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OPERATION AND MAINTENANCE
A case study on
SRI SAI PUMPS AND ENERGY SYSTEMS (P) LTD Administrative Office: House No. 5/11, I Main Road, A-1, Raja Colony, Cantonment, Tiruchirapalli-620001 Email: [email protected] web site: www.saienergy.com
BED COIL FAILURES
OCTOBER/NOVEMBER 2015
Introduction
Based on one of our customers’ request, a team of our technical experts jointly visited the plant along with OEM to study and analyze the cause for frequent failures of boiler bed coils during October 2015.
Type of boiler
The subject boiler is situated in Tamilnadu and is a process steam boiler of bi-drum, natural circulation, AFBC type, coal fired, industrial boiler of capacity 14 TPH, working pressure 21 kg/sq.cm(g). Pressure parts of the boilers are Economiser, Steam and Mud drums and Bank tubes. Required water is outsourced and treated in the boiler system.
Observation
After collecting various data and study of parameters which were being maintained by customer, the following observations were made:-
1. 5 Nos. of Bed coil failures occurred. Sample tubes were visually inspected.
2. Scale formation was found in the bed coil header on inspection after cutting hand hole plate.
3. Mud drum and steam drums were inspected by opening the man hole doors and heavy scale formation was found inside the mud drum.
4. Economiser tubes and outlet headers were found with pitting and corrosion due to scale formation extending the damage up to mud drum.
5. Dosing tank was a top open plastic barrel and heavy contamination was noticed. The tank outlet was connected to the top of deaerator storage tank.
6. No stir arrangement either manual or automatic was available for the dosing. This would have resulted in improper mixing of chemicals used for dosing.
7. Feed water temperature was maintained around 85-90 deg.C at ECO inlet as per log readings.
Root cause
On examination of failed tube samples and other operating data, the team has concluded that the failures were due to:-
1. Hardness of Feed Water was maintained between 10 and 34 ppm in April ’15 and in subsequent months the same was between 2 and 8 ppm. These are as against the recommended values of 0.5 to 1 ppm as per OEM.
2. pH in Feed Water was maintained between 7.9 and 8.8 in April ’15 and in subsequent months the same was between 8.5 and 8.8 as against the recommended value of 8.8 – 9.2.
3. Feed water temperature was 85-90 deg.C as against the recommended value of above 100 deg.C.
Suggestions
The following suggestions and rectification processes were recommended by the team.
1. The boiler shall be chemically cleaned effectively to remove scales and deposits immediately.
2. Existing dosing system shall be replaced with a new skid mounted dosing system with a motorized stirrer for proper mixing of chemicals.
3. Dosing point shall be connected to the pump outlet instead of Deaerator tank top.
4. Feed water sample point shall be provided at Boiler Feed Pump delivery.
5. Feed water and Boiler water quality parameters were submitted to customer which are to be maintained during operation
6. The boiler shall be cleared for operation after chemical cleaning.
Inspection after cleaning
Based on the recommendations, the boiler was chemically cleaned and inspection was carried out after cleaning by the team on 18.11.2015. The findings are recorded as below:-
1. Steam drum was inspected and metal surface found okay. Loose scales were found which were to be removed and cleaned by using RO water before closing.
2. Mud drum was inspected and metal surface found okay. Loose scales were found which were to be removed and cleaned by using RO water before closing.
3. Bed coil header was inspected by opening hand hole plate and found okay. Loose scales were found which were to be removed and cleaned by using RO water before closing.
4. Two numbers of bulged bed coil tubes were cut. Inside surface of the particular coils was found normal.
5. Also it was recommended to customer to take care of all the recommendations given during prior inspection shall be completed and then the boiler shall be cleared for operation.
6. It was also recommended to adhere to the general water chemistry guidelines and to maintain the parameters as given below
Feed Water & Boiler water Characteristics as per IS: 10392-1982
1.FEED WATER Parameters
Upto 20
Kg/cm2
21 Kg/cm2 to
39 Kg/cm2
40 Kg/cm2
to 59 Kg/cm2
Unit
Total Hardness <10 <1.0 <0.5 ppm as CaCO3
pH Value 8.5-9.5 8.5-9.5 8.5-9.5 Dissolved Oxygen 0.1 0.02 0.01 As ppm
Silica 5 0.5 As ppm SiO2
2. BOILER WATER
Parameters
Upto 20 Kg/cm2
21 Kg/cm2to 39
Kg/cm2
40Kg/cm2
59Kg/cm2
Unit
Total Hardness Not Detectable Not Detectable Not Detectable Total Alkalinity 700 500 300 As ppm CaCO3 Caustic alkalinity
350 200 60 As ppm CaCO3
pH Value 11.0 o 12.0 11 to 12 10.5 to 11.0 Residual Sodium Sulphite
30 to 50 20 to 30 -- ppm as Na2SO3
Residual Hydrazine
0.1 to 1.0 0.1 to 0.5 0.05 to 0.3 ppm as N2H4
Ratio Na2SO4/Caustic Alkalinity (as NaOH)
Above 2.5 Above 2.5 Above 2.5
Ratio Na2SO4/Total Alkalinity(as NaOH)
Above 0.4 Above 0.4 Above 0.4
Phosphate 20 to40 15 to 30 5 to 20 ppm as PO4 Total Dissolved Solids
3500 2500 1500 ppm
Silica <0.4 of Caustic Alkalinity
<0.4 of Caustic Alkalinity
15 As ppm SiO2
CONCLUSION:
The team’s observations, recommendations and final inspection data are furnished for the benefit of our engineers in this case study. Photographs taken before and after chemical cleaning are also furnished for information below.
CONDITION OF FAILED BED COIL SAMPLE PIECE
CONDITION OF FAILED BED COIL SAMPLE PIECE
CONDITION OF FAILED BED COIL TUBE INSIDE
FAILED BED COIL INSIDE CHOKED WITH LOOSE SCALES
BEFORE CHEMICAL CLEANING
STEAM DRUM INSIDE CONDITION
CONDITION OF MUD DRUM INSIDE
ECO HEADER CONDITION
COLLECTED LOOSE SCALES FROM BED COIL HEADER
IMPROPER DOSING SYSTEM WITH OPEN BARREL.
AFTER CHEMICAL CLEANING
MUD DRUM INSIDE CONDITION
STEAM DRUM INSIDE CONDITION
BED COIL HEADER INSIDE CONDITION
BED COIL HEADER INSIDE CONDITION