Venezuelan plant completes instrument upgrade

The Lamargas complex, Lake Maracaibo, Venezuela.

The Lamarliquido LPG plant, offshore Lake Maracaibo, Venezuela, has received a major upgrade to its control system after 25 years of pneumatic instrumentation.

The plant is now operating fully remote from a new central control room with a maximum production of 5,500 b/d, expected to go to 6,600 b/d.

The phases of the project included installation and integration of four control systems:

• Distributed control system (DCS)

• Fire prevention, detection, and control system (FCS)

• Refrigeration process turbocompressor control system (TCCS)

• Emergency shutdown (ESD) system.

1960s vintage

The Lamarliquido plant was built in the 1960s for extraction of 12,000 b/d of C₃+, corresponding to a natural-gas feed of 160 MMscfd.

The difference between the actual and design production results from a natural reduction in the reservoir richness that has led to a decrease in the gas-liquid content from 4.5 to 2.9 gal/Mcf.

The extraction is achieved through a propane mechanical-refrigeration process in which gas feed is chilled to -35° F. This stream flows into a three-phase separator for gas, liquid hydrocarbons, and glycol solution separation.

Liquid hydrocarbons are sent to a deethanizer where LPG and residual gas are obtained. Glycol injection is used to avoid hydrates formation.

The maximum production of LPG achieved since completion is 5,500 b/d, with a limited gas feed of 120 MMscfd. A production of 6,600 b/d is expected when plant gas feed reaches 140 MMscfd.

Integration of the four control systems has permitted use of DCS as the main control system, concentration of all operations in the central control room, and reduction of operators in the process area.

The Lamarliquido plant (Fig. 1 [25040 bytes]) is a side-stream installation originally designed to produce LPG from a raw gas feed with a liquid content of 4.5 gal/Mcf of gas.

The feed gas to Lamarliquido can be taken from the first stage of any of three different compression plants which are part of the Lamargas complex: Lamargas, Cincogas III, or Cincogas V.

Today the Lamarliquido feedstock gas and the corresponding LPG production can vary, depending on the associated compression plant, as follows: Lamargas, 160 MMscfd and 7,500 b/d; Cincogas III, 120 MMscfd and 5,500 b/d; and Cincogas V, 130 MMscfd and 6,100 b/d.

The Lamarliquido LPG-extraction process is based on a mechanical propane refrigeration system and a deethanizer column (Fig. 2 [28058 bytes]).

After the extraction process, the deethanized product stream is pumped ashore to a fractionation plant, while the residual gas is returned to the corresponding compression plant's second stage suction.

Gas coming from the compression modules at 435 psig and 125° F. enters the inlet separator which removes any impurities. Outlet saturated gas is cooled in the first heat exchanger to 96° F., with residual gas at 75° F.

In the second heat exchanger, the process-gas temperature is dropped to 42° F. using residual gas at -30° F. Later, process gas enters the third exchanger where its temperature lowers to 21° F. by interchanging with the recovered liquid hydrocarbons at -35° F.

Finally the process gas is chilled to -35° F. in the last two heat exchangers of the cooling section, which work with propane at -3° F. and -43° F., respectively.

To prevent hydrate formation, ethylene glycol at 70% concentration is injected into the process gas at the inlet of the last four exchanger units.

The mixture leaving the last propane chiller flows to a flash tank where the two phases separate. The liquids flow to a liquid-liquid separator which splits the hydrocarbons and glycol.

The glycol is sent to a reconcentrator for its purification. The hydrocarbon liquid at -35° F. is pumped to the deethanizer after first exchanging heat with the inlet gas.

The residual gas from the flash tank at -35° F. joins the deethanizer overhead vapors at 4° F. The combined cold residue gas exchanges heat with the inlet gas and flows at 115° F. to the second stage of the compression unit.

The deethanizer is a 45-tray column with reboiler and reflux sections. The reboiler section is heated with hot oil and operates at 243 F. and 460 psig.

The reflux section working at 27° F. and 455 psig consists of a top gas condenser working with propane, a reflux accumulator, and a reflux pump.

The deethanizer bottom product, with a specification of 1 mol % maximum content of ethane, is pumped ashore for fractionation.

