Plain bearings made of silicon carbide with optimised dry-running properties

Damage to magnetic drive pumps are frequently attributable to running dry; this applies in particular when silicon-carbide plain bearings are used. The frictional heat which is produced in the plain bearings when the pump is activated without medium is so great that the bearings are already damaged after just a few seconds, which results in the complete destruction of the pump if dry running continues.

Newly developed coatings on the plain bearings substantially reduced these problems and have been used with great success for many years. Further research on the entire tribosystem of magnetic pump bearings resulted in new know-how which in turn led to the problems of the dry running of magnetic drive pumps being almost completely eliminated through the use of a recently improved bearing concept.

Dry running
Although every user of centrifugal pumps knows that magnetic drive pumps must not be operated without medium, it repeatedly happens that pumps briefly run dry when the direction of rotation of the prime mover is being checked or when the valves in the suction line are not opened. As no hydraulic forces act on the plain bearings during bone dry running, sliding surfaces are only subjected to the weight forces of the impeller FGL, the pump shaft FGW and the pump rotor FGR (Fig 1). Any forces which may arise due to imbalance or irregular magnetic fields can be disregarded if the production is high, as should be a matter of course with magnetic drive pumps. Thrust forces do not occur during operation without medium and when the pump is installed horizontally. Fig. 1 shows that the arrangement of the bearing points and the rotating components of the pump are important for the loading of the plain bearings bearing distances and the arrangement of the bearing points near the impeller and pump rotor reduce the load. Table 1 contains the bearing forces and the bearing loads during dry-running for some pump sizes of the MNK and MNK-B series. The figures clearly show that the loads during dry running are very low. They are only about 1% of the loads which occur during normal op eration with medium.
In spite of these low loads magnetic drive pumps with plain bearings made of silicon carbide are very quickly destroyed when they run dry due to to the energy produced.

Plain bearings made of silicon carbide
Silicon carbide, SiC, has gained wide acceptance over many years as the bearing material for magnetic drive pumps. Its special properties, such as dimensional stability, great hardness, low wear and almost universal chemical resistance, make it an excellent material for bearings. The high price has fallen substantially in recent years owing to optimise production methods and bearing designs suitable for ceramics. As a result, other bearing materials, such as hard carbon or siliconised silicon carbide, have been almost completely crowded out of the market. However, the high coefficient of friction µ of SiC against SiC of 0.5 to 0.7 means that relatively high frictional energy occurs on the sliding surfaces of plain bearings which are not lubricated or cooled. Table 2 contains frictional energies for various pump sizes MNK and MNK-B series. The forces specified in table 1 at a speed of 2900 rpm and a coefficient of friction of the plain bearings of µ = 0.6 were used as basis. The contact of the bearing bush and the bearing sleeve takes place in flexible bearings along a line; if the bearings are rigid, contact only occurs at a point. The entire frictional energy is converted into heat in this small contact zone. The result is that very high temperatures occur at this point and they in turn can lead to silicon particles flaking off the plain bearing surface and ultimately destroying the entire plain bearing system in a short time.
In the past a lot of research work was conducted to improve the sliding properties of silicon carbide. For example, attempts were made to reduce the coefficient of friction using SiC grades with embedded carbon particles and to maintain brief emergency lubrication with porous SiC. However, the results were not so successful that magnetic drive pumps could be run dry with these materials. Coatings of SiC plain bearings with a soft carbon layer (e.g. graphite) also permitted only a one-off dry start-up of the pump as the layer wears very quickly.

