PENSTOCK CONDITION ASSESSMENT

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1 Northwest Hydro Association May 17, 2012 Technical Seminar PENSTOCK CONDITION ASSESSMENT Thomas L. Kahl, P.E. Kleinschmidt

2 TOPICS Concerns Types of Penstock Failure Modes Steel and Wood Stave Failure Mode Inspection Techniques Penstock Safety Program Recommended Methodology Example Program

3 Concerns Safe, Reliable, and Economical Pestock Performance FERC s Primary Concerns 1. The ability to shut down in the event of a leak or failure is critical. (Penstocks can be physically large structures with a lot of potential hazards difficult to detect.) 2. Inadequate inspection.

4 TOPIC 2 FAILURE MODES 1.Internal Pressure Rupture 2.General Buckling 3.Local Buckling 4.Inadequate Support

5 Failure Mode 1 Internal Pressure Rupture Add photo EPP-8

6 Failure initiated in heat affected zone of 1937 vintage longitudinal weld seam.

8 Galvanic corrosion at 1953 vintage surge tank longitudinal weld seam.

9 Random pitting pattern in 1939 vintage penstock (Evaluate per ASME B31G-2009)

10 Overall Shell Corrosion (typically occurs in moist environments)

11 Wood Stave Penstocks Internal pressure transferred from staves to steel bands.

12 An example of exterior stave wood decay. The ruptured wood at the metal Kelsey butt joint is called brooming.

13 Steel band corrosion

14 Slippage at shoes can release bands

15 Seam failure due to gravity dead weight pulling apart circumferential seam with deteriorated rivet heads.

16 Failure Mode 2 General Buckling

17 2006 collapse of 1995 vintage penstock when ice plug in vent induced vacuum due to external pressure.

18 General buckling caused by uncontrolled release at rupture. Replacement included additional air vacuum valves

19 Blocked vent induced collapse Note classic crack from corner.

20 Pipe collapsed when decayed top wood stave fell.

21 Failure Mode 3 Local Buckling Zick stress deformation at horn of saddle.

22 Local overhead dents in buried pipe probably caused by concentrated loads.

23 Relatively rare shear buckling of very thin (D/t= 614) large diameter steel penstock.

24 Failure Mode 4 Inadequate Support

25 Deteriorated saddles

26 Cracked concrete saddle.

27 . Loss of support at tipped saddle

28 Slope Instability

29 Surge tanks require very stable foundations to prevent tipping.

30 Cracked foundation and buckled anchor bolt gusset.

31 Surge tank anchor bolt corrosion reduces seismic overturning resistance.

32 TOPIC 3 FAILURE MODE INSPECTION TECHNIQUES Shell Thickness and dimensions. Concentrate on shell Connections Supports New ASCE MOP 79 expected to be published Aug 2012 Corrosion at Steel to Concrete Interfaces

33 Types of Inspection Cursory Inspection: Purpose is to note any observable change in condition Typically performed by operation or maintenance personnel weekly, monthly, or quarterly. Changes in condition noted for further investigation Comprehensive Inspection/Evaluation Purpose is to review the condition, safety, and risk of the existing penstock Performed by engineers who understand the design basis and actual condition of penstock If deficiencies are noted they can be resolved by repair, rehabilitation or replacement.

34 Basic Tools

35 Check rivets for loss of material and tightness.

37 TOPIC 4 - Penstock Safety Program Methodology Steps: 1. Establish Baseline Conditions 2. Establish an inspection, monitoring, and documentation plan 3. Implement a documented maintenance and repair program 4. Periodic comprehensive independent condition assessments

38 Penstock Safety Program Effective Program Should Be: Reliably Able to Detect Problems Readily Understood Convenient and Usable by Operating & Maintenance Personnel Cost Effective

39 Ketchikan Alaska Public Utilities (KPU) Penstock Safety Program Ketchikan Lakes Project (4.2 MW) No : concrete: 1,800 : 54 D: 46 head No : ductile iron: 1,800 : 54 D: 46 head No. 3, 4, 5: 1957 steel: 350 : 36 D: 250 head Beaver Falls Project (8.3 MW) No : steel: 342 : 36 D: 321 head No : steel: 3,610 : 42 D: 760 head No : steel: 4,170 : 28 D: 660 head

40 Ketchikan Lakes Beaver Falls

41 Step 1 Establish Baseline Condition Comprehensive System Description Define Design Criteria Define Hazards and Risks Summarize Physical Condition

42 Step 2 Site Specific Inspection, Monitoring, and Documentation Different Portions of a penstock will have different critical components Shell Thickness & Pitting Assessment Concentrate in high humid tunnels and in contact with concrete and soil Vacuum valve testing and inspection Document supports condition Critical to have repeatable observation locations. Inspection checklist can be useful to maintain consistency and completeness.

43 Step 3: Documented Inspection, Maintenance, and Repair KPU already used computerized Express Maintenance program. Only minor modification needed to existing procedures to more reliably document all records for future trending analyses.

44 Step 4: Periodic Comprehensive Condition Review Either external or internal staff independent evaluation/audit. Frequency 5 to 15 years and after special events (e.g. earthquake).

45 Important to Expect the Unexpected

46 Questions / Comments Thomas L. Kahl, P.E. Kleinschmidt Associates Tom.Kahl@Kleinschmidtusa.com (207) , Ext 280

MOCCASIN THIRD PENSTOCK ADDITION: INCREASING POWER BY REDUCING HEADLOSSES

MOCCASIN THIRD PENSTOCK ADDITION: INCREASING POWER BY REDUCING HEADLOSSES By Matthew Gass, Hetch Hetchy Water and Power, Moccasin, California, USA; Mark A. Hammer and David P. Thompson, Harza Engineering