Download - 1483_hiweld_enRuotePeltonSaldate
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VA TECH HYDRO
www.vatech-hydro.com
HIWELD™Optimized Fabrication of Pelton Turbine Runners
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HIWELD™ – Optimized Fabrication ofPelton Turbine Runners
A project team formed by engineers andtechnicians coming from various competencefields from HYDRO VEVEY SA andVOEST-ALPINE MCE has developed a newprocess to manufacture Pelton runners.
The rigorous studies performed to validateeach stage of the fabrication have led to anoperational technique, whose benefits willconvince the power plant owners.
Pelton runners are traditionally casted in one piece,
more often with stainless steel G-X5 CrNi 13.4.
This process leads to long delivery times, especially
due to the mould preparation and runner finishing
tasks.
It is also known that the risk of hidden residual
defects in the casted material, also in high stressed
areas of the runner, cannot be totally eliminated.
In order to assure a perfect control of the severe
quality criteria imposed for Pelton turbine runners,
the method is characterized by an appropriate choice
of materials and processes.
An optimized process
Following economical and technical studies of many
solutions, the idea of separating the runner into
a central disk containing the bucket roots and
bucket pieces fixed
by welding, was
retained as
reaching very
good objectives of
delivery time
reduction, quality
increase and costs
mastery. The
central disk is
machined into a
forged stainless steel CrNi 13.4 block. The choice of
the bucket separation planes allows to fabricate a
narrow disk.
It indeed ensures that high stress areas remain inside
the disk, therefore taking profit of the higher
mechanical properties (ultimate tensile strength) of
forged steel.
The central pieces of the buckets are casted in
CrNi 13.4 steel with an overthickness. Those pieces
are welded on the bucket roots following a sequence
to correct continuously the distortion.
We take profit of the good lateral access to mill the
inner profile of the bucket before welding the lateral
parts of buckets.
The lateral pieces are also casted in CrNi 13.4 steel
with a small overthickness and milled to the final
profile before being fixed on the runner.
Even if it is also possible to manufacture the bucket
pieces in forged steel, the casting of such small
and simple pieces is easy to perform by a great
number of foundries with a quality higher than for a
full casted runner.
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The runner is submitted to a post-weld heat
treatment. All the welding areas are
controlled by non-destructive surface and
volumic testing methods before and after
the heat treatment. After the final machining
of coupling and cut outs, only a final
polishing is necessary.
Demonstrated Quality
Mastering the welding process is a key
point of the technique. A welding test using
an old runner has been performed in order
to check the accessibility for welding and
non-destructive controls, and also to define
the best suited welding sequence
minimizing the distortions. Buckets were
separated in accordance with our studies and the
runner was rebuilt choosing for this test the most
difficult case of the last bucket to weld.
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Beside those tests, in accordance with the
current European standards in use for this kind of
operating mode, the welding procedure was
qualified with a test piece having a similar geometry
to this of a bucket. By those tests, a welding
sequence was defined, allowing to reduce the
distortion to the minimum. 100 % of the welding
volume is controllable and controlled by ultrasonic
method.
The results of non-destructive and destructive
controls performed from this qualification test have
demonstrated that the welding rocedure developed
for this application reached atisfactorily the very
demanding quality criteria imposed for the welding
areas. The exact location of the bucket separation
planes is defined through a Finite Element Method
(FEM) calculation of the stresses, completed in all the
bucket volume. The welding is placed out of the high
stress areas.
Benefits for the power plantowners
With this Pelton runner fabrication process,
mechanical properties of material are increased
in all the runner areas: in the disk and bucket roots
through the forged steel, in the middle and
lateral parts through high quality of casting or
through forged steel pieces, in the welding areas
through an adapted qualified procedure reaching
higher mechanical properties than integral cast
steel, and through a 100 % controllable welding
volume.
Very demanding quality criteria have been defined
for the welding procedure, the testing methods
and the acceptability of welding and constitutive
parts of runner, all this in collaboration with the TVFA
(Technische Versuchs- und Forschungsanstalt,
Technical University of Vienna, Austria), a
neutral institute accredited and renowned in the
material and construction domains.
The bucket profile, completely milled on a
CNC machine, guarantees the exact hydraulic
shape and therefore the high level of runner
performances.
Manufacture time is reduced by 30 % in comparison
with integral casted runners.
Apart from the material supply, all the fabrication
(milling, machining, welding, grinding and testing) is
performed in our own workshop, under our own
responsibility, leading to a perfect mastery of quality
and delivery time.
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Outlook
The process allows to meet quickly the
clients demand, while assuring an optimal
quality through very demanding criteria.
Presented lately on the market, the method
has immediately won a very promising
success as 15 prototype runners are already
ordered from our clients.
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8www.vatech-hydro.com
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