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CONTENT
This instruction outlines the process descriptions of NGL trains. There are three (3) NGL trains, which are identical. The description is mentioned based on train number 1. All instrument tag numbers on this section is prefixed “41-“ for Train 1 and “42-“ and “43-“ for Train 2 and 3 unless otherwise noted.
The text includes:
1. INTRODUCTION 2. DESIGN BASIS
3. PROCESS DESCRIPTION AND CONTROL 4. OVERALL PLANT CONTROL
5. PROCESS CONTROL 6. COMPLEX CONTROL
7. REGENERATION /ABSORPTION LOGIC SEQUENCE DESCRIPTION 8. PROCESS THEORY
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1 INTRODUCTION
The NGL Recovery Trains recover ethane and heavier hydrocarbons. The ethane recovery is limited only by the minimum gross heating value specification on the residue gas of 930 BTU/SCF. The carbon dioxide content of the ethane plus NGL is specified as less than 1500 ppmv.
The NGL recovery unit consists of three identical trains; each designed to handle 33.3% of the peak total inlet gas flow rate. The capacity of each train is 1333 MMSCFD.
Although the Haradh and Hawiyah inlet gas streams to each NGL recovery unit have been dew-point controlled, it contains impurities such as nitrogen, carbon dioxide, mercury and water, which adversely affect cryogenic processing to recover ethane and heavier hydrocarbons (ethane plus).
The Haradh and Hawiyah inlet gas streams to each NGL recovery unit will be mixed inside the train and then pre-cooled before passing through the molecular sieve beds for dehydration and the activated carbon beds for mercury removal. The water is removed using a molecular sieve dehydration system, since cryogenic temperatures are required to recover ethane product and water freezing and hydrate formation shall be prevented. Since brazed aluminum heat exchangers (BAHE) are used in the cryogenic process; mercury that could attack the aluminum material removed in the activated carbon beds.
The ethane and heavier hydrocarbons are extracted from the methane component of the inlet gas by fractionation in a demethanizer column. This is accomplished by liquefying the gas streams using heat exchangers, turbo expanders, or Joule-Thompson (JT) valves before it enters the demethanizer column. The demethanizer column operates at a controlled pressure with heat being added to the bottom by means of reboilers and overhead temperature being controlled by the cold feed stream. The overhead residue gas stream is essentially ethane free and passes to the sales gas compression unit (B68) after
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recompression by brake compressors.
The nitrogen is not removed from the inlet gas and simply passes through the plant with the residue gas unaffected by the process.
The NGL products coming off the bottom of the demethanizer are primarily ethane, heavier hydrocarbon liquids. Major part of carbon dioxide (CO2) in Haradh gas is removed in B65, and remaining CO2 in Haradh gas and CO2 containing in Hawiyah gas pass through the process with the ethane plus product and is send to NGL surge spheres (B67) as part of the ethane plus liquid.
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The following is a brief description of the main facilities included in each NGL Train: • Precondition System
Two Feed Gas enters this system, one from HUG-1 and the other from HDUG-1. The Dry Hawiyah Feed Gas is coming from HUG-1 after filtration in B64. The Hawiyah gas inlet does not have to be treated as the Carbon dioxide (CO2) was removed at the existing Hawiyah Gas Plant. The Hawiyah gas goes through two heat exchangers which will lower its temperature from 139 °F to 80 °F.
The Haradh Gas is wet gas because it is coming from DGA gas treating facility (B65) after removing CO2. The wet Haradh gas enters a heat exchanger and a separator where the temperature is reduced to 80 °F and the condensed liquid separated. Both Gas will then join in a Static Mixer and fed to the Dehydration System.
• Molecular Sieve Gas Dehydration
The Feed Gas Dehydration Beds contain the molecular sieve used to dehydrate the feed gas. There are 6 beds per train, 5 beds are on absorption and one is on regeneration/stand-by at any time.
• Mercury Removal
The Mercury Removal Beds use activated carbon to remove the mercury from the feed gas. The carbon beds are disposable, and when fully loaded with mercury, must be replaced with fresh material.
The dehydrated mercury free gas flows to the dust filters where it is filtered to remove any entrained fine dust and particles.
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Sales gas heated to 550 °F is used to regenerate the beds. The regeneration gas is heated in two Heaters. The first heater utilizes medium pressure hot water to heat the regeneration gas to 400 °F. The second heater utilizes electricity to further heat the regeneration gas to 550 °F.
• Cryogenic System
The gas is cooled through the Brazed Aluminum Heater Exchanger to approximately -55°F, and separated liquids are routed to the Demethanizer.
Gas from the expander feed separator is split into two streams. One stream (28 %vol) is fed to the Demethanizer Overhead Exchanger where it is cooled and passes through an expansion valve where the pressure is dropped to 259 psig. This stream is then fed to the top separator section of the Demethanizer.
The remaining gas (72 %vol) from the expander separator is fed to the turbo expanders. The gas exits the turbo expanders at approximately 260 psig and -124°F and is fed to the top tray of the Demethanizer.
Power recovered from the expanders is utilized in the brake compressor portion to increase the pressure of the residue gas. There are two 50% capacity Turbo-Expanders in each NGL train.
The Demethanizer distills the liquefied NGLs to produce an ethane rich NGL product that meets the required specifications.
It is required to have a CO2 content of less than 1500 ppmv, and no more than 2.5% methane
in ethane. (The NGL liquid produced as bottoms product is pumped to the surge spheres). • Demethanizer
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Aluminum Heat Exchanger and is routed to the suction of the Brake Compressors where those are compressed and sent to the Sales Gas Compression trains (Plant B68).
There are three reboilers on the Demethanizer, two side reboilers and a bottom reboiler with auxiliary reboiler.
• Propane Refrigeration
Propane Refrigeration is used to assist in cooling the feed gas stream. Each NGL train has a dedicated system and consists of two-motor driven, two-stage centrifugal compressors.
2 DESIGN BASIS
The NGL recovery unit consists of three identical trains that include the following main facilities. The description is mentioned based on train number 1. All instrument tag numbers on this section is prefixed “41-“ for Train 1 and “42-“ and “43-“ for Train 2 and 3 unless otherwise noted.
2.1 SYSTEM CAPACITY
The NGL recovery unit consists of three identical trains; each designed to handle 33.3% of the peak total inlet gas flow rate. The capacity of each train is 1,323 MMSCFD. Turndown capability is a minimum of 50% of design capacity.
The HNRP will recover from the inlet gases approximately 310,000 BPD of C2+ NGL (180,000 – 185,000 BPD), whose ethane content is 60 mol%.
Each NGL train consists of the follows:
• six 20% capacity molecular sieve beds per train – 5 beds are on adsorption and one is on regeneration at any point in time.
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• two 50% capacity turbo expander / brake compressors (50% x 2, no spare) • one demethanizer column with reboilers
• two 50% capacity two-stage refrigerant compressors (no spare) • Pre-coolers and Brazed aluminum heat exchangers, etc.
