I can offer one reference for converting a group of BFW pumps to Vespel wear parts. We converted three old Pacific pumps (Model JB 3, 4 stage) to Vespel about 5 years ago. The pumps had a history of low flow operation, cavitation, wear-ring and bushing rubs and seal failures. These very old pumps had long flexible shafts and were originally built with packing. We believed that the conversion to mechanical seals had resulted in greater shaft deflection and made our problems worse. The tighter clearance in the Vespel seems to have helped in that regard. But, we had a number of problems anyway. Even though we exactly followed the design and installation requirements from the maker of the Vespel, we had a set of wear ring failures after a relatively short run. The impeller eye rings had collapsed down onto the rotating rings and been destroyed. Based on some information from another vendor, we think we know the cause.
Vespel has a very low coefficient of thermal expansion in the radial direction, but a very, very high rate in the axial direction. If the Vespel is cut to exactly fit the envelope where the original wear ring had been, it will grow out beyond the holder, axially, once it heats up. The unsupported projection of this ring would normally be toward the high pressure side of the ring. The OD pressure on this unsupported ring causes it to compress inward against the shaft. When the tip touches off on the shaft, it seals off the pressure drop so that the majority of the ID of the ring is now at the lower pressure and the majority of the OD of the ring is at the higher pressure. This is enough to collapse the ring. This can be avoided in a couple of ways. First, a relief cut on the OD that breaks through to the low pressure side can keep the OD from seeing the higher pressure. And second, cutting the ring shorter, so that at full temperature, it does not extend axially beyond the holder, will keep that unsupported projection from deflecting inward to start the failure process. We made both of these changes and have had no further problems.
In this group of pumps, and in a large number of other vertical and horizontal pumps in a wide variety of services, we have had very good results with Vespel and feel it has reduced vibration and shaft deflection, improved low-flow or no-flow survivability, improved overall reliability and increased efficiency. With a little care in the conversion, I would highly recommend it.
Johnny Pellin
