BASIC STUDY GUIDE FOR
REVISION STATUS
REV.NO. DATE DESCRIPTION
R0 92.03.31 --
R1 98.07.24 Generally revised.
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1.0 SCOPE
This basic study guide outlines various methods adopted for the Cooling Tower Make-Up System.
2.0 INTRODUCTION
The water chemistry, treatments and operational control methods discussed in this guide apply specifically to open re-circulating cooling water systems containing cooling towers. The air-water contact in the cooling towers affects, both directly and indirectly, the system water chemistry. Air borne dust is scrubbed from air , increasing suspended solids concentration of the cooling water, and the water is continuously inoculated with air borne micro-organisms. Evaporation of water in the cooling tower causes the dissolved and suspended matter in the make-up water to be concentrated in the recirculating water. These processes influence the corrosion, scaling, deposition and micro-biological fouling potential of the system. The objective of this guide is to give general overview of recirculating cooling water operations, guidelines for calculating make-up water quantity and chemical treatment. For format, structure and other guidelines for preparation of basic study refer TCE.M9-PCS-25.
3.0 SYSTEMS
3.1 The following are the various cooling water systems adopted in chemical, industrial
and power plants etc. : (Refer Appendix 1)
(a) Once-through system
(b) Open or cooling tower re-circulating system
(c) Closed loop system
(d) Recirculation system using spray ponds
3.2 In the case of once-through system, make-up water is not required. Hence this
system is not considered further, in this study. Requirements of items (b) and (c) are studied hereunder. Requirement of (d) is not covered as same is not used now-a-days because of space constraints.
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4.0 INPUT DATA
Following data needs to be collected :
(a) Analysis of the water which is to be used as make-up. Refer TCE.M1-ME-
612-202 Basic Study Guide For Water Treatment System. Analysis shall contain values of Total Dissolved Solids (TDS) in mg/l, total alkalinity in
mg/l as CaCO3, calcium hardness in mg/l as CaCO3. Due to seasonal
variation for ground water, total hardness may be more in summer. Hence total hardness and alkalinity of summer shall be considered.
(b) Maximum cooling water return temperature,
(c) Materials of construction of process plant equipment using the cooling
water,
(d) Range of cooling in cooling tower
(e) Availability of make-up water and
(f) Cooling water quality if given by process collaborator or equipment supplier
5.0 DESCRIPTION
5.1 COOLING TOWER CIRCULATING SYSTEM (CT SYSTEM)
Cooling water is pumped through a point of heat exchange or heat transfer equipment, increasing the temperature of the cooling water from recooled water temperature to hot water temperature. The hot water flows through the recirculating lines to the cooling tower deck , from which the water falls by gravity through the tower packing to the tower basin. Heat is rejected to the atmosphere by means of evaporation as cooling water droplets are broken up in the packing. Air passes upward by means of mechanical or natural draft. The rejection of heat in the recirculating water by evaporation (E) restores the cooling water temperature T1 at which the cooling cycle begins again.
In the operation of cooling tower, water is continuously evaporated and replaced with fresh make-up water. This results in an increase in the concentration of dissolved solids in the recirculating water. The ratio of the total dissolved solids in the recirculating water to the total dissolved solids in the make-up water (TDSR/TDSM) is called “Cycles of Concentration” (COC). Increase in COC results in formation of scale. To prevent the formation of scale, a portion of the
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concentrated recirculating water is blown down or bled from the system and replaced with make-up water. In addition to blowdown some water droplets containing concentrated dissolved solids are carried through the evaporation equipment in the cooling tower and lost to the atmosphere. This windage or drift loss(D) may vary from one cooling tower to another.
Thus, the following contribute to the loss of water from the cooling water system of cooling tower :
(a) Evaporation,
(b) Drift (physical carry-over of the water particles),
(c) Blowdown adopted to control concentration of dissolved solids in the circuit
and
(d) Miscellaneous losses like pump gland leakage etc.
5.2 CLOSED LOOP SYSTEM
Closed loop cooling water system is also called as primary circuit. Cooling of primary circuit water is through heat exchanger. Elements of loss i.e. evaporation and drift are not present. Loss due to occasional blowdown and pump gland leakage etc. is minimal and hence make-up for the system is very low. Since closed loop system is adopted due to process requirement and also since the make-up quantity is small, normally Demineralised (DM) water or soft water or municipal water is used for make-up depending upon process requirements. Also corrosion- cum-scale inhibitors are dosed to prevent corrosion. Make-up water quantity shall be generally 1.5% to 5% of total circulating flow.
