INTERNAL TREATMENT BY BOILER COMPOUNDS:
The residual salts that are not removed by external methods can be removed by adding some chemicals directly into the boiler water.  These chemicals are known as ‘Boiler compounds’.  This method is known as ‘Internal treatment’. 
E.g)  Carbonate conditioning, Phosphate conditioning and Calgon conditioning
a)  Carbonate conditioning:   Used for low pressure boilers.  Here the salts like CaSO4   are converted to easlity removable CaCO3.  But sometimes it produces NaOH, CO2 and hence Carbonic acid.  So it is less preferred.
CaSO4  +  Na2CO3  à  CaCO3  +  Na2SO4

b)Phosphate conditioning:  Used for high pressure boiler.  No risk of CO2  liberation.   
3CaSO4  +  2 Na3PO4  à  Ca3(PO4)2 +  3 Na2SO4
Three types of Phosphate salts are used:
Na3PO4   - Tri sodium Phosphate - Used for highly acidic water
Na2HPO4 – Disodium hydrogen Phosphate – Used for slightly acidic water
NaH2PO4 – Sodium di hydrogen phosphate – Used for alkaline water

c) Calgon conditioning:
Calgon is the trade name of sodium hexa meta phosphate- Na2 [ Na4 (PO3)6].
With calcium ions it forms a soluble complex and prevents scale and sludge formation.  It is used for high and low pressure boilers.
2CaSO4 + Na2[ Na4 (PO3)6]  à  Na2 [Ca2(PO3)6]   +  2 Na2SO4