Page 472 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Lube, Seal and Control Oil System Best Practices Best Practice 7 .34

Best
Best
Best Practice 7.34Practice 7.34Practice 7.34
Use flow-through type bushing seals when contact seal An oil seal flow-through design will ensure that the proper amount
surface speeds exceed 100 m/min (300 ft/min), and/or of seal oil cooling is provided at all times during start-up conditions. In
when suction pressures in the compressor are greater addition, seals with high surface speeds will have a higher MTBF if the
than 1,000 kpa (147 psi). additional cooling of a flow through design is provided.
Note: Many non-flow-through seal systems can be modified to
High seal surface speeds and/or low atmospheric bushing flow
a flow-through option during a turnaround, with proper advance
during the start-up of high suction pressure compressors with non-
planning and OEM involvement.
flow-through bushing seals will result in high bushing seal surface
temperatures. These can exceed the atmospheric side seal Babbitt Lessons Learned

material limit of 150 C (300 F), which will result in premature wear of

Failure to have sufficient oil seal cooling during all phases
the seal, which will in turn cause the auxiliary seal oil pump to start with
exposure to a seal oil system shutdown. of operation has resulted in lower than expected oil seal
Oil seal flow through the atmospheric seal is a direct function of the MTBF and shutdowns due to excessive atmospheric
compressor suction pressure. bushing wear.
Compressors with suction pressures of greater than 10 barg (145
psig) are typically started up with lower suction pressures for reasons Benchmarks
of low starting torque and/or nitrogen purge pressure system limits in The best practice has been used during design phases and field
the plant. auxiliary system audits since 1990 to recommend modification to
If a flow-through option in the seal is not employed, the flow through a flow through bushing seal design. All compressor vendors have the
the atmospheric bushing will be considerably less in proportion to the design capability to modify these seals. Of course, since the mid-
square root of the starting suction pressure divided by the normal 1990s most new compressor designs use dry gas seal system which
operating pressure. eliminates the need for this best practice. See chapter 9 for dry gas seal
best practices.

B.P. 7.34. Supporting Material housing system is to positively contain the fluid in the com-
pressor and not allow leakage to the atmosphere. The seal fluid is
There are numerous types of fluid seal systems since the types of introduced between both seal interfaces, thus constituting
seals utilized, sealing fluids and sealing pressures vary widely. a double seal arrangement. Refer to Figure 7.34.2 for a closer
Regardless of the type of seal used, the function of a critical examination of the seal.
The purpose of the gas side bushing seal is to constantly
equipment seal system is as follows: ‘To continuously supply
contain the reference fluid and minimize sour oil leakage. This
clean fluid to each specified seal interface point at the required
differential pressure, temperature and flow rate’. A typical seal bushing can be conceived as an equivalent orifice. This concept
system for a centrifugal compressor is shown in Figure 7.34.1. is similar to bearings previously discussed, with the exception
The system shown is for use with clearance bushing seals. that the referenced downstream pressure of the gas side
Let’s examine this figure by proceeding through the system from bushing can change. In order to ensure a constant flow across
the seal oil reservoir through the compressor shaft seal and back this ‘orifice’, the differential pressure must be kept constant.
through the reservoir. As previously discussed, the concept of Therefore, every compressor seal system is designed to
sub-systems can be useful here. The seal oil system shown can maintain a constant differential against the gas side seal. The
be divided into four major sub-systems: means of obtaining this objective will be discussed as we
proceed.
A The supply system The other seal in the system is the atmospheric bushing,
B The seal housing system whose purpose is to minimize the flow of seal liquid at an
C The atmospheric drain system amount that will remove frictional heat from the seal. This
D The seal leakage system bushing can be conceptualized as a bearing, since the down-
stream pressure is usually atmospheric pressure. In systems
that directly feed into a bearing, the atmospheric bushing
A The supply system
downstream pressure will be constant (approximately 138 kPa
[20 PSI]). However, the upstream supply pressure will vary
This system consists of the reservoir, pumping units, the ex- with the pressure required by the sealing media in the
changers, transfer valves, temperature control valves, and filters.
The purpose of this sub-system is to continuously supply clean, compressor.
As an example, if a seal system is designed to maintain
cool sealing fluid to the seal interfaces at the correct differential
pressure. a constant differential of 34.5 kPa (5 PSI) between the com-
pressor process gas and the seal oil supply to the gas side
bushing, the supply pressure with 0 kPA (0 PSIG) process gas
B The seal housing system pressure, would be 34.5 kPa (5 PSI) to both the gas side bushing
and atmospheric bushing. Therefore, gas side bushing and the
This system is comprised of two different seals. A gas side atmospheric bushing differential would both be equal to 34.5
bushing, and an atmospheric bushing. The purpose of the seal kPa (5 PSI). If the process gas pressure were increased to 20 psi,

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