RTG – Water &
Equipment Selection Utilities Criteria
Selection Criteria for CTs
Reliance sites have installed all types of Cooling Towers- Cross flow, Counter flow, CTs with wooden structures, concrete structures, mix of concrete and wood/PP and are also provided with various types of fills viz. splash trays, V bars, honey comb or compact PVC fills etc. However a requirement has been felt to standardize the selection criteria to a great extent, if not 100%, for all sites for the forth coming projects and also for any major revamping or augmentations. The counter flow cooling towers made of concrete structures most preferably loaded with splash trays, are best suited for all sites of Reliance and well accepted for large applications. Some of the major points which are required to be considered while freezing the design and selecting a Cooling Tower, are listed below:-
1) Recirculation to Hold Up Ratios:Holding Time Index (HTI) plays an important role in the water conservation at increased cycles of operation with reduced blow downs and overall effectiveness of the chemicals that will be dosed to cooling tower for cooling water treatment. HTI is a function of circulation rates, hold up volumes, cycles of concentration and delta T across the cooling tower. For a CT, say for example, operating at a COC of 10, delta T across the tower at 90 C and a target HTI of 300 hours (Maximum for best operating conditions), the hold up to circulation ratio should be equal to or less than 0.65. The CT hold up capacities (total hold up volume of water that includes sump, basins and pipe network) shall be designed in such a way that the Hold Up volume to Circulation Rate should not be more than 0.65. For example, with 5 pumps of 6000M3/hr each in operation and a circulation rate of 5x6000= 30,000M3/hr, the holdup volume should always be less than 19,500 M3. If the tower is having more than 10 cells in a single line configuration and the consumers are quite away from CT that increases the length of the piping and therefore the holdup volume of the system, it is always advisable to adopt dual cell i.e. back to back cells with a common basin to reduce the length of the
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RTG – Water &
Equipment Selection Utilities Criteria sump and its volume. Or alternatively, instead of running the sump all along parallel to cells, a smaller sump with channels connecting to each cell can also be adopted.
2) CT Axis As far as possible, the location of CT long axis should be in East West direction to avoid direct sunlight and algae growth.
3) Stand Bye Cells If the number of cells is more than 6, the CT shall be provided with 15% additional cells. 4) Fill- System The compact PVC fills, popularly known as Honey Comb or Munter’s type Should Not be selected if the circulating cooling water is having following characteristics :a) SS
: > 10 ppm( Dusty environment, movement of traffic surrounding the CT)
b) LSI
: > (-)0.2, ( high pH, high alkalinity, high Ca and total hardness, high TDS)
c) High levels of microbiological fouling, presence of high algae. d) Chances of frequent HX tube leaks and tower contamination. The Honey Comb type fills are highly susceptible for severe fouling in Industrial applications. These are fit for soft water and a make up with almost zero SS. The circulating water should be free of scaling tendency and the surrounding atmosphere should be dust free. The high efficiency of these type of fill system severely deteriorates with any one of the above factor existing. The Splash Trays made of PP are best suited for counter flow CTs. The hanging wire should be of SS 316 of 3 mm dia. 2.5 mm or even lesser dia wires are susceptible for breakage and cannot take the loads of the fills.
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RTG – Water &
Equipment Selection Utilities Criteria 5) Nozzles The nozzles should be of self cleaning type with broad openings and excellent spray pattern with a provision of water distributor cone at the outlet of nozzles opening. Some manufacturers also provide a small fan that rotates with the falling water and further distributes the water.
6) Side Louvers Side louvers are essential to avoid heavy drift losses. Whenever water is charged and fan is stopped for any reason, either for maintenance or during winter when temperatures are low, heavy drift loss takes place. A side louver collects the falling droplets and diverts them to the basin.
7) Side Stream Filters Each cooling tower shall be provided with side stream filters. The capacity of filters shall be at least 2% of the total cooling water circulation rate.
8) Provision for Chlorine Dioxide dosing A tapping of minimum 2” for circulation rates up to 12000 M3/hr and at least 3” for circulation rates above 12000 M3/hr shall be given from the chlorinated water line for generating ClO2. For the dosing of ClO2 into each cell basin, it shall again be connected to the Chlorinated water line with provision of isolation valves in each line. During ClO2 dosing direct Cl2 dosing shall be suspended.
9) Booster Pumps for Chlorine Addition To meet the requirements of power water for chlorine dosing a separate set of booster pumps (1 w+ 1 s) shall be provided. The water to booster pumps will be from filtered make up water. It should not be provided from CW supply or make up water supply headers.
10) Chlorine Cylinder Storage Sheds Chlorine cylinders shall have suitable storage facility well ventilated and protected from direct sunlight.
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RTG – Water &
Equipment Selection Utilities Criteria The sheds should have extended mono rails to lift the cylinders directly from trucks and to avoid handling by forklifts.
11) CT Fans, Shafts & Vibration Switches Cooling Tower fans shall be made of FRP. The drive shafts shall be of light weight hollow shafts made of Carbon Fiber. The fans shall be provided with accelerator type vibration switches (preferably Bentley Nevada or other equivalent make). The fan motors shall be suitable for Area Classification Zone -2.
12) DGTs/LEL detectors Hydrocarbon Detectors shall be provided at the Cooling Tower top at suitable intervals to detect the presence of any hydrocarbons in the return water from the process side.
13) Return Header The common return header shall not be routed on the top deck of the CT. Each cell shall have individual riser pipes with isolation valves rising from ground level.
14) Flushing Points For pre-commissioning activities, flushing arrangement shall be provided in the return header. There shall be two arrangements- one for flushing of the entire supply and return header and the second arrangement is for pre-cleaning and passivation. For flushing purpose, provision for a valve and an end blind shall be made at the end of the common return header. For pre cleaning and passivation, at least 30 % of the cell risers should have provision for temporary spools those will bypass the fill system and directly terminate into the basins. After pre cleaning and passivation these temporary spools shall be replaced with regular riser pipes.
15) Inspection Windows/Man Holes in the Supply & Return Headers The supply and return headers of 1000 mm and above size shall have inspection windows/man holes
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RTG – Water &
Equipment Selection Utilities Criteria
at suitable intervals for inspection and for the purpose of cleaning and removal of debris from the pipelines.
16) Basin-Sump Screens Two SS Screens first one with 25 mm opening and second one with 10 mm opening for restricting any foreign material in to the sump, shall be provided before Sluice gates.
17) Pump Suction Strainers Conical type of strainers shall be provided in the suction header of the pump to avoid transportation of any fine material to the Heat Exchangers.
18) Sample Manifold A manifold having four sampling points of 1” size and four sampling points of 1/2 “ size with suitable valves shall be provided on the return header at CT end for the purpose of installation of various monitoring devices like corrosion racks, bio film monitoring kits, ORP meters, test heat exchangers etc. The location shall be near CT preferably on the sump or at ground level suitably floored.
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