Water Purification Systems for Process Heating Boilers in Boiler India 2022

Mr Udayan Shrouti, CEO addressing Industrial Boiler users, Boiler manufacturers, Technical Consultants and officials from Directorate of Steam Boilers during recently held event BOILER INDIA 2022 from 14th September 2022 to 16th September 2022 at CIDCO Exhibition Centre, Vashi, Navi Mumbai. The topic of the knowledge session was “Water Purification Systems for Process Heating Boilers”. The session was well received and appreciated by an audience of about 600 plus technical persons working in the field of Industrial Boiler.

Water Purification Systems for Process Heating Boilers

Boilers are heat transfer devices, where in water in the form of either liquid or gaseous steam is commonly employed as a medium for the transport of heat to some distant point of use.

Water is particularly suitable because of its relative abundance, low cost, and high heat carrying capacity. It is generally, the medium of choice in most boiler applications, whether for domestic, commercial, institutional, or industrial purposes.

However, a boiler can only carry out its primary functions of transferring heat to water and (in steam generators) separating steam under pressure from water most efficiently if the quality of the various types of water used (such as makeup water, feedwater, and boiler water) are effectively and continuously controlled. The difficulty in this quality control process is that water is a “universal solvent, ” and as a result, all sources of water contain various natural concentrations of dissolved minerals and gases in addition to suspended solids and biological matter. The relative amounts of each of these impurities tend to vary considerably with geographic location and season. This phenomenon results in countless permutations of water type and quality around the world, each potentially available as a source of makeup supply to boiler plant systems, evaporators, and other forms of water heating and steam generating devices. In many industrial applications, the negative impact of these natural impurities may be further compounded by the presence of small concentrations of process contaminants.

The effect of these various impurities or contaminants is to hinder the heat transfer and steam generation processes, to adversely affect the quality and purity of steam, and to act as primary instigators in the corrosion and wastage of boiler plant system materials of construction. A wide variety of chemical reactions and physical mechanisms can and will take place, including the deposition of various crystalline and non-crystalline scales on the water side of heat transfer surfaces, the formation of sludges, metal corrosion, and carryover of contaminants into the steam. The function of boiler water treatment, therefore, is to control the waterside chemistry of boiler plant systems within certain agreed and relevant parameters and specifications. As these adverse processes are by no means limited to the boiler itself, in practice, boiler water treatment also includes pre-boiler and post-boiler functions and further requires that all the various types of water utilized are controlled through a comprehensive treatment and proactive management program. Clearly, the lack of or the use of an inappropriate boiler water treatment program creates significant operational difficulties and impacts the economics of the entire process, from start to finish.

Water is converted into gaseous form in an industrial boiler to provide heat energy in the process plant. Apart from providing heat energy to the process, the water also keeps the boiler heat exchanger portion cool and healthy. In order to ensure that water acts as a most appropriate cooling medium it is essential to ensure that water without any hindrance continuously keeps the heating area wet, corrosion and scale free.

The various impurities in water are either in the form of dissolved salts, gases or suspended solids. The various impurities that need removal are: – Suspended Solids, Turbidity, Hardness, Acidity, Alkalinity, Oxygen, Total Dissolved Solids, Dissolved Carbon Dioxide, Alkalinity, Organic matter and silica.

IS 10392-1982 is the Specification for feed water and Boiler water for low and medium pressure land boilers.

Sr. No.	Characteristics	Requirement for Boiler Pressure
1.	FEED WATER	Up to 2.0 MN/m2	2.1 to 3.9 MN/m2	4.0 to 5.9 MN/m2
	Total Hardness as CaCO3 mg/l	10.0	1.0	0.5
	pH Value	8.5 – 9.5	8.5 – 9.5	8.5 – 9.5
	Dissolved Oxygen mg/l max	0.1	0.02	0.01
	Silica as SiO2 mg/l max	————	5.0	0.50
2	BOILER WATER
	Total Hardness of filtered sample as CaCO3	——————- NOT DETECTABLE ————————-
	Total Alkalinity as CaCO3 mg/l max	700.00	500.00	300.00
	Caustic Alkalinity as CaCO3 mg/l max	350.00	200.00	60.00
	pH Value	11.0 – 12.0	11.0 – 12.0	10.5 – 11.0
	Residual Sodium Sulphite as Na2SO3 mg/l	30 – 50	20 – 30	————–
	Residual Hydrazine as N2H4 mg/l	0.1 to 1.0 (if added)	0.1 – 0.5 (If added)	0.05 – 0.3
	Ratio Na2SO4/Caustic Alkalinity as NaOH	Applicable to riveted boilers only
	Phosphates as PO4 mg/l	20 – 40	15 – 30	5 – 20
	Total Dissolved Solids mg/l max	3500	2500	1500
	Silica as SiO2 mg/l	Less than 0.4 of caustic alkalinity		15.0

The boiler feed water and boiler drum water confirming to the above standards ensures fairly trouble-free operation of the boiler. In case of the shell type boiler the limit of boiler water TDS can be relaxed up to 5000 mg/l. The total alkalinity in the boiler drum water should be about 20% of the TDS.

The various equipment’s deployed to treat the feed water are as under: –

Water Softening Plant
De Alkalizer Plant
Demineralization Plant
Reverse Osmosis Plant

Water Constituent Vs. Technology

Problem	Softening	De Alkalization	Demineralization	Reverse Osmosis
Hardness removal
pH Change
TDS Change
Dissolved Oxygen
Silica

BLOWDOWN

Even after installation of water treatment plant and regular dosing of the chemicals the concentration of the salts occurring in the boiler is limited by removal of concentrated boiler water. This process of removal of concentrated boiler water is named as blow down. There are two types of blow down in a water tube boiler namely continuous and intermittent. The continuous blow down is given to the boiler drum to ensure maintenance of total dissolve salts at level specified under IS 10392 – 1982. The intermittent blowdown is given to scrub the heat transfer surfaces. Intermittent blow down controls the suspended, dissolved solids and also prevents accumulation of sludge.

Selection Process for New Water Treatment plant

It is vitally important to know the feed water sources in terms of quantity and quality of the water. At the same time, it is prudent to ensure maximum condensate recovery. The condensate temperature is far greater than the ambient water temperature resulting in saving of the fuel. Higher feed water temperature implies lesser fuel to reach the intended steam pressure and temperature. The condensate has very low dissolved solid content. Higher condensate recovery ensures lowering of the boiler feed water TDS and assists in reducing the fuel loss attributable to the blow down.

Any water treatment plant operation has capital cost and operational cost associated with its installation and operation. The operation cost of water treatment plant consists of following: –

Raw water cost
Chemical cost
Electricity / Energy cost
Manpower cost
Wastewater treatment cost
Maintenance cost

The net cost per unit of treated water is calculated on above.

Once the net cost of treated water is arrived at savings if any to reduce the fuel cost associated with the blow down can be worked out and the most appropriate Water Purification plant can be deployed to treat the boiler feed water.

It is of vital importance to treat the boiler feed water including condensate to ensure that all the water parameters are well within the limits. Proper boiler water treatment ensures scale, corrosion and carry over free boiler operation. This leads to best boiler utilization and productivity.

Udayan Shrouti, CEO info@sunirindia.com www.sunirindia.com