5.1 SALES GAS RECYCLE FOR START-UP
During Process Dry-out and Start-up, Sales Gas Recycle is utilized first before introducing Haradh Gas (coming from DGA) to the Unit. To ensure that enough Sales Gas is used, Selective Control method is applied. With this method the low Signal Selector 41-FY-002
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shall select the lower manipulative value between the input signal from pressure controller 41-PIC-003 and flow controller 41-FIC-002 (Both are located at the Sales Gas Line). Output signal from 41-FY-002 is then used to manipulate control valve 41-FV-002 which controls the flow rate of Sales Gas Recycle entering the Unit. Note that this line is normally no flow.
5.2 HAWIYAH FEED GAS FLOW RATE
Actual Hawiyah feed gas flow rate is measured by 4-*FIC-001. To get an accurate flow indication, temperature 4*-TI-008 and pressure 4*-PI-014 is accounted for (calculated in 4*-FY-001A). Hawiyah feed gas flow rate indicated in 4*-FY-001A is pressure and temperature compensated.
5.3 HARADH FEED GAS FLOW RATE
Actual Haradh feed gas flow rate is measured by 4-*FIC-004. To get an accurate flow indication, temperature 4*-TI-015 and pressure 4*-PI-033 is accounted for (calculated in 4*-FY-004). Haradh feed gas flow rate indicated in 4*-FY-004 is pressure and temperature compensated.
5.4 REGEN GAS FROM SALES GAS COMPRESSOR
Sufficient Regeneration Gas Flow is necessary to make sure that the Feed Gas Dehydrators (B66-D-0102A~F) are effectively regenerated within the required Regeneration time. To achieve this, during normal operation, flow controller 4*-FIC-101A will control the regen gas flow rate. If the regen system pressure exceeds the preset value, pressure controller 4*-PIC-228 will reduce regen gas flow rate via low signal selector 4*-FY-101. Output signal from 4*-FY-101 is then used to manipulate control valve 4*-FV-101 which controls regen gas flow rate to B66-E-0103A/B or B66-E-0104. Note that during stand-by period (i.e.
no flow of regen gas), PIC-228 will close FV-101.
5.5 PROPANE REFRIGERANT FEED TO B66-E-0104
Temperature controller 4*-TIC-182 on regen gas line to B66-D-0102A~F is cascaded to
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slave level controller 4*-LIC-033 which manipulates control valve 4*-LV-033 at Regen Gas Cooling Cycle Chiller (B66-E-0104) shell side inlet line. B66-E-0104 outlet Regen gas temperature is controlled by maintaining the propane level and pressure in the exchanger and hence its rate of boiling.
TC-LC cascade will be disconnected by dehydrator sequence via HS-173 except for cooling period.
5.6 PROPANE VAPOR TO COMPRESSOR SUCTION DRUM
The Low Signal Selector 4*-TY-195 shall select the lower manipulative value between the input signal from temperature controller 4*-TIC-195 on the Regen gas line or temperature controller 4*-TIC-196 on the liquid propane line. Output signal from 4*-TY-195 is then used to manipulate control valve 4*-TV-195 which controls the flow rate of propane vapor from Regen Gas/Propane Chiller (B66-E-0107) to Refrigerant 2nd Stage Compressor Suction Drum (B66-D-0113AB). Outlet Regen gas temperature is controlled by maintaining the propane vapor pressure in the exchanger and hence its rate of boiling.
5.7 DEMETHANIZER RESIDUE GAS FLOW RATE
Actual residue gas flow rate from Demethanizer is measured by 4*-FI-289A. To get an accurate flow indication, temperature (average between upstream, 4*-TI-431 and downstream, 4*-TI-364 of FE-289) and pressure 4*-PIC-608 located in Demethanizer overhead line is accounted for (calculated in 4*-FY-289). Residue gas flow rate from Demethanizer indicated in 4*-FI-289B is pressure and temperature compensated.
