Hydrocarbon Processing - May 2021 - 38

Maintenance and Reliability
9

TABLE 3. Tip speed limitation

No. mechanical stages

Molecular weight
< 35

310

< 45

250

< 65

200

< 120

150

5
4
3
2
10

Molecular weight, g/mol
FIG. 2. The change in molecular weight will have an impact on the
number of stages of the compressor since a lower molecular weight
calls for more required head to achieve the desired pressure, leading to
greater pressure coefficient on the impeller.
Anti-surge control line
Compressor rated point

Variable-speed drive
Maximum continuous speed
Speciﬁed
operating
condition
Head

Normal operating condition
100% speed
105% = 100 × 1.05

Predicted surge
limit

B
Any operating speed

98%
(assumed)
Speciﬁed operating condition

Miminum operating speed C
Maximum ﬁrst critical ﬂexible shaft

Predicted capacity limit
Operating stability
Turndown range

Inlet volume ﬂow capacity

FIG. 3. The diagram presented in API 617 outlines the compressor
stability requirements.

in which machines are operated at more than machine mach
number without any issues-the important factor is the inlet
relative mach number, which is proportional to the machine
mach number and flow coefficient. Speed shall be optimized
with respect to lower/upper critical speeds. The maximum
speed is limited by aerodynamic and mechanical limitations.
* Rule of thumb 12: Impeller maximum speed is limited by
mach number, material strength (yield stress at maximum
continuous speed) and critical speed (resonance).
* Rule of thumb 13: Some original equipment
manufacturers (OEMs) set the overload limit based on
the relative Mach number of 0.96 or lower.
Utilization of design margin. Utilizing the existing over-

load limits is a common method of optimization. However,
such limits are based on experience and function of machine
mach number limitations, gas composition, number of stages,
etc. It is well known that a compressor running at a high machine mach number or handling high molecular weight gases
will have less overall flow range than a compressor that runs at
a low machine mach number or handles low molecular weight
38

Average tip speed m/sec

MAY 2021 | HydrocarbonProcessing.com

gases. Therefore, the allowable overload margin may vary. For
example, the high molecular weight compressor with an overload margin of 120% will have a 140% design margin when it
handles low molecular weight.
Adding additional trains. The addition of parallel compres-

sor trains is quite common in the oil and gas industry; however,
deciding the parallel configuration option-a series-parallel
(each casing will have independent driver) and tandem-parallel (single driver for multiple casings)-plays a key role in plant
reliability, turnaround capabilities and efficiency.10,11,12
In general, a tandem-parallel arrangement requires more
casing compared to a series-parallel arrangement for the given
flowrate, resulting in higher increased fixed capital. More casing calls for more piping, sealing, lube system and accessories,
which adds to the cost in a tandem-parallel arrangement. The
loss of a compressor in a series-parallel arrangement is associated with variation in the inter-stage pressure, and the impact
should be studied during the engineering stage. The loss of
driver in the series-parallel will result in flow instabilities in the
other stage. Loss of driver in the tandem-parallel will result in
capacity reduction. The tandem-parallel represents a simple
control system. Careful analysis of the configuration during the
revamp/installation of additional trains should be exercised
with respect to reliability, availability, safety, capital cost, space
constraint and turndown flexibility.
Number of impellers. The number of impellers can be estimated by dividing the total polytropic head by the maximum
head per impeller. The inlet flow coefficient can be improved by
improving the eye diameter and impeller diameter ratio. Normally, it is preferred to have the same diameter for smooth flow.
However, the maximum number of impellers in a single casing
is normally limited to 10 (or 10 impeller stages) due to rotodynamic, aerodynamic, operational and design constraints.
Casing. Horizontally split casings are typically used for lower
pressure applications (up to approximately 40 bar discharge
pressure), while vertically split (barrel type) casings have successfully been used for discharge pressures up to 800 bar. During rerating/restaging, casing will not usually be modified.
Compressor stability. Both operational and aerodynamic
stability should be considered during the rewheeling/rerating
of compressors. FIG. 3 presented in API 617 outlines compressor stability requirements.
Suction boosters. Gas density can be increased by many
ways, including suction boosting and refrigeration. Simple suction boosting is considered as an additional stage.

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Hydrocarbon Processing - May 2021

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