Lay-up Protection for Boilers and HRSGs APPA E&O Conference

Transcription

1 Lay-up Protection for Boilers and HRSGs APPA E&O Conference APRIL

2 FOR SALE: 250 MW 2-on-1 Natural Gas Fired Combined Cycle Plant Completed in 2000, lightly used, left full of water with no lay-up or preservation. Make Offer 2

3 Goals of Proper Lay-up Minimize Downtime Corrosion Eliminate Corrosion Product Transport on Startup Reach Chemical Operating Limits Quickly 3

4 Lay-up the Entire Steam Cycle! Vent HP Turbine IP Turbine LP Turbine Condenser Deaerator Vent Boiler HP Heaters LP Heaters Condensate Polisher 4

5 Lay-up is Critical for HRSG s Cycle frequently Faster heating cycles Complex circulation Often stand-alone (no source of aux. steam) Chemical cleaning more complicated 5

6 Your Choice Rust and Corrosion 6

7 Your Choice Passivation and Protection 7

8 Failure Mechanisms Affected by Poor Lay-up Oxygen Pitting Corrosion Fatigue Under Deposit Corrosion Hydrogen Damage Caustic Gouging 8

9 Oxygen Pitting Causes: Downtime Oxygen Dissolved oxygen on Startup Corrosion Products Location: Anywhere where water and oxygen coexist 9

10 Oxygen Pitting Starts with a deposit Driven by oxygen in the water/air versus no oxygen under the deposit Heating increases corrosion rate Chloride increases corrosion rate 10

11 Normal Passivation Half Reactions Fe Fe e - 1 / 2 O 2 + H 2 O + 2 e - 2 OH - Fe OH - Fe (OH) 2 2 Fe(OH) 2 + O 2 Fe 2 O 3 + H 2 O (<300 F) 3 Fe(OH) 2 Fe 3 O 4 + H 2 (>300 F) Fe 2 O 3 O 2 OH - Fe +2 Fe(OH) 2 Fe 3 O 4 Fe 0 Anode 2e - Steel Cathode 11

12 Normal Passivation Random anodic and cathodic sites create a passivation layer AC AC AC Fe 0 AC AC 2e - AC AC AC AC AC AC AC AC AC AC AC AC AC AC AC AC A- Anode C- Cathode Steel 12

13 Oxygen Pitting Deposits create a localized cathode and anode that form a corrosion pit Fe 2 O 3 Oxygen-deficient area High dissolved oxygen O 2 OH - Fe(OH) 2 Anode Steel Cathode 13

14 Corrosion Fatigue 1. Stress cracks protective oxide 2. Oxygen pits form Fixed point 14

15 Corrosion Fatigue Causes: Oxygen plus Stress DO on startup Affects: LP Economizer and Evaporator Fixed points, headers 15

16 Under-deposit Corrosion Steam out Conc. chemicals Heat Boiler water in 16

17 Underdeposit Corrosion Causes: Deposits High heat flux Improper Chemistry 17

18 Lay-up Considerations Not Time Dependent Good Lay-up must work for a day or a year 18

19 Lay-up Considerations Quick return OR Scheduled return 19

20 Lay-up Considerations Breech OR No Breech 20

21 Corrosion Triangle 21

22 Dry Lay-up Exclude Water 22

23 Wet Lay-up Excludes Oxygen 23

24 Lay-up Options Dry Lay-up Hot Drain Dehumidified Air Desiccants Wet Lay-up Nitrogen Steam 24

25 Design Issues Affecting Layup Can all headers and piping be drained completely? How quickly can it be done? How accessible are drain valves? 25

26 Design issues: headers Cannot be drained : Wet Lay-up only Dry Lay-up will work 26

27 Dry Lay-up Advantages Only option when repairs are required Requires less time and cost to maintain 27

28 Dry Lay-up Disadvantages Need to get systems dry and keep them dry Requires considerable time and water on startup 28

29 Steps for Dry Lay-up Drain HRSG hot (some drain under nitrogen pressure) Drain out associated feedwater equipment and piping Drain and dry out condenser Make sure ALL drains are opened, including header drains 29

