Dry Running Tests Utilizing Silicon Carbide Bearings And Polymer Lubricating Strips With Conductive And Nonconductive Containment Shells In An ANSI Magnetic Drive Pump

Abstract

As users continue to turn to sealless pumps to solve their leakage (emission) problems they are demanding robust designs that are more tolerant of system upsets and have the ability to survive short periods of dry running. Silicon carbide bearings are known to be the least forgiving during dry running, but are still the best universal choice. The purpose of this study is to demonstrate means to extend the life of silicon carbide bearings in a dry running environment. A test rig was designed to simulate broken prime and drained pump dry run conditions. The test pump was a 1 ½ x 1 x 8 ANSI magnetic drive pump driven by a 15hp 3600 rpm motor. Both sintered silicon carbide and 70 micron controlled porosity silicon carbide with Teflon lubricating strips were tested in conductive and nonconductive containment shells. Damage assessment was made after each test. Results of water tests show the robust pump design and proper bearing alignment is essential. Under broken prime conditions with a nonconductive containment shell, controlled porosity silicon carbide with Teflon lubricating strips ran for 4 ½ hours without overheating, failure or damage. The broken prime dry run time for conductive containment shells is partly dependent upon the magnitude of eddy current heat generated and the time required to boil off the product lubricant; for the models tested, this time was approximately 32 to 35 minutes. Drained casing tests with nonconductive containment shells show a doubling of dry run time with controlled porosity silicon carbide and Teflon strips over that of sintered silicon carbide. A drained casing test with conductive containment shell configured with controlled porosity silicon carbide and Teflon lubricating strips had a dry run time of 36 minutes and underwent much less bearing damage than a broken prime test with sintered silicon carbide and conductive containment shell. The addition of controlled porosity silicon carbide and Teflon lubricating strips can add substantially to dry run life and help reduce damage to the product lubricated bearings.