There are other issues of adding make-up water to the deaerator instead of to the condensate tank. ONe is the retention time and how you are going to control putting condensate into the deaerator and how you are going to control putting make-up water into the deaerator? Typically, if water is coming from two different sources identical set of controls need to be added to the deaerator where only one set is required. This would include an additional level control and valve. On larger deaerators especially, this requires a higher purchase cost and more equipment means more spare parts and maintenance. THe added pumping cost described above is negligible. Deaerators usually have modulating feedwater valves with pumps that run continuous on the surge tank feeding the deaerator. The pump is running anyhow and would not add any additional pumping cost.
Another disadvantage is if a large amount of make-up water is added to a hot deaerator it can cause a vacuum in the tank. Although all deaerators should have some sort of vacuum protection it is not good for the system or the installation. Some vacuums can cause violent reactions with piping. Spray deaerators, generally, cannot react fast enough to properly change from hot condensate to cold make-up water for proper deaeration. Tray types are a little better but not recommended. Even the PTC for deaerators recommend steady loads. THis makes the deaerator work more efficient and keeps the system steady. Adding a large amount of cold make-up also takes its toll on required steam that comes from your main steam line. All of a sudden the dearator is demanding more steam. If it is drastic enough the deaerator will lose pressure and possibly vacuum which will make worsen the steam demand instantaneously. THis steam has to come from somewhere and will rob another place for it. The boiler certainly cannot act fast enough for an istantaneous demand. What I am trying to say is that it can effect the entire system. The best thing for a steam system is trying to keep it as steady as possible and this is true throughout the system.
As far as coatings on surge tanks... the returned condensate will also have dissolved oxygen once exposed to atmospheric conditions. Return condensate at 133F, for example, will have approximately 5 ppm of dissolved oxygen at atmospheric conditions. While it is true the make-up water will have more the tank is still being attacked. It does not take much dissolved oxygen to cause a pin hole in carbon steel. Personally, I am in favor of lining boiler feed or surge tanks. If applied properly it makes the unit last longer. Maybe the key is to have a manufacturer that can apply it properly. There are several out there with 10 year warranties. I have seen tanks not lined take 1-2 years to become pitted with oxygen.
