All data acquisition, signal conditioning, signal scaling and process control is performed by each individual Distributed Processing Unit. This solution guarantees that local operation is available even if the Operator Stations are not functioning.
2.7.1 Stand-by pumps
General
Pump control is located in the Distributed Processing Units.
Remote control is available from the Operator Stations in two ways:
• Through the stand by pump display (mimic diagrams).
• Through a (mimic) diagram included in the monitoring and control system.
Stand by start at low pressure
When the pressure drops below the set limit the stand by pump automatically starts. The limit may be adjusted if an analogue sensor is used.
Inhibit stand by start
Starting of the stand by pump is blocked after a blackout or at a group start, until the pressure has been built up. The stand by logic can also be inhibited by an external condition, such as “main engine stopped”, “aux engine stopped” or a similar condition.
Restart after blackout
Some units have to be automatically restarted after a blackout.
These units are included in a start sequence. The delay time defined in the Distributed Processing Unit’s pump logic determines the start sequence. The operator can adjust the sequence timing.
Group start of pumps
Remote controlled pumps can be started in sequence.
Alarms from the stand by pump logic
• Stand by start alarm
• Auto start fail or trip
• Non stand by alarm (indicates that a stopped pump is no longer in stand by mode).
Limits: All limits are downloaded from the Operator Station as part of its database. The limits may be adjusted at the Operator Station or the Midi Operator Station.
Configuration: To maintain redundancy in a two or three pump system, the signals from the motor starters and the process are normally routed to different Distributed Processing Units.
This configuration allows the system to work even if one of the Distributed Processing Unit fails. The stand by pump logic is also active when the Operator Station is down.
2.7.2 Valve control
General
Valve control is located in the Distributed Processing Units.
Remote control is available through the displays on the Operator Stations.
Valve Types
Single Acting Valve: This valve type is controlled by one digital output signal. The valve-position (closed or open) is given by the signals by one or two limit switches.
Double-Acting Valve: This valve type is controlled by two digital output and two input signals. The valve is opened or closed by pulsing a relay.
Double-Acting Valve with Position Control: This valve type is controlled by two digital outputs; two digital inputs and an analogue input signal. The valve position is set within a defined dead band by pulsing a relay.
Additional functions
In addition, the following functions are available as part of the valve control system:
• Pulsed output.
• Automatic closing of a set of valves.
• Automatic opening or closing of valves.
• Close on an analogue setpoint, using deadband.
• Low-level Auto CLOSE Valve, remote OPEN/CLOSE function. Close when a “low level” switch opens.
• Automatic OPEN/CLOSE of Valve Remote OPEN/CLOSE function. Close when a “low level” switch opens.
• Valve Change-Over on Alarm with Reset.
2.7.3 PID controllers
The PID controller system is a set of software modules located in the appropriate Distributed Processing Unit. All combinations of PID controllers are available. Cascade control is accomplished by connecting two controllers together. The PID controllers can be configured from the Operator Station or the Midi Operator Station. The following parameters are available:
• Tuning parameters (gain, integration time and derivation time)
• Controller setpoint
• Output mode (true control mode, AUTO, or valve position control mode, SEMIAUTO)
• Controller types (P, PI, PD or PID)
• Selection of input and output signal type
• Input and output channel number
• Sampling time
2.7.4 Compressor control
The compressor control system is a set of software modules located in the appropriate Distributed Processing Unit. The main purpose is to maintain the air receiver pressure at a constant value. This function includes remote manual and automatic start and stop of compressors, restart after blackout and automatic drain control.
The compressor control system communicates with the Operator Station or the Midi Operator Station and can be monitored or controlled from here.
2.7.5 Purifier control
The purifier control system is a set of software modules located in the appropriate Distributed Processing Unit. The purifier control program is specially written to operate Alfa Laval’s self cleaning oil purifiers, and includes operation of the following equipment:
• Crude oil booster pump (optional)
• Crude oil temperature controller
• Purifiers
• Sealing valves
• Discharge valves
The equipment may be controlled from the Operator Station or the Midi Operator Station. Purifier configuration is selected manually by operating different by-pass valves directly. The
display by showing the oil path as green lines. Purifier no. 1 will always run as purifier, whereas purifier no. 2 may be selected to run as purifier or clarifier.
2.7.6 Boiler control
The boiler control system is a set of software modules located in the appropriate Distributed Processing Unit. The program consists of a standard PID controller and is adaptable to different types of boilers. The boiler control system uses fail checking extensively, and employs a program for controlled shut down, and provides visible alarms.
Boiler system parameters are available on the Operator Station or the Midi Operator Station. All parameters are found in the steam system display. System information such as alarms and analogue values are part of the normal K-Chief 500 alarm and monitoring system. The boiler control involves total control of the following equipment:
• Fuel oil stand-by pumps
• Burner motor
• Fuel pre-heater
• Nozzles
• Ignition unit
• Air damper
• Steam release valve
These units are connected to and controlled by the appropriate Distributed Processing Unit.