For the refrigeration system, the main equipment is a three-stage centrifugal pro pane compressor driven by a gas turbine.

In this closed cycle, propane is first compressed to approximately 205 psig in the centrifugal compressor. Then six parallel water-cooled atmospheric coil units condense the propane which flows to a surge tank operating at 105° F. and 200 psig.

Next, liquid propane is flashed into a high-stage flash tank at 80 psig and 53° F. Vapors from this vessel return to the high suction stage of the compressor, and high-stage liquid is flashed into the first chiller of the cooling train and the deethanizer overhead condenser, both operating at 20 psig and -3° F.

Vapors generated in these two units return to the intermediate stage of the compressor. An excess of liquid propane is flashed into the deethanizer chiller shell and then flows to the second chiller of the cooling train which operates at atmospheric pressure and -43° F.

Vapors from this chiller return to the low suction stage of the compressor, thus completing the propane circuit.

For the hot-oil system, the oil is heated with an unfired heater installed beside the turbine exhaust stack and connected to it by a duct. The hot-oil temperature is controlled by a damper located in the stack which diverts hot gases to the heater.

The hot oil supplies heat to the deethanizer reboiler, glycol reconcentrator, and a propane make-up unit. The oil is pumped from a surge tank, through the heater where temperature is raised to 475° F., and then to the units previously mentioned.

It returns to the surge tank at approximately 350° F.

Objectives, phases

The modernization project's objectives are to:

• Increase flexibility and ensure the continuous and successful operation of the plant

• Extend the life of the plant

• Reduce risks of accidents

• Improve plant product quality

• Ensure environmental protection

• Reduce costs of maintenance and plant operations

• Operate the plant from a remote control room reducing the number of operators in the process area

• Improve the plant's control technology to state of the art.

The project was developed in seven phases:

1. Design and construction of a new unit control room and a new central control room

2. Change of instrumentation from pneumatic to "smart" electronic

3. Installation of a DCS for process monitoring and control

4. Installation of a fire prevention, detection, and control system (FCS)

5. Installation of a dedicated system for the automatic operation of the refrigeration process turbocompressor (TCCS)

6. Installation of a triple modular redundant system for commanding the plant emergency shutdown and the refrigeration process turbocompressor protection

7. Integration of the four control systems mentioned previously.

A unit control room was designed for all the cabinets and panels of signals for the new systems, taking into account the needs of maintenance flexibility and expandability of these systems.

It was built next to the process area and is pressurized, air locked, and air conditioned (Fig. 3 [21218 bytes]).

A new central control room was designed to reduce to the minimum the number of personnel in the plant process area.

Placed 500 ft from the unit control room, this room was designed for the DCS operation stations, the engineering work station, and all the supplementary equipment that permit a complete remote operation of the plant (Fig. 3 [21218 bytes]).

In the plant reinstrumentation phase of the project, all the original pneumatic instrumentation was changed to "smart" electronic. This included instrument selection and installation, in addition to design and construction of all the infrastructure to connect the new instruments to the unit control room.

Distributed control system

The DCS was installed at the central control system for all the different processes that take place in the plant during normal operation, including pumps and air compressors start-up and shutdown.

Additionally, the DCS supervises the performance of the other three control systems. Functionally, the system is divided in four parts (Fig. 4 [25930 bytes]):

• Man-machine interface. The MMI is a fully redundant interface consisting of two operation stations located in the central control room.

  Each operation station includes an electronic console, which masters two 19-in. color visual display units, two operator-oriented membrane model keyboards, and the necessary printers.

  As part of the DCS configuration, 65 dynamic diagrams were designed: 17 for monitoring and controlling the plant processes, 8 for monitoring the DCS/TCCS integration, 3 for plant emergency shutdown and turbocompressor protection shutdown, 4 for alarm managing, 9 for process pumps operation, 4 for general DCS information, 20 for interconnection diagrams.

• Engineering workstation. The engineering workstation permits the DCS configuration.

  For this application, the workstation is in the central control room and holds the configuration software and the operating system.

  Additionally, it supports data historian and console trend display software.

