large valve single device isolation problems & … is single device isolation? ... spherical...
Post on 12-Mar-2018
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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.
% Cracked Welds vs. Valve HeadGraph 1 - Crack % vs. Head on Valve
0
20
40
60
80
100
0 100 200 300 400
Head (ft)
Cra
ck
%
% Cracked Welds vs. Valve Dia.Graph 2 - Crack % vs. Valve Diameter
0
20
40
60
80
100
0 5 10 15
Valve Disc Diameter (ft)
Cra
ck %
% Cracked Welds vs. Valve AgeGraph 3 - Crack % vs Valve Age
0
20
40
60
80
100
1920 1930 1940 1950 1960
Year
Cra
ck
%
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.
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
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
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