The advancement and application of magnetic drive pumps in Industry has seen constant growth and successes in many Industrial areas where we need to safely and effectively handle corrosive, toxic and generally dangerous liquids. With Magnetic drive pumps now being able to handle solids safely, the single biggest cause of failure is that of dry-running.
Dry running can occur in many circumstances, closure of valves, emptying supply vessel of liquid or poor operation of the pump on site. The effect of dry running can be catastrophic within a matter of seconds with the traditional slide bearing design.
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Traditional Silicon Carbide bearings
Silicon Carbide (SiC) has been used as the preferred bearing material for magnetic drive pumps for a number of years, due to its great hardness, low wear, dimensional stability and almost universal chemical resistance in the most arduous of applications, with this material often outlasting other pump components in the harshest of applications.
If we look very closely at bearing designs and tolerances, the contact of the bearing bush and sleeve takes place along a line; if the bearings are rigid, contact only occurs at local points, the entire frictional energy turning to heat in the small contact area. The result is that very high temperatures occur in localised points, resulting in particles flaking from the main bearing surface, and ultimately destroying the entire bearing component.
Bearing in mind this occurrence, needs us to be very careful in certain applications with some fluids. If we are pumping products with very low vapour pressures, or particularly temperature sensitive, it could be the case that we cause localised dry running just on the bearing faces causing failure, even though it would appears that the pump is flooded and primed.
Many attempts to improve the sliding properties of Silicon Carbide have been trialled in the past, with minimal or limited success. Impregnation of Carbon material (which has a lower co-efficient of friction and better wear characteristics) into the SiC, or coating with carbon layers has worked, but with limited success.
‘RunSafe’ coatings
The CDR Pumps ‘RunSafe’ bearing design consists of a standard Silicon Carbide bearing, undergoing a process to add an amorphous layer to the surface, made of diamond like carbon, just a few microns thick.
This layer maintains the same wear characteristics, optimum surface qualities, good thermal conductivity and universally resistant to chemicals – even giving temperature resistance up to 350ºC. The property which makes it most important for the pump design is that of the drastically reduced co-efficient of friction reducing from 0.4 to 0.7 to as little as 0.04.
This drastically lower co-efficient of friction means that the point contact that we discussed earlier, does not turn to frictional energy and so damage is eliminated and temperature rise is significantly reduced.
Several field trials have been successfully carried out on both metallic and lined pump units (metallic pumps we highlight have added restrictions for eddy current), involving complete dry running, regular stop/start, sudden application of liquid (to provoke thermal shock) as well as air entrainment in process liquid. On inspection, all components and units have been found to be is an as built state with no sign of any damage.
The ‘RunSafe’ bearing system is a real safety feature for ALL mag-drive pumps, dry running problems can happen in a matter of seconds, and catastrophic failure happen just as quick. This system provides assurances that if all else fails, short term dry running can be absorbed without potential for leakage of product to atmosphere or the environment.
CDR Pumps (UK) Ltd – 01933 226200
www.cdrpumps.co.uk
