Page 431 - Centrifugal Pumps 2E

Mechanical Seals 395

The measured leakage from each end of this horizontal pump was 10 to
120 ppm. This installation is a rotating seal head with a tungsten carbide
insert running on a silicon carbide mating ring. A steam quench is being
used on the atmospheric side of the seal.
Hydropads are also used to aid the lubrication process on the refinery
application shown in Figure 17-32. The pump is used in a hydrofluoric
alkylation unit to move hydrofluoric acid. To protect the sealing system,
a clean injection of isobutane is flushed in at the seal faces. The original
seals experienced short seal life due to chemical attack from the hydro-
fluoric acid. The operating conditions for this application are:

• Liquid sealed: isobutane/hydrofluoric acid
» Stuffing box pressure: 240 psig
• Temperature: 89°F
• Specific gravity: 0,945
• Speed: llSOrpm
» Seal size: 3.5 inches

This hydropadded seal has added years to the service life of the unit,
The materials of construction for the seal faces are solid silicon carbide
running on solid silicon carbide. This material combination was selected
for its excellent corrosion resistance to hydrofluoric acid, because corro-
sion is a common cause for seal failure on these types of applications.
Hydropads or lubrication recesses when properly applied will result in
improved seal life. This design feature is the first step in the process of
advancement to non-contacting seals.

Figure 17-32. Mechanical seal installation for refinery HF Alkylation Unit (cour-
tesy John Crane).