• Communications subsystem. Communication functions for this DCS application are performed through a local area network. This is the physical redundant medium used to interconnect the controllers, the data acquisition unit, the operation consoles, and the engineering workstation.

  The first two are at the unit control room; the last two at the central control room.

  The communication protocol for this network is the token-pass. This kind of protocol permits a continuous communication among the different units using the primary medium or the secondary medium, even if any one of the units connected to the network fails.

  For the DCS, there is also a communication highway based on a coaxial cable, which is in charge of handling the information from and to the field connecting the I/O (input/output) drivers with the I/O card racks. This medium is called the I/O bus.

• Controlling and monitoring subsystem. All the control loop functions and pumps and air compressors start-up and shutdown sequences are performed by the DCS integrated function controller (IFC).

The IFC installed in this case is a multi-loop redundant controller that can handle up to 500 logic points.

The IFC controller as a whole consists of three different types of cards: the microprocessor unit, the highway communication, and the I/O driver.

The driver card controls the information flow in a maximum of 16 I/O card racks that can be connected 5,000 ft away and which can accommodate 14 I/O cards each.

For this application, 4 of the 16 racks of maximum capacity are connected: 1 for the I/O analog signals (14 cards), 1 for the I/O temperature signals (4 cards), and 2 for the I/O discreet signals (21 cards).

Data collection from the other control systems (TCCS and ESD system) is performed for this application by the DCS multiplexer unit (MUX). This device was implemented as part of the equipment required for remote operation of the refrigeration turbocompressor.

Fire-control system

In the Lamarliquido plant, the fire prevention, detection, and control security system is based on gas, fire, heat, and smoke detection in the different fire sections or equipment (Fig. 5 [24504 bytes]).

The main devices are a detection and control panel, a graphic announcer (GA), and a manual remote-control panel.

The first is located at the unit control room, and the other two are located at the central control room.

The detection and control panel manages all incoming field signals from fire, gas, heat, and smoke detectors, and provides outlet signals for sound diffusers, automatic deluge valves, and local visual alarms.

In order to send to the central control room the necessary information about the different fire sections of the plant, the detection and control panel is interconnected with the GA, using a communication card and a redundant communication line.

Additionally, the detection and control panel is interconnected with the ESD system.

Gas, fire detection

When gas detectors receive a gas concentration signal of 20% of the lower limit of inflammability, the detection and control panel initiates an alert condition that includes the following actions:

• Sound and visual alarms in both the unit and the central control rooms that identify the fire section and equipment related.

  If the gas-concentration signal rises to 40% of the lower limit of inflammability, the system initiates an emergency condition that includes:

• Sound and visual alarms in both the unit and the central control rooms that identify the fire section and related equipment.

• Communication with the ESD system initiates a general plant shutdown following its own programmed logic.

  When a fire or a heat detector activates, the detection and control panel initiates an emergency condition which includes:

• Sound and visual alarms in both the unit and the central control rooms identify the fire section and related equipment.

• The automatic deluge valve associated with the affected fire section is opened.

• Communication with the ESD system initiates a general plant shutdown following its own programmed logic.

FCS equipment

The fire detectors combine two sensors, one for ultraviolet (UV) rays detection and the other one for infrared (IR) rays detection.

Only when both sensors are activated is the fire signal sent to the detector controllers. This condition virtually eliminates all possible false alarms.

The fire detector controllers constantly monitor the outlet signals from all of the UV/IR fire detectors connected to them. In the case of a fire alarm, these units transmit a signal to the detection and control panel, activating the logic programmed in this device.

In Lamarliquido, three fire-detector controllers and 21 UV/IR fire detectors were installed.

The gas detectors installed for this application have an active sensor and a reference temperature compensatory unit. In the presence of fuel gas, resistance of the active element rises in proportion to the gas concentration detected. This change compared to the reference is employed to determine the percentage of gas concentration.

Heat detectors consist of a bimetallic element which detects the environment temperature. When this temperature surpasses the nominal or operation temperature, the detector closes its contacts, generating an alarm signal.

The FCS has two kinds of smoke detectors:

• Photoelectric, which activate when the smoke entering it deviates the detector photoelectric cell ray.

