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

560 Centrifugal Pumps: Design and Application
Because mechanical seal failures can be expensive, there is no excuse
for using second rate materials. In many refinery and chemical plant services,
premium grade carbon and tungsten carbide or silicon carbide
faces have proven reliable. Springs are frequently Alloy 20. Glands,
sleeve, shells, adapters, etc. are made from 316SS. In order to reduce
inventories and avoid costly mix-ups, these same materials are also used
in. easy services—even cold water,
Hangup. Loss of axial movement, or seal hangup, is caused when leakage
accumulates and hardens beneath the flexible element (see Figure 21-1). In
most seals, this is the rotating dement. Hangup is a particularly troublesome
problem in high temperature seals because any leakage tends to form "coke,"
There are four approaches to solving hangup problems:
• Change the liquid (external flush that will not solidify).
• Reduce leakage (higher balance ratio and/or narrow face seal).
• Remove leakage (steam quench).
• Remove opportunity to hangup (change to a stationary seal).
Some stationary seals are designed so that there is a relative motion
between the "floating" element and the rotating shaft. This relative motion
makes it difficult for solids to accumulate and cause hangup. Because
stationary metal bellows seals include this feature as well as narrow
faces and are rated for high temperatures, they are often used in
services that tend to cause hangup of conventional seals.
Fretting. Fretting is the damage caused by the constant rubbing of the
flexible element against its drive lugs or the dynamic gasket against the
sleeve. The movement may be less than a thousandth of an inch, but
eventually the sleeve or drive lug is damaged. The damage may cause
leakage directly or may restrict axial movement so that leakage occurs
between the faces.
The solution to fretting problems usually involves one of the following
approaches:
• Improve seal alignment during installation.
« Reduce shaft axial float (adjust bearings).
• Use O-rings for gaskets.
• Change to a bellows seal.
Corrosion. Most mechanical seals are made of relatively corrosion-resistant
materials. For example, a typical off-the-shelf seal might be made of
316 stainless steel, carbon graphite and tungsten carbide with fluoroelastomeric
gaskets. Metal parts are also available in Alloy 20, Hastelloy,
and other corrosion resistant materials. Carbon graphites are available