Page 433 - Compression Machinery for Oil and Gas

412 SECTION III Applications

FIG. 10.8 Representation of velocity triangle for inlet relative Mach number (Mr1s¼W1S/a 0 ).

FIG. 10.9 The operating range is affected by the peripheral and inlet relative Mach numbers.

Impeller Head per Stage
For centrifugal compressors operating with high molecular weights and low-
temperature gases, users and manufacturers have traditionally tended to limit
the head per impeller within the range of 3200–3400m based on considerations
that stages with high head could have a low pressure rise to surge and a narrow
operating range between the design point and the surge. Current experience
shows it is possible to design centrifugal impellers with heads in the range of
5000–6000m with head coefficient above 0.54 keeping satisfactory character-
istics of flow stability and operating range.
Design trade-offs come into play here as lower head per impeller means
more impellers resulting in an increase in bearing span, which could lead to
rotordynamics issues. Higher head per impeller requires increased rotational
speed resulting in operation at higher Mach number levels which as stated ear-
lier can restrict the operating range and reduce efficiency. High head coefficient
stages provide a narrower operating range and lower rise to surge. Low head
impellers provide much greater head rise to surge (HRTS), are easier to control,
and provide a wider operating range.