• Ionization, which activate when the entering smoke produces an ionization and makes an electric current circulate through the unit.

The manual/remote panel permits the manual remote start-up of the fire water pumps, and the direct opening of the automatic deluge valves. It is located in the central control room and works as a backup of the detection and control panel.

The graphic announcer (GA) of the system is located at the central control room and is equipped with a microprocessor, I/O signal cards, communication cards, and an indication front panel with a matrix of lights.

In a fire or gas-detector activation, the GA permits a quick reference of the affected fire section and related equipment.

The detection and control panel includes a power source, motherboards, a control card, and functional cards.

The motherboard provides eight slots for functional cards location, each one associated with one terminal strip. The control card supervises and directs system functions. One control card can command a maximum of four motherboards.

This device communicates with the ESD system using dry contacts. In the case of a communications failure, the detection and control panel continues with its alarm and security functions independently because the operating system parameters are stored in no volatile memory.

Turbocompressor system

The dedicated refrigeration turbocompressor control system controls the entire operation of the propane refrigeration process turbocompressor.

The TCCS is functionally divided into three parts which are connected as shown in Fig. 6 [22267 bytes] and are all installed in the unit control room: turbocompressor start-up and shutdown sequence unit, turbine fuel regulator, and compressor surge controller.

The system also includes a local control panel and a display station.

The turbocompressor start-up and shutdown sequence unit commands the start-up and shutdown sequences of the turbocompressor, monitoring its performance variables, and generating the corresponding alarms.

These functions are accomplished by a programmable controller with a base memory of 6K and a capacity of 512 I/O points.

This controller can communicate with other controllers, processors, and programming terminals on the data highway link. This capability was used for integration with the DCS.

The turbine fuel regulator subsystem controls the turbocompressor speed by developing an algorithm that determines the outputs to regulate the pressure and volume of fuel gas to the turbine and the position of the variable angle nozzle ring that separates the high and low pressure turbines.

The surge controller subsystem is installed to protect the compressor against surge in its three suctions. Main components are the antisurge controller, the final control elements, and transmitters and flow elements.

The antisurge controller has two algorithms, one for calibration and one for control.

The calibration algorithm determines where the compressor is operating in relation to its surge limit line and provides set point and measurement values to the control algorithm.

These calculations are based in the inlet flow, the suction pressure, the differential pressure across the compressor, and the load setpoint from the capacity controller.

The control algorithm manipulates the final control elements.

These elements move the compressor operating point away from the surge line. They are two recycle valves which reduce discharge pressure (or prevent it from increasing), thereby allowing the compressor to maintain sufficient flow to avoid surge.

Transmitters and flow elements send information to the calibration algorithm.

The surge events are detected and counted and, after a pre-established number of surges, the surge controller sends a signal to the turbocompressor start-up and shutdown sequence unit to initiate a turbocompressor shutdown.

Emergency shutdown system

The ESD system installed in Lamarliquido is a triple modular redundant system and was designed and installed with two purposes: refrigeration process turbocompressor protection and plant emergency shutdown. These two functions are performed simultaneously.

During the normal operation of the propane refrigeration process, different events exist that incite the shutdown sequence of the refrigeration turbocompressor. Any of these events generates a signal that activates the ESD system which immediately carries out the turbocompressor shutdown, following the corresponding programmed logic.

The signals that initiate the turbocompressor shutdown are distributed as shown in the accompanying box. The discreet signals must be sustained for 0.5 sec except for low hydraulic pressure that must be sustained for 3 sec and surge detection in which the signal must repeat a pre-established number of times.

Failure or opening of any of the thermocouples activates the shutdown sequence, except for the exhaust gas temperature for which the six thermocouples installed must fail simultaneously.

Once a signal for the turbocompressor emergency shutdown has originated, the actions shown in an accompanying box occur immediately and simultaneously.

The logic of the emergency shutdown control system is on line at any plant operating condition. This logic is activated by one or more of the signals shown in the accompanying box.

Plant shutdown logic

Once a signal for the plant emergency shutdown has originated, the following actions occur immediately and simultaneously:

• The same actions indicated for the turbocompressor emergency shutdown, but the audible alarm has a different push button for recognition, and the visual alarm at the DCS screen indicates a plant emergency shutdown.