2.2 FEED AND PRODUCT DATA
The ethane recovery is limited only by the minimum gross heating value specification on the residue gas of 930 BTU/SCF. The CO2 content of the C2+ NGL is specified as less than 1500 ppmv.
The design parameters of the NGL unit are as follows: • Capacity of each train: 1,323 MMSCFD
• Turndown capability: to a minimum of 50% of design capacity
1) Feed Gas Pre-cooling
• Dry inlet gas temperature of Hawiyah gas stream: 140 ºF
• Wet inlet gas temperature of Haradh gas downstream of gas treating: 146 ºF (water saturated)
• Pre-cooling temperature for dehydration: 80 ºF
2) Molecular Sieve Gas Dehydration
• Design water content of the wet gas for molecular sieve: 20 lb/MMSCF at 1,323 MMSCFD and saturated at 680 MMSCFD (Haradh gas only case) • Water content of the dehydrated gas: less than 0.1 ppmv.
• Designed bed life is 3 years
• Regeneration gas temperature: 550 ºF (heating) and 90 ºF (cooling) • Regeneration gas flow rate: Maximum 60 MMSCFD
3) Mercury Removal
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gas.
• Designed bed life is 3 years
4) C2+ NGL recovery
• Heat content of residue gas 930 BTU/SCF (minimum gross heating value) • NGL liquid produced as a bottoms product when processed at Saudi
Aramco’s NGL Fractionation plants to meet the following ethane specifications:
- CO2 content of less than 1500 ppmv
- No more than 2.5% mole methane in ethane.
5) Propane Refrigeration
• Refrigeration design levels: (1) 1st suction: -14 ºF at 13 psig
(2) side stream: economizer 52 ºF at 83 psig, chillers 55 ºF at 83 psig and 75 ºF at 118 psig
(3) condenser 140 ºF at 302 psig
• The heat gain in the propane refrigeration system is 2% of the chiller duties in summer operation and 1% of the chiller duties in winter operation.
3 PROCESS DESCRIPTION AND CONTROL
The following description is based on train 1 (Unit 41), but is also applicable to train 2 & 3 (Unit 42 & 43).
3.1 FEED GAS PRE-COOLING
The pre-cooling area equipment is used to lower the temperature of the feed gas stream to 80 °F, which is based on hydrate formation temperature of approximately 62 °F plus 18 °F margin, before it passes into the Molecular Sieve Dehydrators. The feed gas streams are Hawiyah gas and Haradh gas. Haradh gas is coming from gas tearing trains (B65) and water
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The Feed Gas/Residue Gas Exchanger (B66-E-0101 A/B) is the first set of shell-and-tube type heat exchangers on Hawiyah feed gas lines in the NGL recovery process. It is used to cool the Hawiyah inlet gas to 90 °F. Due to the presence of mercury in the gas, this exchanger is a shell-and-tube type heat exchanger of steel material.
After the Feed Gas/Residue Gas Exchanger, the Hawiyah gas is fed to the Hawiyah Gas Chiller (B66-E-0102). It is a steel tube bundle inside a propane-bath kettle-type heat exchanger and is used to cool the gas to 80 °F. The propane refrigerant for this chiller is fed from the propane sub-cooler (B66-E-0117 A/B). The outlet gas from this chiller is fed to the static mixer where it is mixed with the chilled Haradh gas.
The Haradh Gas Chiller (B66-E-0108) is a steel tube bundle inside a propane-bath kettle-type heat exchanger and is used to cool the wet Haradh gas from 146 °F to 80°F and condense the majority of the water vapor. The propane refrigerant for this chiller is fed from the propane sub-cooler (B66-E-0117 A/B).
Gas from the Haradh Gas Chiller is fed to Haradh Gas Chiller Separator (B66-D-0101) to remove the water/entrained DGA condensed in the Haradh Gas Chiller. The condensed is recycled back to the gas treating trains (B65) where the water/entrained DGA is reused by adding it to the Diglycolamine (DGA) solution to minimize DGA loss.
The chilled Hawiyah gas and the chilled Haradh gas are commingled and mixed in the static mixer (B66-SM-001). The purpose of this mixer is to ensure the mixture is homogeneous and is without stratification, which could cause unequal loadings of water in the molecular sieve dehydration beds.
If temperature of Hawiyah gas is lower than that of Haradh gas, Condensation of water may take place and, thereby, performance of molecular sieves dehydrator worsens. Therefore, the differential temperature between Hawiyah and Haradh gas at mixer inlet is monitored
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detected.
3.2 MOLECULAR SIEVE GAS DEHYDRATION
The Feed Gas Dehydration Beds (B66-D-0102A/B/C/D/E/F) contain the molecular sieve used to dehydrate the feed gas to 0.1 ppmv (0.1 ppmV water content corresponds to less than -174degF at 263psig which is the operating condition of demethanizer reflux line). This low water content is required to prevent hydrate formation in the cryogenic section of the plant. There are 6 beds per train – 5 beds are on adsorption and one is on regeneration or stand-by at any point in time. Each bed is sized to process 20% of the inlet gas flow. Adsorption is done with the gas flowing vertically down through the bed. Regeneration gas is lean sales gas and taken from the discharge of each sales gas compressors (B68) and flowing vertically up through bed.
Once a bed has adsorbed as much water as the molecular sieve can handle, it is placed on regeneration. Sales gas heated to 550 °F is used to regenerate the beds. Following regeneration, the bed is cooled down to 90 °F, which is the normal operating temperature plus 10 °F, using cooled lean sales gas.
Sequencing of the dehydration bed switching valves is to be controlled by timer sequencers programmed into the DCS control system.
The water content of the dehydrator outlet gas will be monitored with an on-line moisture analyzer. The time span the beds are on adsorption shall be adjusted to ensure that the beds are on adsorption as long as possible while ensuring that water break-through does not occur.
The outlet gas from the Feed Gas Dehydration Beds is fed to the Mercury Removal Beds. During the regeneration heating cycle, the sales gas is heated in two regeneration gas heaters. The first of the heaters, the Regen Gas Hot Water Heater (B66-E-0103A/B), utilizes medium pressure hot water to heat the regeneration gas to 400 °F. The second heater, the
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Regen Gas Electric Heater (B66-E-0105 A/B), utilizes electricity to further heat the regeneration gas to 550 °F.
The regeneration gas can be automatically heated in two steps by activating or deactivating the electric heater and by controlling the hot-water flow rate.
During the regeneration cooling cycle, the Regen Gas Cooling Cycle Chiller (B66-E-0104) cools the sales gas to 90 °F using propane refrigerant.
Two coolers in series cool the regeneration gas from the gas dehydration beds. The first cooler is the Regen Gas Air Cooler (B66-E-0106), a fin-fan cooler that cools the gas to approximately 145 °F. The second cooler, the Regen Gas/Propane Chiller (B66-E-0107), cools the gas to 80 °F using propane refrigerant. This is to condense as much water vapor as practicable. Hydrate formation temperature is approximately 51 °F. The gas is then routed to the Regen Gas Separator.
