Assessing Rubber Expansion Joints

Contractors, maintenance personnel, and engineers have used rubber flexible expansion joints on the suction and discharge of pumps to reduce vibration levels, while also providing an easy means to install and disassemble pumps. A theme used by rubber joint manufacturers to promote these joints is that they “relieve pipe and anchor stresses, compensating for misalignment.” Rather than...

At the startup of two different facilities, pump vibration levels exceeded ANSI/HI 9.6.4 vibration criteria for vertical end suction wastewater pumps. Pump startup technicians, contractors, and pump manufacturers were puzzled as to the root cause of the excessive vibration. Several corrective methods, such as loosening nuts on the anchor bolts, installing rubber washers under the pump and motor...

At the first facility, a systematic step-by-step investigation was performed, starting with evaluation of the pumping station hydraulic characteristics. The suction specific speed was investigated to determine if there was a potential for cavitation. The Hydraulic Institute Standard, ANSI/HI 9.6.1 (HI) Rotodynamic Pumps Guidelines for NPSH Margins — 2012 recommends suction specific speed to...

At another facility with rubber expansion joints on the discharge, similar vibration issues were encountered. Vibration readings taken on the pump upper bearing were 15.01 mm/s (0.591 ips), which significantly exceeded the HI maximum allowable for this wastewater pump. Figure 5 (right) is a plot of the vibration measurement on the pump.

Vibration spikes were noticed in both installations when the pumps operated at their blade pass frequency. Issues that contributed to blade pass vibration were