A CASE STUDY FAILUR OF COOLING TOWER, TROUBLE SHOOTING & MODIFICATIONS
AUTHOR
PREM BABOO Sr. Manager (Prod) National Fe rtilize rs Ltd. India
[email protected] Mob. +919425735974 Sr. Advisor & Expert, www.ureaknowhow.com Fellow of Institution of Engineers (India)
ABSTRACT This paper intended to the damage of cooling tower of our pla nt a detail case study of the i nci dent. East side of Urea Cooling Tower of Line-II suddenly collapsed at 7.30 PM on 18.06.2007As per an eye witness mi ddle porti on of the tower collapsed first and within seconds the entire extended portion of the tower along wi th the return header collapsed. All the five fans were running after the collapse and were stopped manually. So me of the pictures of Urea Cooling Tower taken after the failure as shown in the main front page and figure No.6.The one side of cooling water collapse with totally return header of urea cooling tower of ureaII , resulted in huge production and financial loss.”What was the reason of failure behind i t?” National Fertilizers Limited has reviewed the practices fo llo wed in operation and maintenance to avoid any catastrophe in future. Some possible causes behind this collapse, like heavy vibration of return header, water hammering, failure of wooden colu mn, failure o f wood due to biological attack, failure of structure due to non-conformity standard, shifting of support column fro m RCC support, etc. Previously the return header was placed on deck. “ Later it has been modified & down to ground fl oor.” To avoi d vi bration & addi tional l oad on deck.
INTRODUCTION The cooling tower of Urea line-II plant was collapsed and plant stopped due to cooling water return header and one side of cooling tower were badly damaged. After that within week the interconnection after making preparations for Inter-connection of A mmonia Cooling Tower -II & Urea cooling Tower-II headers and for bringing down the return header, the plant was started within the week at low load operation. A ll the possible causes of damaged were well studied and remedies of these causes. Some modifications were also carried out.Main aspects to be addressed to increase the life and to avoid catastrophe. Cooling To wers used for reject ing the process heat to atmosphere. Cooling Towers in Nat ional Fertilizers Ltd. Vijaipur un its are field erected, heavy duty, splash filled, cross flow and wooden construction designed to serve cooling water system. M/S Paharpur cooling tower on inception of the projects erected all these Towers of Marley Model 6000. Normally in fertilizer industry maintenance time available is very limited. The Cooling Towers are last to stop and first to start. MAJOR COMPONENTS OF COOLING TOWER 1. 2. 3.
Wooden components like colu mns, Girt, Diagonals, Pipe supports Etc. Fasteners and anchors, connectors, Louvers, Stud assemblies, FRP Fan Cylinder, Fan Blade assembly including Gearbo x, Motor, Torque Tube and supporting structure. 4. Concrete Su mp, Concrete Pillars etc. 5. Cooling water return header WOODEN COMPONENTS Wood continues to be used in construction of cooling towers. Species selection, strength properties and preservation aspects are important criteria fo r selection of cooling tower timber. The standard specifies the strength of particular specie and preservation treatment to be followed on timber fo r cooling water towers. The standard also specifies the permissible defects for particular grade of t imber; Wood is composed of 40-50% cellu lose, 20-35% hemi cellulose, and 25-35% lignin and 5-30% natural ext ractives. Long fibers of cellulose give wood its strength whereas lignin acts as cementing agent or binder for cellulose. The extract ives are not part of structure but contribute properties such as color, odor, resistance to decay and density. These extractives contain chemicals like tannin, poly phenolics, oils, fats, waxes and resins. The maintenance of wooden column may be done as figure No.-1.