"safeglide" coating
In 1990 the problems of dry running were greatly reduced by ITT Richter Chemie-Technik with the aid of a "safeglide" coating of the plain bearings. It consists of an amorphous layer made of diamond-like carbon just a few microns thick which is applied to silicon carbide bearing by a special process. The layer is characterised by the following properties:
- amorphous, isotropic, readily adhesive layer
- great hardness (HVO.05 4000-6000)
- good elasticity in spite of great hardness
- high wear resistance
- optimum surface qualities
- good thermal conductivity
- temperature-resistant up to 300°C
- universally resistant to chemicals; cannot be destroyed by wet chemicals
The property which is most important for its use as a friction-reducing layer in plain bearings is the coefficient of friction. It is 0.02 to 0.04 in the non-1ubricated, i.e. dry, state "safeglide" against "safeglide". The frictional energy during dry running falls dramatically with these low coefficients of friction.Table 3 contains the relevant figures. This coating made it possible for the first time to let magnetic drive pumps (Fig. 2) run bone dry for a perio d of up to 5 minutes at 2900 rpm. While it was not possible to measure the development of the temperature in non-coated plain bearings at a speed of 2900 rpm owing to the very rapid destruction of the bearings, the temperature curve illustrated in Fig. 3 was measured on coated plain bearings. The test bed results were so impressive that ITT Richter, the manufacturer, very quickly refined these coated plain bearings for launching onto the market in order to effectively counteract the problems of dry running in magnetic drive pumps.

Experience
After the introduction of "safeglide"-coated plain bearings for the magnetic drive pumps of the MNK and MNK-B series the statistics of the users showed that pump damage caused by improper operation, which resulted in dry running, fell dramatically within a very short time. After the initial excellent results with coated plain bearings the range of operonditions was increasingly extended. The pumps were now al for media with a permanently ids content and for low-boiling near the evaporation point. M operation, e.g. pumping co completely empty, monitored "dry-run" signal became possi the new plain bearings. Expensi itoring facilities, such as filli monitors or flow meters with a 1 play, could be dispensed with. 1 low-cost motor load monitors w still are frequently used in conjunction with "safeglide".
The now more than six year perience have shown that caused by dry running almost n curs in @ Richter magneti pumps. Studies on the plain b used demonstrated that the cc still fully functional even after pr service in highly corrosive medi
One example of this excellent r the plain bearing system of an M 160 shown in Fig. 4 which was i ation for roughly 40,000 hours. over, one problematic aspect w the product crystallised out and t ing surfaces of the bearings we contaminated with crystalline The examination of the sliding s showed that the coating was inta after this lengthy period of opera

Refinement
The design and production pro the plain bearings have been furt timised in recent years. The parameters of the coating process were also improved, which means that today a very uniform, defect-free "safeglide" coating can be manufactured in large series. Dry-running tests were continiously conducted with these plain bearings, which are optimal for magnetic drive pumps.
It is very difficult to reliably observe the temperature which is produced on the sliding surfaces during dry running. The temperature must remain so low that the surrounding plastic-lined components are not damaged.
Moreover, the pump user must bear in mind that high temperatures occur with prolonged dry running. If the suction valve is suddenly opened after dry running, the product which is to conveyed and which also flows through the plain bearings is heated. This can cause undesirable reactions with sensitive products. The situation becomes even more critical when an explosive mixture is in the pump during dry running or the medium to be conveyed has a low ignition temperature. In this case it is very important to limit the dry running time to a few minutes, e.g. by using appropriate monitoring facilities like a motor load monitor.
In order to be able to make a statement about the temperatures which are produced on the plain bearings, attempts were made to determine the limiting temperature theoretically and to confirm the values in tests. If the energy flow during dry running of the plain bearings in a magnetic drive pump is examined, it becomes clear that the thermal energy produced in the sliding surfaces of the bearings must be dissipated to the atmosphere. A very simple model in which the heat is only transferred through the plain bearing bush and the plain bearing pedestal to the outside produces a good result. The heat dissipated by convection was disregarded. Only the stationary state was examined in which the amounts of heat produced and dissipated are in equilibrium. The heat transferred from the sliding surface to the outside diameter plain bearing pedestal is thus calculated according to the following formula:
= amount of heat (W)
l = thermal conductivity (W/(m.K)
A = area covered by flow (m²)
t = wall temperature (°C)
s = wall thickness [m]
Fig. 5 shows the simplified calculation
model with which the temperatures produced were determined.
This model and the frictional energy of 13.26 J/s specified in table 3 for the rotor plain bearings of an MNK 32-160 thus result in the temperature plotted in Fig. 6.
The problems encountered with the dry running of plastic-lined magnetic drive pumps are illustrated in Fig 6. Although the corrosion-resistant fluoro-plastic lining of PFA is only a few mm thick between the plain bearing bush and the plain bearing pedestal, the heat insulation is so strong that the temperature difference in this lining is roughly 75% of the total temperature difference between the sliding surface and the outside temperature. This becomes clear when the thermal conductivities listed in table 4 for the materials used are exemined.
The thermal conductivity of the lining cannot be changed unless filler materials are used to the detriment of the chemical resistance. As a result, it is necessary especially for plastic-lined pumps to keep the coefficient of friction of the plain bearings and thus the resultant amount of heat as low as possible. If the theoretical temperatures sliding surfaces are calculated for the frictional energies of the rotor "safeglide" bearings given in table 3, it can be seen that even with the largest pump 80-200 with a magnetic drive rating of 65 kW a temperature rise of only (338°F) is calculated at 2900 rpm (table 5).
With these relatively small temperature rises, which are uncritical both for the layer, the silicon carbide as well as the PFA lining, the smaller pump sizes in particular up to 50-200 should in theory be able to run bone dry at a speed of 2900 rpm over a very 1ong period. The temperature rises must be taken into account depending on the application in question.
The entire tribosystem was examined and optimised in a 2-year research project. Different layer variations based on the "safeglide" were manufactured and tested. The latest coating processes, e.g. the so-called "multi-layer coating process", were also included in the studies. However, the use of soft graphite layers was intentionally omitted in order to guarantee a long-life coating and thus the capability to repeatedly run dry.
In addition, various details of the bearing geometry and bearing mount were changed in order to eliminate the negative effects of the temperature rise during dry running.
All the optimisation aspects related to the bearing situation of the magnetic drive pumps of this manufactur are being launched onto the market as the "safeglide Plus".

