A case of corrosion in accelerator cooling water systems - MKK

“A case of corrosion in
accelerator cooling water
systems: the Daphne
Wiggler magnet”
Luigi Pellegrino
Accelerator Division
Our Lab in Frascati (Roma)
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Daφne: the Main Rings
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COPPER CORROSION IN DEMINERALIZED
WATER
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In the accelerator cooling systems the copper
corrosion in demineralized water is
unavoidable but can be limited;
The corrosion rate depends on conductivity,
dissolved O2 and CO2, pH, temperature, flow
velocity, imposed electrical potential
difference, galvanic potential difference
(different metallurgy).
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Corrosion vs pH and O2
From: E. Maughan, “Case study-copper based coolig water system”, MEDSI 2002.
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Water treatment
(what we know we should do)
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Conductivity and other parameters control by 1.
polishing on line and 2. controlled make up;
Limiting the oxigen inlet by closing the water circuit
(sealed or inert gas cushion);
Taking advantage by O2 pumping away with inert
gas flow;
Monitoring of parameters with continuous data
acquisition;
Vacuum deaeration;
UV hygienization.
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In addition:
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Avoid stops and unnecessary opening of
circuits;
Avoid dead leg in circuitry;
Avoid different metallurgy;
Check any possible depression in the
piping;
Check unwanted electric potential
difference.
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Typical cooling schematic
N2
POLISHING
ON-LINE
T
MAKE-UP
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WATER TREATMENT
PROCESS TANK
MAKE-UP
POLISHING
ON LINE
³
³
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Nytrogen pressurization
HYDRAULIC
GUARD
5 kPa
N2
TO THE PROCESS
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Typical parameters values
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Conductivity < 0.2 µS/cm
Dissolved oxigen <20 ppb
6.8 < pH < 7.2
But here follows an example of
what happens when they are far
away from their best....
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Wiggler magnet cooling distribution
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....an uncomfortable workplace!
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Electric and hydraulic connections
A series of 4 Wiggler, each one made by
10 poles in series (5 upper and 5 lower),
each pole is a series of 5 coils.
All the 50 coils are an hydraulic parallel.
The inlet water is at 32°C, the outlet
reaches 65°C. Except coils, made of
OFHC copper, and EPDM rubber hoses,
all other components are in AISI 304L ss.
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Manifold-coil connection
copper coil ID 4mm
Rubber hose ID 6 mm
Water flow
S.Steel spigot ID 4 mm
S.Steel manifold
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Zoom again to the
internal of rubber
hose between the
manifold spigot and
the coil end.
Here is a section
enlargement, ID 4 to
6 and again to 4mm,
as a little “chamber”.
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The failure history
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Solid pebbles (D>5 mm) formed in the “chambers”
inside the rubber and there moved back and forth,
obstructing sometime the flow in one or more coils.
Therefore the coils heated until safety thermal device
switched and shutted down the whole chain of four
wiggler magnets (~1 MW electric power).
After a while, the pebble moved, the flow went right
and we was not able to find the cause of the
failure!
Only once a persistent obstruction permitted us to
catch the problem and start finding a solution.
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The causes
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The relatively high conductivity, the relevant dissolved
oxigen contents of cooling water and the high electric
potential of some coils with respect to the grounded piping
yield the production of copper oxides and their migration
according to potential difference.
Actually we found a coal-black powder in several circuit points.
Probably sulphide-reducing bacteria cathalized the
agglomeration process, starting on the lips of the inlet copper
spigots, in a slower flow region.
The deposits growed until detached and continued growing,
probably without leaving the “chamber”. At the end its
dimensions (>5mm) were greater than the spigots throat: at
least the final growing phase was surely in the place where
we found the pebbles.
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Growth of the copper oxide
pebble
WATER
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The analysis (collaboration with
“Centro Sviluppo Materiali” di
Pomezia)
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Analyzed water at inlet and return of magnets
circuit, before and after the polishing system
(conductivity, O2 contents, pH, dissolved
metals);
Checked the surface of pieces of copper
conductor with some months of operating life
(micrographia, “energy dispersion” X-ray
surface analysis);
Analyzed the pebble and the powder (SEM
microscopy with EDS spectrometer).
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Analysis results (sample taken
before and after the polishing)
O2
conductivity
temp.
Cu
mg/l
microS/cm
°C
microg/l
before
12.50
0.8
7.55
23.1
92
after
12.40
0.3
7.87
23
4
before
4.35
0.5
7.4
32.5
78
after
4.30
0.3
7.65
31.5
0.9
before
3.85
0.5
7.86
33.5
81
after
3.89
0.2
7.65
33.1
0.5
sample 1
pH
sample 2
sample 3
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The copper oxide pebble
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Copper oxide powder
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EDS analysis of the powder
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The corrective actions
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We modified the polishing circuit and its
location in the plant, to assure a continuus
treatment of the whole water mass with the
proper flow in the mixed bed bottles, and
added new instruments and a datalogger;
The N2 cushion system was reviewed and
frequently checked;
The whole plant pressurization scheme was
reviewed, to ensure a positive pressure
everywhere.
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Typical water quality recovery from
a maintenance stop of a week
CONDUCTIVITY AFTER A MAINTENANCE STOP (24-11-03)
CONDUCTIVITY (microS/cm)
2.5
2
1.5
1
0.5
0
12.00
0.00
12.00
0.00
12.00
0.00
12.00
0.00
TIME
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Another failure case...
Septum magnet
coil
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