Page 219 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .22

Fig 3.22.7 Compressor contact seal (Courtesy of Elliott Co.)

system problems have resulted from improper attention to for most bushing seals. This fact will be discussed in the next
the above facts. section.
Contact seals Restricted bushing seals
Figure 3.22.7 shows a typical compressor contact seal. As The last type of seal to be discussed is a restricted bushing type
mentioned, these seals are similar in design to pump seals. In seal. This type of seal is shown in Figure 3.22.8.
order to remove the heat of friction for this type of seal, This particular type of restricted bushing seal utilizes a small
asufficient differential pressure above the reference gas must pumping ring in the opposite direction of bushing liquid flow to
be maintained. Typical differentials for contact seals vary compensate for the relatively large leakage experienced with
between 240 and 345 kPa (35 and 50 psi) differential pres- bushing seals by introducing an opposing pumping flow in the
sure. Leakage rates with a properly installed seal can be opposite direction. Seals of this type can be designed for prac-
maintained between19to38liters(5to 10gallons)per day tically zero flow leakages. However, it must be pointed out that
per seal. in variable speed applications, the pumping capability of the
A limitation in the use of contact seals is the shaft speed; trapped seal ring must be calculated for both minimum and
since the contact seal operates on a surface perpendicular to maximum speeds. Failure to do so can result in the actual
the axis of rotation, the rubbing speed of the seal surface is pumping of gas from the compressor into the sealing system. It is
critical, and as a result, contact seals are speed-limited. Typical recommended that such seals be designed to leak a small
maximum speeds are approximately 12,000 revolutions per amount at maximum operating speed. Any retrofits of equip-
minute. Above those speeds, bushing seals are used, since the ment employing this type of seal should be investigated when
sealing surface is maintained at a lower diameter and corre- higher operating speeds are anticipated. A restricted bushing
spondingly lower rubbing speed. The maximum limit of dif- seal is used exclusively for gas side service.
ferential pressure across contact seals is controlled by the In summary, the basic types of liquid seals used for com-
materials of construction, and is approximately 1,380 kPa (200 pressor applications can be either; open bushing types, contact
psi) differential. As a result, contact seals are usually used for types, or restricted bushing types. Contact types are used pri-
gas side seal applications. They are very seldom utilized for marily on the gas side. Liquid bushing types are used on either
atmospheric seal applications since they are differential pres- the gas or atmospheric side. Restricted bushing types are used
sure limited. exclusively on the gas side.
Since the differential pressure required across the seal face We will now investigate various seal system designs using
is relatively high compared to a bushing seal, contact seals various seal combinations employing the types of seals that have
utilize differential pressure control as opposed to level control been discussed in this section.

193