The Regen Gas Separator (HP) (B66-D-0105) separates any water that condenses out of the regeneration gas stream as it is cooled. The gas from B66-D-0105 is then routed to sales gas pipeline via the regeneration gas blowers KO drums and Blowers (B64). The water from B66-D-0105 is flashed to the Regen Gas LP Knock Out Drum (B66-D-0106) to depressurize it to 5 psig. Condensed water is routed to the Oily Water Sewer (OWS) and the flashed gas is routed to LP flare.
3.3 MERCURY REMOVAL
The gas from the Dehydration Beds passes through the Activated Carbon Beds (B66-D-0107A/B/C/D/E). Flow direction is vertically down through the beds. The beds contain an activated carbon that removes the mercury from the feed gas. The activated carbon is disposable, and when fully loaded with mercury, the bed must be replaced with fresh material.
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The Carbon Bed Outlet Dust Filters/Basket Strainers (B66-D-0103A/B/C/D/E and B66-D-0104 A/B/C/D/E) are used to remove the fine dust and particles to prevent foiling or plugging of the downstream brazed aluminum heat exchangers. Each dust filter/basket strainer is dedicated to one of the activated-carbon beds.
3.4 C2+ NGL RECOVERY
The NGL Recovery Area equipment is used to lower the temperature of the gas stream to liquefy and distill the NGLs.
The Feed Gas Exchanger (B66-E-0110 A/B) receives gas from the molecular-sieve dehydration unit. The gas is fully dehydrated and is free of mercury. The exchanger is a brazed aluminum heat exchanger (BAHE) and has three sections (passes) – the feed gas section (Pass A), a residue gas section (Pass B), and the Demethanizer reboiler section (Pass C). It is used to cool the feed gas to approximately 30 ºF.
Two 50% units (A &B) have been provided to allow one unit to be taken out of service for maintenance (back puffing) without shutting down the entire train.
Following this exchanger, the gas is split into two streams, approximately 42% being fed to the Second Stage Feed Gas Chiller (B66-E-0114), and the remaining 58% being fed to the Warm Gas Exchanger (B66-E-0111).
The Second Stage Feed Gas Chiller (B66-E-0114) is a BAHE (core) inside a propane-bath kettle-type heat exchanger and is used to cool the gas to approximately -9 ºF. The propane refrigerant for this chiller is fed from the Refrigerant Economizer (B66-D-0115).
The Warm Gas Exchanger (B66-E-0111) receives gas from the Feed Gas Exchanger. The exchanger is a brazed aluminum heat exchanger (BAHE) and has three sections (passes) – the feed gas section (Pass A), a residue gas section (Pass B), and the Demethanizer bottom side reboiler section (Pass C). It is used to cool the inlet gas to approximately -9 ºF.
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The feed gas fluid stream from the Warm Gas Exchanger is commingled with the feed gas fluid stream from the Second Stage Feed Gas Chiller and is routed to the Chiller Separator. The Chiller Separator (B66-D-0110) receives a two-phase fluid stream from the Second Stage Feed Gas Chiller and from the Warm Gas Exchanger. The separator is designed with a single inlet, a single liquid outlet and two vapor outlet connections, one vapor outlet at each end of the vessel. Each of these vapor outlet connections is provided with a mist eliminator pad. Liquids separated are routed to the Demethanizer as a side feed. The remaining gases are fed to the Cold Gas Exchanger.
The Cold Gas Exchanger (B66-E-0112) receives feed gas from the Chiller Separator. The exchanger is a brazed aluminum heat exchanger (BAHE) and has three sections (passes) – the feed gas section (Pass A), a residue gas section (Pass B), and the Demethanizer top side reboiler section (Pass C). It is used to cool the inlet gas to approximately -56 ºF.
The Expander Feed Separator (B66-D-0111) receives a two-phase stream from the Cold Gas Exchanger. The separator is designed with a single inlet, a single liquid outlet and two vapor outlet connections, one vapor outlet at each end of the vessel. Each of these vapor outlet connections is provided with a mist eliminator pad. Liquids separated are routed to the Demethanizer as a side feed.
Following this separator, the gas is split into two streams, approximately 28% being fed to the Demethanizer Overhead Exchanger (B66-E-0113), and the remaining 72% being fed to the Turbo-Expanders.
The Demethanizer Overhead Exchanger (B66-E-0113) receives feed gas from the Expander Feed Separator. The exchanger is a brazed aluminum heat exchanger (BAHE) and has two sections (passes) – the feed gas section (Pass A), and a residue gas section (Pass B). It is used to cool the feed gas to approximately -159 ºF.
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expansion valve where the pressure is dropped to 264 psig and the temperature is decreased to -174 ºF. The fluid is then fed to the top packed section of the Demethanizer.
The Turbo-Expander/Brake Compressor Units (B66-K-0110A/B) receive gas from the Expander Feed Separator. The two-phase fluid stream exits each of the expanders at approximately 276 psig and -123 ºF, and are fed individually to the top tray of the Demethanizer. Power recovered in the expander portion of the Turbo-Expander is utilized in the compressor portion to increase the pressure of the residue gas.
There are two 50% capacity Turbo-Expanders in each NGL train; there are no spare or stand-by units. The Turbo-Expander/Brake Compressor units are connected in parallel. The Demethanizer (B66-C-0110) is used to distill the recovered liquid NGLs and produce an ethane rich NGL product, which meets the methane and carbon dioxide content specifications. The Demethanizer receives a two- phase (gas and liquid) feed from the Demethanizer Overhead Exchanger and from the two Turbo-Expanders. It also receives flashed liquid NGLs from the Expander Feed Separator and from the Chiller Separator. The demethanizer smaller diameter section has 34 Flexitrays (Valve trays) with four chimney trays and larger diameter section has random packing with two chimney trays. There are three reboilers on the Demethanizer. The bottom reboiler is heated by the Feed Gas Exchanger, the bottom side reboiler is heated by the Warm Gas Exchanger, and the top side reboiler is heated by the Cold Gas Exchanger.
The NGL liquid produced as a bottoms product shall meet the ethane product specification when further processed at Saudi Aramco’s NGL Fractionation plants.
A Demethanizer Auxiliary Reboiler (B66-E-0118) is provided for one JT / one Turbo-Expander operation mode so that the produced NGL liquids meet the required specifications.
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Propane Refrigeration is used to supply the cold source for heat removal. The Propane Refrigeration system in each NGL train has two 50% (Unit A & Unit B) capacity compressors, suction drums, condensers and accumulators. The sub-cooler and economizer are common to both Unit A and Unit B.
The Propane Refrigerant Compressors (B66-K-0111A/B) are centrifugal compressors and have two stages. The low pressure stage takes propane vapors from the Second Stage Feed Gas Chiller (B66-E-0114) and the high pressure stage takes propane vapors from the Refrigerant Economizer (B66-D-0115), Hawiyah Gas Chiller (B66-E-0102), Regen Gas Cooling Cycle Chiller (B66-E-0104), Regen Gas/Propane Chiller (B66-E-0107), and Haradh Gas Chiller (B66-E-0108).
