SPM-3510 Turbo-Expander

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Process Description

Simtronics’ Turbo Expander program illustrates the principles and operation of a typical turbo expander unit. The turbo expander unit consists of a gas expander (turbine) directly coupled to a centrifugal compressor (recompressor) and is the heart of a simple Gas Subcooled Process (GSP) that condenses natural gas liquids from high pressure raw natural gas and then recompresses the purified natural gas back to typical pipeline pressure. Turbo expander units are commonly used in the natural gas, air separation and bulk gas chemical industries to achieve very low process temperatures without the need for external refrigeration systems.

In the Turbo Expander program, the feed to the unit is raw natural gas gathered from wells that has been treated for removal of hydrogen sulfide, water and other detrimental impurities. The feed mainly contains a range of alkane hydrocarbons: methane through hexane. Nitrogen and carbon dioxide are also present in small concentrations. The Turbo Expander unit separates out most of the propane and nearly all of heavier hydrocarbon compounds from the feed gas. A portion of the ethane in the feed is also removed. Almost all the methane in the feed ends up in the product gas from the unit and is recompressed and sent to a pipeline along with all the nitrogen in the feed and the uncondensed ethane in the feed. The product gas is termed sales gas. The separated components from the feed are recovered as a liquid product called NGL (natural gas liquids). These are sent offsite to a fractionation unit for separation into ethane, propane, butane and light gasoline.

A full range of operations can be learned and practiced on the Turbo Expander simulator. These include normal, startup, shutdown, and emergency shutdown procedures.

The Turbo Expander program represents a simple Gas Subcooled Process (GSP) with a turbo-expander. The feed to the unit is raw natural gas gathered from wells that has been treated for removal of hydrogen sulfide, water, and other detrimental impurities. The feed mainly contains a range of alkane hydrocarbons: methane through hexane. Nitrogen and carbon dioxide are also present in small concentrations. The purpose of the Turbo Expander process is to separate out some of the ethane and nearly all of the propane and heavier hydrocarbon compounds from the feed gas. Nearly all the methane in the feed ends up in the product gas from the unit and is recompressed and sent to a pipeline along with all the nitrogen in the feed and a fraction of the ethane in the feed. The product gas is termed sales gas. The separated components from the feed are recovered as a liquid product called NGL (natural gas liquids). These are sent offsite to a fractionation unit for separation into ethane, propane, butane and light gasoline.

To effect separation, the feed gas is cooled down using:

  • cold sales gas from the top of the Demethanizer
  • auto-chilling by gas expansion and work extraction through an expander (turbine)
  • auto-chilling by liquid expansion through valves (auto-refrigeration and Joule-Thomson effects)

As the feed gas cools down, the heaviest components (some propane, butane and light gasoline) condense first in Feed/Sales Gas Exchanger (E-801). The liquid is separated in Feed Separator (D-801) and sent to the lower part of Demethanizer T-801. The remaining feed gas is chilled further by a large pressure reduction (expansion) in the Feed Gas Expander (KT-801). Like most gases, as natural gas expands it will also drop in temperature. This is known as the Joule-Thomson effect. In the Feed Gas Expander, additional cooling takes place due to the extraction of work by the Expander which acts as a turbine for the shaft-connected Sales Gas Recompressor (K-801). As a consequence of the gas expansion and work extraction, the gas becomes cold enough so that lighter components condense (the balance of the feed propane and a portion of the ethane). The cold, two-phase stream at the outlet of the KT-801 is sent to the top of the Demethanizer (T-801) to serve as reflux.

The feed cooling section operates close to the feed gas supply pressure while the Demethanizer overhead operates at a much lower pressure. This pressure difference is the driving force which causes the self-chilling in the process. The larger the pressure difference, the deeper the chilling. Thus, the Turbo Expander plays the central role in causing the separation process to occur.

In the simple Gas Subcooled Process in the Turbo Expander program, the Demethanizer column performs two functions: it strips out most of the dissolved methane in the liquid feeds to the Demethanizer while washing down as much propane and heavier hydrocarbons to the bottom. Ethane does not condense in significant rates in this simpler process and, therefore, most of it is stripped along with the methane.

Feed Chilling Warm

Treated feedgas at high pressure from battery limits is first cooled in Feed/Sales Gas Exchanger No. 1 E-801 using cool Sales Gas from E-803. The feedgas then passes on to Feed Chiller E-802 which uses propane refrigerant from battery limits. The pressure of vaporized refrigerant on the shell side is controlled to adjust the temperature of the boiling refrigerant which, in turn, controls the outlet temperature of the feedgas from E-802. The vaporized refrigerant is returned back to battery limits.

