How do you calculate Boiler HP dosing system capacity ???

Phosphate treatment is preferred in drum type boilers. In this Phosphate especially Tri sodium phosphate is added at the boiler drum which is operating at high pressure (so called HP dosing system) through HP dosing system.

If it is dosed at feed pipe, it may react with impurities and sludge may be deposited at the feed line.

Tri sodium Phosphate (TSP) is generally used for HP dosing. This is directly dosed into steam drum to remove carbonate & bicarbonate scale sludge & to increase pH. Phosphate reacts with calcium and magnesium forms less sticky, loose and non-adherent sludge instead of hard scale. This sludge remains in the boiler water in suspended condition and then, removed from the boiler through blow down Other than TSP, disodium phosphate (Na2HPO4) and monosodium phosphate (NaH2PO4) are used in boiler for dosing.

Questions & Answers on AFBC Boilers

CONSIDERATIONS & ASSUMPTIONS FOR SYSTEM DESIGN:

1. Boiler capacity, operating pressure & temperatures be 100 TPH, 110 kg/cm2 & 530 deg C respectively

2-Residual Phosphate in drum water be 8ppm (Acceptable range 5 to 10ppm)

3-Residual Hydrazine in feed water collected at BFP outlet be 0.02ppm

4-Make up water requirement is 5%

5-Boiler maximum possible TDS be 50ppm

7-Feed water TDS be 5ppm

Let us calculate phosphate consumption

Percentage of blow down =Make up % X Feed TDS / (Boiler TDS-Feed TDS)

                                                = (5/100) X 5 X 100 / (50-5) =0.55%

Water loss in blow down = 100 X 0.55/100 =0.55 TPH =550 kg/hr

Loss of phosphate in blow down water = (Residual phosphate X Blow down) / 1000

                                                                       =8 X 550 / 1000 =4.4 Gm/hr

Consumption of TSP in Gm/hr = Residual phosphate X Phosphate loss = 8 X 4.4 =35.2 gm/hr

Consumption of TSP in a day in kg = 35.2 X 24 /1000 =0.85 kg/day

That is we need to dose 0.85 kg of TSP in a day to maintain phosphate level & pH in drum water

So phosphate dosing in mg/liter or ppm = 0.85 X 10⁶ / (100*1000 X 24) =0.35

So per hour we have to dose (0.35 X 100 X 1000) / 1000 =35 gram of TSP into steam drum.

Pump selection

Considerations:

1-Pump type is positive displacement plunger type

2-Discharge pressure = Drum operating pressure X 110% =115 X 110% = 126.5 kg/cm2, select 130 kg/cm2

3-Pump will be operated at 50% (Can operate up to 75%) stroke to avoid frequent maintenance issues.

4-TSP solution is prepared twice in a day (0.88 / 2 =0.44 kg / half day)

Now we have to choose a pump for dosing TSP

There for Total solution of TSP to be dosed in Boiler water at 15 LPH @ 50% stroke (Standard dosing required is 12-15 LPH for 100 TPH boiler)

So pump rated capacity = 15 X 2 = 30 LPH (At 100% stroke)

Note:

Selection of pump stroke depends on pump capacity & maintenance reliability. So better to operate the pump at 50% stroke

Tank selection:

Select a cylindrical tank

Capacity of the tank = Maximum operating stroke X 24 =30 X 24 =720 liters

Optimum size of the tank = Operating stroke X 24 hours X 115% =15 X 24 X 120% = 432 liters

Select tank capacity = 450 litres

Note:

Pump can be operated at higher stroke by reducing the concentration of TSP in tank

Pump can be operated even at less stroke (<50%) if TSP concentration in tank is increased

But standard practice is to dose chemical at higher flow rate with less chemical concentration to get good result.

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