The discharge from the compressors is routed to the Refrigerant Condensers (B66-E-0115 A/B).
The Refrigerant Compressors are equipped with anti-surge flow-control loops on the first and second stages. Each recycle-gas stream is temperature controlled by quenching (i.e., mixing) with liquid refrigerant.
The Refrigerant First Stage Compressor Suction Drums (B66-D-0112A/B) and the Refrigerant Second Stage Compressor Suction Drums (B66-D-0113A/B) are intended to prevent any liquid propane from entering the suction of the compressors. Each of the drums is equipped with a mist eliminator pad and heating coil in the bottom of the drum used to vaporize any liquid propane which accumulates in the drum. The coils are heated using hot refrigerant compressor discharge gas.
Each compressor discharges into a Refrigerant Condenser (B66-E-0115A/B). This is a battery of air cooled heat exchangers. Condensed propane refrigerant from the condenser is combined and fed to the Refrigerant / Water Sub-cooler (B66-E-0117 A/B).
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A Refrigerant Accumulator (B66-D-0114A/B) is installed between the Refrigerant Condenser and the common sub-cooler. System liquid fluctuations will be accommodated in the accumulator. In order to ensure there is always sufficient pressure in the accumulator to allow flow through the chillers, a hot gas by-pass is installed around the Refrigerant Condenser. This maintains pressure in the Refrigerant Accumulator when ambient temperatures decrease.
A Refrigerant / Water Sub-cooler (B66-E-0117 A/B) is installed downstream of the Refrigerant Accumulators. The Refrigerant / Water Sub-cooler uses chilled cooling water that has been further cooled by the export NGL in the Product Surge area (B67).
The compressors are equipped with a common Refrigerant Economizer (B66-D-0115) to reduce the overall load on the compressors.
Propane refrigerant transferred from the storage facility in Plant B67 to Plant B66 is filtered through a Refrigerant Filter (B66-D-0116) before being distributed into the refrigerant circuit. During a unit shutdown, propane refrigerant is transferred from Plant B66 to B67 via the Refrigerant Return Pump (B66-G-0114) from economizer and accumulators.
During Haradh gas only operation, refrigerant compressors' side stream duty is almost same as that of normal operation, however, 1st suction stream duty is reduced to less than 45% of normal and possible minimum flow recycle is required for 1st stage only. To prevent minimum flow recycle and save energy consumption, side stream flow is bypassed to 1st suction via letdown valve.
4 OVERALL PLANT CONTROL
On the main gas line, there are five control valves for overall plant control as below: 1. Hawiyah Feed Gas Line, 41/42/43-FV-001
2. Haradh Feed Gas Line, 31/32-FV-003
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4. 41/42/43-PV-537 (DeC1 overhead)
5. 61/62/63/64-FV-001, Sales Gas Compressor Inlet
The detail of control is mentioned the following section and B74-J-BE530023 (S-741-1371-101), Specification for Complex Control Strategies, para 6.
4.1 PURPOSE
The priority of purpose is the follows.
a. To maintain inlet pressure to turbo expander (K-0110A/B, K-0210A/B, K-0310A/B).
b. To maintain constant pressure in both inlet pipelines (to match the plant processing rate to the supply from both pipelines).
c. To provide flexibility of evenly splitting both Hawiyah and Haradh gas flows between all three NGL recovery trains.
d. To limit gas flow through each NGL train to the train design capacity and to allow any single train capacity testing during multi-train operation, also to limit Hawiyah gas flow through a single line train.
e. To automatically perform all the above independently of number DGA, NGL and sales compression trains in operation.
f. To ensure priority of operation with Haradh gas, over Hawiyah gas, in case of reduced plant capacity.
g. To perform all the above with minimum pressure drop to the gas flow.
Demethanizer overhead pressure is controlled by 4x-PV-537 during normal operation. During JT operation, Demethanizer overhead pressure is depend on sales gas
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4.2 STRADDLE CONTROL (PLANT BYPASS): (GTCC PORTION.)
This control loop operates the plant bypass on the Haradh & Hawiyah sales gas lines. In the event of a plant trip or emergency, gas feed is bypassed around the Hawiyah NGL plant. This loop is controlled by continually monitoring the Pressure in the HUG-1 and HDUG-1 pipelines In and Out of the plant.
If the Hawiyah gas supply header pressure exceeds its preset value, Hawiyah Gas supply header high pressure protection controller 21-PIC-005 opens pipeline bypass valves, 21-PV-005A/B to reduce the header pressure.
Differential Pressure 21-PDI-007 will provide permissive-to-open bypass valves, 21-PV-005A/B via DCS logic. If the pressure difference between supply header pressure and return pressure becomes less than preset value, this DCS logic will close the pipeline bypass valves to avoid reverse flow. A 48 inches check valve is also available to mechanically ensure that no back flow will occur.
If the Haradh gas supply header pressure exceeds its preset value, Haradh Gas supply header high pressure protection controller 22-PIC-105 opens pipeline bypass valves, 22-PV-105A/B to reduce the header pressure.
Differential Pressure 22-PDI-008 will provide permissive-to-open bypass valves, 22-PV-105A/B via DCS logic. If the pressure difference between supply header pressure and return pressure becomes less than preset value, this DCS logic will close the pipeline bypass valves to avoid reverse flow. A 48 inches check valve is also available to mechanically ensure that no back flow will occur.
4.3 HAWIYAH GAS AND HARADH GAS FLOWRATE CONTROL
Nitrogen component of Hawiyah feed gas is higher than that of Haradh feed gas. If Hawiyah gas is only feed to DeC1, Nitrogen content of sales gas is increased and possible off-spec sales gas occurs. In view of keep the heating value of sales gas as minimum 930
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For C2 recovery rate, the difference between the Hawiyah gas maximum case and Haradh gas maximum case is +/- 0.3% and negligible.
Therefore, ratio control for Haradh gas and Hawiyah gas flow rate is not required and Haradh feed gas flow to each NGL train is controlled by balance of NGL train Maximum total flow controller (41/42/43-FIC-237) and Hawiyah gas flow controller (41/42/43-FIC-001).
Haradh gas and Total NGL feed gas flow ratio of each NGL train is calculated (41/42/43-FY-009) and indicated in DCS (41/42/43-FI-009) as a reference for operator.
4.3.1 Maximum Flow Limit Control on Total Hawiyah and Total Haradh Feed Lines
The maximum total feed gas flow from Hawiyah can be set through 40-HIC-004. When measured total gas feed flow from Hawiyah gas pipeline exceeds this limit, 40-HIC-004 will override signal from 21-PIC-006 via low signal selector, 40-PY-036 and thus limit the setpoint of 40-FIC-002.
