Assignment No. A200114-S00
Assignment Provision of Front-End Engineering Design (FEED) for Dolphin A
Produced Water Upgrade
Client BG International Limited (Trinidad & Tobago)
Subject Cetco DGF Relief Rate Assessment
Calc. No. A-200114-S00-CALC-028 Rev R01
Calc. by Stuart Kerr Date 28/08/2014
Checked by Date
Approved by Date
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AIM
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BASIS
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2.1 Fire Relief Rate 3
2.2 Blocked Gas Outlet Relief Rate 3
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METHOD
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3.1 Fire Relief Rate 4
3.1.1 Case 1 - Water Filled DGF Fire Relief Rate 4
3.1.2 Case 2 - Oil Filled DGF Fire Relief Rate 4
3.1.3 Case 3 - DGF Filled 50% Water & 50% Oil Fire Relief Rate 4
3.2 Blocked Gas Outlet 4
3.2.1 Case 4 - Blocked Gas Outlet (Fuel Gas Purge) Gas Relief Rate – Extracted from Ref. 2 4 3.2.2 Case 5 - Blocked Gas Outlet (Fuel Gas Purge) Displaced Liquid Relief Rate 5
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RESULTS
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CONCLUSION
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REFERENCES
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Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 2 of 9
1 AIM
The objective of this calculation is to determine the governing relief case, required capacity and subsequent relief valve size for the Cetco DGF vessel design. Previous relief work had been undertaken based on the Siemens DGF vessel design but due to significant changes in size and operability, a new calculation is required.
Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 3 of 9
2 BASIS
The DGF has a design pressure of 250 psig (17.2 barg) [Ref. 1]
2.1 Fire Relief Rate
The basis for the fire scenario is as follows;
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External pool fire>
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No credit taken for any passive fire protection>
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No credit taken for active fire protection or adequate drainage>
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121% over pressure allowed.>
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Produced Water composition taken from A-20011-S00-CALC-021 [Ref. 2].>
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A 20% margin was added to the wetted surface area to account for piping.>
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DGF vessel assumed to be full of produced water – Case 1.>
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DGF vessel assumed to be full of oil – Case 2. This is highly unlikely but has been included as it is considered the most onerous case [Ref. 1].>
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Since it is considered highly unlikely that the DGF would be completely filled with oil, an alternative 50% Water / 50% Oil Split has also been considered – Case 3.>
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Cetco DGF Vessel Dimensions– Ø 1219.2mm x 4851.4mm T/T (Ø 48” x 191”) [Ref.1].>
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Fire relief stream properties taken from A-200114-S00-CALC-021 [Ref. 2].>
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It is assumed that the time required to heat the system from the relief-device set pressure to the relieving conditions allows for the relief of the liquid displacement prior to reaching the relieving conditions. Therefore, liquid relief has not been considered in this case.2.2 Blocked Gas Outlet Relief Rate
The basis for the blocked gas outlet is as follows;
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Nitrogen system (primary purge gas) has a design pressure of 200 psig (13.8 barg) [Ref. 3] and as such cannot over pressurise the DGF.>
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Fuel Gas system (secondary purge gas) has a design pressure of 275 psig (19.0 barg) [Ref. 4] and as such can over pressurise the DGF. This case was considered in the previous calculation (refer to Case 6 in Reference 2) and is unaffected by the changes in design. Refer to previous calculation [Ref. 2] for full details.Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 4 of 9
3 METHOD
3.1 Fire Relief Rate
3.1.1 Case 1 - Water Filled DGF Fire Relief Rate
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Fire relief stream properties were taken from the previous work [Ref. 2].>
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The fire relief load was calculated using the Xodus standard calculation spreadsheet for fire relief for the Cetco vessel design (Attachment 1).>
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The fire relief PSV size was calculated using Xodus standard gas PSV spreadsheet to find the required PSV orifice size (Attachment 2).3.1.2 Case 2 - Oil Filled DGF Fire Relief Rate
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Fire relief stream properties were taken from the previous work [Ref. 2].>
