Boulden supplies non-metallic composite materials like Vespel® CR-6100 and Boulden B-Series which can be used to fabricate pump shaft stabilizing bushings. Our materials are dimensionally stable, non-galling, non-seizing, easily machined, and very durable
Why do you need to stabilize the pump shaft?One of the main causes of mechanical seal or packing failure is excessive pump shaft deflection. Pump shaft deflection creates uneven loads at the sealing areas, premature wear, and reduced component life. Excessive pump shaft deflection can also result in high vibration which reduces bearing life. In extreme cases, excessive shaft deflection can even cause pump shaft failure.
Why does a centrifugal pump suffer from excessive shaft deflection?There are two primary reasons for excessive pump shaft deflection. The first is the pump design. Many pump designs incorporate long, thin shafts. The shaft stiffness ratio, or L/D ratio, provides an indication of the flexibility of the pump shaft. A simple way to think of it is that the longer and thinner the pump shaft, the more susceptible it is to excessive pump shaft deflection. The second cause of shaft deflection is operational. Centrifugal pumps generate radial forces during operation. These forces are related to the flow rate. When a pump is operating near its BEP (best efficiency point) the radial force is at its minimum level. As the pump operates further away from BEP, these forces increase. To simplify this aspect, low flow operation creates high radial forces which can cause excessive shaft deflection.
The Shaft Stabilizing Bushing SolutionA common method of addressing pump shaft deflection is to install a pump shaft stabilizing bushing in the bottom of the pump stuffing box or mechanical seal chamber in place of the original throat bushing. The idea is to install a longer throat bushing with reduced clearance to reduce shaft deflection at the mechanical seal or packing.
- The bushing should be as long as possible to create maximum bearing support.
- The clearance at the bushing should be reduced sufficiently to minimize pump shaft deflection.
- Conversely, the clearance needs to allow the mechanical seal flush (assuming API Plan 11) to maintain a sufficient flow rate.
- If it is an abrasive service, a clean flush for the pump stabilizing bushing may be needed.
- The material you use needs to be capable of withstanding operating temperature and chemicals, rubbing contact with the shaft, and can be machined and installed into your existing geometry.