FIELD OF THE INVENTION:
The present invention relates to a method of clearing of re-heater tube in boiler or more specifically relates to method of cleaning of re-heater tube for removal of whitish deposition formed inside in a simple and economic method.
BACKGROUND OF THE INVENTION:
In steam generator, steam/water is flowing through various section of tubes viz. Economizer, Water wall, roof tubes, steam cooled wall, low temperature superheater, Platen superheater, final superheater and rehater. Demineralised water is introduced to steam generator at high pressure through economizer to furnace water wall and then generated steam goes to high pressure turbine at high temperature and pressure. After passing through high pressure turbine, steam is again sent to reheater for raising temperature and then to intermediate pressure turbine then to condenser and then back to economizer. This cycle is continued for the steam generator. For maintaining the Steam quality (pH, total dissolved solid etc.) dosing is being done in boiler drum. For maintaining silica level, blowdown is to be kept open as per requirement. Make up water also carries some impurities. Reheater tubes are subjected to high temperature (>540° C) and pressure of steam (35-45 Kg/cm2) during operation. In one of the boilers, repeated boiler tube failure was faced in Reheater bottom bends. Each failure could have caused generation loss of about Rs. 2.5 Crores to 3 Crores. This huge loss due to repeated reheater failure is the need for invention/investigation to find out a process or method to avoid such occurrences. Fig.4 shows the failed and Fig. 2 illustrated the unfailed reheater tubes with deposits. The failure was due to overheating of the tube. On investigation it was found that water formed deposits inside the reheater tubes is restricting the flow, as well as retarding the heat transfer thereby causing overheating. Carryover of deposits in reheater and insufficient blow-down from drum was the reason for the deposit in pendent section. Deposits was also observed in re-heater headers (Fig. 3) upon inspection.

Prior art
JP 2016161255, the above patent describes the chemical cleaning of the boiler tubes. Oxygen treatment is done for water supply system.
However the above is not relevant to present invention as there is no chemical cleaning is used, instead the hot water is used.
Hence, there is always a need for providing a simple yet effective method which can easily clear all the contamination from reheater tube in boiler.
The present invention meets the above-mentioned need.
SUMMARY OF THE INVENTION:
A method for cleaning of reheater boiler tube comprises the steps of: -flushing reheater at a specific temperature where the hot water passed through reheater circuits such as desuperheater spray line to cold reheater header to reheater circuit tubes to hot reheater header to hot reheater pipe to hot reheater Strainer and lastly drain to high pressure drain flash tank (HPDFT) (9); wherein the cold reheater isolation valve is kept closed; - collection of inlet and outlet of water samples by initial increase of the suitable conductivity and pH values and again decreasing; - repeated flushing until the conductivity and pH at inlet and outlet become same; -performing hydrotest of reheater coils, where water samples were collected from hot reheater line at drain level; - filling the reheater circuit with demineralised water for a suitable duration and suitable pressure.
OBJECTS OF THE INVENTION:
It is therefore, the primary object of the present invention to provide a method for clearing of re-heater tube in boiler to eliminate all the whitish deposition or contamination formed inside due to overheating of tubes.
Another object of the present invention to provide a method of cleaning of re-heater tube in boiler, which eliminates the restriction of flow and retardation of the heat transfer.

Yet another object of the present invention to provide a method of cleaning of re-heater tube in boiler, which incorporates the step of soaking of the contamination for removal of the deposits.
Further object of the present invention to provide a method of cleaning of re-heater tube in boiler, which checks the pH and conductivity for complete cleaning of re-heater tubes.
Another object of the present invention to provide a method of cleaning of re-heater tube in boiler, which is simple economic and environment friendly.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
Fig. 1 illustrates structural component of reheater tube.
Fig.-2 illustrates unfailed reheater tube with inside deposits.
Fig.-3 illustrates reheater header inside deposits.
Fig.-4 illustrates failed reheater tube with inside deposition.
Fig.-5 illustrates scheme of reheater rinsing process.
Fig.-6 illustrates variation of Conductivity and pH during first cleaning.
Fig.-7 illustrates variation of Conductivity and pH during second cleaning.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The subject matter of the present invention relates to a novel method of cleaning of header reheater boiler tube or more specifically at the pendent

section of reheater tube where a whitish type deposition is formed (as illustrates in Fig 3).
The method for cleaning of reheater tube is developed to overcome the repeated failures of reheater boiler tube.
Fig 2 illustrates the unfailed reheater tube with deposits inside which will subjected to cleaning.
After the thorough research and analysis in laboratory it has found that the failure of reheater tube sample is due to the overheating and the cause of such overheating is deposition of water laden particles along with other compounds which causes substantial blockage or chocking of the boiler tubes.
The constituents which causes such blockage are given in Table 1:



The flashing or cleaning process is carried out with hot water at temperature between 80°C to 90°C.
The flashing process is carried out 35.5kg/cm2 for one hour with the hot water (as shown in Fig 1).
Fig 1 illustrates the structural component of the boiler pipe through which the hot water passes through.
The hot water is passes through reheater circuit (please provide the reference numerals and indicate at Fig 1), then to desuperheater spray line (1), cold reheater (2) and also rehetater circuit tubes (2). After that, the hot water enters shot reheater header (3) and then hot reheater pipe (4) and subjected to hot reheater strainer (5) and finally drain to high pressure drain flash tank (HPDFT) (9).

