperature and the location of the introduction of the make up water. For example, if the makeup water is entering the DA at ISO conditions and the DA is operating at a fixed pressure, the DA could be undersized. There are modifications to the DA that can be done but I would start first with a control volume around the current DA; if you need help with this let me know.

 

[cub3bead]

DAs are relatively cheap, buy a new one sized for the max steaming rate of the new boiler.

Perhaps you will be able to shove enough boiler makeup through the existing DA but your stripping steam supply maybe inadequate to achieve the proper heating. If this happens you won't get the outlet DO2 needed to operate your boiler so you scavenger costs go up and you run the risk of O2 pitting.

The spray section of the DA accounts for about 95% of the DO2 removal and if you flood that section by increased flow your removal efficiency will drop.

 

[TBP]

Don't forget to check the rest of the system - feedpumps, make-up water system, water treatment equipment etc.

 

[gr2vessels]

engcentral,

Whilst in agreement with the previous replys, would you consider, for your new system the concept of controlled oxygen injection?
It is now the unic chance to upgrade your system and possibly get rid of the obsolete deaerator alltogether. Its not a simple change, but is in line with modern concepts and you are in control of all the corrosion issues, you can plan your maintenance, replacements, etc..(Just a suggestion..)

cheers,

gr2vessels

 

[rmw]

DA's and oxygenation are two different concepts done at different locations.

While the "A" in DA is euphamistically called air, it is in reality any dissolved gas that will be driven off within the temperature range of the DA. Sure Oxygen (from air) is the most prevalent, but there is also CO2 and other bad actors that you want to get out of the system. Then you can think about oxygenation if it suits your process.

rmw

 

[Keith53]

To answer your question, , The DA does not have to exceed the max capacity of the boiler, but it does have to meet it.

First, the DA heater section must be sized to handle the calculated rate of feedwater consumption. Second, the storage area of the DA must be sized to store a certain minimum ammount of treated feedwater.

So, determine the rate of evaporation in LBS/HR for the new boiler at 100% fire. From there establish the max flow rate required to sustain the firing rate indefinately....That is the required rating you will need for the heater section.

Then size the storage section using an old rule of thumb where the storage section must contain enough treated water to support 20 minutes of boiler evaporation at 100% fire.

You will end up with two minimum parameters....One is how many GPM the heater section must handle, and the other is how many gallons the storage section must hold when at normal controlled level(which is usually half full).

 

[Keith53]

Sorry for the incomplete first reply, but I was interrupted before I could complete it.

The DA heater section as stated above, is sized to handle GPM requirements based on max evaporation rate of boiler....The ammount of make-up has no bearing on this, so ignore it when spec'ing the DA heater size and capacity of the storage tank.

The ammount of make-up required does eventually come into play however, but that comes later when you make final 'tweeks' to the DA after it is installed.

One 'tweek' involves the heater vent line and the size of the orifice that must be drilled in the vent line gate valve disc.

On a steam heated pressurized DA, there is a vent pipe installed that must include a gate valve before the pipe exits the roof....That gate valve will be shut in normal operation, but it will have a hole drilled thru it's gate to vent the dissolved O2 that is removed in the heater section.

The hole in the gate valve is sized based on expected ammmount of dissolved O2 and other non condensable gases contained in the water entering the DA heater....It must be large enough to allow venting of those gases, but small enough to conserve heating steam....

If you are consuming a large ammount of make-up, then the hole must be larger....If you later embark on a condensate recovery project and begin reducing the ammount of make-up, then you can also reduce the size hole in the gate valve and thereby also reduce the ammount of heating steam wasted thru the vent pipe.

The best way to determine valve gate hole size is to start with an 1/4" hole, then test the feedwater leaving the DA for dissolved O2....If O2 is less than 5-7 PPB, you are good to go...If more, then drill the next large sized hole in the gate and retest....You get the idea.