ASSESSMENT

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

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

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)

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

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
New ASCE –MOP 79
expected to be
published Aug 2012
• Corrosion at Steel
to Concrete
Interfaces

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