Page 224 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 3 .23 Compressor Best Practices
control valve were installed, the accuracy of the valve would be In conclusion, remembering the concept of an orifice will
measured in inches of oil instead of psi. Any level control system help in understanding the operation of these systems. Re-
could control the level within two inches, which would be only member, the gas side bushing is essentially zero flow, the at-
0.41 kPa (0.06 psid) variation in pressure differential! mospheric side bushing flow varies with changing differential
This example shows that the most accurate way of controlling across the seal and any seal chamber through flow will change
differential pressure for systems requiring control of small dif- either as a result of differential across a fixed orifice or the
ferential values is to use level instead of differential control. This repositioning of the control valve.
system would be designed such that the combination of the
atmospheric flow and the through flow through the seal would
be equal to the flow from the pump. Seal liquid leakage system

Example 5: Trapped bushing gas side seal: This seal system sub-system’s function is to collect all of the
atmospheric side bushing seal with flowthrough leakage from the gas side seal and return it to the seal reservoir at
design specified seal fluid conditions. Depending upon the gas condi-
tion in the case, this objective may or may not be possible. If the
This system follows exactly the same design as that described in gas being compressed has a tendency to change the specification
Example 4. The only difference would be in the amount of flow of the seal oil to off-specification conditions, one of two possi-
registered in the seal oil drainer. A trapped bushing system is bilities remain:
designed to minimize seal oil drainer pot leakage. Typical values
can be less than five gallons/day. - Introduce a clean buffer gas between the seal to ensure
proper oil conditions
Seal supply system summary - Dispose of the seal oil leakage
In most cases, the first alternative is utilized. Once the seal oil
All of the above examples have dealt with a system incorporating is in the drainer, a combination of oil and gas are present. A vent
one seal assembly. It must be understood that most systems may be installed in the drainer pot to remove some of the gas, or
utilize two or more seal system assemblies. Typical multi-stage a degassing tank can be incorporated.
compressors contain two seal assemblies per compressor body, This concludes the overview section of seal oil systems. As
and many applications contain upwards of three compressor can be seen from the above discussion, it is evident that the
bodies in series, or six seal assemblies. Usually each compressor design of a seal oil system follows closely to that of a lube
body is maintained at the suction pressure to that body, therefore system. The major difference is that the downstream reference
three discreet seal pressure levels would be required and three pressure of the components (seal) varies, whereas in the case of
differential pressure systems would be utilized. The concepts a lube system it does not. In addition, the collection of the ex-
discussed in this section follow through regardless of the amount pensive seal oil is required in most cases and a downstream
of seals in the system. Sometimes, the entire train, that is, all the collector, or drainer system, must be utilized. Other than these
seals referenced to the same pressure. In this case, one differ- two exceptions, the design of the seal system is very similar to
ential seal system could be used across all seals. that of a lube system and the same concepts apply in both cases.

Best
Best Practice 3.23Practice 3.23
Define all dry gas seal and system requirements in the Failure Analysis. All issues were the result of the end user
specification and ensure they are complied with for seal not properly specifying the system for the plant operation
MTBFs greater than 100 months. during the design phase.
Use site, company and industry dry gas seal and system best In all cases, the failures were in critical un-spared services and
practices (see supporting material). resulted in a minimum of 3 days of downtime for each case.
Attach the DGS best practices to the data sheet and include a P&ID
of the required system. Benchmarks
Do not allow any exceptions, since the best practices required will This best practice has been used since the mid-1990s in all pro-
result in optimum seal system safety and reliability for the life of the jects and has resulted in dry gas seal MTBFs in excess of 100
process unit. months.

Lessons Learned
In 2006, FAI experienced over 50 dry gas seal and system
reliability issues by being involved with the Root Cause

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