Page 432 - Centrifugal Pumps 2E

396 Centrifugal Pumps: Design and Application

True non-contacting seals are being applied to pumping equipment and
will change the way a seal is viewed for a given application. When pump-
ing a light hydrocarbon near its vapor pressure, it is more efficient to
allow the liquid to flash to a gas in the stuffing box and use a dry running
seal to seal the gas. This is done with a non-contacting seal design. This
most efficient method in achieving non-contact in operation is through
the use of the spiral groove face seal. This patented concept was devel-
oped by John Crane Inc. for use in sealing high performance gas com-
pressors and is illustrated in Figure 17-33. The spiral groove pattern is a
series of logarithmic spirals recessed into the hard mating ring. The un-
grooved portion of the seal face below the spiral is called the sealing
dam. The spiral groove pattern is designed to rotate only in one direc-
tion. As the seal face begins to rotate from the outside diameter of the
seal faces to the groove diameter, gas is compressed and then expanded
across the sealing dam. This will generate sufficient opening force to
separate the seal faces by a few micro-inches. Seal balance, face width
and spiral groove diameters are all critical and determine the static and
dynamic operation of the seal during periods of startup and shutdown
when the seal faces contact. A comparison of hydropadded and spiral
groove faces is made in Figure 17-34,

Figure 17-33. Dry running face design (courtesy of John Crane).