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The fire relief load was calculated using the Xodus standard calculation spreadsheet for fire relief for the Cetco vessel design (Attachment 3).>
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The fire relief PSV size was calculated using Xodus standard gas PSV spreadsheet to find the required PSV orifice size (Attachment 4).3.1.3 Case 3 - DGF Filled 50% Water & 50% Oil Fire Relief Rate
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Fire relief stream properties were taken from the previous work [Ref. 2].>
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The fire relief loads calculated above are halved assuming that exactly half of the surface area of DGF and associated pipework will be in contact with water and the other half oil.>
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The fire relief PSV size was calculated using Xodus standard gas PSV spreadsheet to find the required PSV orifice size for the water case (Attachment 5) and the oil case (Attachment 6).>
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These orifice areas were added together to establish the total PSV orifice area required for this composition.3.2 Blocked Gas Outlet
3.2.1 Case 4 - Blocked Gas Outlet (Fuel Gas Purge) Gas Relief Rate – Extracted from
Ref. 2
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Relief Rate Calculated: 1,168 kg/h>
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Required Orifice Area: 94 mm2>
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Selected PSV Size: 1E2>
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Selected PSV Area: 126 mm2>
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Rated Capacity: 1,563 kg/hCalc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 5 of 9
3.2.2 Case 5 - Blocked Gas Outlet (Fuel Gas Purge) Displaced Liquid Relief Rate
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Relief stream actual volume flow of fuel gas taken stream ‘7’ in Reference 5 - See Attachment 7.>
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It is assumed that the produced water will be displaced.>
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Convert produced water flowrate from Am3/h to L/min.>
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Produced water properties were taken from stream ‘PW @ 25.72oC’ which was defined from Prod water-0250 and adjusted to the relieving conditions in Reference 5 - See Attachment 8.>
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These were used to populate the Xodus Standard Liquid PSV Sizing tool to find the required PSV orifice size (Attachment 1).Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 6 of 9
4 RESULTS
Case 1 2 3 4 5
Description Fire – Water
Case
Fire – Oil Case Fire – 50% Water/ 50% Oil Case Blocked Gas Outlet – Gas Relief Blocked Gas Outlet – Liquid Relief Applicable References / Attachments
Att. 1, 2 Att. 3, 4 Att. 2A, 4A Ref. 2 Att. 7
Relief Rate 1,912 kg/h 12,616 kg/h 956 kg/h Water
6,308 kg/h Oil
1,168 kg/h 1,365 L/min
Required Orifice Area 176 mm² 909 mm² 88 mm² Water
454 mm² Oil 542 mm² Total
94 mm² 575 mm²
Selected PSV Size 1.5G2.5 3K4 2J3 1E2 2J3
Selected PSV Area 325 mm² 1,186 mm² 830 mm² 126 mm² 830 mm²
Rated Capacity 3,524 kg/h 16,469 kg/h Note 1 1,563 kg/h 1,969 kg/h
Table 4.1 DGF PSV Sizing Results
Note 1.: The relief valve would be operating at approximately 65% capacity during this relief scenario.
It is considered highly unlikely that the DGF would be completely filled with oil; therefore sizing the relief valves on this basis seems overly conservative. As such, the blocked gas outlet - liquid relief case is considered the governing case.
Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 7 of 9
5 CONCLUSION
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The governing relief case for the Cetco DGF Vessel is Case 5 - Blocked Gas Outlet (Fuel Gas Purge) Displaced Liquid Relief Rate.>
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A 2J3 relief valve has been selected for this duty.>
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This relief valve can accommodate the scenario where the DGF is half filled with Water and half filled with oil.Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 8 of 9
6 REFERENCES
1. Dissolved Gas Flotation Unit Construct P&ID, A200114-S00-P-PIDS-026 / TRI-EC-DBE-PR-12-151-4015-1-P2 Rev 0.
2. DGF and Water Skimmer PSV Sizing Excluding Production Separator Gas Blow By, A-200114-S00-P-CALC-021 R01.
3. Nitrogen Generator Package P&ID, TRI/EC/DOL/PR/12/03/4041 Rev 12. 4. Fuel Gas System P&ID, TRI/EC/DOL/PR/12/03/4016-1 Rev 12.
5. HYSYS Model - DOLPHIN - BG COPY 450MMSCFD_600BBLDWATER_MAX
Calc. No. A-200114-S00-CALC-028 Assignment
No. A200114-S00 Sheet 9 of 9
7 ATTACHMENTS
1. Fire Relief Load Calculation - Water 2. Fire Relief PSV Sizing Calculation – Water 3. Fire Relief Load Calculation - Oil
4. Fire Relief PSV Sizing Calculation – Oil
5. Fire Relief PSV Sizing Calculation – 50% Water 6. Fire Relief PSV Sizing Calculation – 50% Oil 7. HYSYS Model Stream ‘7’ Properties
8. HYSYS Model Stream ‘PW @ 25.72oC’ Properties