• General ac power cut off.

• Direct current voltage to all the solenoids of the plant block valves is cut. Therefore, all these valves go to their safe position.

• When the plant emergency shutdown is initiated by a fire detection, the system generates a signal for automatic plant blowdown. This action has a delay of 5 min (Fig. 9 [24136 bytes]).

The delay time has been designed to permit operators to isolate the plant.

If the operators control the emergency within this time, they can inhibit the blowdown action manually from the control room. Additionally, the operators can blow down the plant manually from the control room whenever they consider it necessary and independently from the ESD system.

The delay time will also permit the plant gas feed and gas return block valves to close and plant bypass valves to open, isolating Lamarliquido.

All the ESD system conditions are presented in the DCS screens, including the remaining time for automatic blow down in the case of fire detection.

The low dc voltage is detected by the ESD system from its own supply. When this signal gets out of the normal range, the system adopts its safe fail position, de-energizing all its outlet signals.

This action will cause an emergency plant shutdown with an immediate plant blow-down.

After a refrigeration turbocompressor emergency shutdown or a plant emergency shutdown, the ESD system must be repositioned in order to start up the equipment or the plant. This action is accomplished from the control room by the reposition push bottom.

At this moment, the system will verify that all the initiation signals are within the normal range, with the exception of those signals which should be inhibited and that will be activated progressively: turbine lube oil low pressure, starting turbine lube oil low pressure, compressor lube oil low pressure, and seal oil low differential pressure.

Each of these signals reactivates when its value reaches and sustains for 20 sec the reference value established for the turbocompressor shutdown: flame loss of Combustor No. 5 and flame loss of Combustor No. 6. These signals reactivate when flame is detected in Combustor Nos. 5 and 6.

The signal for low hydraulic pressure reactivates when the turbine gets to 60% of its velocity.

After this verification has been accomplished, all the ESD system outlet signals will energize, and the operators will be able to manipulate the associated elements following the best practices. At the same time, the TCCS will be ready to restart the turbocompressor.

From now on, the ESD system will be operative and ready to operate in any subsequent emergency situation.

System integration

To guarantee complete and remote plant operation from the central control room, including general start-up and shutdown, the four systems installed were integrated (Fig. 7 [21365 bytes]).

For this integration, the DCS is the highest level control system and is directly interconnected with the ESD system and the TCCS, in this way centralizing the entire plant operation.

At the same time, the TCCS, the ESD system, and the FCS are also interconnected.

Integration of the DCS/TCCS (Fig. 4 [25930 bytes] and Fig. 8 [25667 bytes]) was accomplished to monitor and operate the refrigeration turbocompressor from the central control room.

The remote actions include the following:

• Turbocompressor start-up and shutdown

• Change of turbine load setting

• Monitoring of the TCCS elements' signals and alarms (start-up and shutdown unit, turbine fuel regulator, and surge controller)

• Monitoring of turbocompressor and auxiliary equipment operation parameters.

The DCS/TCCS integration was achieved using the architecture shown in Fig. 8 [25667 bytes] and involves connection of the TCCS and the MUX of the DCS and connection of the TCCS and the IFC of the DCS.

For the first connection, it was necessary to install two new PLCs, one acting as a data collector, the other one as a data concentrator.

The data collector gathers information from the turbine-fuel regulator and surge controller of the TCCS and sends this information to the data concentrator. This receives at the same time the information of the start-up and shutdown sequence unit of the TCCS.

The data concentrator transfers all this information to the MUX of the DCS. The data concentrator also transmits the start-up, shutdown, and alarm recognition signals from the DCS to the start-up and shutdown sequence unit.

The surge controller, the turbine fuel regulator, and the data collector communicate through a serial communication channel, with the necessary communication cards in each one of these units.

The data collector, data concentrator, and start-up and shutdown sequence unit communicate over a data-highway communication system.

The data concentrator and the MUX of the DCS communicate with a serial communication channel with the necessary communication cards in each one of these units.

The second connection consists in sending those analog signals, which are essential for the remote operation of the turbocompressor, directly from the field instrumentation to the IFC of the DCS, with the normal arrangement inside the DCS system.