A portion of the feedgas (approximately 32% of the plant feed rate by weight) is condensed in E-802 and the liquid and vapor are separated in Warm Separator D-801. The collected warm liquid is routed to tray 13 of the Demethanizer T-801. This liquid contains a significant amount of methane which will be stripped out in the lower section of T-801.

Feed Chilling Cold

The vapor stream from D-801 flows through Feed/Sales Gas Exchanger No. 2 E-803 to cool the feedgas further using cold Sales Gas from E-804. Additional liquid is produced from E-803 (approximately 8% of the plant feed rate by weight) and is separated out in Cold Separator D-802.

The cold liquid from D-802 is combined with warm liquid from D-801 and fed to tray 13 of Demethanizer T-801.

A portion of the vapor from D-801 (approximately 24% of the plant feed rate by weight) is further chilled and condensed against Sales Gas directly from the top of Demethanizer T-801 in Feed/Sales Gas Exchanger No. 3 E-804. This stream is depressured to T-801’s pressure across FV-808. The flashing of liquid and the Joule-Thompson effect from depressurization causes the stream temperature to drop. Approximately 33% of the stream, by weight, flashes to vapor. This cold stream, which is rich in methane, is routed to the top of T-801 (tray 40) to provide reflux for washing ethane and propane down the Demethanizer.

Heat is added at the base of the Demethanizer (T-801) column with Reboiler (E-805) that uses hot oil as its heat source. This ensures removal of most of the dissolved methane (and ethane) reaching the bottom of the Demethanizer. The NGL collected in the bottom of the Demethanizer is sent to an NGL Distillation Unit at battery limit.

Finally, the overhead gas from the Demethanizer – known as sales gas – is compressed in two stages before it is sent to a pipeline at battery limits. The first compression stage is the Sales Gas Recompressor (K-801) which is a centrifugal type and takes warmed sales gas from Feed/Sales Gas Exchanger (E-801) at its suction. The compressor is driven by a shaft connecting the Feed Gas Expander (KT-801). The second stage of compression is the Sales Gas Compressor (K-802), also a centrifugal compressor, and is driven by an electric motor.

Feed Cooling

Treated feed gas at high pressure from battery limits is first cooled in Feed/Sales Gas Exchanger (E-801) using cool sales gas from Demethanizer (T-801). The feed gas then passes on to Feed Separator (D-801). The collected cool liquid is routed to tray 13 of the Demethanizer. This liquid contains a significant amount of methane which will be stripped out in the lower section of Demethanizer.

Feed Chilling Expander

The balance of vapor from Feed Separator (D-801) is normally routed through Feed Gas Expander (KT-801). Guide vanes on the inlet of the Feed Gas Expander allow control of the flow rate through the unit. The expander drives the Sales Gas Recompressor (K-801) which is an integrated unit. The Feed Gas Expander/Sales Gas Recompressor normally operates at 12,531 RPM but will dynamically change according to changes in expander- and recompressor-side conditions. The outlet flow from Feed Gas Expander is routed to tray 40 of Demethanizer (T-801) to provide reflux for washing propane and butane down the Demethanizer. Because of both gas expansion and work performed by the expander in turning the Recompressor, the temperature drops across Feed Gas Expander and produces a modest amount of liquid.

A bypass valve is provided around Feed Gas Expander for startup and shutdown operations. This valve, FV-801B, is known as the JT Valve as it will provide chilling due to just the Joule-Thomson effect. Without the Feed Gas Expander in operation, it will take more power consumption by the Sales Gas Recompressor to maintain plant capacity to offset the loss of the Recompressor’s compression stage. NGL production will also decrease because the of the loss of work extraction from the feed gas, resulting in an increase in the temperature of the stream to the top of Demethanizer.

In case of a trip of Feed Gas Expander, the control system will automatically open the bypass valve on a one-shot basis to a predefined opening that depends on the current feed flow rate.

Demethanizer

The Demethanizer (T-801) is a distillation tower consisting of 40 trays. Its main functions are to strip out any methane and ethane from the liquid feeds to the tower and to wash down most of the propane and all the higher hydrocarbons to the bottom of the column. Cold gas feed from Feed Gas Expander (KT-801) is sent to the top tray (tray 40) to wash down propane and butane rising with the vapors from the lower sections of the tower. Cool liquid from Feed Separator (D-801) is fed to tray 27.