30 Maintaining Dry Lay-up Use continuous dehumidified air circulation OR Close drains/drums and use desiccant 30

31 Dehumidified Air Air must circulate though all equipment High flows to achieve dew point of 10F as quickly as possible Continue to circulate 1% system volume per hour 31

32 Silica Gel Use 5 lbs. silica gel/100 ft 3 of panel volume Place in each accessible drum Use indicating type Check regularly At least twice during first week Once per week for first month Once per month thereafter 32

33 Vapor Phase Inhibitors Used on clean metal surfaces Addition rate- ~3 lbs per 1000 gallons capacity 33

34 Vapor Phase Inhibitors Volatile at ambient temperatures Destroyed in steam blow or operation Treatment can be repeated if needed 34

35 VPI Preservation After 30 days After 106 days 35

36 Wet Lay-up Advantages Quick restart Minimizes corrosion product transport 36

37 Wet Lay-up Disadvantages Requires freeze protection More time and cost to maintain properly 37

38 Wet Lay-up Hot or Cold Uses Auxiliary Steam Reduces Thermal Cycling Stresses Faster Starts Nitrogen Simple to maintain No fuel costs 38

39 Nitrogen Cap Cap should be applied to: Steam Drum(s) Deaerator (if separate) Condenser 39

40 Danger:Nitrogen Nitrogen lay-up is designed to prevent oxygen from entering the boiler Nitrogen will NOT support life All confined spaces MUST be clearly marked Confined Space Entry Procedures MUST be followed 40

41 Nitrogen Addition Points Vent HP Turbine IP Turbine LP Turbine Condenser Deaerator Vent Boiler HP Heaters LP Heaters 41

42 IP SH LP SH DA LP Drum Feedwater Preheater From HP FW Pump 42 HP Economizer IP Economizer HP Economizer LP Evaporator IP Drum To IP Turbine To LP Turbine HP Evaporator HP Econmizer IP Evaporator HP Drum --Nitrogen Addition Points Drain Drain To HP Turbine HP SH

43 Nitrogen Cap- Drums Enters the drum through connection on vent line Nitrogen feed started while drum is still hot (~212 F) Less than 5 psig of pressure is applied 43

44 Nitrogen Cap - Condenser Addition starts while while steam turbine is still spinning down Added quickly at first, then slowly as the vacuum approaches zero Constant flow of 20 SCFH is maintained while the unit is down 44

45 Wet Chemical Lay-up ph of water 9.8 with ammonia or amines Hydrazine or other scavenger may be added Need to circulate and test 45

46 IP SH LP SH DA LP Drum Feedwater Preheater From HP FW Pump 46 HP Economizer IP Economizer HP Economizer LP Evaporator IP Drum To IP Turbine To LP Turbine HP Evaporator HP Econmizer IP Evaporator HP Drum --Nitrogen Addition Points --Chemical Sampling Points Drain Drain To HP Turbine HP SH

47 Wet Chemical Lay-up Requires significant amounts of chemical Must add nitrogen or steam provide adequate protection above the water line May create an environmental issue if you have to drain the HRSG (particularly if hydrazine is used) 47

48 Transitions Dry To Wet Need a source of deoxygenated water Add chemical treatment during fill Cap with Nitrogen 48

49 Transitions Wet to Dry (Cold) Drain under nitrogen pressure Continue to blow nitrogen until water is gone Dry out with dehumidified air Use dry air or silica gel to maintain 49

50 Transitions Wet to Dry (Better) Fire boiler to 25 psig Drain under nitrogen pressure Dry out with dehumidified air Use dry air or silica gel to maintain 50

51 Startup Chemistry Dry Lay-up Boiler/HRSG fill-water should be: Hot Deaerated Chemically Treated 51

52 Feedwater Startup Chemistry Treatments Amine Scavenger Boiler Phosphate Caustic (if needed) 52

53 Startup Chemistry Wet Lay-up (Nitrogen) Chemistry from the lay-up predominates Treat with a heavier than normal dose of amine and scavenger to account for high make-up rates 53

54 Reference Document ASME CRTD-Vol. 66 Consensus for the Lay-up of Boilers, Turbines, Turbine Condensers and Auxiliary Equipment Available at the 54