Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Method for Manufacturing Shaft Seals
This invention relates to a method for manufacturing
shaft seals. The shaft seal in question comprises a seal member
rotating with the shaft and a stationary seal member, the
opposing seal surfaces of which form the seal.
This kind of seal is commonly used in pumps. They can
be either so called hydrostatic seals, in which case there is
a thin fluid membrane between the surfaces, or, so called
mechanical seals, wherein the surface of non-rotating part is
pressed against and held in contact with the rotating surface.
In both cases, parallel and even seal surfaces are required
conditions for satisfactory operation.
When selecting the material for hydrostatic seals for
exacting applications, as for example, in main recirculating
water pumps used in nuclear power plants, one must first pro- ;
vide a material which is resistant to corrosion, even at the
cost of some strength. This leads to the necessity to use
austenite steel types as the material for the seal members of
the shaft seal. These steel types have, however, a negative
property in that permanent deformation occurs when small
stresses are applied to them.
When stresses are applied to seals made of this type of
material, either under normal working conditions, ~r, due to
disturbances in operation, the seal surfaces change their shape
permanently even due to small stresses. The fact that the seal-
ing surfaces are thus distorted leads to an inferior sealing
capability and in operation subjects the seal to disturbances
or damages.
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`The object of the invention is to provide an improved
manufacturing method for shaft seals, by which seals are man-
ufactured which better endure the stresses occurring in actual
use, e.g. due to rapid changes in temperature virtually with-
out distortions and loss of sealing efficiency.
The aim of this invention is reached when the sealmembers are exposed to heat treatment which causes permanent
deformations. The heat treatment must be at least twice the
magnitude of stresses to which the seal members will be
exposed during operation. The seal surfaces are then pre- ;;
cision finished e.g. by lapping. Virtually no permanent
deformations arise anymore within the range of stresses of
equal or smaller magnitude.
It is a characteristic feature of the method according
to the invention that the heat treatment comprises at least
two alternate heating-up and cooling-down phases and the
alteration in temperature of each heating-up and cooling-
down pbase is greater than that of the preceding heating-up
or cooling-down phase.
According to the invention, when heating up or cooling
down the seal member rapidly, it is exposed to an uneven
temperature distribution which causes a stress condition and
permanent deformations, if the stresses are greater than the
yield point of the material.
Due to a local yield of the material, it becomes
harder, and therefore a greater alteration in temperature
is used in the following heat treatme~t phase so that stresses .,~
exceeding the yield point and permanent deformations will
occur.
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When the heat treatment is carried out by way of
several alternate heating-up and cooling-down phases, cracking
can be avoided by gradually increasing the difference in -
temperature.
The speed of alteration in temperature and thermal
barriers depends on the seal member material, size, and
shape as well as on the prevailing load conditions. A
specialist can easily determine the right estimation for
~ each separate case. The essential feature is that the
temperatures are so low, e.g. below 100C, that the defor-
mations caused by them bè so small that the seal members
need not be entirely straightened by machining after the
treatment, so that only their seal surfaces have to be
machined after the treatment.
As a result, the treatment can be carried out on a
ready machined seal member. The heat treatment does not
cause any structural alterations of the material, which
means that the material properties as e.g. its corrosion
resistance remain unchanged.
The invention is explained in more detail below with
reference to the enclosed drawing, which shows a typical
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seal assembly in longitudinal section and, to which the method
of this invention may be applied.
The figures show a rotating shaft 1 having a locating
collar 2, which supports a seal member 3 with the assis~ance
of springs 4. The opposing surfaces 7 and 8 of the seal
members form a narrow gap 9, wherein an extremely thin fluid -~
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membrane is formed, separating the seal surfaces from each
other. The fluid is charged between the planar surfaces under
pressure from outside the gap 9 as well as through passages
10 and channel 11 in the seal member 3. The fluid which flows
through the gap 9 exits at a lower pressure into the housing
through an annular space 12 between the stationary seal member
5 and the shaft 1. As the gap between the planar surfaces 7 and
8 is extremely narrow, even small deformations of the seal
surfaces cause an unacceptable increase in the flow of fluid.
The heat treatment is preferably carried out by placing
the seal surfaces of the seal members in heat exchange contact
with a heat transfer medium such as water which is caused to
flow through the seal when it is mounted in a special pilot
plant or pump, wherein one seal member is rotated. The heat
treatme~: comprises a heating-up phase, during which the
temperature is changed so rapidly that permanent deformations
result, and a cooling-down phase during which the temperature
is changed so slowly that no further permanent deformations
will result. The heat treatment is made to cause at least
equal or greater stresses in the seal members than those to
which the seal members are to be eventually exposed to in
operation. The heat treatment may be required to comprise
several alternately occurring heating-up and cooling-down
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phases, in which case the stress caused by every new heating-up
phase is higher than that of the foregoing phase. The heat
treatment can e.g. comprise three heating-up phases and three
cooling-down phases, in which case the temperature is regu-
lated during the treatment as follows: 20C - 40C - 20C -
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60C - 20C - 90C - 20C. The speed of the temperature
alteration in the heating-up phases should be minimum 20C/min,
and that of the cooling-down phases maximum 10C/min. In any
case, the speed of the temperature alterations which are selected
in each specific case, depends among other things on the seal
member material, size, and shape, the fluid used for the treat-
ment, as well as, upon mechanical loads and thermal stresses to
which the seals are expected to become subjected.
According to an example carried out in practice, where
the outer diameter of the seal member was 270 mm, the inner
diameter 192 mm, and the thickness 33 mm, and where the heat
treatment was carried out by passing fluid through the sealing,
causing the seal surfaces in turns to come in contact with the
hot and the cold fluid, the temperature changed as follows:
20C - 40C - 20C - 60C - 20C - 90C - 20C, the speed of
the temperature alteration in the heating-up phases being
20C/min and that of the cooling-down phases 8C/min; the dis-
tortion of the seal surfaces was approximately 5 mm. The
sealing surfaces were straightened by lapping and the sealing
has operated faultlessly in practice under variable loads.
When re-examined, no changes in the alignment of the seal
surfaces have been found.
It is evident that the heat treatment can also be '
accomplished in the opposite way, i.e. a rapid cooling-down
is first applied to cause permanent deformations, whereafter
a slow heating-up follows, which does not cause further per-
manent deformations. This treatment is used when the seal is
to be exposed to corresponding thermal stresses in operation.
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