Shaft Deflection. How to Reduce It
Here are some things you can do to help reduce the deflection caused by operating off the BEP. (best efficiency point):
- Shorten the shaft if possible.
- Go to a larger diameter shaft. You can do this by either replacing the present power end with a larger diameter shaft or in some cases you can replace the sleeved shaft with a solid version.
- Remove the packing and substitute a sleeve bearing in its place. A mechanical seal can then be installed between the face of the stuffing box and the bearing case.
- To prevent operating the pump too far in the throttled direction, you can install a recirculation line between the pump discharge and a low pressure point in the system. This will work for throttled applications if you are prepared to lose some of the pump’s efficiency. This is a common arrangement in boiler feed pump applications.
- Switch to a double volute pump design. The slight loss in efficiency is worth it. The problem is trying to find a supplier of double volute pumps in the smaller impeller sizes of less than fourteen (14) inches (355 mm)
- If the pump’s primary head is friction head a variable speed motor would make sense.
- Tell the operator to operate the pump at its best efficiency point. (Good luck with that one!)
You will notice that I did not recommend “up grading” to a different shaft material. Unfortunately all of the common shaft materials have approximately the same modulus of elasticity, so they will all have the same bending problem.
- Moving a mechanical seal outside of the stuffing box and closer to the pump radial bearing will not reduce shaft deflection, but it will help to preserve the seal life when the shaft does deflect. You can do this with a split seal, some newer cartridge seal designs or an extension to the existing stuffing box.
- On February 17, 2018