CHEMICAL CLEANING, CLADDING & CHEMICAL DOSING SOLUTIONS FOR POWER PLANT CONDENSERS
BY
CLADDING
CHEMICAL CLEANING, CLADDING & CHEMICAL DOSING SOLUTIONS FOR POWER PLANT CONDENSERS
BY
CLADDING
Why Cladding? Condenser is a heat-exchange apparatus and is vital for a thermal power station because the operational capacity of the thermal power plant is based on the operational efficiency of the condenser. condenser. The performance of the condenser is controlled by the various parameters such as coolant used and the vacuum achieved. Hence, the design of condensers has become complex in view of the requirement of identifying suitable materials having both good thermal conductivity and corrosion resistant. Even though a comprehensive design, however this is prohibitively expense. With this background, the epoxy claddings have become the best and cost-effective solution. The following types of corrosion take place during the op eration of the condenser: 1.) Galvani Galvanicc Corro Corrosion sion
The design of the condenser requires selection of suitable material for the tubes, having both good thermal conductivity and corrosion resistant properties. However, the material for the condenser shell shall have good mechanical properties and be compatible for welding. Consequently, the condensers are manufactured in bi-metallic bi-metallic or even tri-metallic configurations. configurations. Hence, the phenomena of galvanic corrosion occur in all parts of the condenser such as the water box, tube sheet and tubes, which are therefore eroded.
The tube sheet and tube essentially e ssentially being different metals and surrounded by hard water or sea water, galvanic corrosion takes place and the tube sheet or tube(as the case maybe) ma ybe) is eroded and crevices are formed between the tube and tube sheet. Consequently, the sea/hard water leaks through the tube sheet and mixes with the condensate.
Besides, corrosion, mechanical damages are also common, which can be caused due to poor flaring of the tube ends on the tube sheet, vibration fatigue and erosion of peripheral tubes. In-leakage of water into the steam space through tube sheet can necessitate load shedding or even emergency shut-down of the turbine. 2.) Under Deposit Corrosion
Eroded Materials from the water box due to galvanic corrosion are deposited in the condenser tube, which not only affects the heat transfer but also triggers under deposit corrosion. Consequently, d e-zincification of tubes below the deposit takes place, which become porous and pin-holes are formed. The cooling water leaks through the pin holes and affects the feed water purity, forcing the unit to shutdown. 3.) Microbial Corrosion
This type of corrosion occurs due to the presence of bacteria in the water being used in the condenser. Biocorrosion and bio-fouling can strongly influence the integrity and functionality of metallic materials employed in cooling circuits of power plants. Another important negative phenomenon affecting the biofouled metallic surfaces is the marked reduction of their heat ex change capacity, which has a direct effect on the efficiency of the thermal cycle of power plants. 4.) Stress induced corrosion
Stress Corrosion is caused by turbulent flow within the condenser. Therefore this form of corrosion is intense at the tube inlet and up to a length of 150-200 mm, as the flow around this section of the tubes is turbulent. Subsequently, the tubes get punctured and many tubes are even cut. Figure 3 shows the effect of stress erosion in which the cut tubes close to the sheet can be seen. As a result water will leak and mix with the main condensate, forcing the boiler to shutdown. The frequency of such forced shut-downs depends on the condition of the condenser tubes. Howeve r, the failure rate of the condenser tubes exponentially increases as the tubes age.
Effects of Fouling a.) Under-deposit corrosion of condenser tube is initiated, which causes puncturing of tubes b.) Layers of fouling will reduce the heat transfer, consequently the condenser vacuum will also reduce c.) Partial and sometimes total fouling of condenser tubes, restricts flow of cooling water due to which vacuum will be reduced
Materials used in Condensers The Condensers in the power plants are generally made of multi metal design. Mild Steel is used for Water Box, Partition or Deflection Plates, Coolant Inlet and Outlet pipes and the Door o r manhole covers. Mild Steel, Admiralty Brass/Bronze, Cupronickel or other special alloys are used for the Tube Sheet Construction. Mild Steel, Titanium, Cupronickel, Alumina Brass or Duplex Steel are used for the tubes. The coolant is required in huge quantities hence sea water, lake or dam water or river water is used without any treatment or with minimal treatment. Due to the above design parameters and the huge heat loads and load swings severe corrosion, abrasion, erosion, cavity formation, scoring, severe metal loss and other problems are encountered in the water box and other mild steel areas. Cavity formation between the tube ends and tube sheets, severe corrosion and scale formation are the common problems faced in the tube sheets. Due to the above problems frequent shutdown and maintenance work is necessary. These frequent shutdowns are a very costly exercise because of the following a) Loss of Power Generation in the unit during the shutdown b) Cost associated with cooling and reheating the entire system
c) Power generation loss due to inefficient operation of the condenser before shut down d) Cost of replacement materials and service charges for rebuilding the damaged areas. More importantly, the reliability and trouble free operation of the system is not ensured because of the faulty working condition of the condenser. To improve this condition and to get a trouble free operation and reliability from the condenser we suggest the adoption of the Engineered Polymer Matrix Cladding of the Condenser interiors namely Water Box, Partition plates, Tube Sheets and Tube inlets.
