Functional Specification Export Gas Compressor PDF [PDF]

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SHELL PETROLEUM DEVELOPMENT COMPANY

CONCEPTUAL DESIGN FOR CAWTHORNE CHANNEL GAS INJECTION / SUPPLY PROJECT

FUNCTIONAL SPECIFICATION FOR EXPORT GAS COMPRESSION AT CAWTHORNE CHANNEL GAS PLANT

B1

15.09.99

ISSUED FOR EPC TENDER

J.B. NITZSCHE

A3

29.06.99

RELEASED

A2

17.06.99

A1

04.05.99

REV

DATE

F. J. SHAW

E. A. AKINSIPE

A. LENEVEU

A. LENEVEU

P. KERGUSTANC

E. W. OBOTT

ISSUE FOR APPROVAL

A. LENEVEU

A. LENEVEU

P. KERGUSTANC

E. W. OBOTT

ISSUE FOR COMMENT

A. LENEVEU

A. LENEVEU

P. KERGUSTANC

E. W. OBOTT

REASON FOR ISSUE

NETCO/TECHNIP GEOPRODUCTION NTPG 2 AJOSE ADEOGUN STREET PO BOX 74173 VICTORIA ISLAND LAGOS 2 AJOSE ADEOGUN STREET PO BOX 74173 VICTORIA ISLAND LAGOS

PREPARED

CHECKED

DISCIPLINE ENGINEER

DISCIPLINE CHECK

APPROVED PROJECT MANAGER

T. DU-FRAYER

APPROVED QA

SPDC APPROVAL SIGNATURE:

Document No.: CCGP-SF-P-011

REV B1

FUNCTIONAL SPECIFICATION FOR EXPORT GAS COMPRESSION AT CAWTHORNE CHANNEL GAS PLANT

TABLE OF CONTENTS

1.0

GENERAL

2.0

PROCESS DESCRIPTION

3.0

SYSTEM LIMITS & INTERFACES

4.0

RELIEF AND BLOWDOWN

5.0

PERFORMANCE REQUIREMENTS

6.0

INSTRUMENTATION AND ELECTRICAL

7.0

MATERIALS REQUIREMENTS

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FUNCTIONAL SPECIFICATION FOR EXPORT GAS COMPRESSION AT CAWTHORNE CHANNEL GAS PLANT

1.0

GENERAL

1.1

Introduction This specification describes the functional requirements for the export gas compression system of Cawthorne Channel Gas Plant in the Cawthorne Channel Gas Injection/Supply Project. The main objective of the function is to compress associated gas from 2 pressure levels (LP: 3.5 bara, HP: 9.1 bara) up to 111 bara for export via a manifold to various users.

1.2

Reference Documents SPDC Specifications SPDCE-AGP-001 DEP and SPDC standards to be used for Associated Gas Projects SPDCE-AGP-009 Common Design Philosophy Project Specifications/Documents COMM-ST-P-002 Process Control Philosophy CCH-SP-M-005 Mechanical design Specification for GT driven Centrifugal Compressor Packages Process Flow Schemes – Cawthorne Channel Gas Plant CCGP-DW-P-101 Compression Facilities CCGP-DW-P-101 Dehydration & Export Process Engineering Flow Schemes – Cawthorne Channel Gas Plant CCGP-DW-P-211 1st Stage Suction Scrubber & Compressor CCGP-DW-P-212 1st Stage Aftercooler CCGP-DW-P 213 2nd Stage Suction Scrubber & Compressor CCGP-DW-P 214 2nd Stage Aftercooler CCGP-DW-P 215 3rd Stage Suction Scrubber & Compressor CCGP-DW-P 216 3rd Stage Aftercooler & Disch. Scrubber CCGP-DW-P 217 4th Stage Suction Scrubber & Compressor CCGP-DW-P 218 4th Stage Aftercooler & Disch. Scrubber

