Pump School: Pump and driver alignment

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 

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