Page 73 - Compression Machinery for Oil and Gas

64 SECTION II Types of Equipment

require the use of stainless steels and sometimes exotic claddings. Low-
temperature service requires the use of metals with good low-temperature
toughness such as nickel alloy steels, or in the case of ultralow-temperature ser-
vice, austenitic stainless steels. API 617 defines low-temperature service as
having metal temperature below 29°C( 20°F), impact testing is required
by the API 617 standard for materials used in such low-temperature pressure
containing components including nozzles, flanges, weldments, and casing.
The material selection and processing are based on meeting the service require-
ments and economics.
FEA is often used to analyze and design the casing. In this case, the analysis
is performed in accordance with Section VIII, Division 2 of the ASME Code.
The casing is hydrotested at 150% of the maximum allowable working pressure,
which is based on the customer provided relief valve setting or is determined as
125% of the maximum discharge pressure. The limiting factor for horizontally
split casing designs is typically deflection that could result in a failed hydrotest
(Fig. 3.31). The limiting factor for vertically split casing designs is typically
stress under hydrotest conditions, not just at the barrel, but also at the endwall
and in the DGS cavity. This is also true for extrusion gaps in barrel-style com-
pressors of very high pressure. In these applications, meeting the required extru-
sion gap requires substantially thicker casing walls than the allowable stresses
in the cross sections.

Lateral Rotordynamics
Lateral rotordynamics of centrifugal compressors must be considered to ensure
acceptable lateral vibration levels and long-term machine reliability. High
levels of lateral rotor vibration can cause operational issues, unplanned shut-
downs, or even a lack of operability. The most common lateral rotordynamic
issues are often related to critical speeds (synchronous excitation of a natural
frequency) and rotordynamic stability (subsynchronous vibration). This section
discusses lateral rotordynamic modeling and analysis for a centrifugal compres-
sor along with acceptance criteria. The discussion here is limited to a basic dis-
cussion of lateral rotordynamics for centrifugal compressors. Numerous books
and other material have been written on the topic of rotordynamics and machin-
ery vibration [6, 7, 7a].

FIG. 3.31 Casing hydrotest FEA showing potential split-line leakage.