The signal for changing the turbine-load setpoint was also connected, directly from the IFC of the DCS, to the TCCS marshaling panel.

The ESD system and the DCS (Fig. 9 [24136 bytes]) were connected via a serial communication channel, using the respective communication cards for each unit. This connection permits monitoring of the status of the initiators and alarms of the ESD system, at the DCS operation stations.

The fire detection and gas-detection signals are sent from the FCS to the ESD system via hard wiring. This link permits the ESD system to initiate the particular plant emergency shutdown for each one of these cases.

For TCCS/ESD system integration (Fig. 9 [24136 bytes]), the ESD system receives the necessary signals to protect the refrigeration turbocompressor directly from the field instrumentation and from the TCCS marshaling panel.

At the same time, the necessary ESD system outlet signals to shut down the turbocompressor and lock up the TCCS are cabled directly between the two systems' marshaling panels.

Based on a presentation to 75th Annual GPA Convention, Mar. 11-13, Denver.

Signals initiating turbocompressor shutdown

Discreet signals

• Suction scrubber high level.
• First side load suction scrubber high level
• High stage flash tank high level
• Compressor gas discharge high pressure
• Turbine fuel regulator failure
• Compressor antisurge controller failure
• Surge detection Suction scrubber low pressure
• First side load suction scrubber low pressure
• High stage flash tank low pressure
• Turbine fuel gas high pressure
• Turbine fuel gas low pressure
• Turbine lube oil low pressure
• Starting turbine lube oil low pressure
• Compressor lube oil low pressure
• Seal oil low differential pressure
• Turbine/compressor vibration
• Low-pressure turbine overspeed
• High-pressure turbine overspeed
• Starting turbine overspeed
• Low hydraulic pressure
• Flame loss of Combustor No. 5
• Flame loss of Combustor No. 6
• Local/remote manual trip
• Fuel-gas scrubber high level
• Instrument air low pressure
• Lube-oil tank high level
• Lube-oil tank low level
• Plant emergency shutdown

Thermocouples signals

Compressor discharge high temperature
• Compressor lube-oil high temperature
• Turbine exhaust high temperature
• Turbine lube-oil high temperature
• Starting turbine lube-oil high temperature
• Lube-oil header high temperature

Turbocompresor protection logic

• Audible alarm switches on at both the unit and central control rooms.
• Visual alarm appears at the DCS screen, indicating the turbocompressor emergency shutdown.
• Turbine fuel-gas block valve closes.
• Block valves of the three compressor suctions close.
• Compressor discharge block valve closes.
• Suction-recycle liquid propane injection valve closes.
• First side load recycle liquid propane injection valve closes.
• Suction recycle block valves open.
• First-side load suction recycle opens.
• Glycol feed pumps shut down.

Plant emergency shutdown signals

• Low instrument air pressure
• Manual shutdown stations
• Gas detection
• Heat detection
• Fire detection
• High temperature in the exhaust stack of the hot oil heater in excess of 750° F.
• Simultaneous failure of the two thermocouples installed to measure the above mentioned temperature.
• Low dc voltage
• Loss of ac voltage
• Loss of unit control room pressurization

The Authors

Heberto Martinez works in gas processing, infrastructure, and technology developments for the gas engineering department of Petroleos de Venezuela S.A. (Pdvsa) subsidiary Maraven S.A. Between 1986 and 1990, he worked as LPG plant quality control engineer, LPG plant process engineer, and chief of El Tablazo and Bajogrande LPG plants.

Martinez holds a bachelor's degree in mechanical engineering from the University of Zulia, Venezuela, and an MS (1992) in chemical engineering from the Illinois Institute of Technology.

Cesar Ortega Garcia has been control systems advisor in Maraven, S.A., since 1989. He also served as control projects leader (1981-1989) and as head of electrical and instrumentation maintenance C.V.P. for Moron Venezuela (1973-75).

Ortega Garcia holds a BS (honors; 1981) in computer and control systems from Coventry Polytechnic (U.K.). He is a member of the Technical & Professional Association for the Venezuela Oil Industry and the International Society for Measurement & Control.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.