Fig.-1
WOOD DETERIORATION Cooling tower wood experiences chemical, bio logical and physical deterioration. Usually all three attack simu ltaneously. FUNGAI/ CHEMICAL ATTACK (SOFT ROT) Fungal Attack(soft Rot) /Chemical attack on wood normally results in delignification. Chemical attack on wood caused by oxidizing agent and alkaline substances present in water and its treatment. Chlorine reacts with most of the organics dissolving the lignin binder. Reactivity of various chemicals used in cooling water treat ment with lign in is as given below: O3 >Cl 2 .>Br2 >ClO2 Delignification is severe when high chlorine (more than 1pp m) and high alkanity concentrations(PH more than 8). So chlorine and alkanity deteriorates the wood affecting life of cooling tower. The fungal attack normally occurs in water distribution section. BIOLOGICAL ATTACK Bacterial Attack or Interior Attack (white rot)- So me organisms use cellulose as their source of carbon for growth and development. These organisms degrade cellulose part of wood and leaves wood rich in lignin. The wood loses its strength. Biological attacked wood becomes soft. Biological attack timber in non-flooded section i.e. Plenum area. The three types of fungi responsible for decay are: Imperfect Fungi
:
Soft Rot or Surface Attack
Ascomycetes
: So ft Rot or Surface Attack
Basidio mycetes
: Deep Rot or Internal attack
Algae such as Photosynthesis, Uni/Multicellular, Diverse Forms, Filamentous, Colonial, Plantlike
Diatoms such as A Group of Algae, Organic walls impregnated with silica. Blue Green Algae, Photosynthetic bacteria, Fungi Aerobic growth above the waterline, do not contain chlorophyll, Protozoa, Diverse group of unicellular Microorganisms. Bacteria such as Unicellular, Cells may grow attached to each other in clusters , chains , rods or filaments Require carbon source for growth Different shapes, Rods- Bacillus, SphericalCoccus,Spiral- Spirill. IRON B ECTERIA Iron bacteria also called brown rot. This phenomenon occurs at locations, the fasteners like nut and bolts are used for tightening the wooden members. The nut and bolts corroded and appears brown due to brown rot. The life expectancy of a cooling tower can be materially increased through a good program of tower maintenance. Early detection of both biological and chemical attack is mandatory for effective control. The presence of biological decay both soft and deep rot can be detected by microscopic examination that can reveal the degree of erosion, surface structure, and depth of surface attack. To prevent chemical and biological attack on wood free residual chlorine shall be less than 1 ppm, preferab ly to a range of 0.3-0.7 ppm. Chlorine should be supplemented by non-oxidizing antimicrobials to control biological attack. FASTEN ERS, CONNECTORS, ANCHORS, LOUVERS AND S TUD ASS EMB LIES Fasteners used shall be compatible with the cooling water treatment. Nor mally galvanized fasteners are being used. Once the zinc layer damage in any part of fastener become weak due to galvanic corrosion. Hence it is preferable to use SS 304 / 316 fasteners to achieve better life. The fasteners shall be tightened regularly to prevent vibrations. The stud should be push fit in the wooden hole. If need arises higher shank dia stud with proper size washer shall be used. It is preferable to use bigger size washer to increase the load area on wood. This shall reduce the end split in wood. FAN B LADE
ASSEMB LY,
GEAR BOX,
MOTOR,
TORQUE
TUB E AND S UPPORTING
STRUCTURE Fan vibration shall be monitored fortn ightly. The increase in vib ration due gearbox, FRP b lades, shall be attended on priority. Fan supporting structure shall be checked for any corrosion. The mounting structure of Fan shall be galvanized to prevent atmospheric corrosion. Stain less steel 304 structure should not be used for temperature more than 50 degrees, as the air contains chlorine also that is being dosed to remove algae. The studs can fail due to stress corrosion cracking in presence of chlorine. However SS -316 material can be used if needed to increase the life. The cost of SS-316 will be very high as compared to Galvanized / SS-304 studs. Fan blade angle should be changed keeping in view the quantity of droplets escaping to atmosphere. The Fan angle can be adjusted for lesser flo w in winter to save energy.
CONCRETE SUMP AND COLOUMNS Concrete colu mns supporting wooden columns shall be checked. The concrete colu mns should b e healthy and wooden columns should be in centre. Sump shall be checked for deterioration of concrete and exposure of reinforcement. Necessary repair shall be done if needed. In order to increase the life of concrete structure epoxy coating can be done.
COOLING TOWER RETURN HEADER Vents shall be provided on cooling water return header. This will help to vent the entrapped gases or air that enters due to exchanger leakages. NRV installed in cooling tower pump shall be healthy. Looseness of flapper on spindle leads to water hammering in system. Hammering in the return water header can lead to shifting of supports from header. “The major modification have been carried out the B oth side return Header shifted from top(deck) to ground floor now no vi bration observed on deck and this was the additional load on header with filled of water.” AS S HOWN IN THE FIG.-2
FIG.-2 EFFECT OF WATER CHEMISTRY ON WOOD Natural water is an excellent heat removal medium. Natural water contains impurities in form of dissolved gases, dissolved solids and suspended solids. Natural water is either corrosive or scale forming. Untreated water in cooling water systems results in corrosion, deposit formation, and fouling. All the three problems are interred- related. This results in reduced operating efficiency of cooling tower and heat exchangers, reduced production capacity, increased maintenance cost and sometimes-significant downtime loss. Cooling water is treated to reduce above problems of corrosion of metals, the formation of deposits, and the growth harmful microorganism. Cooling water is treated to reduce water related problems and maintain efficient system. The desired cooling water parameters are monitored precisely. The parameters maintained are as follows table No.1: -
Parameter
Unit
Normal
Maxi mum
6.8 - 7.5
8.0
PH
--
Turbidity
NTU
5.0
20
Total Hardness as CaCO3
PPM
700-800
1000
Calciu m Hardness as CaCO3
PPM
520
--
Magnesium Hardness as CaCO3
PPM
280
Total Dissolved Solids
PPM
2200Max
3000Max
Silica as SiO2
PPM