2.0

PROCESS DESCRIPTION The gas compression facilities at Cawthorne Channel consist of two compression trains (2 x 50% capacity) driven by gas turbines of approximately 13MW ISO rating each. The trains each consist of four stages with associated aftercoolers and scrubbers. Between third and fourth stages, gas is routed through two dehydration units (2x100%) which are described in functional specification for Gas Dehydraton at Cawthorne Channel Gas Plant CCGP-SF-P-013. Following dehydration, the (rich) gas is routed to the nearby Global Energy Recovery Ltd. (GERL) facility, where condensates are removed from the gas. The lean gas is subsequently returned from GERL to the 4th stage suction of the export compressors, at essentially the same pressure level as at 3rd stage

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FUNCTIONAL SPECIFICATION FOR EXPORT GAS COMPRESSION AT CAWTHORNE CHANNEL GAS PLANT

discharge. The GERL facility can be bypassed such that the gas from downstream the TEG contactor can be routed directly to the 4th stage suction of the export compressor. The 4th stage suction gas mole weight can thus vary very considerably, and quite instantaneously, due to a trip at GERL. The gas separated in the existing LP and HP separators at the Cawthorne Channel flow stations CCH1, CCH2 and CCH3 is routed to respectively the inlet of the 1st and 2nd stages of the export compressors. Each stage, including the fourth, has a suction scrubber to remove entrained liquids from the suction feed. Suction scrubbers will be single stage vane pack separators with schoepentoeter inlet devices. The 3rd and 4th stages have a discharge scrubber fitted with demisters. The gas from each compressor discharge is cooled by an air-cooled aftercooler, rated to cool the compressed gas to 410C (a 100C approach to the mean maximum ambient temperature of 310C). Each aftercooler will cool the gas to a specified temperature to prevent hydrate formation within the process and, for the 3rd stage, to operate the TEG contactor at a constant temperature of 45°C, at the maximum ambient temperature of 35 0 C. The 1st, 2nd and 4th stage aftercoolers will achieve this by means of a temperature-controlled bypass around the cooler. The 3rd stage aftercooler will use temperature controlled variable speed fans, as hydrate formation across the cooler is of concern at reduced flows through the cooler. The compression ratio of each stage will be selected such that discharge temperatures do not exceed 1700C during normal operation. Although slightly higher temperatures can be tolerated under transient conditions, prolonged operation at temperatures above 1700C is not desirable and is not permissible if coking is predicted. The compressor is to be fitted with tandem dry-running gas seals, which will use process gas taken from the compressor at a suitable pressure level as the primary buffer gas. Secondary (barrier labyrinth) buffering will be by air from the plant instrument air system. Compressor capacity control will be based on suction pressure control with discharge pressure override. It will be achieved by means of speed control of the gas turbine driver, with corrections for imbalance between LP and HP gas availability being made by individual stage recycle. Each stage of compression of each train will have a dedicated capacity control recycle valve, except the 2nd and 3rd stage, which may have a shared capacity control recycle valve. The compressor discharge pressures of the 3rd and 4th stages are maintained at a constant value by a pressure controller downstream of the TEG contactor and at the export gas metering respectively. Capacity control is achieved primarily by speed control, supplemented by recycle, and overridden by discharge pressure control. If 1st, 2nd or 3rd CCGPSFP011B1

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stage suction pressure increases, the relevant separator, and upstream stage, supply more gas than is being compressed. The recycle valve, if open, will close and, if suction pressure is still rising, the speed will increase until either maximum speed or maximum discharge pressure is reached. If suction pressure still increases, the gas available exceeds the compressor capacity and the spillover valve to flare on the relevant separator outlet will open to flare the excess gas. In case of falling suction pressure, the spillover valve to flare closes, the speed reduces and, on reaching minimum, the recycle valve opens. The performance controller will determine the compressor speed based on the highest supply available from any of the separators. For instance, if the HP separator (2nd stage suction) pressure is falling (due to shutting in of some HP wells), the speed will not be reduced (to avoid unnecessary flaring of LP gas), but only the 2nd stage recycle will open. A station performance controller will ensure that the required compression load is shared equally by the two export trains. Each compressor stage is protected by a dedicated anti-surge controller and valve. Surge and capacity control of the 4th stage of compression will merit particularly careful review in view of the possibility of sudden mole weight changes due to a process upset at GERL. The temperature at the outlet of the capacity and surge control recycle valves is controlled by upstream injection in the recycling line of hot gas taken at compressor discharge before cooling. 3.0

