Boiler LP and HP dosing
1.
Why chemical dosing required in Boiler?
Corrosive
components, especially O2 and CO2 have to be removed, usually by use of a
deaerator. Remnants can be removed chemically, by use of
oxygen scavenger. Feed water has to be alkalized to a pH of 9 or higher,
to reduce oxidation and to support the forming of a stable layer of magnetite
on the water-side surface of the boiler, protecting the material underneath
from further corrosion. This is usually done by dosing alkalis into the feed
water, like sodium hydroxide (caustic soda) or volatile ammonia.
Conditioning
of feed water / steam condensate cycle, to prevent corrosion and scale
formation in boiler tubes and headers.
The boiler feed water
treatment is normally done with the following chemicals:
·
Hardness stabilizers
·
Deflocculating agents
·
Corrosion inhibitors
·
Oxygen binding agents (e.g.
hydrazine)
·
pH value regulation (caustic soda,
ammonia)
·
Defoaming agents
Dosing of these chemicals into
the tank of boiler feed water is done using dosing systems.
(I)H.P.
(High Pressure) dosing and (II)
L.P. (Low Pressure) dosing
Corrosion
rate of M.S. surface is lowest, when pH is between 9 - 10. Dosing is therefore done,
to maintain pH. Generally solution dosed into drum under full boiler pressure.
Chemical
used - Phosphate solution (combination of di-sodium and tri-sodium phosphate)
is generally used, in concentrations from 2.5 % to 7.5 %.
·
Exact
dosage rate depends on pH variation of water in drum.
·
It
prevents scale formation from low remaining levels of hardness which may have
escaped treatment.
·
Sodium
phosphate causes the hardness to precipitate to the bottom of the boiler where
it can be blown down.
L.P. dosing –
Dissolved
oxygen in feed water causes serious corrosion. Hence "oxygen
scavengers" or "de-oxygenators" are dosed into feed water. Dosing
done into feed water stream, either at BFP suction, or at deareator, at low
pressures. Other chemicals may be also added for pH adjustment of DM water.
Chemical
used – Hydrazine Hydrate solution (concentrations of about 0.10%) and Sodium
Sulphite solution is used for oxygen scavenging purposes. Ammonia
solution (about 1 - 2 %), Morpholine and Caustic soda are used for pH control
of feed water. Other proprietary chemicals are also available for same purposes.
|
Parameters |
|||
|
Steam
generation rate Nett |
G |
kg / hr |
10000 |
|
TDS
in feed water |
T |
ppm |
0.1 |
|
TDS
permitted in boiler water |
Tp |
ppm |
25 |
|
Oxygen
in feed water |
O |
ppm |
0.02 |
|
Oxygen
permitted in feed water |
Op |
ppm |
0.002 |
|
Residual
phosphate level in boiler water |
P |
ppm |
4 |
|
Concentration
of phosphate solution |
Pc |
gm / 100 ml |
0.3 |
|
Residual
sulphite in feed water |
S |
ppm |
15 |
|
Concentration
of sulphite solution |
Sc |
gm / 100 ml |
0.04 |
|
Residual
Hydrazine in Feed water |
H |
ppm |
0.02 |
|
Concentration
of hydrazine solution |
Hc |
% |
35 |
|
HP
CHEMICAL DOSING CALCULATION |
Chemical dosed |
|
TRI SODIUM PHOSPHATE |
|
Percentage
Blow Down |
BD = T x 100/(Tp-T) |
% |
0.40 |
|
Actual
Blow Down Rate |
ABD = G x Blow down % |
kg / hr |
40.16 |
|
Loss
of phosphate in blow down water |
Pl = (P x ABD)/1000 |
gm/hr |
0.16 |
|
Loss
of TSP in blow down water |
|
gm/hr |
0.643 |
|
Tri
sodium phosphate consumption per day |
|
Kg/Day |
0.015 |
|
LP
CHEMICAL DOSING CALCULATION |
Chemical dosed |
|
SODIUM SULPHITE |
|
Oxygen
to be removed |
Or = (O - Op) x G x .000001 |
Kg/hr |
0.00018 |
|
Oxygen
to be removed in moles per hr |
Or/Mol Wt |
Kg/hr |
0.000005625 |
|
Sulphite
required in moles per hr |
|
Moles/hr |
0.00001125 |
|
Sulphite
required in gms per hr |
molecular wt (Na2SO3)x moles/h |
gm/hr |
1.4175 |
|
Rate
of dosing |
|
ml/hr |
3543.75 |
|
Sodium
sulphite consumption per day |
|
Kg/Day |
0.03402 |
|
LP
CHEMICAL DOSING CALCULATION |
Chemical dosed |
|
HYDRAZINE |
|
Oxygen
to be removed |
O - Op |
ppm |
0.018 |
|
Hydrazine
reqd (30 times depending on reaction kinetics) |
|
ppm |
0.54 |
|
Residual
hydrazine to be kept |
|
ppm |
0.02 |
|
Total
hydrazine required |
|
ppm |
0.56 |
|
Total
hydrazine required in kg/kg of evaporation |
|
Kg/Kg |
0.00000056 |
|
Hydrazine
required per hour |
|
Kg/hr |
0.0056 |
|
Hydrazine
to be dosed |
|
Kg/hr |
0.016 |
|
Hydrazine
consumption per day |
|
Kg/Day |
0.384 |
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