Page 155 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.7
B.P. 3.6. Supporting Material Maintenance costs, excessive pulsation and associated safety
and mechanical issues have resulted in a great aversion to the use
In the last 10 years, the use of medium (400-1000 rpm) and of lubricated compressors operating above 400 rpm. Typical
high speed (1000-3600 rpm) lubricated reciprocating com- component MTBFs for high speed (greater than 1000 rpm) are:
pressors has gained popularity with project personnel. Granted,
the capital costs are lower, and installation can and has been - Packing e less than 12 months
skid type in many cases. - Piston rings e less than 12 months
Field personnel experience is significantly different and has - Valves e less than 12 months
resulted in many hard lessons learned that now prohibit the use - Shutdown to repair pulsation related issues e less than 6
of this type of reciprocating compressor. months.

Best
Best Practice 3.7Practice 3.7Practice 3.7
Best
Do not use lubricated screw compressors in sour gas Lessons Learned
services or when C6D components are present in the gas The use of lubricated screw compressor types in sour gas
analysis. services and/or where C6D components are present has
Accurately determine the gas sample in the pre-FEED phase and resulted in the following consequences:
note if the gas is sour (contains any H 2 S (hydrogen sulfide)) and/or has Need for daily lube oil filter changeover
carbon components C6þ. Shutdown of compressors weekly for bearing change and complete
If either of the two characteristics noted above are present, do not oil system cleaning (one week duration)
use a lubricated screw compressor. Legal suits costing $MM
Lube oil in these services will deteriorate (mineral or synthetic will be
equally affected), hence reducing the life of bearings, and quickly
plugging the lube oil filters, requiring frequent attention (often once per Benchmarks
day!). I have used this best practice since 2000 when multiple lubricated
screw compressor field issues were experienced. All of these issues
caused the end user significant costs (revenue and maintenance), and
in one case resulted in legal action that lasted for over three years and
did not produce positive results for the end user or vendor.

B.P. 3.7. Supporting Material volume. As a result, pulsations are minimized, suction and dis-
charge valves and troublesome unloaders are not required, nor is
Screw compressors are the newest type of compressors. The high maintenance packing. Since there is only rotary motion, all
dry screw compressor was developed in the late 1940s and of the conventional sealing alternatives are available (including
did not experience wide use until the 1960s for low to well proven dry gas mechanical seals).
medium flow plant air services. On the other hand, re- In addition, the rotary motion significantly reduces the
ciprocating compressors were developed 100 years before ‘footprint’ of the screw compressor unit compared to other
(circa 1850), and centrifugal compressors at the turn of the positive displacement alternatives. The result is an efficient,
last century. In the 1980s, the concept of the dry screw reliable compressor type that is very competitive from an initial
compressor was modified by continuously injecting a liquid cost and installed cost standpoint. The advantages of screw type
(usually lube oil), which enabled much higher compression compressors are presented in Figure 3.7.3.
ratios and simplified the mechanical design by eliminating the As a result of their many advantages, dry screw and flooded
timing gears. Figure 3.7.1 and 3.7.2 illustrate the two major screw compressors have become a dominant force in low to
screw compressor designs. medium flow process applications. They have also continued to
Screw compressors have many inherent advantages over their grow as the preferred type of plant and instrument air com-
competition in the low to medium flow range. Since they are pressor in this flow range.
positive displacement compressors, like the reciprocating type, In the ‘upstream’ exploration and production industry, and in
they will draw a constant inlet volume (assuming constant gas plants, the screw compressor has become ‘the type to use’.
speed), can meet the varying differential pressure requirements For gas gathering, the depleted fields of North America have
of the process and are not significantly affected by gas density utilized the advantages of the screw compressor to provide
changes. a highly reliable, cost-effective service. Many ‘upstream’ appli-
However, unlike their reciprocating cousins, they can ac- cations utilize gas turbines as prime movers for power genera-
complish the above tasks by drawing a continuous, non-pulsating tion, large pump and compressor drives. The screw compressor

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