5.8 DEMETHANIZER BOTTOMS
Level of Demethanizer (B66-C-0110) Bottoms is maintained by level controller 4*-LIC-127A by manipulating control valve 4*-FV-341 by cascade control with flow controller 4*-FIC-341. 4*FV-341 located at Demethanizer Bottoms Pumps B66-G-0113A/B discharge line controls the flow rate of NGL going to Surge Spheres
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At process upset, with Demethanizer bottoms level exceeding LIC-127A high set point, level controller 4*-LIC-127B will open control valve 4*-LV-127B to divert NGL to Cryogenic Burn Pit. Further increase in liquid level (exceeding LIC-127B high set point), level controller 4*-LIC-127C will open control valve 4*-LV-128C to divert NGL to Cryogenic Burn Pit.
5.9 DEMETHANIZER CHIMNEY TRAY #3
Level Controller 4*-LIC-123 cascades onto flow controller 4*-FIC-303 with positive bias (calculated in 4*-FY-303). This controls the level in Demethanizer Chimney Tray 3 by manipulating control valve 4*-FV-303. 4*-FV-303 located at Demethanizer Top Side Reboiler Pumps (B66-G-0112A/B) discharge line controls the HC Liquid flow rate going to Cold Gas Exchanger, B66-E-0112.
5.10 DEMETHANIZER CHIMNEY TRAY #2
Level Controller 4*-LIC-124 cascades onto flow controller 4*-FIC-316 with positive bias (calculated in 4*-FY-316). This controls the level in Demethanizer Chimney Tray 2 by manipulating control valve 4*-FV-316. 4*-FV-316 located at Demethanizer Bottom Side Reboiler Pumps (B66-G-0111A/B) discharge line controls the HC Liquid flow rate going to Warm Gas Exchanger, B66-E-0111.
5.11 DEMETHANIZER CHIMNEY TRAY #1
Level Controller 4*-LIC-125 cascades onto flow controller 4*-FIC-329 with positive bias (calculated in 4*-FY-329). This controls the level in Demethanizer Chimney Tray 1 by manipulating control valve 4*-FV-329. 4*-FV-329 located at Demethanizer Reboiler Pumps (B66-G-0110A/B) discharge line controls the HC Liquid flow rate going to Feed Gas Exchanger, B66-E-0110A/B.
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___________________________________________________________________________________________________________________________________________________________________________________________ 5.12 CHILLED COOLING WATER FOR B66-E-0117A/B
50-TE-018 is installed on NGL/Water Exchanger B67-E-0101A~E in order to have stable NGL Product temperature. 50-TIC-018 output signal is split range:
a) X% - This signal is cascaded to slave flow controller 4*-FIC-404 to manipulate control valve 4*-FV-404 located on the Chilled Cooling Water line from Refrigerant / Water Subcooler, B66-E-0117A/B to B67-D-0103. This controls the temperature of propane from B66-E-0117A/B by varying the flow rate of Chilled Cooling Water.
b) X-100% - This signal is used to control the Chilled Water (Warm) Supply bypass control valve 50-TV-018 to the chilled water return header.
On Temperature increase, first close 4*-FV-404 to reduce water flow to minimum flow of B67-G-0105ABC, then open bypass control valve 50-TV-018.
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5.13 HAWIYAH GAS CHILLER B66-E-0102 OUTLET TEMPERATURE CONTROL (BA-543210.005)
The 41-TIC-003 cascades onto 41-LIC-002 to control 41-LV-002. Also 41-TIC-003 process value is passed to 41-TY-005 to compare with 41-TIC-016. The temperature difference between 41-TIC-003 and 41-TIC-016 will be shown on 41-TDI-005. If the temperature of the Haradh stream to the mixer gets significantly warmer than the temperature of the Hawiyah stream, condensation of water may take place. Any liquid water going to the dehydrator beds will damage the mol sieve material. Therefore, the temperature difference between the Hawiyah and Haradh streams going to the mixer is monitored by 41-TDI-005 and an alarm signal is initiated when the Haradh stream is more than 9 degF warmer than the
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Hawiyah stream.
5.14 HARADH GAS CHILLER B66-E-0108 OUTLET TEMPERATURE CONTROL (BA-543210.005)
Either lower MV of 41-TIC-018 or 41-TY-016 is selected at 41-TY-005 to control 41-TV016. During ESD condition, 41-TV-016 and 41-TXV-016 behave as well as control description for control valve with SOV in 2.15.
5.15 TEMPERATURE CONTROL WITH WARM GAS EXCHANGER (B66-E-0111) &