Northwest Hydro Association

May 17, 2012 Technical Seminar

PENSTOCK CONDITION ASSESSMENT

Thomas L. Kahl, P.E.

Kleinschmidt

• Concerns

• Types of Penstock Failure Modes

▫ Steel and Wood Stave

• Failure Mode Inspection Techniques

• Penstock Safety Program

▫ Recommended Methodology

▫ Example Program

TOPICS

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.

TOPIC 2 – FAILURE MODES

1.Internal Pressure Rupture

2.General Buckling

3.Local Buckling

4.Inadequate Support

Failure Mode 1 – Internal Pressure

Rupture

Add photo

EPP-8

Failure initiated in heat affected

zone of 1937 vintage

longitudinal weld seam.

Galvanic corrosion at 1953

vintage surge tank longitudinal

weld seam.

Random pitting pattern in 1939 vintage penstock –

(Evaluate per ASME B31G-2009)

Overall Shell Corrosion (typically occurs in

moist environments)

Internal pressure transferred from staves to steel bands.

Wood Stave Penstocks

An example of exterior

stave wood decay.

The ruptured wood at the metal “Kelsey”

butt joint is called “brooming”.

Steel band corrosion

Slippage at shoes can release bands

Seam failure due to gravity dead weight pulling apart

circumferential seam with deteriorated rivet heads.

Failure Mode 2 – General Buckling

2006 collapse of 1995 vintage penstock when ice

plug in vent induced vacuum due to external

pressure.

General buckling caused by uncontrolled release

at rupture. Replacement included additional air

vacuum valves

Blocked vent induced collapse

Note “classic” crack from corner.

Pipe collapsed when decayed top wood stave fell.

Failure Mode 3 – Local Buckling

“Zick” stress deformation at horn of saddle.

Local overhead “dents” in buried pipe probably

caused by concentrated loads.

Relatively rare shear buckling of very thin

(D/t= 614) large diameter steel penstock.

Failure Mode 4–Inadequate Support

Deteriorated saddles

Cracked concrete saddle.

Loss of support at tipped saddle

.

Slope Instability

Surge tanks require

very stable

foundations to

prevent tipping.

Cracked foundation and

buckled anchor bolt gusset.

Surge tank anchor bolt

corrosion reduces

seismic overturning

resistance.

TOPIC 3 – FAILURE MODE

INSPECTION TECHNIQUES • Shell Thickness

and dimensions.

• Concentrate on

shell Connections

• Supports

• Corrosion at Steel

to Concrete

Interfaces

New ASCE –MOP 79

expected to be

published Aug 2012

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.

Basic Tools

Check rivets for loss of

material and tightness.

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

Penstock Safety Program

Effective Program Should Be:

• Reliably Able to Detect Problems

• Readily Understood

• Convenient and Usable by Operating & Maintenance Personnel

• Cost Effective

Ketchikan Alaska Public Utilities (KPU)

Penstock Safety Program

• Ketchikan Lakes Project (4.2 MW) ▫ No. 1 1957: concrete: 1,800’: 54” D: 46’ head ▫ No. 2 1997: 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. 1 1966: steel: 342’: 36”D: 321’ head ▫ No. 2 1952: steel: 3,610’: 42”D: 760’ head ▫ No. 3 1953: steel: 4,170’: 28”D: 660’ head

Ketchikan Lakes

Beaver Falls

Step 1 Establish Baseline Condition

• Comprehensive System Description

• Define Design Criteria

• Define Hazards and Risks

• Summarize Physical Condition

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.

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.

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).

Important to

“Expect the Unexpected”

Questions / Comments

Thomas L. Kahl, P.E.

Kleinschmidt Associates

Tom.Kahl@Kleinschmidtusa.com

(207) 487-3328, Ext 280