HLS Help - does one need to add soda ash for "carbonate deficiency"?

[Coolergy]

Help - I am not a water expert however I am worried that our operators are being taken for a ride. We are running a Hot Lime Softener (HLS) to reduce silica and harness for boiler feed water and currently adding both lime and Magox.

The inlet water has no non-carbonate hardness - does this not mean that we need little to no soda ash per the Nalco Water handbook?

It is being argued that we will need 20t/d soda ash even though we are only using 9t/d lime in order to make up for a "carbonate deficiency".

I do not understand this given that if we are reducing the hardness and silica levels using the right amount of lime and MagOx we should not have a carbonate deficiency, no?

The only thing I can think of is maybe that as we add more lime to drive up the pH that we need caustic soda to account for the excess lime given that all the hardness in the inlet water has been "consumed"? Will soda ash even be effective for this given that we have no non-carbonate hardness?

Help! Any help would be greatly appreciated. Also anyone out there have a nice spreadsheet for HLS operation?

 

[bimr]

Adding soda ash allows you to decrease the hardness further. You are removing what is known as non-carbonate hardness.

Soda ash is Na2CO3. The calcium in the raw water will react with the CO3 of the soda ash and will then precipitate out of solution as CaCO3. The net result is to lower the hardness.

Lime is CaOH. Adding lime will not precipitate additional hardness.