Consequences of using "non-deaerated" demin water in condenser. 1

[Tedops]

I have powerplant that uses demin water to make-up to the deaerator. What would be the conseqences, as far as water quality and metalurgy, for the water system if we were to make-up to the condenser/hotwell instead?

 

[bimr]

Addition of undearated makeup water directly to the condenser causes extra oxygen to be introduced into the cycle. Corrosion is likely to occur. Corrosion products may also be introduced. The extent of corrosion depends on the metalurgy. In a peaking unit, the consequences are more severe than for a base load system.

 

[Tedops]

Thanks bimr....I assume corrosion would be caused by Oxidation or would the introduction of Oxygen cause a low PH in the condensate? This unit is not a peaking unit but does cycle quite a bit. If corrosion were occuring I assume I would see a rise in iron,.... decrease in PH?.... especially in the condensate or feedwater. Any other things I should be looking for?

 

[bimr]

The oxygen should not effect the pH. The corrosion caused by introduction of oxygen is from the oxygen attacking the metal.

The boiler pH is usually on the alkaline side of the pH scale.

Sometimes in industrial facilities, the return condensate has a low pH from absorption of CO2. This is not normally the case in a power plant.

Some in the power industry advocate the use of a vacuum degasifier on the demin makeup to further minimize the introduction of oxygen into the system.

It is difficult to make any further comments without knowing more about the system.

 

[cub3bead]

Introducing cycle makeup into the condenser is actually common. If it is a powerplant with a <1-3% makeup rate it is common to just add the makeup to the condenser hotwell. If it is a cogen plant with higher makeup rates then the makeup is sometimes sprayed into the vapor space above the tubes to remove the DO2. The condenser air removal system is actually pretty good at deaerating the makeup.

The Graham DO2 system incorporates a vacuum DA with the condenser to eliminate the need for a pressurized DA in the cycle. Ecolaire makes a competing design. In combined cycle units eliminating the integral DA saves money in the HRSG and improves efficieny by offering the potential of a lower stack temperature. If the S in your fuel is very low this can improve project economics significantly.

 

[bimr]

Oxygen in condensate and feedwater aggravates the corrosion of steel and copper alloys in the preboiler cycle. The best way to keep oxygen out of the cycle is to keep it out of the system with deaeration.

If demin water is introduced at the condenser, it should be sprayed in at a location that ensures good deaeration and venting.

Air leakage into the system must generally be kept to a minimum because the ability of the condenser to eliminate oxygen is fixed by design. Most condenser tube bundles can meet guaranteed oxygen levels only over the upper range (50%) of the load.

Oxygen scavengers are also typically introduced at the discharge of the condensate pumps.

 

[bimr]

http://www.graham-mfg.com/downloads/32.pdf

 

[Dodeca]

you could try spraying the water into the gas space of the condenser and any oxygen should be taken out by the vacuum raises equipment