Hydrocarbon Processing - May 2021 - 36

Maintenance and Reliability
TABLE 1. Effects of composition/molecular weight changes
Causes

Primary impact

Effects on compressor

Change in
composition

Higher average molecular weight

1. Polytropic head increases
2. Pressure ratio increases
3. Reduction in surge margin (stability region)
4. Increased stage efficiency

Lower average molecular weight

1. Pressure ratio decreases
2. Lower efficiency
3. Raises required head to meet the discharge pressure

Low suction
pressure

1. Lower density

1. Lower discharge pressure

2. Compressibility factor (Z) increases

2. Lower power requirement

3. Lower NRe and higher friction loss

3. Slightly higher polytropic head at same pressure ratio
4. Lower rotational speed to avoid surge
5. Lower overall efficiency
6. Compression ratio through impeller is reduced

must be evaluated to find the suitable solvent injection requirements to avoid costly, time-consuming offline cleaning. Normally, the amount of solvent required should not be more than
3% of the total flowrate, and excessive flow can lead to serious
erosion problems.
Settle-out pressure should be estimated for a rerated/
restaged compressor system and the design adequacy of the
existing system-including the knockout drum (KOD), interstage coolers, piping system, pressure relief valve, blowdown
system and associated systems like the flare system-should
be checked. Vibration analysis of the piping system is required
to avoid acoustic-induced vibration/flow-induced vibration.
Requirements of alarm and trip schedule changes should be
addressed based on the rerated or restaged compressor operating conditions and composition. The adequacy and requirements of the lube oil system, sealing system, bearing cooling water system, inter-stage cooler load and KOD must be checked.
Brief adequacy check requirements are highlighted in TABLE 1.
While rerating with material of different gas composition,
compatibility with respect to the new composition should be
studied. As per API 617,1 casing should be radially split when
the partial pressure of hydrogen (H2) at maximum allowable
working pressure (MAWP) exceeds 200 psig. The presence of
hydrogen sulfide (H2S) in the gas composition should be considered to check the material compatibility, as per NACE MR
0103, NACE SP 0472 and NACE MR 0175. Gas service where
the partial pressure of H2 exceeds 100 psig or H2 concentration
exceeds 90 mol% at pressure should be considered as hydrogen
service, and the compatibility should be checked during the rerating of compressors.
Compressor discharge temperature should be limited to
150°C (302°F) for natural gas services to avoid polymerization
of hydrocarbons (fouling in the impeller), possible decomposition/cracking, auto-ignition hazards and seal/lube system limitations, even though the compressor components can withstand
higher temperatures. Discharge temperature should be estimated based on the new composition and operating conditions.
The following rules of thumb have been deduced for ease
of understanding and application:
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MAY 2021 | HydrocarbonProcessing.com

* Rule of thumb 1: The head of a compressor varies
as the square of the tip speed and flow handled by the
compressor varies with tip speed and impeller diameter.
* Rule of thumb 2: A 1% increase in speed corresponds
to a 3% increase in flowrate.
* Rule of thumb 3: A velocity of 38 m/sec-45 m/sec is
generally considered as the upper limit for an inlet nozzle
operating on air or gases with acoustic velocities like air.
* Rule of thumb 4: Higher molecular weight or lower
temperature may reduce the allowable flow through
the nozzle.
* Rule of thumb 5: Volumetric flow and molecular weight
define the size of the compressor.
* Rule of thumb 6: In general, reduced gas molecular
weight reduces pressure ratio surges at lower flowrates.
* Rule of thumb 7: Lower suction pressure affects the
overall efficiency of the compressor.
* Rule of thumb 8: Excessive fluctuation in the molecular
weight causes change in the incidence angle to the entry
of the diffuser vane.
Compared to an isentropic process, the discharge temperature at the impeller exit is greater in the polytropic process.
Therefore, the provided work by the impeller is higher in the
case at constant discharge pressure. Even though the exit pressure of both polytropic and isentropic processes are equal, the
discharge temperature and enthalpy differences are greater in
the polytropic process. Discharge temperature in both isentropic and polytropic processes is a function of the pressure ratio,
suction temperature and process exponent. The polytropic process is a function of inlet temperature, pressure ratio, compressibility factor and molecular weight.
Fouling/scaling. Fouling is normally accompanied by lower

efficiency and decreased head due to changes in aerodynamic
performance and flow restrictions. One of the early warning
symptoms is a decrease in the amount of turndown to surge,
i.e., an increase in the minimum flow. This is normally caused
by deposits in the diffuser/guide vane, which offers flow restrictions that often result in surging. Online chemical wash-

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Hydrocarbon Processing - May 2021

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