5.3 QUALITY OF WATER USED FOR CT SYSTEM
Quality of water in cooling water circuit shall be such that it does not have tendency to form any scale on heat transfer surfaces or elsewhere. Scale forming characteristics of the circulating water can be predicted by calculating the Puckorius Scaling Index (PSI). It is also called Predictable Scaling Index. PSI is worked out for various COC based on :
(a) Total dissolved solids (TDS) X COC - Factor A
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(c) Calcium hardness X COC - Factor C
(d) Alkalinity X COC - Factor D
Factors A, B, C and D are derived from the appendices 2, 3, 4 and 5 respectively. If soft water is used, normally calcium hardness of 5 mg/l is to be considered. Calcium carbonate saturation index (pHs) and PSI are worked out as follows :
(a) pHs = 9.3 + A + B - (C + D)
(b) PSI = 2 pHs – pHeq
pHeq is taken from the Appendix 6.
If the PSI is less than 6.0, the water has scale forming tendency. Characteristics of water versus PSI can be read from Appendix 7. While designing the system, PSI shall be around 6.5.
5.4 QUANTITY OF MAKE-UP WATER FOR CT SYSTEM
5.4.1 For Chemical Plants, Industrial Plants and Oil Fired Power Plants
(a) If as per 5.3, the untreated raw water for COC upto 1.3 has scale forming
characteristic, soft water shall be used as make-up to the system. In the case of both raw water and/or softened water, used as make-up, highest cycle of concentration is selected, after calculation of PSI with cycles of concentration 1.5, 2.0, 2.5 etc. on trial basis such that the characteristics of circulation water remains non-scale forming. COC is generally limited to 3.0 to 3.5 as beyond this make-up quantity changes marginally.
(b) Cycles of concentration is related to blowdown by the following formula
E + B + D
COC = , where
B + D
COC = Cycles of concentration
E = Evaporation - taken as 1% of circulation flow rate for
every 5.56 0C (10 0F) temperature range across
cooling tower - M3/Hr.
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D = Drift taken as 0.1% of circulation water flow rate -
M3/Hr.
Make-up water to cooling tower system is the sum of E + B + D in M3/Hr. A
margin of 10% is considered for design purposes.
5.4.2 For Coal Fired Power Plant
The blowdown from the system is governed by the end use of the blowdown. For instance, in case of coal fired power plants, blowdown is utilised for ash handling system. Hence, depending upon the water requirement for ash handling system, the blowdown shall be determined and for this blowdown, concentration ratio shall be calculated and with this concentration ratio the PSI shall be determined to ascertain need or otherwise of the treatment.
Following two (2) alternatives may be considered.
(a) If, for example, the concentration ratio with the blowdown as above is 1.8
and this results in slight scaling, a lower concentration ratio of upto 1.3 may be considered and if this concentration ratio does not call for any treatment, same may be adopted, provided adequate quantity of make-up water is available to meet the increased blowdown.
(b) If lower concentration ratio as suggested in (a) above cannot be adopted
due to non-availability of water or even the lower concentration results in scaling, adopt make-up water treatment. Once the type of treatment is decided, adopt as high a concentration ratio as permissible which leads to minimum blowdown. Additional water (over this blowdown) required for the ash handling system shall be supplied from other source.
5.4.3 While selecting the maximum concentration ratio, care shall be taken to ensure that
the silica, chloride and sulphate values of circulating water do not exceed the limits given in IS 8188.
5.4.4 Normally, cooling tower blowdown is led to effluent treatment plant. Any treatment
and/or dilution to meet the pollution standard criteria at the final disposal point shall be carried out in effluent treatment plant.
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5.5.1 Based on PSI, if raw water without treatment is used, it is advisable to use filtered
water for make-up especially for chemical plants (considering small quantity of make-up).
