Page 225 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .23
B.P. 3.23. Supporting Material Comparison of a liquid and gas
sealing system
The reliability of critical equipment is dependent on the
reliability of each component in every auxiliary system that
Figure 3.23.1 shows a liquid sealing system. Compare this
is connected with it. How do we maximize critical equip-
ment reliability? The easiest way is to eliminate the auxiliary system to Figure 3.23.2, which shows a gas seal system, if the
systems. Imagine the opportunity to eliminate all of the same compressor were retrofitted for a gas seal. WOW e what
components; pumps, filters, reservoirs, etc. and thereby in- a difference. Why are there such a small number of components
crease reliability and, hopefully, the safety of the equipment. for the gas seal system? As an aid, refer to Figure 3.23.3, which
The gas seal, as used in compressor applications, affords the shows a typical pump liquid flush system as specified by the
opportunity to achieve these objectives. However, the gas American Petroleum Institute. This system incorporates a liquid
seal is still part of a system and the entire gas seal system mechanical seal and utilizes pump discharge liquid as a flush for
must be properly specified, designed, maintained and oper- the seal. Refer now to Figure 3.23.2 and observe the similarities.
ated to achieve the objectives of optimum safety and re- It should be evident that a gas seal system is simpler in com-
liability of the critical equipment. In this chapter, the pressor applications than a liquid seal system merely because the
gas seal utilizes the process fluid. This is exactly the same case
principles of gas seal design will be discussed and applied to
various gas seal system types. In addition, best practices will for a pump. By using the process fluid, and not a liquid, one can
eliminate the need to separate liquid from a gas, thereby totally
be discussed for saturated gas systems as well as shutdown
philosophies. eliminating the need for a liquid supply system and the need for
a contaminated liquid (sour oil) drain system.
Referring back to Figure 3.23.1, we can see that the following
major components are eliminated:
System function
1. The seal oil reservoir
The function of a gas seal system is naturally the same as a liquid 2. The pumping units
seal system. The function of a fluid seal system, remembering 3. The exchangers
that a fluid can be a liquid or a gas, is to continuously supply 4. The temperature control valves
clean fluid to each specified seal interface point at the required 5. The overhead tank
differential pressure, temperature, and flow rate. Therefore, one 6. The drain pot
would expect the design of a gas seal and a liquid seal to be very 7. The degassing tank
similar, which, in fact, they are. Then why are their systems so 8. All control valves
different? 9. A significant amount of instrumentation

NOTE GAS REF.UNE BUFFER
LEVEL IN OH TANK IS OH GAS
MAINTAINED APPROX 10° TANK S.O. SUPPLY
ABOVE CENTRIFUGAL OF LT
PI
COMPRESSOR SHAFT LAEY
ATMOSPHERIC SEAL
BUSHING
TI
CONFAL SHIFT
PROC. GAS
FILT FILT
GAS SIDE
FLOW THROUGH
PDI PDI OIL SUMH VENT
TRANS. VALVE VENT DEVASTER GAUGE
(S) (S) SOUR OIL
TCV TCV LCV
LEVEL
DPANER
POT
TANK
EXCHANGER EXCHANGER DEGASER
MANWAVENT

SO RESERVOIR
LEVEL
GAUGE
M
T TI
MAIN PUMP ALDC PUMP NORMAL LEVEL
HEATER
Fig 3.23.1 Typical seal oil system for clearance bushing seal (Courtesy of M.E. Crane, Consultant)

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