Alignment in the pump business means that the centerline of the pump is aligned with the centerline of the driver.
While this alignment was always a consideration with packed gland pumps, it is critical with mechanically sealed pumps, especially if you are using rotating seal designs where the springs or bellows rotate with the shaft.
A little misalignment at the power end of the pump results in a lot of misalignment at the wet end, and unfortunately that is where the seal is located (in most pump applications).
Problems caused by misalignment:
- It can cause rotating mechanical seals to move back and forth axially two times per revolution. The more the seals move the more opportunity for the lapped faces to open
- Packing could support a misaligned shaft. A mechanical seal cannot
- Misalignment will cause severe shaft or sleeve fretting if a spring loaded Teflon as a secondary seal is used in the mechanical seal design
- The pump bearings can become overloaded
- The misalignment could be severe enough to cause contact between stationary and rotating seal components
- The wear rings can touch and rub
- The shaft can come into contact with the restriction bushing (often found at the end of the stuffing box)
- The shaft or sleeve can contact the stationary face of the mechanical seal.
- The shaft can contact the disaster bushing in an API (American Petroleum Institute) gland
- The impeller could contact the volute or back plate
You must start with a pump and driver in good repair, irrespective of the alignment method you select. A thoroughly aligned piece of garbage is still a piece of garbage.
The following checks should also be made:
- A straight shaft that has been dynamically balanced
- Good wear rings with the proper clearance
- The correct impeller to volute, or backplate clearance
- The elimination of “soft foot”
- Eliminate all pipe strain
- Good bearings installed on a shaft with the proper finish and tolerances
- A good mechanical seal set at the proper face load.
Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters” Phone: 1300 789 466 or visit www.kelairpumps.com.au