5.5.2 In other process plants and power plants with splash type fill for cooling tower and
where large quantity of make-up is required, raw water without treatment is used as make-up based on PSI. In process and power plants where film type fill is used for cooling tower, clarified water shall be used. Side stream filtration may be adopted for both the above cases. (Appendix 1)
5.5.3 Cooling water system is dynamic system, due to variation in make-up water
analysis, temperatures and possible contamination from process and atmosphere etc. Hence in order to make sure that the system remains non- scaling and non- corrosive, scaling and corrosion inhibitors may be required to be dosed. These inhibitors are of proprietory nature and manufacturer shall be consulted for dosage etc. Manufacturer invariably carries out actual tests at site to finalise the dosage, etc.
5.5.4 In case good quality water is available in adequate quantity in or nearby the plant ,
this water can be used directly or after some treatment instead of using raw water. For example, if clarified water is to be used for process requirement, this can be used instead of raw water.
5.6 SCALE INHIBITORS
Based on PSI if raw water is to be treated, for controlling scaling, the following methods are available :
(a) External side stream or full flow treatment of the circulating water,
(b) Chemical treatment
The optimum scale control programme developed for any specific system must depend upon the make-up water composition and its availablity, operating parameters in the cooling system , the number of COC to be carried in the circulating system and sometimes on effluent considerations.
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(a) External Pre-Treatment of the Make-Up Water
This can be through ion-exchange, de-alklisation or cold lime clarifier methods. Ion-exchange method removes all the hardness and minimises scaling. However, the use of very low or zero hardness make-up is generally not desirable as the treated water tends to be corrosive and may need corrosion inhibitor. However, for small to medium cooling water systems, this can be blended with raw water.
Cold lime clarifier involves large reaction vessel and hence high initial cost. This removes most of bicarbonate hardness, which is the aim of treatment for cooling tower make-up and reduces turbidity and suspended solids also. The treated water is high in pH and may need pH control. Hence this method is normally adopted for very large cooling water systems only.
For medium and large systems, where the water contains very high bicarbonate hardness, de-alkalisation may be adopted. This is a form of external acid dosing, and consists of cation exchanger or cation-cum- softener combination and a decarbonator. A suitable corrosion inhibitor and pH conditioning equipment is needed.
(b) Chemical Treatment
In this method bicarbonate content of water is converted to sulphate form, by dosing sulphuric acid. Use of corrosion inhibitor is a must. In this method number of COC in cooling tower circuit is decided based on solubility limit of calcium sulphate. It is necessary to incorporate sophisticated pH control equipment with automation and skilled operational staff. This method is generally not considered since handling of sulphuric acid is difficult. Hence it is advisable to adopt this method only for medium to large systems.
Scale suppressant can be used without softening with due care that very high concentration of hardness and pH does not occur. In this system poly phosphates, with or without acid dosing for reduction of alkalinity, or synthetic organic material like phosphonates may be used.
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Corrosion inhibitors are almost universally used to control deterioration of carbon steel and other alloys in cooling systems. Various corrosion inhibitors
such as polyphosphates, orthophosphates, chromate, zinc, organic phosphorous compounds, molybdate are generally employed as corrosion inhibitors for cooling tower system. Mixture of corrosion inhibitors are often synergistic and provide excellent protection at costs far lower than would be required for individual components. Selection of a specific corrosion inhibitor must be based on compatibility of the inhibitor with other chemicals in the system, materials of construction, operating conditions in the system and formulation considerations in preparing the complete inhibitor package. Many of these products are protected by patents.
5.8 MICRO-BIOLOGICAL GROWTH CONTROL
Chlorine is generally used for micro-biological growth control. It is effective for the control of bacteria, algae and fungi. Chlorine may be purchased as a compressed gas, solid calcium hypochlorite or sodium hypochlorite soluion.
Refer following design guides for further details :
(a) Design Guide for Water Treatment Plant TCE.M6-ME-612-203
(b) Design Guide for Bulk Chemicals Storage TCE.M6-ME-612-205
and Handling
5.9 OTHER RECENT CONCEPTS
Pollution control regulations and shortage of water has prompted industries to adopt zero blowdown concept and recycling of water for reuse and near zero drift of cooling towers.
Recycling is done on case to case basis, by segregation of various waste water streams including cooling tower blowdown and further treatment such as clarification, reverse osmosis etc.
Zero blowdown concept is further step to the recycling i.e. not only water is recovered for reuse but the concentrated stream is totally converted to water for reuse and solid waste. This results in zero waste water from the plant.
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APPENDIX 1 SCHEMATIC DIAGRAMS FOR