Tests with "safeglide PL
This optimised bearing system repeatedly produced the temperature curves shown in Fig. 7 for the impeller and rotor plain bearings during dry running tests. The result shows that it is possible to let magnetic drive pumps with "safeglide Plus" plain bearings run dry for over 75 minutes. This outstanding result was repeatedly confirmed in many tests and on pumps of various sizes.
Further test series were conducted in order to check whether the layer properties are changed after such a long dry- running period. In these tests the plain bearings which had previous been subjected to a dry run of 75 minutes were used. Then the same bearings were operated 19 times with a dry run of 10 minutes and the temperature was measured on the plain bearings (table 6).
This test provides much more information as the objective of the developrnent of plain bearings suitable running cannot be a centrifugal pump which runs dry once over several hours but rather the proof that plain bearings provided with "safeglide Plus" also survive frequent dry runs without suffering any damage. 10 minutes are normally enough for the operator to detect a functional error and to take appropriate action.
As a final test, a dry run over 75 minutes was conducted again. In this case the same temperature curve was determined as is shown in Fig. 9. A subsequent evaluation of the plain bearings showed that no damage to the components had occurred.
It is understandably very important for industrial practice that the dry-running properties are also retained after a long period of operation and that this dry-running safety feature is still present when the pump is repeatedly swiched off and started up again.
To this extent other coating systems which have an additional soft carbon layer on a hard support material to achieve as long a dry running time as possible are not viewed as being favourable. The reason for this is that the soft layer wears down during pump operation and then after deactivation and renewed start-up of the pump it might possibly no longer satisfy the requirements. This aspect would be even more important for solids-containing media.
Finally, another test was conducted to determine whether, after a dry run followed by a sudden supply of cold medium, the silicon carbide bearings with "safeglide Plus" are damaged by the temperature shock. In this test an MNK 50-200 was allowed to run dry until the temperature had risen to 110’C on the impeller plain bearings and to 150’C on the rotor plain bearings. Then the suction valve was opened while the pump was running so that 20°C coldwater could flow directly into the pump. The temperature at the bearing points fell within 3 seconds to about 20°C. A subsequent examination of the plain bearing components showed that all the parts had survived the test without suffering any damage.