Cladding and allied solution The solution for the above problem is carried out primarily in three steps: 1. Chemical Cleaning 2. Application of Engineered Ploymer Matrix (Cladding) 3. Application of Chemical Dosing 1.) Chemical Cleaning Here the surface of the condenser (water-box, separation plate, tube sheet etc.) is chemically cleaned. Thereby, removing all loose particles and preparing the surface for th e application of the Cladding. This is done to ensure that the surface is smooth, even, and free of all loose particles so that the condenser can operate at higher efficiency. 2.) Application of Engineered Ploymer Matrix (Cladding) Under this process, the Cladding is applied to resist corrosion, erosion and scale formation on the surface of the condenser, post chemical cleaning during operations. The Cladding material is an epoxy based polymer matrix, which is resistant to all types of corrosion and erosion. The Cladding is applied with varying thickness across different areas of the condenser, based on the relevant effects of erosion and corrosion. With its application, the effect of corrosion and erosion of the base metal is arrested. 3.) Application of Chemical Dosing To help preserve the Cladding, it is recommended that a chemical dosage. The dosage is selected based on the water analysis report, which would be carried out prior to recommending the dosage. The chemical constituent of the dosage is plant specific and is based thoroughly on the type of water being used in the plant. The chemical dosing effectively keeps the surface free of impurities and thereby effectively extends the Claddings life as well are reduces the formation of slag on the surface.
Procedure followed for Chemical Cleaning, Cladding & Chemical Dosing 1. The entire condenser interior is thoroughly water washed and cleaned free of debris, foulants and other loose particles. 2. All the Iron portions like the partition plate, water box, Sea water inlet and outlet are inspected for integrity and suitably repaired. 3. The damaged and leaking tubes are identified and plugged suitably. 4. The necessary modifications are made for the Chemical recirculation cleaning of the entire condenser interiors. 5. By chemical recirculation cleaning the entire condenser interiors are thoroughly cleaned to satisfactory levels. This operation removes all the corrosion deposits, scales and other foulants present in the water box, tube plate and also inside the tubes safely. After draining the circulating chemical with debris and dissolved impurities, thorough water wash is given by circulation and high p ressure water jet washing. 6. The tube plate and tube ends are inspected for cleanliness and any leftover dirt is thoroughly cleaned by suitable means. 7. Now the entire condenser inside is thoroughly dried by a suitable hot air blowing arrangement to bring the moisture levels to accepted levels. 8. After thorough drying the tube ends are primed suitably to the required depth of 150 mm. and the final coating given to the required thickness. 9. Tube cover plugs are installed and the entire tube plate, Water box, Partition plate and Doors are cleaned with suitable Chloride free Garnet or Copper Slag blasting without damage to the tube interiors or tube ends. 10. Garnet or Copper Slag blasting is done with the following parameters. a. Blasting to get SA 2.5 standard cleanliness and contour. b. Temperature and Humidity controlled above dew point to prevent condensation. c. Proper isolation techniques are used to control the dust and environmental impact. 11. After the blast cleaning operation, all the blast particles are cleaned by vacuum and the cleaned surface is degreased after removing the tube cover plu gs. 12. All the areas were cladding is to be applied are primed with a specially prepared Epoxy based Primer to arrest the flash corrosion and to act as a superior penetrating bondin g agent. 13. Tube end form plugs are installed to get the right tube end profile in the cladding and the Specially prepared Lamellar Ceramic Epoxy based Cladding is applied ev enly by suitable techniques to get the required thickness to cover the flared tube ends uniformly. 14. After allowing to cure for 24 hours, the excess cladding material is cleaned and all the tube end form plugs are removed.
