Slurry Pump Tungsten Carbide Coating
HVAF equipment is utilised to protect slurry pumps' wet parts from erosion and abrasion as a result of hard solid particles. The HVAF coatings extend component life which in turn enables longer mean time between failures and significantly reduces maintenance costs.
A slurry is a mineral feedstock to be processed in a mining concentrator plant or a waste stream of tailings including hard particles. In oil and gas refining, slurries are usually catalyst streams, including limestone or zeolites. Under unfavourable circumstances of corrosive media, the combined effect of corrosion and erosion can lead to dramatically increased wear rates.
The mechanical and chemical forces are extremely demanding for the material integrity of pump parts — especially when temperature, pressure or flow volume increases.
Background: A customer asked us to coat a slurry pump backing plate to protect it from the abrasive slurry passing through the pump. The slurry consists of gypsum, aluminum oxide and water. The impeller came from the manufacturer already coated with tungsten carbide, presumably by a HVOF process. The pump housing was uncoated and the customer didn’t want the housing coated as, in previous experiences with this pump in this particular application, the backing plate was the only part adversely affected by the pumped slurry.
Results: Figure 1 shows the backing plate with the applied HVAF tungsten carbide coating after one year of service.
After measuring the coating thickness, we found that the thickness after one year of service was identical to the original applied coating thickness. As we began to examine the backing plate we found an interesting area (see Figure 2).
The backing plate seal to the housing wasn’t absolute and some erodent made it past the coated surface of the backing plate and eroded material from under the coating. It is hard to see in the photo but the coating is undercut and is still intact forming a sharp overhang of coating. Figure 3 shows a close up of the eroded area.
The other main parts of this slurry pump didn’t fare as well. The impeller shown in Figure 4, lost much of the applied coating and the blades were wearing away fast. If the pump didn’t begin leaking the impeller would have failed after little additional service. In the close-up view you can see where the HVOF coating has completely gone (Figure 5). In those areas where the coating is worn away, the metal is being worn away quickly due to the blades being significantly shorter than the adjacent blades that still have coating remaining on the surface. This can be seen using a straight edge and a little back lighting (Figure 6).
The pump case didn’t do well either. Here is a look at it as received (Figure 7).
The damage is readily apparent in Figures 8, 9 & 10, where closeup views shoe the extent of the damage.
The customer decided that the best course of action was to line everything with a WC-10Co-4Cr tungsten carbide coating, applied with a HVAF system. We wholeheartedly agreed with this decision.
