Hydrocarbon Processing - May 2021 - 37

Maintenance and Reliability
TABLE 2. Dimensionless number analysis
Dimensional number

Signiﬁcance

Normal range

Note

Heat coefficient or
pressure coefficient

Relates head capabilities of a
wheel with its peripheral velocity

Flow coefficient

Relates ﬂow, tip speed and
impeller diameter

Mach number

Relates velocity of gas to velocity
of sound at operating conditions

0.6-0.7 (at inlet)

Reynolds number

Relates inertial and viscous forces

Turbulent ﬂow but
less than mach velocity

0.48-0.54

It can be increased by increasing the number of blades
in the impeller

0.01-0.1

Capacity enhancement. Existing compressor system capac-

ity can be enhanced by the following methods or a combination of the following methods:
* Utilization of design margin
* Addition of stages/impellers
* Optimizing impeller tip speeds
* Adding a parallel compressor train
* Optimizing the impeller design
* Suction boosters
* Eliminating the compressor losses.

DIMENSIONLESS PARAMETERS ANALYSIS
AND FEASIBILITY ANALYSIS
The following dimensional number analysis (TABLE 2) will
provide the scope for restaging/rewheeling of the compressor.
A rationalized approach to describe the aerodynamic characteristics of compression machinery can be used for analysis.
Polytropic head per impeller. As per Simmon's method,2

the maximum polytropic head can be approximately 30 kJ/
kg (3,058 m) for an operating pressure < 100 bar, whereas the
maximum head is limited to 20 kJ/kg (2,038 m) when the discharge pressure exceeds 100 bar (FIG. 1). In low-pressure services (> 100 bar), the maximum head values must be corrected
based on molecular weight using the Brown method.
As per the Brown method, the value of 30 kJ/kg (3058 m) is
to be used for molecular weight between 28 and 30. For molecular weight above this value, 0.3 kJ/kg (31 m) is to be subtracted
from this head value for every unit increase in molecular weight,
whereas 0.6 kJ/kg (61 m) head is to be added for every unit decrease in molecular weight. In general, a single-stage closed impeller compressor can raise the head up to 42 kJ/kg (4,281 m).
The change in molecular weight will have an impact on
the number of stages (FIG. 2) 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.3,4,5,6 It is always better to review the number of im-

Many compressors are operating above mach number,
but the OEM shall be consulted. It should be limited to 0.9
Used to characterize the ﬂow regime and is useful
to ﬁnd friction loss

50
45
40

Max. polytropic head/impeller, kl/kg

ing/offline removal techniques improve the performance of
the compressor. This requirement can be identified during the
design stage and appropriate provisions can be provided. Water-based or petroleum-based solvents can be used to dissolve
the contaminations.
* Rule of thumb 9: Generally, petroleum-based solvents
are not useful to remove salty deposits.
* Rule of thumb 10: The amount of injected solvents
should not be more than 3% of the total flowrate.

Minimum discharge coefficient is 0.008-0.01
Peak efficiency occurs around mid-range 0.04-0.05

Disharge pressure
< 100 bara

35
30
25
20

Disharge pressure
> 100 bara

15
10
5
0
0

Molecular weight , g/mol
FIG. 1. The maximum polytropic head can be approximately 30 kJ/kg
(3,058 m) for an operating pressure < 100 bar, whereas the maximum
head is limited to 20 kJ/kg (2,038 m) when the discharge pressure
exceeds 100 bar.

pellers/stages required with respect to all possible operating
scenarios and molecular weight to avoid unnecessary cost and
time delay at a project's later stages.
* Rule of thumb 11: Polytropic head per impeller and
the number of stages of a compressor depend on the
molecular weight of the gas being compressed.
Optimum impeller tip speed. Optimum tip speed depends

mainly on the type of impeller, molecular weight, material
strength and tip mach number. TABLE 3 can be used for preliminary design. However, for corrosive and low-temperature service
[below -50°C (-58°F)], maximum tip speed is normally limited
to 250 m/sec even though the molecular weight is < 35. Apart
from these criteria, rotational stress, critical speed (mechanical
resonances) and driver capabilities must be considered.7,8,9
During rerating, tip speed can reach up to 274 m/sec with
careful engineering. Gas acoustic velocity is directly related
to specific heat ratio and temperature and inversely propositional to molecular weight. Higher molecular weight gases are
associated with lower acoustic velocities and lower allowable
tip speed. It should be noted that the impeller material stress
level is directly proportional to the square tip speed. Material
strength often plays a critical role when low molecular weight
gas is being handled with high head requirements. Cases exist
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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