15. Now the entire cladded surface is coated with specially prepared Lamellar Ceramic Epoxy. The Final Cladding thickness in various parts of the condenser will be as follows: a. Over the tube sheet minimum…….. 4.00 mm ( 4000 microns) b. Over the tube ends upto 150 mm depth 0.25 mm ( 250 microns) c. Over the other areas minimum…… 1.50 mm ( 1500 microns) 16. A sample of the water being used in the condenser is collected and analyzed. Based on the report a specialized proprietary chemical dosing is recommended to maintain the c oating integrity as well as prevent future build up of slag, silt and other ph ysical impurities that develop during the operation of the plant.
Method of Application of Cladding & Chemical Dosing •
Chemical Cleaning & Cladding – Opening the doors and pressure jet washing it to remove all micro-organism growth, foulants, silt, and other foulants – The water-box is insolated from inlet and outlet mains & suitable inlet outlet sockets are welded for chemical circulation purpose. – The cleaning chemical is added along with water to fill the water-box and condenser tubes, and the circuit is closed with external circulation pump. Th is chemical circulation loosens all the scales and corrosion deposits. – Doors are opened and individual tubes are thoroughly cleaned by pressure jet washing with suitable special nozzles – The entire water-box and tubes are thoroughl y dried with hot air and then all the tubes are closed with rubber plugs to prevent the blasting medium from entering the tubes – The water-box is prepared for garnet blasting and blasted – The blasted area is cleaned with dry air to remove all the loose particles and then specially prepared Primer is applied to prevent flash rusting. – The tube plugs are removed and the tubes are coated to a depth of about 150mm with suitable primer and lining material to a thickness of 50 microns – The tube sheet is primed and cladded with S-CLAD999 to a thickness of 4000microns (4 mm). The tube ends are given a profiled contour to reduce the turbulence an d to facilitate laminar flow – The entire water-box, partition plates, doors, and manhole covers are finished with S-CLAD999 to a thickness of 1500microns (1.5 mm) – The isolated parts of the condenser are reconnecte d to – Chemical Dosage – The chemical dosing pump is supplied along with the first supply of chemicals for effective continuous dosing of chemical on the condenser inlet – Approximate Chemical requirement for 200/210 MW condenser will be 14 4litres per day diluted three times with water for effective dosing Flow rate is assumed at 30000 m3/hr and the chemical dosing is 200ml 1m3, i.e. 6litres p er hour When the above procedure is followed the coolant side problems in the condenser are completely eliminated and the cooling efficiency & back pressure of the condenser are (or in other words the differential temperature between the inlet and outlet coolant) dramatically improved. This Cladding System of the Water box and Tube Sheet of the condenser are expected to last for a minimum period of five years even under very aggressive conditions.
The above works have been successfully carried out in the following Plants: 1.) Ennore Thermal Power Station 2.) Tuticorin Thermal Power Station 3.) North Chennai Thermal Power Station
Before and after Cladding
Before Cladding
After Cladding
Water Box after Cladding
Water Box after Cladding (without chemical dosing)
Effect of Cladding on Condenser Efficiency The performance increase in the three plants is elaborated below: Plant Back Pressure Increase ∆T Increase North Chennai TPS 93% - 89% = 4% 12 – 9 = 3˚C Ennore TPS 92% - 87% = 5% 13 – 9 = 4˚C Tuticorin TPS 94% - 90% = 4% 13 – 10 = 3˚C With the above increase in performance of the condenser the payback period will be around less than one year. This is because the plant will be able to operate at full load after applying the coating, as opposed to the restricted load because the condenser will be performing at a lower operational efficiency. •
Additional Intangible benefits –
Lower frequency of maintenance shut-downs
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Increase in the Reliability of operation of the plant
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R&M costs are reduced due to less corrosion
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Increased Generation due to improved operational parameters
Pictures of Tube-sheet and Tubes Before Chemical Cleaning
Pictures of Condensers, Tube-sheet and Tubes After Chemical Cleaning
Pictures of Condenser during Application of Primer (Yellow) & Cladding (Blue)
WORK ORDERS