Below the bottommost tray of Demethanizer (tray 1) is a full trap-out pan that routes liquid from tray 1 to the shell side of Demethanizer Reboiler (E-805). Hot oil from battery limits is used to heat the liquid to drive off most of the methane and ethane that reaches the bottom tray. The vapor from Demethanizer Reboiler is returned to the bottom of the Demethanizer and flows through chimney vents in the trap-out pan and then flows up into tray 1. Warm liquid on the shell side of Demethanizer Reboiler spills over a weir and flows by gravity into the base of the Demethanizer. This liquid is product NGL.

Product NGL in the base of the Demethanizer flows by pressure differential to the Deethanizer tower at battery limits.

Sales Gas Compression

Cold sales gas is produced at the top of the Demethanizer (T-801) and is mainly methane and ethane. It also contains almost all the nitrogen contained in feed gas as well as a portion of the carbon dioxide contained in the feed gas. Sales gas is heated up in the Feed/Sales Gas Heat Exchanger (E-801) prior to compression back to pipeline pressure.

The sales gas from the Feed/Sales Gas Heat Exchanger is first compressed by the Sales Gas Recompressor (K-801) which is driven by the Feed Gas Expander (KT-801). The Sales Gas Recompressor is a centrifugal compressor. A check valve is installed in the bypass line around the Sales Gas Recompressor to allow Sales Gas flow to continue in case of a trip of the Feed Gas Expander. Because the pressure ratio of the Sales Gas Recompressor is fairly low, the temperature of the gas leaving recompressor is low enough where it does not have to be cooled prior to final compression in Sales Gas Compressor (K-802).

Sales Gas Compressor is a motor-driven centrifugal compressor and takes suction from the discharge of Sales Gas Recompressor. The flow through the Sales Gas Compressor is adjustable by changing the position of the inlet guide vanes. The Sales Gas Compressor discharges into the sales gas pipeline at battery limits. A check valve on the discharge of the Sales Gas Compressor prevents back-flow from the pipeline.

At startup and shutdown, the discharge of Sales Gas Recompressor can be vented to the flare system at battery limits.

Instrumentation

Feed Cooling Section Controls

The temperature of feed gas from battery limits is indicated on Feed Gas Temperature Indicator (TI-801) and its pressure is indicated on Feed Gas Pressure Indicator (PI-801). The ethane content of the feed gas is indicated on Feed C2 Analyzer Indicator (AI-801). The feed gas flow, FIC-801, is normally controlled by adjustment of the Feed Gas Expander (KT-801) inlet guide vane position. Refer to the Advanced Controls section below for more details of the control strategy. Feed Gas Hand Controller (HIC-801) is used to adjust the position of feed gas valve HV-801. Normally, HIC-801 is fully open.

E-801 Feed Gas Out Temperature Indicator (TI-802) indicates the outlet temperature of the feed gas from Feed/Sales Gas Exchanger (E-801). E-801 Sales Gas Outlet Temperature Indicator (TI-823) indicates the sales gas outlet temperature.

The two-phase mixture of feed gas leaving the Feed/Sales Gas Exchanger is separated in Feed Separator (D-801). The pressure of the Feed Separator is indicated on D-801 Feed Separator Pressure Controller (PIC-803). PIC-803 can be used to control the position of inlet guide vane of the Sales Gas Compressor (K-802). Refer to the Advanced Controls section below for more details of the control strategy. Normally PIC-803 is in manual mode. The temperature of feed gas leaving Feed Separator is indicated on D-801 Gas Outlet Temperature Indicator (TI-804). The level of liquid in Feed Separator is controlled by D-801 Feed Separator Level Controller (LIC-803) which adjusts the opening of control valve LV-803. The flow of liquid flowing through LV-803 is indicated on D-801 Liquid Flow Indicator (FI-803).

D-801 HH Trip Level Alarm (LAH-803) is an independent level instrument for Feed Separator and is used as a trip sensor for Interlock I-801. LIC-803 will alarm before LAH-803 to give the operator warning of an impending high-level trip (refer to section Interlock I-801 below for details).

Expander & Recompressor Controls

Cool gas from Feed Separator (D-801) enters Feed Gas Expander (KT-801) by passing through expander trip valve XV-801. XV-801 is controlled by Expander Trip Interlock I-801 (see section Interlock I-801 below for details).

