40- thumb rules/standards you must know in water treatment plant

1-Clarifier (HRSCC) outlet pH is 1 to 1.5 units lesser than inlet pH or out let pH is 10 to 15% lesser than inlet pH

2-Clarifier outlet water conductivity increases by 115 to 125% than its inlet water conductivity

3-Hardness of water at clarifier outlet decreases slightly (1 to 3 ppm) than its inlet water

4-Losses in clarifier = Inlet water X 2%

5-Detention time in clarifier =The time it takes for a drop of water to travel from inlet to outlet

Detention time = Clarifier tank volume M³ / Flow rate M3/day

Actually clarifiers are designed for detention time for 3 to 4 hours. If clarifier inlet flow increases detention time reduces & vice versa

6-For reverse osmosis

• Permeate water pH=Inlet water pH X 80-85%
• Permeate water conductivity =Inlet water conductivity X 5-15%
• Reject water pH =Inlet water pH X 105 to 110%
• Reject water conductivity = Inlet water conductivity X 325 to 350%

7-Pressure drop in RO cartridge filter will be 0.1 to 0.2 kg/cm2
Water treatment plant chemicals & their functions

8-RO membrane pores opening will be 0.0001 microns

9-UF membranes pores opening will be 0.01 to 0.1 microns

10-Ro plant recovery rate = (Feed water flow-Reject water flow) X 100 / Feed water

11-Salt passage in RO = Concentration in permeate water X 100 / Concentration in feed water

12-Salt rejection in RO = (1-Salt passage) X 100

13-In a RO plant reaction time for chemicals is minimum 15-20 seconds, so dosing taps should be at least 20 meter before the RO unit (RO inlet line)

14-For SAC: Outlet water pH = Inlet water pH X 70-75%

15-For SAC: Outlet water conductivity = Inlet water conductivity X 130-150%

16-For SBA: Outlet water pH = Inlet water pH X 140-150%

17-Feed water or Boiler water TDS = Conductivity X 50-60%

18-Standard pressure drop in DM plant vessels (SAC, SBA, MB etc) is 0.4 to 0.6 kg/cm2

19-Pressure drop in UF membranes is 0.1 to 0.3 kg/cm2

20-Whatman paper required for SDI is of cellulose nitrate membrane having pore size 0.45 microns & 47 mm diameter

21-Colloidal silica is in un-dissolved form & non reactive, it is removed in clarifier

22-Reactive silica is in dissolved form, is removed in RO, SBA & MB

23-A change of 1 pH value represents the change of 10 times in relative acidity or alkalinity.

For example: pH 4 is 10 times acidic than pH 5

24-Higher pH of water causes scaling & lower pH causes corrosion

25-Acids & alkalis have tendencies of increasing or decreasing the conductivity of water

For example: Water of pH 10 has more conductivity than that of pH 7

26-In DM plant SBA is always kept after SAC:

The cat ion resin is usually located before the anion resin. If the water contains any hardness traces, then it reacts with OH- of anion resin forming Hydroxide. And hydroxide has less solubility; it precipitates in the alkali environment.

27-For DM plant if DM plant Degasser is provided if inlet water has bicarbonates > 150 ppm, if it is less then there is no necessary of degasser tower.

28-On every 1 degree rise in water temperature its conductivity increases by 4 to 5 micS/cm

29-M alkalinity should be less than 20% of TDS

30-Silica in drum water = 0.4 X OH alkalinity

31-2 X P-M = 0 or negative, indicates no hardness in water

32-Conductivity = 6 X PO₄ in drum water

50-Selected QnA for WTP operators & Chemists

33-Standard parameters of water in steam drum at various operating pressure

Sl No.	Parameter	UOM	P = up to 35 Kg/cm2	P=35-45 kg/cm2	P = 45-67 Kg/cm2	P =67-100 kg/cm2	P = >100 kg/cm2
1	pH in drum		9.5-10.2	9.5-10.2	9.5-10.2	9.5-9.8	9.5-9.8
2	Conductivity in drum	MicS/cm	500	250	150	100	50
3	TDS in drum	ppm	250	125	75	50	25
4	Hardness in drum	ppm	Nil	Nil	N	N	N
5	Silica in drum	ppm	<15	<10	<5	<1	<0.6
6	Chloride in drum	ppm	Nil	Nil	Nil	Nil	Nil
7	Oil & Grease in drum	ppm	Nil	Nil	Nil	Nil	Nil
8	Hydrazine (N2H4) in feed water	ppm	0.01-0.2	0.01-0.2	0.01-0.2	0.01-0.2	0.01-0.1
8	Residual phosphate in drum	ppm	18-20	15-18	10-15	6-10	5-7

34-Standard parameters of feed water/Condensate water

Sl No.	Parameters	UOM	Values
1	pH		8.5-8.8
2	Conductivity	MicS/cm	3-5
3	TDS	ppm	1.5-2
4	Hardness	ppm	Nil
5	Silica	ppm	0.02
6	Chloride	ppm	Nil
7	Oil & Grease	ppm	Nil

Also read Power plant thumb rules

Top-36 Thumb rules for steam Turbine

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 8 ppm (Acceptable range 5 to 10 ppm)

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

4-Make up water requirement is 5%

5-Boiler water maximum possible TDS be 50 ppm

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.

10-Tips to reduce LOI in Boilers

Boiler Feed Pumps Design factors & Pump Capacity calculation