Relief Valve Calculations

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

1

AIM

2

2

BASIS

3

2.1 2.2

3 3

3

Fire Relief Rate Blocked Gas Outlet Relief Rate

METHOD

4

3.1

4

Fire Relief Rate

3.1.1 3.1.2 3.1.3

3.2

Case 1 - Water Filled DGF Fire Relief Rate Case 2 - Oil Filled DGF Fire Relief Rate Case 3 - DGF Filled 50% Water & 50% Oil Fire Relief Rate

Blocked Gas Outlet

3.2.1 3.2.2

Case 4 - Blocked Gas Outlet (Fuel Gas Purge) Gas Relief Rate – Extracted from Ref. 2 Case 5 - Blocked Gas Outlet (Fuel Gas Purge) Displaced Liquid Relief Rate

4 4 4

4 4 5

4

RESULTS

6

5

CONCLUSION

7

6

REFERENCES

8

7

ATTACHMENTS

9

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;

> External pool fire > No credit taken for any passive fire protection > No credit taken for active fire protection or adequate drainage > 121% over pressure allowed. > Produced Water composition taken from A-20011-S00-CALC-021 [Ref. 2]. > A 20% margin was added to the wetted surface area to account for piping. > DGF vessel assumed to be full of produced water – Case 1. > 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].

> 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.

> Cetco DGF Vessel Dimensions– Ø 1219.2mm x 4851.4mm T/T (Ø 48” x 191”) [Ref.1]. > Fire relief stream properties taken from A-200114-S00-CALC-021 [Ref. 2]. > 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;

> 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.

> 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 > Fire relief stream properties were taken from the previous work [Ref. 2]. > The fire relief load was calculated using the Xodus standard calculation spreadsheet for fire relief for the Cetco vessel design (Attachment 1).

> 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 > Fire relief stream properties were taken from the previous work [Ref. 2]. > The fire relief load was calculated using the Xodus standard calculation spreadsheet for fire relief for the Cetco vessel design (Attachment 3).

> 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 > Fire relief stream properties were taken from the previous work [Ref. 2]. > 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.

> 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).

> 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 > Relief Rate Calculated:

1,168 kg/h

> Required Orifice Area:

94 mm2

> Selected PSV Size:

1E2

> Selected PSV Area:

126 mm2

> Rated Capacity:

1,563 kg/h

Calc. 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 > Relief stream actual volume flow of fuel gas taken stream ‘7’ in Reference 5 - See Attachment 7. > It is assumed that the produced water will be displaced. > Convert produced water flowrate from Am3/h to L/min. > 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.

> 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 Description Applicable References / Attachments Relief Rate

1

2

3

4

5

Fire – Water Case

Fire – Oil Case

Fire – 50% Water/ 50% Oil Case

Blocked Gas Outlet – Gas Relief

Blocked Gas Outlet – Liquid Relief

Att. 1, 2

Att. 3, 4

Att. 2A, 4A

Ref. 2

Att. 7

1,912 kg/h

12,616 kg/h

956 kg/h Water

1,168 kg/h

1,365 L/min

176 mm²

909 mm²

88 mm² Water

94 mm²

575 mm²

6,308 kg/h Oil

Required Orifice Area

454 mm² Oil 542 mm² Total

Selected PSV Size

1.5G2.5

3K4

2J3

1E2

2J3

Selected PSV Area

325 mm²

1,186 mm²

830 mm²

126 mm²

830 mm²

3,524 kg/h

16,469 kg/h

Note 1

1,563 kg/h

1,969 kg/h

Rated Capacity

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 >

The governing relief case for the Cetco DGF Vessel is Case 5 - Blocked Gas Outlet (Fuel Gas Purge) Displaced Liquid Relief Rate.

>

A 2J3 relief valve has been selected for this duty.

>

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-12-151-40151-P2 Rev 0. 2. DGF and Water Skimmer PSV Sizing Excluding Production Separator Gas Blow By, A-200114-S00-PCALC-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 COND_BOD_510PSIG_54DEGF_20% (Ref. 1 in A-200114-S00-CALC-021).

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 9. Displaced Liquid Relief Calculation – Blocked Gas Outlet Case