The flow of gas to the Feed Gas Expander is controlled by the position of the inlet guide vane FV-801A. HIC-801A is a hand controller that is normally in cascade mode and receives its setpoint from selector FY-801.

HIC-801B is a hand controller that adjusts the expander bypass control valve FV-801B. HIC-801B is normally used at startup and shutdown and is automatically adjusted in the event of a trip of KT-801 by I-801 (see section Interlock I-801 below for details). Normally HIC-801B is in manual mode with an output of 0% to close FV-801B.

Expander Gas Flow Indicator (FI-805) indicates the total flow of gas to Feed Gas Expander and its bypass.

Switch HS-803 with selector FY-801 form an “A/B switch” used to select the control signal destined for HIC-801A and HIC-801B. Normally HS-803 is in the “A” position to select the output of feed flow controller FIC-801. When it is in the “B” position, it selects the output of T-801 Top Pressure Controller (PIC-814B). The unselected controller of FY-801 is forced into manual mode and its output tracks the selected controller’s output. This ensures bumpless control when switching the position of HS-803.

The speed of the shaft connecting Sales Gas Recompressor (K-801) and Feed Gas Expander is indicated on Expander Speed Indicator (SI-801). A second speed indication, SAH-801, is used as a trip sensor for interlock I-801. SI-801 will alarm before SAH-801 to give the operator warning of an impending overspeed trip (refer to section Interlock I-801 below for details).

The outlet temperature from Feed Gas Expander is indicated on KT-801 Outlet Feed Temperature Indicator (TI-807). The temperature of mixed gas from Feed Gas Expander and its bypass is indicated on T-801 Cold Feed Temperature Indicator (TI-812).

XI-801A indicates the status of the machinery monitoring system of the Feed Gas Expander/Sales Gas Recompressor unit. In case of trouble (e.g. excessive vibration, high bearing temperature, low lube oil pressure) XI-801A will produce an alarm. If the trouble is severe, XA-801A will alarm and cause a trip of I-801 to protect the machines.

HS-801 is a switch used to trip and reset interlock I-801. XA-801 generates an alarm that the Expander/Recompressor train has been tripped.

K-801 Suction Pressure Indicator (PI-823) indicates the pressure of Sales Gas entering Sales Gas Recompressor. K-801 Discharge Temperature Indicator (TI-824) indicates the discharge temperature of Sales Gas Recompressor.

K-802 Suction Pressure Controller (PIC-824) controls the discharge pressure of Sales Gas Recompressor in the event of a stop of downstream Sales Gas Compressor (K-802). PIC-824 controls the position of control valve PV-824 to route gas to the flare system at battery limits.

Demethanizer Controls

The cold vapor feed from Feed Gas Expander (KT-801) to the top of Demethanizer (T-801) is indicated on T-801 Cold Feed Temperature Indicator (TI-812). The flashed liquid feed to Demethanizer is indicated on T-801 Liquid Feed Temperature Indicator (TI-813).

The temperature of the overhead vapor leaving the top of the Demethanizer is indicated on T-801 Overhead Temperature Indicator (TI-814). The temperature of tray 33 in the upper section of the Demethanizer is indicated on T-801 Tray 33 Temperature Indicator (TI-815). The temperature of tray 20 in the center section of the Demethanizer is indicated onT-801 Tray 20 Temperature Indicator (TI-816). The temperature of tray 7 in the bottom section of the Demethanizer is indicated on T-801 Tray 7 Temperature Indicator (TI-817).

The differential pressure across the trays in the upper section of the Demethanizer is indicated on Top Section T-801 Pressure Differential Indicator (PDI-811). The differential pressure across the trays in the lower section of the Demethanizer is indicated on Bottom Section T-801 Pressure Differential Indicator (PDI-812).

The overhead pressure of the Demethanizer is normally controlled by T-801 Top Pressure Controller (PIC-814A) which adjusts the position of the guide vane on Sales Gas Compressor (K-802). An alternate mode of pressure control is to use T-801 Top Pressure Controller (PIC-814B) to control the inlet guide vane of Feed Gas Expander (KT-801). To avoid unstable control, only one of these two controllers is permitted to be in automatic mode at any time. The advanced control system will reset the second controller that is placed in automatic mode back to manual mode.

The outlet temperature of Reboiler (E-805) is controlled by E-805 Vapor Outlet Temperature Controller (TIC-818) which adjusts the setpoint of the hot oil flow controller Hot Oil to E-805 Flow Controller (FIC-818). The pressure of the hot oil supply is indicated on Hot Oil Supply Pressure Indicator (PI-001) and its temperature is indicated on Hot Oil Supply Temperature Indicator (TI-001). The temperature of the bottoms liquid leaving the Demethanizer is indicated on T-801 Bottom NGL (TI-819).

