Page 156 - Compression Machinery for Oil and Gas

144 SECTION II Types of Equipment

damping to ensure a stable rotor design. For the bull gear bearings, the
limiting factors are shaft strength, lateral rotordynamics, and torsional rotor-
dynamic considerations.
l Step 6: Determine total compressor power and mechanical losses. After pre-
liminary selection of the bearings and gears, it is possible to estimate par-
asitic losses that will be present. These include losses in the gear meshes
and the bearings, which are evident in the increase in oil temperature.
The parasitic losses are added to the stage power, plus additional power
margin, to determine the required driver power.
l Step 7: Evaluate secondary gas path leakage. A multistage IGC has more
seals than a comparable inline compressor. As previously noted, an inline
compressor must only seal at the inlet of the first stage and the discharge
of the last stage, while each stage of an IGC requires its own shaft seal.
The leakage from the gas path must be assessed to accurately estimate
the performance of the unit. The leakage rate will depend upon the type
of seal applied and the pressures. The greater the pressure, the greater the
sealing challenge. Once the leakage rate is known, this can be compared
to original estimates of leakage to determine if additional iteration is
required.

Standards
When considering the oil and gas industry, the primary design concerns are
related to safety and reliability. IGCs and other IG turbomachinery, while hav-
ing superior performance, do have increased mechanical complexity as related
to inline machines. In the design process, the need to balance safety and reli-
ability constraints with capital and operating expenditures drive many design
decisions. To achieve an optimal design requires balancing the aerodynamic
considerations previously mentioned with mechanical concerns.
API 617 [4], Chapter 3 is applied to critical applications. API 672 [5] is a
more general design criteria, still relevant to the oil and gas industry, but limited
more to air machines as opposed to other process gases. In more detail, the
ASME Boiler and Pressure Vessel Code [6] is applied to pressure boundaries
and the casing, and AGMA 6011 [7] is applied to high-speed gear units. To
a great extent, each manufacturer applies their own design criteria to the impel-
ler static and fatigue life assessments.

Drivers
Since the central bull gear of an IGC rotates at relatively low speed, the most
common drivers for IGCs are directly coupled electric motors. Depending on
the machine size, motors with two, four, or six poles are used. Both induction
motors and synchronous motors are chosen as drivers for IGCs, with induction
motors preferred up to 12–15MW and synchronous motors for higher power