SYSTEM LIMITS AND INTERFACES Content and limits of the gas compression packages, for what is directly related to process are shown on PEF’s quoted in paragraph 1.2 hereabove, and are as follow:  LP gas inlet  HP gas inlet  Gas to the dehydration manifold  Gas from the dehydration manifold/from GERL  Gas to the export gas manifold  Vent/relief/blowdown to the HP flare header  Vent/relief/blowdown to the LP flare header  Drains to the process drains system  Drains to the closed hazardous drains system Site conditions are described in document No.SPDCE-AGP-009 “Common Design Philosophy”. Fuel gas treatment system is described in functional specification No CCGP-SF-P-016.

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Electric power is provided from the electrical distribution system for the GT starter motor (if gas expanders are not used) and compressors and GT auxiliaries. Emergency source to be provided for the pre/post lube pumps as needed. Main shutdown valves (UZV’s) at inlet to each stage (apart from third) of the gas compression trains, and at the outlet of 3rd and 4th stages, are to be located externally of the gas compression skids. 4.0

RELIEF AND BLOWDOWN Blowdown will result automatically from an ESD signal or it can be initiated manually from the control room. Emergency depressuring system shall be designed according to Shell DEP 80.45.10.10-Gen. Relief provisions are included in the gas compression package, as shown on PEFS for fire protection of the vessels, or overpressure due to blocked line as necessary.

5.0

PERFORMANCE REQUIREMENTS The compression system shall be able to compress the full range of process fluids specified in the Process Flow Schemes quoted in Section 1.2 above, between 0% and 100% of the rated capacity. For the 4th stage, this shall include the gas composition for all cases, with and without condensate recovery by GERL. The required discharge pressures specified in the above mentioned PFS’s shall be attained at 100% speed, as defined per API 617 paragraphs 1.4.4 and 1.4.15, for any of the gas compositions specified. The compressor package shall deliver gas from the 3rd stage at a constant temperature of 45 °C at a maximum ambient temperature of 35 °C in order to satisfy TEG contactor requirements.

6.0

INSTRUMENTATION AND ELECTRICAL The instrumentation requirements are shown in the PEFSs referenced in Section 1.2 and the control philosophy is described in the Process Control Philosophy (COM-ST-P-002). All process control instrumentation is generally routed via the central control room, but local loops incorporating local controls may be used where applicable.

6.1

ESD/OSD Functioning In addition to the normal ESD 1.0 signals, e.g. fire and gas detection, loss of instrument air etc., operational shutdown of the compressor itself will occur in the following situations:    

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High-high liquid level in any of the scrubbers High-high temperature at the discharge of each compression stage High-high temperature at the outlet of 3rd and 4th stage aftercoolers Low-low pressure at the suction of each compression stage Page 6 of

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 High-high pressure at the discharge of each compression stage  Low-low temperature at the discharge of the 3rd stages of intercooling  Equipment protection system 6.2

Hazardous Area Classification The compressors create a Zone 2 hazardous area – all auxiliary equipment installed within 3 meters shall be suitable for use in a Zone 2 classified area, Gas Group IIA, Temperature Class T2 minimum. In addition, instrumentation must meet the requirements of Zone 1, Gas Group IIC, Temperature Class T3 minimum.

7.0

MATERIAL REQUIREMENTS All materials of construction within the compression package should be adequate to meet the project’s 20-year lifecycle requirement for the full range of process fluids and environmental conditions specified. For full details of piping material requirements, see the Project Specification for Piping and Valves CCGP-SP-L-001 and PEFS’s referenced in section 1.2.

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