The level of NGL in the bottom of the Demethanizer is controlled by T-801 Bottom Level Controller (LIC-819) which adjusts the position of LV-819 to regulate the flow of NGL to the Deethanizer at battery limits. The flow of NGL through LV-819 is indicated on NGL to Deethanizer Flow Indicator (FI-819). The methane content in the product NGL is indicated on NGL Methane Analyzer Indicator (AI-819).

Sales Gas Compressor Controls

K-801 Discharge Temperature Indicator (TI-824) indicates the sales gas temperature from Sales Gas Recompressor (K-801). K-802 Suction Pressure Controller (PIC-824) controls the discharge pressure of Sales Gas Recompressor/suction pressure of Sales Gas Compressor (K-802) in the event of a stop of Sales Gas Compressor. PIC-824 controls the position of control valve PV-824 to route gas to the flare system at battery limits.

K-802 Suction Pressure Indicator (PI-825) indicates the suction pressure of Sales Gas Compressor. K-802 Discharge Pressure Indicator (PI-826) indicates the discharge pressure. The ratio of the two pressures (first converted to absolute pressures) is indicated on PY-826. PYH-826 generates a trip signal for I-802 Sales Gas Compressor interlock based on the PV of PY-826. PY-826 will alarm before the trip point to warn the operator of an impending trip condition (refer to section Interlock I-802 below for details).

The discharge temperature of Sales Gas Compressor is indicated on K-802 Disharge Temperature Indicator (TI-826). A second, independent temperature is indicated on TAH-826 which serves as a trip sensor to I-802. TI-826 will alarm before the trip point of TAH-826 to warn the operator of an impending trip condition (refer to section Interlock I-802 below for details).

The flow of sales gas from Sales Gas Compressor to the pipeline is indicated on Sales Gas Pipeline Flow Indicator (FI-826). The ethane content of the sales gas is indicated on Sales Gas Ethane Analyzer Indicator (AI-826).

HIC-827 is used to isolate Sales Gas Compressor from the pipeline by adjusting the position of control valve HV-827. The pressure of the pipeline is indicated in Sales Gas Pipeline Pressure Indicator (PI-827).

XI-802A indicates the status of the machinery monitoring system of Sales Gas Compressor. In case of trouble (e.g. excessive vibration, high bearing temperature, low lube oil pressure) XI-802A will produce an alarm. If the trouble is severe, XA-802A will alarm and cause a trip of I-802 to protect the machine.

HS-802A is a switch that operates the electric motor of Sales Gas Compressor. The electric power used by the motor of K-802 is indicated on KM-802 Power Joules Indicator (JI-826). HS-802B is a switch used to trip and reset interlock I-802. XA-802 generates an alarm that the Sales Gas Compressor has been tripped. When I-802 is in the trip state, HS-802A is locked in the STOP state (refer to section Interlock I-802 below for details).

Switch HS-814 with selector PY-814 form an “A/B switch” used to select the control signal for the inlet guide vane PV-814 of Sales Gas Compressor. Normally HS-814 is in the “A” position to select the output of Demethanizer overhead pressure controller T-801 Top Pressure Controller (PIC-814A). When it is in the “B” position, it selects the output of D-801 Feed Separator Pressure Controller (PIC-803). The unselected controller of PY-814 is forced into manual mode and its output tracks the selected controller’s output. This ensures bumpless control when switching the position of HS-814.

Interlock I-801

This interlock protects the Feed Gas Expander (KT-801) and Sales Gas Recompressor (K-801) from mechanical damage. It is a latched interlock and is activated by any of the following:

  • the speed of the Expander SAH-801 exceeds the trip setting of 15,000 RPM
  • mechanical trouble signal XA-801A
  • LAH-803 exceeds the trip setting of 95%
  • switch HS-801 is manually changed from the OK state to the TRIP state

This interlock has the following effects:

  • closes the inlet valve XV-801 to the Feed Gas Expander
  • places HIC-801A (expander guide vane controller) into manual and locks its output to 0.0
  • performs a one-time reset of the output of Feed Gas Flow Controller (FIC-801) (or T-801 Top Pressure Controller (PIC-814B)) if it is automatic mode. This is done to automatically open the Feed Gas Expander bypass valve to a preset position when the Feed Gas Expander trips. This system minimizes upsets to the unit (see section Special Controls below for other details)
  • causes a high alarm on XA-801.