The maximum total feed gas from Hawiyah and Haradh to NGL train is limited by 41/42/43-FIC-237. 41/42/43-FIC-237 setpoint is about 1,400 MMSCFD in consideration of the Demethanizer, B66-C-0*10 capacity. When this value is exceeded, 41/42/43-FIC-237 will manipulate the expander Inlet guide vane to close via low signal selector, 41/42/43-PY-499A and override signals from 41/42/43-PIC-499A.
The maximum total Haradh feed gas to DGA units is around 1,700 MMSCFD in consideration of the DGA unit capacity of 816 MMSCFD per DGA unit. This maximum limit can be set through 22-HIC-004. When this limit is exceeded, 22-HIC-004 will override signal from 22-PIC-004 via low signal selector, 22-PY-004 and thus limit the setpoint of 22-FIC-003.
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calculated by 40-FY-004. When 41/42/43-FIC-004 process value read lower than a preset value, 40-FY-004 will consider that train to be blocked/shutdown and reduce 22-FIC-003 setpoint via low signal selector 22-PY-004 and override signal from 22-PIC-004 and 22-HIC-004.
No. of Running Trains Max. Total Haradh Gas Feed Flow Rate 0 or 1 900 MMSCFD
2 or 3 1700 MMSCFD
The set point of maximum feed gas rate of total feed gas (Hawiyah and Haradh) and Haradh gas is calculated from the number of operating trains as below:
NGL train # Hawiyah + Haradh Gas output Haradh Gas (10-FY-004) 1 1,400 MMSCFD 900 MMSCFD 2 2,800 MMSCFD 1,700 MMSCFD 3 4,380 MMSCFD 1,700 MMSCFD
Logic for 40-FY-004 output is as follows:
Three NGL Trains in operation
If 41-FIC-004.PV >= X and 42-FIC-004.PV >= X and 43-FIC-004.PV >= X Then 40-FY-004.MV= 1700
Two NGL Trains in operation
If 41-FIC-004.PV < X and 42-FIC-004.PV >= X and 43-FIC-004.PV >= X or If 41-FIC-004.PV >= X and 42FIC004.PV< X and 43-FIC-004.PV>=X or If 41-FIC-004.PV >= X and 42-FIC-004.PV >= X and 43-FIC-004.PV < X
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Then 40-FY-004.MV= 1700 (MMSCFD)
One NGL Train in operation
If 41-FIC-004.PV < X and 42-FIC-004.PV<X and 43-FIC-004.PV>= X or If 41-FIC-004.PV < X and 42-FIC-004.PV>= X and 43-FIC-004.PV<X or If 41-FIC-004.PV >= X and 42-FIC-004.PV< and 43-FIC-004.PV<X Then 40-FY-004.MV= 900 (MMSCFD)
Where “X” is low flow value to consider that the train is blocked or shutdown.
The DCS logic shall be provided to set the Max Total Haradh Gas Feed Flow Rate to 22-HIC-004 as one shot action in case of the number of running NGL train becomes less than 2.
A rate of change limiter in “SCFD/ sec” shall be provided for 22-HIC-004 output signal to change gradually when number of NGL trains running is changed.
4.3.2 Equal Distribution of Total Haradh Flow on NGL Trains
NGL trains are to have same flow rate for all 3 Hawiyah feed flow to NGL trains, and all 3 Haradh feed also at the same flow rate which is mixed with Hawiyah feed gas on each Train. The ratio of Hawiyah to Haradh gas feed is not really a main concern but an unevenly distributed Hawiyah & Haradh feed flows may shorten the running time of Dehydration system on NGL Trains, and may cause operation problem.
At normal operation, where 3 NGL trains are operating, to avoid overloading the dehydration systems by uneven distribution of Haradh feed, Hawiyah ratio setter, 41/42/43-FY-001B will be automatically adjusted by 41/42/43-FDC-010 to maintain a set value on Haradh flow to each NGL train. Process value of 41/42/43-FIC-004.PV will be averaged, 40-FY-003 and the calculated value will then be compared to individual NGL
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train process value, 41/42/43FIC004.PV. The difference between process value and average value (41/42/43-FIC-004.PV-Average) will then be used to automatically adjust the ratio set via 41/42/43-FY-001B on corresponding Hawiyah flow.
DCS logic will disable this control by changing the mode of 41/42/43-FY-001B Ratio Setter from “CAS” to “AUTO” when one of NGL trains is shutdown and keep the last ratio setpoint.
4.3.3 Hawiyah Gas Feed Flow Control & Supply Header Pressure Control
During normal plant operation, Hawiyah gas supply header pressure controller, 21-PIC-006 will control the header pressure by manipulating the setpoint of total Hawiyah feed gas flow controller, 40-FIC-002 via low signal selector, 40-PY-036. The total Hawiyah feed gas flowrate is then equally distributed by controller, 40-FIC-002 by manipulating the setpoint of 41/42/43-FIC-001 for each NGL train via ratio setter, 41/42/43-FY-001B. Hawiyah feed gas flow to each NGL train is controlled by 41/42/43-FIC-001 that manipulates control valve, 41/42/43-FV-001. The total Hawiyah gas feed flow rate is calculated from the PV value of 41/42/43-FIC-001 and used as process value of total Hawiyah gas feed flow rate controller 40-FIC-002.
If operator would like to limit the maximum total flow rate through the NGL trains, operator can set the maximum flow limit to the 40-HIC-004 connected with the total Hawiyah gas feed flowrate setpoint via low signal selector, 40-PY-036.
21-PIC-006 to be “non-linear gain PI controller” will minimize the change of feed flow to NGL unit under small Hawiyah Gas header pressure changes. The non-linear gain PI controller will lower the proportional gain to moderate the 41/42/43-FV-001 movement when the pressure variation is within a range (Gap). This Gap will be decided at later and can be modified during plant commissioning.
The total gas flowrate, Hawiyah and Haradh gas to NGL train is limited by total feed gas max flow limit controller (DCS) described in section 4.2.
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If the operator would like to operate one or two NGL trains in FIC (Stable feed flow) mode and the remaining trains by PIC - FIC cascade control, the DCS operator can disconnect the cascade connection of each train to be run in FIC mode by changing Hawiyah gas 41/42/43-FIC-001 on each NGL train to Auto mode.
If the expander suction pressure exceeds the preset value, the High Pressure protection controller, 41/42/43-PIC-499B will reduce feed gas flow rate from Hawiyah gas header via 41/42/43-HIC-014 and low signal selector, 41/42/43-FY-001. Even if the gas feed from Hawiyah pipeline is reduced but the expander suction pressure still exceeds its preset value, 41/42/43-PIC-499B will then reduce feed gas from Haradh gas supply header.
If the Hawiyah gas supply header pressure exceeds its preset value, Hawiyah gas supply header high pressure protection controller 21-PIC-005 opens pipeline bypass valves to reduce the header pressure via low signal selector.
