Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Mechanical Seal Reliability 563
Seal Failures Related to Pump Hardware
Sometimes seal failures are so strongly related to the pump and its performance
that reliability can be improved only by modifying the pump.
The pump may have a true design problem or may simply have been misapplied.
Some of the more common pump hardware related seal failures
are;
« Excessive shaft deflection.
• Narrow annulus of stuffing box.
• Shaft axial movement.
• Excessive piping loads.
Excessive Shaft Deflection. If the shaft deflects, the seal must move
axially each revolution to compensate. API Standard 610 for centrifugal
pumps specifies a maximum of 0.002-inch shaft deflection at the location
of the seal faces. Some older pumps and non-API pumps may not meet
this specification. In particular, older pumps designed for packing may
have excessive shaft deflection. Shaft deflection is reduced by increasing
the shaft diameter and/or reducing the bearing span or shaft overhang.
Narrow Annulus. In spite of the fact that the vast majority of centrifugal
pumps are equipped with mechanical seals, most older pumps are actually
designed for packing. This is particularly true for heavy duty API
type pumps manufactured prior to the seventh edition of API Standard
610.
A pump designed for packing has a cylindrical "stuffing box" through
which the shaft passes. Packing is compressed within the annulus between
the OD of the shaft and the ID of the stuffing box. This annulus
may be from 3 /s inch to 1 inch in cross section. If the pump is actually
equipped with a seal, the seal must fit inside this annulus. The result is
that very little liquid surrounds the seal.
Seal manufacturers know that their products are more reliable when
operated within a "seal chamber" that contains more liquid than a stuffing
box. However, many pump users insist on the ability to convert from
seals to packing. Also, pump manufacturers prefer to use existing designs
and patterns that are interchangeable with older models. The result
is that most pumps are designed for packing but use seals. It should be no
wonder that the seal is the most common cause of a pump repair.
In actual laboratory tests, seal face temperatures have been observed to
decrease when the stuffing box bore is increased. For example, a stan-