Page 435 - Compression Machinery for Oil and Gas

414 SECTION III Applications

HRTS and Operating Range (Surge to Choke Margin)
A positive continuous head rise to the compressor surge point at constant speed
is crucial to stable compressor operation and reliable responsive process con-
trol. The achievable rise to surge is directly influenced by the required compres-
sor operating range. The HRTS is a measure of the rise in the head (at constant
speed) when the operating point moves toward the surge line. Other common
terms related to HRTS include pressure rise to surge, and pressure ratio rise
to surge. In some cases, specifying a hard number such as 5% may result in
a reduced efficiency and also a reduced “overload” capacity. Selecting a lower
HRTS may result in a higher efficiency and an increased operating margin.
With refrigeration machines, therefore, specification of excessive HRTS and
surge margin criteria may result in a moderate overload capacity.
The refrigeration process requires operation at nearly constant discharge
pressure due to condensing temperature. Either the driver speed or suction pres-
sure must reduce as flow moves toward the surge. Reduction of suction pressure
below atmospheric pressure is usually not desirable. Due to various aerody-
namic laws, the surge to stone-wall/choke flow range for refrigeration service
is less than that seen on various other types of applications, hence, imposition of
an excessive HRTS and/or surge margin criteria may result in only minimal
overload capacity. A recent application has shown a compressor working well
with HRTS below 2% on the guarantee point and lower on alternate operating
points.

Head per Section
This is an area where there is an interaction between mechanical design and
aerodynamic design. A larger number of impellers may cause an excessively
long shaft and thus a high critical speed ratio. Fewer impellers per section would
result in higher head per stage. This, in turn, calls for higher speed operation at
higher Mach numbers. Operation at higher Mach numbers will cause a restric-
tion in operating range, and cause the head rise curve to flatten and may reduce
the efficiency. Further, as the flow reduces, either the driver speed or the suction
pressure must be reduced.

Aerodynamic Mismatching of Stages
As more impellers are added to the compressor, “off-design” operation gets
affected and stability inherently reduces. This occurs because the individual
section performance maps dictate the overall surge point. The situation is fur-
ther complicated by the large side-load flows and the complexities of uniform
mixing at the sidestream mixing plane. Impellers must be carefully selected in
concert with the other impellers of the compressor section and interactions
between different compressor stages must be well understood to ensure that
the overall compressor has an acceptable operating range under both design