The interlock can be reset after all process trip inputs are cleared by placing switch HS-801 into the OK state.

Interlock I-802

This interlock protects the Sales Gas Compressor (K-802) from mechanical damage. It is a latched interlock and is activated by any of the following:

  • the discharge temperature TAH-826 exceeds 302 DEG F for more than 10 seconds
  • the pressure ratio PYH-826 exceeds 3.5 for more than 10 seconds
  • mechanical trouble signal XA-802A

This interlock has the following effects:

  • locks the compressor motor switch HS-802A in the STOP state
  • causes a high alarm on XA-802

The interlock can be reset after all trip inputs are cleared by placing switch HS-802B into the OK state. Once the interlock has been reset, Sales Gas Compressor can be started by placing switch HS-802A into the RUN state.

Advanced Controls

This section describes the special controls of the Turbo Expander unit.

Expander Guide Vane Control

The output signal from Feed Gas Flow Controller (FIC-801) is connected to the “A” input signal of switch FY-801 which, in turn, is connected to the external setpoint signal of both Expander Guide Vane Hand Controller (HIC-801A) and Expander Bypass Hand Controller (HIC-801B) which are configured with a ratio of 1.0. Neither HIC will back-initialize any controller when the HIC is not in cascade mode.

An alternate mode of expander guide vane control uses the T-801 Top Pressure Controller (PIC-814B) output to the “B” input of FY-801. HS-803 is used to switch between the A and B inputs. To ensure bumpless transfer when switching HS-803, the unselected controller to FY-801 is locked in manual mode and its output tracks the selected controller.

Normally, HS-803 is in the “A” position and Feed Gas Fow Controller (FIC-801) will control the inlet guide vane position of the Feed Gas Expander (KT-801) while HIC-801A is in cascade mode. The Feed Gas Expander bypass valve will normally be closed with HIC-801B in manual with an output of 0.0.

In case the Feed Gas Expander is taken out of service, the bypass valve can be regulated from FIC-801 (or PIC-814B) by placing HIC-801B into cascade mode.

In case of a Feed Gas Expander trip, special logic is required to do a one-shot initialization of controllers FIC-801 (or PIC-814B), HIC-801A and HIC-801B in order to minimize an upset to the unit. This one-shot occurs when:

  • FIC-801 (or PIC-814B) is in automatic mode, and
  • HIC-801A is in cascade mode, and
  • HIC-801B is not in cascade mode, and
  • I-801 has just transitioned from OK to TRIP (one-shot)

When this happens, the following takes place:

  • FIC-801 (or PIC-814B) is placed in manual for two seconds
  • FIC-801 (or PIC-814B) output is initialized to a value calculated from a lookup table that is a function of the feed flow (FIC-801’s PV before the trip) (one-shot)
  • HIC-801B is placed into cascade mode (one-shot)
  • HIC-801A is locked by interlock I-801 (see above)
  • After a delay of three seconds, FIC-801 (or PIC-814B) is placed into automatic mode (one-shot)
  • After a delay of four seconds, the setpoint of FIC-801 (or PIC-814B) is restored to the value just before the trip (one-shot)

Sales Gas Compressor Guide Vane Control

The flow of sales gas to the pipeline is controlled by adjusting the position of the guide vane on Sales Gas Compressor (K-802). The control signal to the guide vane position is selected by switch PY-814 which has the T-801 Top Pressure Controller (PIC-814A) output as its “A” input and the D-801 Feed Separator Pressure Controller (PIC-803) output as its “B” input. The selection of the guide vane control signal is made using handswitch HS-814. To ensure bumpless transfer when switching HS-814, the unselected controller to PY-814 is locked in manual mode and its output tracks the selected controller.

Normally HS-814 is in the “A” position so that PIC-814A controls the guide vane position of the Sales Gas Compressor.

Demethanizer Overhead Pressure Control

There are two controllers, PIC-814A and PIC-814B, for controlling the Demethanizer overhead pressure. Both controllers share the same pressure transmitter. PIC-814A can be used to control the inlet guide vane of the Sales Gas Compressor while PIC-814B can be used to control the inlet guide vane of the Feed Gas Expander (KT-801). Logic prevents both controllers from being in automatic mode at the same time. The first controller put in automatic mode will remain in automatic mode while the second controller will be set back to manual mode. To switch control, both controllers must first be placed into manual mode.