4.3.4 Haradh Gas Feed Flow Control & Supply Header Pressure Control
During normal plant operation, Haradh gas supply header pressure controller 22-PIC-004 will control the header pressure by manipulating the setpoint of the total Haradh gas feed flow controller, 22-FIC-003 via low signal selector, 22-PY-004. The total Haradh feed gas is then equally distributed to two DGA trains by controller, 22-FIC-003 by manipulating the setpoint of controller, 3*-FIC-003 to each DGA via ratio setter, 3*-FY-036/039. Haradh feed gas flow to each DGA train is controlled by 3*-FIC-003 that manipulates control valve, 3*-FV-003. The total Haradh gas feed flow rate is calculated from the PV value of 3*-FIC-003 and used as process value of total Haradh gas feed flow rate controller 22-FIC-003.
If operator would like to limit the maximum total flow rate through the DGA trains, operator can set the maximum flow limit to the 22-HIC-004 connected with the total Haradh gas feed flowrate setpoint via low signal selector, 22-PY-004.
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22-PIC-004 to be “non-linear gain PI controller” will minimize the change of feed flow to DGA units under small Haradh Gas header pressure changing. The non-linear gain PI controller will lower the proportional gain to moderate the 3*-FV-003 movement when the pressure variation is within a range (Gap). This Gap will be decided at later and can be modified during plant commissioning.
The total Haradh gas max flow rate to NGL trains via DGA trains is limited by total Haradh gas max flow limit controller (DCS) described in section 4.3.1.
If operator would like to operate one DGA train in FIC (Auto mode) and other train by PIC - FIC cascade control, DCS operator can disconnect the cascade connection by changing the mode of Haradh gas 3*-FIC-003 on DGA train to Auto mode which the operator would like to operate the train in FIC mode, i.e. stable feed flow rate.
If the expander suction pressure exceeds the preset value, the expander suction header high pressure protection 41/42/43-PIC-499B will reduce feed gas flow rate from Hawiyah gas header first, and if the header pressure is still higher than the preset value, the 41/42/43-PIC-499B will reduce the gas flow rate from Haradh gas pipeline via 3*-HIC-AAA and low signal selector, 3*-FY-003.
If the Haradh gas supply header pressure exceeds its preset value, the High Pressure protection controller 22-PIC-105 opens pipeline bypass valves to reduce the header pressure via low signal selector. Refer to Section 4.2.
4.4 EXPANDER INLET HEADER PRESSURE CONTROL AND NGL TRAIN
MAXIMUM TOTAL GAS FLOW LIMIT
At normal operation, the turbo expander inlet header pressure is controlled at a pseudo setpoint value via 41/42/43-PIC-499A. 41/42/43-PIC-499A output goes through low signal selector, 41/42/43-PY-499A to manipulate the 2 set of Turbo Expanders’ IGVs (41/42/43-HV-311, 330) or Expander bypass JT valves (41/42/43-FV-237A, C) via
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expander/compressor control system.
The Demethanizer overhead feed flow controller, 41/42/43-FIC-238 receives its setpoint from total feed flow 41/42/43-FIC-237 via fixed flow ratio setter, 41/42/43-FY-238. By this ratio setter, the Demethanizer overhead (OVHD) feed flow through the DeC1 OVHD exchanger (B66-E-*13) via 41/42/43-FV-238 is maintained at a preset flow ratio of total gas flow to the NGL train.
The 41/42/43-PIC-499A pseudo pressure set point value is calculated via 41/42/43-PY-499C which consider the following:
P499A setpoint = Higher Signal ((P499A (PFD Value) - PDEH), PFIC004.MV)
1. P499A (PFD Value) = 748 psig as per PFD
2. PDEH = a manual term to allow bias for aging desiccant higher pressure drop
If Pressure drop on dehydrators is increased, the operator have to compare PDEH with
other trains and determine a PDEH bias value (from zero to a certain value) in order to
have same operating pressure at mixing point of Hawiyah and Haradh Gas on three NGL trains. Same pressure at mixing point of Hawiyah and Haradh Gas on three NGL trains means that the gas flow from Haradh gas pipeline will be equalized.
3. PFIC004 = 41/42/43-FIC-004 output value in psig
If 41/42/43-FIC-004.PV exceed its set point (Maximum Haradh Gas flow limit per train), 41/42/43-FIC-004 output value will be increased. If 41/42/43-PIC-499A.SP is increased and 41/42/43-PIC-499A.PV is increased, the Haradh gas flowrate will be decreased because the Haradh flowrate is determined by the pressure balance between DGA outlet header pressure and mixing point pressure between Hawiyah and Haradh
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Haradh gas distribution to each NGL trains is by hydraulic balancing by the piping and resistance (dehydrator pressure drops). Hawiyah flow to each NGL train is controlled by 41/42/43-FV-001. The Haradh gas must be balanced to avoid overloading the dehydrators in the units (Max = 680 MMSCFD @ Haradh only case). The overall flow distribution must be biased to prevent possible crushing the dehydrator beds as the desiccant ages.
Total gas flow rate to Demethanizer (DeC1) is also limited by NGL train total flow controller (41/42/43-FIC-237) via low signal selector, 41/42/43-PY-499A. When the limit setpoint around 1,400 MMSCFD is exceeded, 41/42/43-FIC-237 will override 41/42/43-PIC-499A to manipulate the Turbo expander IGVs (41/42/43-HV-310,330). This is to protect DeC1 from operating at over the design capacity.
If the sales gas (SG) compressor suction pressure exceeds its preset value due to one or more of SG compressor trip, the SG suction header high pressure protection 90-PIC-016 via SG compressor master pressure validation controller will override the signals from 41/42/43-PIC-499A and 41/42/43-FIC-237 via low signal selector, 41/42/43-PY-499A. In this case, 60-PIC-016 will manipulate the opening of the turbo expander IGVs to reduce DeC1 overhead flow.
If the expander suction pressure exceeds the preset value, the Header High Pressure protection, 41/42/43-PIC-499B will reduce feed gas flow rate from Hawiyah gas pipeline first and if the header pressure is still higher than the preset value, the 41/42/43-PIC-499B will reduce the gas flow rate from Haradh gas pipeline (especially for Haradh Gas Only Operation).
The control action of the SG suction Header High Pressure protection 60-PIC-016 and expander suction header High Pressure protection 41/42/43-PIC-499B are reverse (the output of the controller will decrease if suction pressure measurement increases) and the expander suction header pressure controller 41/42/43-PIC-499A is direct (output of the
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Operator can limit the NGL train total gas flow rate during start-up or during normal operation by changing the controller 41/42/43-FIC-237 setpoint.
4.5 DEMETHANIZER OVERHEAD PRESSURE CONTROLLER
In normal operation, i.e. two expanders in operation, Demethanizer overhead pressure is controlled by manipulating 41/42/43-PV-537 at common suction line of brake compressors. If one of turbo expander/ compressor tripped, the Demethanizer will operate at a higher pressure. The Demethanizer overhead pressure controller 41/42/43-PIC-537 kept in AUTO mode will fully open 41/42/43-PV-537. In this case, Demethanizer overhead pressure is not controlled by 41/42/43-PIC-537, rather it would be determine by the hydraulic balance from the suction pressure controller of the Sales Gas Compressor. All Demethanizer overhead gas can be sent to SG compressor via running brake compressor and bypass line.