Fig 1 also indicates a cold reheater isolation valve (7) which was kept closed, and is used for cleaning the reheater circuit, where deposits are present inside the tube.
The auxiliary steam from working unit is used for heating the feed water in de-aerator.
The water of inlet is collected at BFP suction line drain (pleas indicate in Figure) and the outlet water sample are collected at hot equalization line vent (please indicate in Fig).
During the cleaning and flashing process, the optimum conductivity and pH values have been reached which are 180 µs/cm and 9.6 respectively. After reaching such values, they were found to be decreased (as shown in Fig 10).
Washing with water is continued until the conductivity and pH values of both inlet and outlet become same. Hence, pH and conductivity of hot water at boiler feed pump and hot reheater equalizer line vent were measured to ensure the complete cleaning of water of deposits.
The scheme has been shown in flow diagram in Fig 5.
Flushing was carried out for the second time and the parameters maintained were as that of the first time. Flushing was done for about 3.5 to 4 hours and during that time not much variation in inlet and outlet parameters (conductivity and pH of water) was observed. For outlet, two samples were collected, one from left hot reheater drain line at drum level and the other at hot reheater equalizer line vent.
The difference in data for variation of conductivity and pH during the first and second cleaning are illustrated in Figs 6 and 7.
Due to the dissolution properties of different deposits, hot water is used for cleaning as the dissolution is faster at higher temperature.
Deposits such as Na2O and P2O5 are highly alkaline in nature. High silica retards heat transfer leading to overheating of reheater tubes.

The reheater section is filled with hot water and soak it and keep the continuous circulation till attaining the same pH and conductivity of samples both at inlet and outlet of reheater section.
Fig 3 illustrates the reheater heater inside the deposits.
During shutdown, reheater flushing was carried out at 35.5 kg/cm2 for one hour. The water temperature was maintained at around 80°C to 90°C. Auxiliary steam from working unit was used to heat the feed water in de-aerator. Water samples at inlet were collected at boiler feed pump suction line drain and outlet samples were collected at hot reheater equalizer line vent. During the flushing operation the conductivity and pH values were observed to be increased up to 180µS/cm and 9.6 respectively and then they were found decreasing.
Different values of conductivity and pH of inlet and outlet water are given in tables 3 and 4:

Reheater cleaning was planned after unit shut down due to reheater boiler tube failure. After replacing the failed reheater tubes, hydro test of reheater coils was carried out. During draining after hydro test, water samples were collected from hot reheater line at drum level. Samples collected from hot reheater drain line showed the conductivity of 445 µS/cm and pH of 10.0.
Demineralised water was filled in reheater circuit and kept for a period of 1 hour for soaking (with pressure of 30 kg/cm2). Then water was drained. The quantity of demineralised water required for one filling of reheater circuit was about 110 tons.
Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the method of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.

WE CLAIM:
1. A method for cleaning of reheater boiler tube comprises the steps of:
-flushing reheater at a specific temperature where the hot water passed through reheater circuits such as desuperheater spray line (1) to cold reheater header (4) to reheater circuit tubes (5) to hot reheater header (6) to hot reheater pipe (7) to hot reheater Strainer (8) and lastly drain to high pressure drain flash tank (HPDFT) (9);
wherein the cold reheater isolation valve is kept closed;
- collection of inlet and outlet of water samples by initial increase of the suitable conductivity and pH values and again decreasing;
- repeated flushing until the conductivity and pH at inlet and outlet become same;
-performing hydrotest of reheater coils, where water samples were collected from hot reheater line at drain level;
- filling the reheater circuit with demineralised water for a suitable
duration and suitable pressure.
2. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the temperature of hot water maintained at 80°C to 90°C during reheater flushing.
3. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the inlet water samples are collected at boiler feed pump (BFP), (not shown) suction line drain and outlet water samples were collected at hot reheater equalizer line vent (not shown).
4. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the conductivity and pH values were increased up to 180µs/cm and 9.6 respectively.

5. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the duration of flushing operation is 3.5 to 4 hours.
6. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the demineralised water was kept for a duration of 1 hour with a pressure of 30kg/cm2.
7. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the pH and conductivity of hot water at boiler feed pump and hot reheater equalizer line vent were measured for checking of complete cleaning of water formed deposits.
8. The method of cleaning of reheater boiler tube as claimed in claim 1, wherein the reheater flushing was carried out at 35.5 kg/cm2 for a duration of one hour.