Large Valve Single Device Isolation Problems & Solutions

Large Valve
Single Device Isolation
Problems & Solutions

What is Single Device Isolation?
• Gates, stoplogs and valves are used to
isolate a work area.
• Single device isolation is the certification
by a Professional Engineer that the device
is safe to work behind.
• In some jurisdictions certification is
required in lieu of providing a double block
and bleed isolation system.

What is required for SDI
Certification?
• Risk assessment.
• Design review.
• Condition assessment.
• Maintenance review.
• A written certification provided by a PE

What is KCBL experience with
SDI?
Gates Valves Stoplogs
52 28 19

Typical SDI Valve Problems
• Cracked Casting Hole Cover Plate Welds.
• Leaking Seals – particularly metal to metal
seals
• Corroded Surfaces
• Inadequate lockout schemes – blocking
devices

Cover Plate Potential Problems
• Many older hydro plants have butterfly and
spherical valves with cast steel rotors.
• Castings for disks and plugs often included
holes to allow the sand to be removed after
casting.
• The holes were plugged with welded plates.
• Some of these welds may be cracked.
• Worst case - the plate can fall out.

13 Ft. Dia. Butterfly Valve

5 Ft. Dia. Spherical Valve

Failed plug in a Spherical Valve

Typical Weld Detail

Typical Cracked Weld

Crack %
% Cracked Welds vs. Valve Head
Graph 1 - Crack % vs. Head on Valve
100
80
60
40
20
0
0 100 200 300 400
Head (ft)

Crack %
% Cracked Welds vs. Valve Dia.
Graph 2 - Crack % vs. Valve Diameter
100
80
60
40
20
0
0 5 10 15
Valve Disc Diameter (ft)

Crack %
% Cracked Welds vs. Valve Age
Graph 3 - Crack % vs Valve Age
100
80
60
40
20
0
1920 1930 1940 1950 1960
Year

Preliminary Observation
• Cast valves built between 1928 and
1960, with welded-in cover plates, may
have cracks in more than 40% of the
cover plate welds.

How Not To Fix The Crack!

How To Fix The Crack!

6 Ft. Dia. Butterfly After Repair

13 Ft. Dia. Butterfly Valve

13 Ft. Dia. Butterfly After Repair

Cover Plate Weld Conclusions
• Cast valves may have cracked cover plate welds,
• The cracks may result in the cover plates falling out,
• Cracked welds pose a potential safety hazard to
personnel working behind the closed valve,
• The cracks can be detected relatively easily with
magnetic particle NDT,
• Partial repair of a cracked weld has been problematic,
whereas complete replacement of the cover plate and
new welding has proven to be completely successful.

Seal Upgrade
• SDI failed twice on two 6 ft dia. butterfly
valves due to excessive seal leakage

Original seal design

Redesigned Seal

New Seals Being Installed

New Seal After Installation

Leakage Through New Seals

Summary
• Cast valves have issues, but they can
often be solved at a fraction of the cost of
a new valve
• Casting hole cover plates can be replaced
• New seals can be designed
• Corrosion coatings can be renewed

The End (time to wake up!)
Dan Campbell, P.Eng.
Manager Hydro Projects
Klohn Crippen Berger Ltd.
500 - 2955 Virtual Way
Vancouver, BC
Canada, V5M 4X6
Email: DCampbell@klohn.com