If two set of Expander / compressor trip, it is possible to produce off-specification of NGL, operator should take appropriate action, e.g., shut down the train or reduce the NGL feed gas rate.
4.6 ONE NGL RECOVERY TRAIN SHUTDOWN
When one of NGL trains is shutdown, Hawiyah and Haradh feed gas rate controlled by the Total Feed gas flow limit controller. Refer to section 4.3.1.
5 PROCESS CONTROL
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
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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.
Close Open TC-018 TV018 FV404 0 X 100
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) & 2ND STAGE GAS CHILLER (B66-E-0114) (BA-543212.001/003/004)
The 2nd Stage Gas Chiller (B66-E-0114) outlet temperature (41-TI-312) cascade on outlet temperature (41-TIC-292) of Warm Gas Exchanger (B66-E-0111) to control 41-TV-292. During ESD condition, 41-TV-292 and 41-TXV-292 behave as well as control description for control valve with SOV in 2.15.
6 COMPLEX CONTROL
6.1 NGL RECOVERY AREA B66 (3 TRAINS)
NGL Trains High Pressure Protection Control
Refer to P&ID: B66-A-BA-543210-001, B66-A-BA-543213-001, B65-A-BA-540691-002 and Figure 4.13.1 of this document.
Objective
Protect each NGL Train feed gas header from over-pressure by reducing the feed gas flow rate from Hawiyah gas pipeline first and if the header pressure is still higher than the set value, reduce the feed gas flow rate from Haradh gas pipeline.
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Functional Description
The controls description that follows is for the NGL recovery Train1 only, but the control scheme is the same for NGL Train 2 and NGL Train 3 only with different prefix for instrument tag identifications.
The NGL train feed gas header High Pressure protection controller 41-PIC-499B receives its PV from the Expander suction header pressure measurement 41-PT-499. The output of 41-PIC-499B is split via a split range calculator. Two outputs of the split range calculator are connected to blocks 41-HIC-014 and 40-PY-499B.
Higher range split control signal to reduce feed gas flow rate from Hawiyah gas pipeline will adjust setpoint of Max Hawiyah train feed flow limit controller, 41-HIC-014, when it is in Cascade mode. Output of 41-HIC-014 will then pass through low signal selector, 41-FY-001 to finally control 41-FV-001.
When 41-HIC-014 is in Auto mode, the operator can set the maximum feed gas flow rate for NGL train 1 at the SP of 41-HIC-014.
When 41-HIC-014 is in Manual mode, the operator can manually change HIC output signal to override the output signal from 41-FIC-001 via low signal selector 41-FY-001.
Lower range split control signal from 41/42/43-PIC-499B to reduce feed gas flow rate from Haradh gas pipeline will pass through low signal selector, 40-PY-499B. Output of 40-PY-499B will then adjust the SP of both 31-HIC-003 and 32-HIC-003 when it is in Cascade mode. Output of 31/32-HIC-003 will then pass through low signal selector switch 31/32-FY-003 to finally control 31/32-FV-003.
When 31/32-HIC-003 is in Auto mode, the operator can set the maximum feed gas flow rate for DGA train 1/2 at the SP of the HIC.
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signal to override the output signal from 31/32-FIC-003 via low signal selector 31/32-FY-003.
Operational and Implementation Aspects
NGL Train Header High Pressure Protection control scheme is required to be operational during start up and normal operation.
In normal steady state, for Hawiyah pipeline feed gas control, low signal selector 41-FY-001 is cascaded to 41-HIC-014, which in turn cascaded to NGL Train High pressure controller, 41-PIC-499B. While for Haradh pipeline feed gas control, low signal selectors 31/32-FY-003 are cascaded to low signal selector 40-PY-499B via 31/32-HIC-003 respectively. Low signal selector 40-PY-499B will select the lower value from 41/42/43-PIC-499B lower range output signals to control the feed gas rate to DGA trains when one or more NGL Trains header pressure is too high.
NGL Train Header High Pressure Protection Controller: Operator to be able to decrease SP down to an enforced low limit. The operator shall not have access to 41-PIC-499B controller’s mode or set point enforced low limit. Only engineer can change the controller mode or set point enforced low limit.
Crippled Mode Operation
Failure of the NGL Train gas inlet pressure input signal from 41-PIT-499, identified as a “Bad Value” status in DCS, will cause an alarm and high pressure protection controller 41-PIC-499B to go to MANUAL mode, with the output remaining at the last good value previous to transferring to MANUAL mode.
_ ___________________________________________________________________________________________________________________________________________________________________________________________ F igu re 4.13.1: H N G L T rain H igh Pressu re Pr ot ectio n C o n tr o l Feed Gas fr om Ha wiy ah AO /A FS 42 FV-00 1 43 FV-00 1 41 FY 00 1 42 FY 00 1 43 FY 00 1 41FIC 001 42F IC 001 43FIC 001 31 FV-003 32 FV-00 3 DGA -1 B6 5-C -1 01 DGA -2 B65 -C -X X X 31F Y 003 32 FY 003 31FIC 00 3 32FIC 003 Feed Gas fr om H ar ad h 41 PI C 49 9A 41 PI C 49 9B 41 H IC 01 4 42 H IC 01 4 43 H IC 01 4 SR (N O TE 1 ) 42PIC 499B 43PIC 499B 31 H IC 00 3 32 H IC 00 3 HNGL-1 B6 6-C -1 10 0-X % X-10 0 % NO T E S : 1. T ypi ca l f or HNG L Tra in 1, 2 an d 3 Hi gh P res su re P ro te ct ion C on tr ol ler 4*P IC -49 9B. Valv e Op en in g [% ] 0 10 0 100 4* P IC -49 9B MV [% ] x 4*FV -0 01 31 /3 2FV -0 03 SR (N O TE 1 ) SR (N OT E 1) 40 PY 49 9B HNGL Trai n-2 HNGL Trai n-3 HNGL Tr ain-2 HNGL Tr ain-3 X-10 0 % 0-X % X-10 0 % 0-X % SP SP SP SP SP 41 FV-00 1 AO /A FS AO /A FS AO /A FS AO/ AF S Re v
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Calculation
41-PIC-499B Output Split Range Calculation
When 41-PIC-499B.PV is less than the setpoint value, the High Pressure protection controller, 41-PIC-499B, output signal is on the high side, 100%.
When process value is greater than or equal to the set value, Split range calculation block should convert 41-PIC-499B higher range output signal, X% ~ 100% to 0% ~ 100% as input signal to 41-HIC-014, and lower range output signal, 0% ~ X% to 0% ~ 100% as input signal to low signal selector switch, 40-PY-499B.
Initializations
NGL Train Header High Pressure Protection controller, 41-PIC-499B SP shall not track PV when it is in MANUAL mode.
