Page 436 - Compression Machinery for Oil and Gas

Downstream Chapter 10 415

and off-design conditions. Careful staging will allow the designer to take advan-
tage of each impeller’s best operating range.

Risks of Overload Operation
It is not recommended that compressors be operated outside their defined map
envelope. While the surge line is considered as a firm limit line, where it is com-
monly known that operating in surge is dangerous, operating outside the perfor-
mance map to the right at or near choke is often considered “acceptable” by
operators. Apart from the associated loss in efficiency, there are potential
mechanical risks in operating in overload related to impeller vibration and
distress. Sorokes et al. [7] and Kushner [8] have addressed this issue.

Rotordynamic Considerations
Rotordynamics play a very important role for large LNG centrifugal compres-
sors where the rotational speeds may be low but bearing spans can be very large
(up to 5850mm). In general these large rotors are quite rigid and have good sep-
aration margins. API 617, stability analysis (Level 2) must be always carried out
and cross-checked with manufacturer experience.
Rotor displacement, deflection, amplification factors, and separation mar-
gins must be evaluated. Many LNG compressors have “drive through” arrange-
ments with large couplings on both ends of the machine, and these overhung
weights would affect the rotordynamic model.

Mechanical Run Tests
These are tests specified in API 617 and are essentially a 4-h endurance test run
at full speed in a no-load condition. This is done under vacuum with the inlet and
discharge flanges blanked off. Acceptable vibration characteristics including
the mechanical and electrical run-out levels should be defined before the equip-
ment is purchased based on the recommendations of API 617 and the supplier’s
experience. Generally, the larger the compressor, the lower the operating speed
and therefore the higher the allowable peak-to-peak unfiltered vibration and the
higher the allowable electrical and mechanical run out. An unbalanced rotor
response test should be performed as part of the mechanical running test per
API 617. This unbalanced response test should only be performed on the first
rotor tested. The results of the mechanical run including the unbalance response
verification test should be compared with those from the analytical model.
Spare compressor rotors should also be given a mechanical run test. Testing
with the contract couplings is preferred. If this is not practical, the mechanical
running test should be performed with couplings or simulators that have over-
hung moments within 10% of the contract couplings. Consideration should be
given to using the contract coupling guards if supplier experience is problematic