Split ring seal of a centrifugal pump

A split ring seal of a centrifugal pump comprises two rings that consist of a sintered material and that are respectively connected in a frictional or positive manner to a front cover disk of the impeller and to the pump housing which are made of a cast polyamide as produced by the activated anionic polymerization of monomeric laurinlactam.

FIELD OF THE INVENTION 
This invention relates to a split ring seal of a centrifugal pump. 
BACKGROUND ART 
In the past few years it has become more and more clear in the field of 
centrifugal pumps, which covers a relatively wide range of application in 
industry, that for the achievement of application-oriented maximum service 
lives the essential wearing parts of the pump must be made from materials 
which guarantee the correspondingly optimimum material properties with 
regard to the delivery medium concerned. Apart from the consideration of 
the flow rates of the delivery media through the pump, which are usually 
very high, other criteria mainly regarding the causes of damage leading to 
the destruction of the materials are of importance to this correspondingly 
individual selection of materials which must be adapted to the most 
different delivery media. These criteria can be substantially subsumed 
under the influences to be taken into account here, namely corrosion, 
erosion, cavitation and/or a combination thereof, the classification 
depending on the predetermination of specific parameters of the delivery 
media, especially the pH-value, the solids content, the temperature and 
the flow rate of the media. 
When being employed, for instance in connection with highly aggressive 
delivery media, the materials which have mostly been used for heavily 
loaded centrifugal pumps when the aforementioned criteria are to be 
complied with are a martensitic chromium-molybdenum alloyed cast iron with 
the DIN designation C-X 250 CrMg 153. With regard to this material it is 
stated in the journal "cav 8/86", pages 87 to 90 that as far as hardness 
and wear resistance are concerned it is only surpassed by pure metal 
oxides/carbides or also by hard surfacings, but that in comparison 
therewith it has the advantage of being machinable in the soft-annealed 
state. Apart from the customary casting materials GG-25, GGG-40.3, etc. 
which are also in current use, other corrosion-resistant special materials 
which are also suited for delivery media with a high content of solids are 
-according to this literature - an austenitic ferritic steel with the DIN 
designation C-X 3 CrNiMoCu 24 6 and a chill casting which is also 
chromium-molybdenum alloyed and has the DIN designation G-X 170 CrMo 25 2. 
Since all of these special materials are relatively expensive, their use 
has substantially been restricted to the impeller and apart from that - 
but only in practice - to component parts which, like the two rings of the 
split ring seal which is disposed on the front cover disk of the impeller 
in every centrifugal pump, turn out to be replacement parts which are 
especially susceptible to wear. Furthermore, the journal 
"Wekstofftechnik", Vol. 17, October 1986, pages 378 to 384, shows the 
results of tests in which a comparison is made between the aforementioned 
austenitic ferritic steel with the DIN designation G-X 3 CrNiMoCu 24 6 and 
a cast polyamide made by the activated anionic polymerization of monomeric 
laurinlactam and in which the destruction of the materials by cavitational 
loading is examined. In the preliminary period of the present invention 
this cast polyamide already proved to be a highly resistant alternative 
material to a high-quality nickel chromium steel when it was used for a 
wear disk in centrifugal pumps intended for delivering very abrasive media 
occurring in sugar industry. 
This invention which is characterized by the patent claims solves the 
problem of providing a split ring seal of the stated type which is also 
very susceptible to wear in centrifugal pumps and in the case of which the 
knowledge gained in the construction of pumps is broadened with regard to 
a possible use of the aforementioned cast polyamide as an alternative 
material for other critical pump parts of this type. 
SUMMARY OF THE INVENTION 
The split ring seal of this invention has the advantages that due to the 
characterized composite arrangement of the cast polyamide with a sintered 
material the mainly hard, wear and temperature resistant material 
properties thereof are now made use of in an optimum way, as the impact 
strength which, on the other hand, is only minimum because of the great 
hardness is optimally compensated by the cast polyamide. This compensation 
which is consequently accomplished with regard to a more or less great 
risk of breakage is here simultaneously favored by the possibility that 
due to a cast polyamide melt which shows a relatively high fluidity under 
casting conditions the composite arrangement of said polyamide with the 
two rings can virtually be made without any closer consideration of the 
constructional details applying to the entire split ring seal. Of course, 
the manufacturer can here also modify the melt of the cast polyamide prior 
to its integral casting with the rings by using the customary reinforcing 
agents, such as glass fibers or glass spheres, fillers, such as kiesalouhr 
or metal powder, lubricants, such as graphite or molybdenum sulfate, or 
other additives added prior to polymerization. Finally, due to this 
composite arrangement with the cast polyamide, each of the two rings seems 
to be less critical with regard to the tolerances which are relevant to 
the production thereof because it is virtually only the gap size which 
must now be observed when they are arranged with respect to each other 
with the usual size of about 0.1 mm being practically uncritical.

The centrifugal pump comprises a pump shaft 1 on which an impeller 2 is 
disposed for rotation therewith. The impeller 2 is surrounded by a pump 
housing 3 which is sealed with respect to the pump shaft 1 by an axial 
face seal or, alternatively, by a soft packing 4. At the suction side 5 of 
the centrifugal pump the impeller 2 is provided with a front cover disk 6 
which has fastened thereto an inner ring 7 of a split ring seal 8 which 
additionally includes an outer ring 10 fastened to the adjacent portion 9 
of the pump housing. 
The two rings 7,10 of the split ring seal 8 consist of a sintered material 
based preferably on a metal silicide or of a metal carbide based on 
silicon carbide. Furthermore, the front cover disk 6 and at least the 
portion 9 of the pump housing 3 which is adjacent to the outer ring 10 of 
the split ring seal 8 consist of a cast polyamide which is made by the 
activated anionic polymerization of monomeric laurinlactam and which is 
respectively cast with the two rings 7,10. With regard to the inner ring 
7, shrinking onto a hub which is coaxially formed on the cover disk 6 for 
this purpose may be an alternative to this type of integral casting, said 
shrinking being effected by undercooling the hub part of the wear disk. As 
for the outer ring 10, it can only be connected in a frictional manner to 
the adjacent portion 9 of the pump housing 3 by means of a screw or claw 
connection. 
When the disk cover 6 is made from the aforementioned cast polyamide, it 
seems expedient to make the whole impeller 2 from said cast polyamide 
provided that this is possible with casting techniques. The same applies 
also to the whole pump housing 3; the aforementioned restriction to the 
portion 9 alone, which is directly adjacent to the outer ring 10 of the 
split ring seal 8, must here be understood such that this also includes an 
easy repair of centrifugal pumps in the case of which the whole pump 
housing consists of another material. When there is such a differently 
formed centrifugal pump, the repair of a differently formed damaged split 
ring seal can only necessitate the provision of a corresponding composite 
arrangement of the outer ring 10 consisting of a sintered material and of 
a housing portion adapted to the existing pump housing as a corresponding 
replacement part. In this case, however, the replacement should be 
expediently combined with a simultaneous replacement of the existing pump 
wheel by a replacement part in the case of which at least the cover disk 6 
is also made from the cast polyamide and provided with the associated inner 
ring 7 of a split ring seal 8.