Output of 41-PIC-499B should be configured to track the 41-HIC-014 setpoint, when HIC is placed in MANUAL or AUTO mode. Upon switching 41-HIC-014 to CASCADE mode, 41-PIC-499B output should be initialize to 41-HIC-014 setpoint to prevent bumping of the process.
No output pushback shall be configured for low signal selector 40-PY-499B when any of the 31/32-HIC-003 switched from AUTO or MAN mode to CASCADE. Upon switching 31/32-HIC-003 from AUTO or MAN mode to CASCADE, HIC SP should ramp slowly to match MV of 40-PY-499B.
Anti reset windup (ARWU) shall be configured for NGL train feed flow controller 41-FIC-001 and feed flow override controller 41-HIC-014 to ensure bumpless transfer in case HIC is overriding the FIC as follows:
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Normally, 41-FIC-001 in control, 41-HIC-014 MV shall be initialized to the low selector output value (41-FY-001.MV). When 41-HIC-014 in control, the NGL train feed flow controller 41-FIC-001 MV shall be initialized to the low selector output value (41-FY-001.MV)
Anti reset windup (ARWU) shall be configured for DGA feed flow controller 31/32-FIC-003 and Maximum DGA feed flow limit controller 31-HIC-003/32-HIC-003 to ensure bumpless transfer in case HIC is overriding the FIC as follow:
Normally, 31/32-FIC-003 in control, 31/32-HIC-003 MV shall be initialized to the low selector output value (31/32-FY-003.MV). When the 31/32-HIC-003 in control, the DGA train feed flow controller 31/32-FIC-003 MV shall be initialized to the low selector output value (31/32-FY-003.MV)
Special Consideration
Upon 41-FY-001 selecting signal from 41-HIC-014 for more than 10 seconds, and the mode of 41-HIC-014 is in CASCADE, 42-FIC-001 and 43-FIC-001 will set to AUTO mode (one shot signal) if it is in CASCADE. The Operator is responsible to switch 42/43-FIC-001 back to CASCADE when the operating condition is back to normal.
Maximum Hawiyah train feed flow limit controllers, 41/42/43-HIC-014, and Hawiyah train feed flow controllers, 41/42/43-FIC-001 must have the same process variable range. Similarly, Maximum DGA feed flow limit controller, 31/32-HIC-003, and DGA feed flow controller, 31/32-FIC-003 must have the same process variable range.
6.2 NGL TRAIN EXPANDER INLET PRESSURE CONTROL
Refer to P&ID: B66-A-BA-543213-001/ 002/003/004, B68-BA-540838-003 and Section 9.2, 9.3 & 9.4: Appendix , Figure 7.2.2 & Attachment 1 & 2 of this document.
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The anti-sure control system for Brake Compressor will be implemented in the Unit Control Panel (PLC) by compressor vendor (MTC) and the control narrative for its compressor will be described in the vendor’s documents.(V-2158-201A-050).
Refer to P&ID: B66-A-BA-543213-001/ 002/003/004, B68-BA-540838-003 and Figure 4.13.2 of this document.
Objective
To maintain constant Inlet pressure to each NGL Train turbo expander by regulating the total gas flow to expander and Demethanizer overhead feed flow through DeC1 Overhead Exchanger via fixed flow ratio setter.
Functional Description
The controls description that follows is for the NGL recovery Train1 only, but the control scheme is the same for NGL Train 2 and NGL Train 3 only with different prefix for instrument tag identifications.
41/42/43-PIC-499A output goes through low signal selector, 41/42/43-PY-499A to manipulate the 2 set of Turbo Expanders’ IGVs (41/42/43-HV-311, 330) or Expander bypass JT valves (41/42/43-FV-237A, C) via expander/compressor control system.
The Demethanizer overhead feed flow controller, 41/42/43-FIC-238 receives its setpoint from total feed flow 41/42/43-FIC-237 via fixed flow ratio setter, 41/42/43-FY-238. By this ratio setter, the Demethanizer overhead feed flow through the DeC1 OVHD exchanger (B66-E-0*13) via 41/42/43-FV-238 is maintained at a preset flow ratio of total gas flow to the NGL train.
Total gas flow rate to Demethanizer (DeC1) is also limited by NGL train total flow controller (41/42/43-FIC-237) via low signal selector, 41/42/43-PY-499A. When the limit setpoint around 1,400 MMSCFD is exceeded, 41/42/43-FIC-237 will override 41/42/43-PIC-499A to manipulate the Turbo expander IGVs. This is to protect DeC1 from
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If the sales gas (SG) compressor suction pressure exceeds its preset value due to one or more of SG compressor trip, the SG suction header high pressure protection 90-PIC-016 via SG compressor master pressure validation controller will override the signals from 41/42/43-PIC-499A and 41/42/43-FIC-237 via low signal selector, 41/42/43-PY-499A. In this case, 90-PIC-016 will manipulate the opening of the turbo expander IGVs via expander/compressor control system to reduce DeC1 overhead flow.
The control action of the SG suction header High Pressure protection 90-PIC-016 and expander suction header High Pressure protection 41/42/43-PIC-499B are reverse (the output of the controller will decrease if suction pressure measurement increases) and the expander suction header pressure controller 41/42/43-PIC-499A is direct (output of the controller will increase if pressure measurement increases).
Operator can limit the NGL train total gas flow rate during start-up or during normal operation by changing the controller 41/42/43-FIC-237 setpoint.
The turbo expanders inlet header pressure is controlled at a pseudo setpoint value via 41/42/43-PIC-499A.
_ ___________________________________________________________________________________________________________________________________________________________________________________________ Expander B66-K -110A /B De C1 O V H D E xc h B66-E-11 3 B66-D-11 1 Exp F eed S epa TR3 SG C o m p re ss o r B 68-K -101A~ D CCS Loa d Sh ar in g (H O L D ) 34 23 19 Chi m 1 T ray 1 10 Ch im 2 Tr ay 11 18 Ch im 3 Ch im 4 Ch im 5 #1 #6 #10 #10 #11 #15 #20 41P IC 499B 499A 41P IC 237 41FI C 238 41FI C 238 41FY RS 016 90P IC TR2 PV SP 41-F IC -004 .M V H IG H PR ESSU R E PR OT EC TI ON 41FV -238 41FV -2 37A 41F V-237C A B B reak Com p ressor B68-K-110A /B B 6 6 -E-101A/ B B 6 6 -E-110A/ B B66-E-11 1/ 112 Re v Di r Re v AC/ AF O AO /A FC AO /A FC 499A 41P Y Ma x T ot al F eed G as 300 41H IC 301 41H IC PDE H BI AS PPF D Ha ra dh Ma x Fl ow L imi t No te 1 SP N O TE 1: P Y 499C. M V = PPFD - PDEH bias w her e: P PFD = set P ressur e as per PFD PDEH = Bi as due to pr essur e dr op on de hydrator (-) (+) Re v 41H V-311 41H V-330 499C 41P Y 41P Y Fig u re 4.13.2: H N GL T rain Expand er Inlet Pr e ssu re C o n tro l
