Page 230 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .23 Compressor Best Practices
entry of lube oil into this port must be provided, in order to given to control of the inter-stage pressure between the primary
prevent the loss of lube oil, or prevent the ingestion of lube oil and backup seal. Experience has shown that low differentials
into the compressor if this line is referenced back to the com- across the backup seal can significantly decrease its life. As in the
pressor suction. A suitable design must be incorporated for this case of liquid seals, a minimum pressure in the cavity between
bushing. Typically called a disaster bushing, it serves a dual the seals of 172e207 kPa (25e30 psi) is usually specified. This
purpose of isolating the lube system from the seal system and is achieved by properly sizing the orifice in the vent or reference
providing a means to minimize leakage of process fluid into the line back to the suction to ensure this pressure is maintained. All
lube system in the event of a gas seal failure. In this system, instrumentation and filtration are identical to that of the pre-
a pressure switch upstream of an orifice in a vent line is used as vious system.
an alarm and a shutdown to monitor flow. This switch uses the
concept of an equivalent vessel, in that increased seal leakage Dual seal and system options for toxic and/
will increase the rate of supply versus demand flow in the
equivalent vessel (pipe) and result in a higher pressure. When or flammable gas applications
a high flow is reached, the orifice and pressure switch setting are
thus sized and selected to alarm and shutdown the unit if nec- There are many field proven options available today for use in
essary. As in any system, close attention to changes in operating toxic and/or flammable gas applications. In this section we will
parameters are required. Flow meters must be properly sized discuss the following systems:
and maintained clean such that relative changes in the flows can - Tandem seals for dry gas applications
be detected in order to adequately plan for seal maintenance.
- Tandem seals for saturated gas applications
- Tandem seals with interstage labyrinth and nitrogen
High pressure applications
separation gas
In this application, for pressures in excess of 6,895 kPa (1,000 - Double seal system for dry gas or saturated gas applications
psi), a tandem seal arrangement or series seal arrangement is
usually used. Since failure of the inner seal would cause signif-
icant upset of the seal system, and large amounts of gas escaping Tandem seals for dry gas applications
to the atmosphere, a backup seal is employed. Refer to The tandem seal arrangement for this application is shown in
Figure 3.23.9, which shows a triple gas tandem seal. For present Figure 3.23.10, and a schematic of this seal in the compressor
designs up to 17,250 kPa (2500 psi), double tandem seals are seal housing is shown in Figure 3.23.11. Gas from the com-
proven and used. pressor discharge enters the port that is closest to the com-
The arrangement is essentially the same as low/medium pressor (labyrinth end), and the majority of the gas enters the
pressure applications, except that a backup seal is used in place compressor through this labyrinth. To ensure that process gas,
of the disaster bushing. Most designs still incorporate a disaster which is not treated by the dry gas system, does not enter the
bushing between the backup seal and the bearing cavity known seal chamber, velocities across the labyrinth should be main-
these days as the barrier seal. Attention in this design must be tained between 6e15 m/sec (20e50 ft/sec). Taking labyrinth

Fig 3.23.9 Dry gas seal: tandem dry
gas seal arrangement (Courtesy of Dresser-
Rand Corp.)

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