Attachment 4_How to trim LP stage flow limits for 2-stage compressions
Final stage T2
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By: CangTo CHEAH (1st
June 2015), Senior Rotating Equipment Engineer (Technip)
Project: SLIC HCL recycle project
Equipment / type: Chlorine gas compressor / centrifugal
Document title: Evaluation of compressor discharge temperatures when chilled water supply fails at inter-stage
cooler, based on MDT’s proposal.
Introduction
The chlorine compressor proposed by MDT consists of two process stages; and inter-stage coolers will be supplied
by MDT. The inter-stage cooling itself consists of two shell-and-tube coolers configured in series, note that coolant
for 1st
cooler is cooling water (supply & return temperatures are 33 deg. C and 43 deg. C, respectively) and coolant
for 2nd
cooler is chilled water (supply & return temperatures are 5 deg. C and 10 deg. C, respectively). 1st
cooler
discharge temperature for process gas is 55 deg. C per MDT’s preliminary proposal. The LOPA (layer of protection
analysis) performed in the earlier stage of SLIC HCL recycle project places an action item for EPC contractor to look
at mitigation measures to limit compressor discharge temperature below 120 deg. C, during failure of chilled water
supply. There are various ways to reduce discharge temperature for centrifugal compressor, namely:
a) Improved efficiency b) Reduced head c) Reduced inlet temperature
Noting that item a) can be realized in two ways. Efficiency can either be improved by varying suction volume flow
rate along the x-axis of performance curve “efficiency vs. inlet volume flow rate”, or in case if inlet volume flow
rates cannot be varied (due to process restrictions), then re-selection of impeller family may be considered. This
study is based on the former (with combinations of b and c), as maintaining inlet volume flow rate at abnormal
process conditions (e.g. failure of chilled water supply) is not a mandatory requirement.
Basis of compressor performance calculation
1) Thermodynamic model
Lee-Kesler-Plocker corresponding state is used, with binary interaction coefficients extracted from HYSYS.
Polytropic temperature and polytropic volume exponents are used to calculate discharge temperature and
discharge pressure, respectively.
2) Aerodynamic model
MDT’s performance curves (polytropic head vs. actual inlet volume flow rate and polytropic efficiency vs. actual
inlet volume flow rate) are modeled with 4th
order polynomial curve-fitting technique (i.e. least square method).
Inputs are: Inlet volume flow rate & inlet pressure for LP stage, suction temperature for both LP and HP stages,
inter-stage pressure loss (assumed 0.15 bar, per MDT’s data sheet), and project specific gas mixture.
Calculated outputs are: Polytropic head, polytropic efficiency, discharge temperature, discharge pressure,
compression power, polytropic exponents, compressibility factor, and mass flow rate.
Conclusion: On page 8
CangTo Cheah
2015.06.01
18:12:48 +08'00'
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Normal: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 116 deg. C. Okay
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Maximum: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 119 deg. C. Okay
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Case A: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 95 deg. C. Okay
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Case B: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 123 deg. C. Not okay.
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Case B: 50 deg. C cooling water cooling discharge temperature
Remark: Final stage discharge temperature approx 120 deg. C. Okay
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Case C: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 129 deg. C. Not okay
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Case C: 50 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 119 deg. C. Note that inlet volume flow rate 4191 m^3/hr (per
MDT’s Q_inlet) on LP stage would yield final stage discharge temperature 124.7 deg. C (based on 50 deg. C inter-
stage temperature); Therefore a larger Inlet volume flow rate, i.e. 4550 m^3/hr is used to estimate HP stage T2.
With Q_inlet 4550 m^3/hr, discharge temperatures on both LP and HP stages are reduced (LP stage T2 is reduced
with improved efficiency (from 82.58% to 83.60%) & reduced head; HP stage: even though efficiency is reduced
(from 82.65% to 81.60%), low T2 is however compensated by reduced head, as T2 is directly proportional to
compression ratio). See Appendix 1 for more details (page 10 to 15).
Conclusion: It is proposed to reduce 1st
cooler discharge temperature (from 55 deg. C down to 50 deg. C), so that
HP stage discharge temperature can be controlled below 120 deg. C during a chilled water supply failure, and
chlorine compressor train can be coast-down in a safe manner.
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Case D: 55 deg. C cooling water cooler discharge temperature
Remark: Final stage discharge temperature approx 115 deg. C
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Appendix 1 (page 10 to 15)
Case C with Q_inlet 4191 m^3/hr and inter-stage temperature 50 deg. C
Performance curves: Refer to page 11 and 12 for LP and HP stages, respectively.
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Case C with Q_inlet 4550 m^3/hr and inter-stage temperature 50 deg. C
Performance curves: Refer to page 14 and 15 for LP and HP stages, respectively.
