08 January 2001
Gas transmission compressor station delivers
Developers find that avoiding higher-level programming, while a bit slower, is more than adequate and less costly.
Thirteen million people throughout Northern and Central California count on Pacific Gas and Electric Co. (PG&E) to deliver their energy. PG&E's parallel gas transmission pipelines, L400 and L401, carry 1.98 billion standard cubic feet of natural gas from the Oregon border to the San Francisco Bay area every day.
Four compressor stations situated strategically along the 400-mile-long pipelines boost the pressure of the gas to keep it moving down the line to the customers. The stations are typically unmanned and must be remotely monitored and controlled by several layers of supervisory control and data acquisition system (SCADA).
The largest of the four stations, the Delevan compressor station, has three gas-driven centrifugal compressors and is arguably the most critical station on the line. A scheduled shutdown at Delevan costs as much as $10,000 per day, and unplanned shutdowns are even more costly.
An upgrade for the station control system at Delevan overcame significant challenges to meet new requirements to increase remote control capabilities and improve system performance while maintaining safe, reliable operation.
Delevan compressor station
The Delevan compressor station consists of three gas-driven centrifugal compressors: K1, K2, and K3. K1 and K2 units rated at 9,250 horsepower, and K3 is a 14,000-horsepower unit.
They compress incoming gas from the 36-inch transmission lines and discharge it into 36- and 42-inch pipes. The station piping allows for the three units to discharge into each of the two pipelines at different discharge pressures and different flow rates.
Control valves, in the suction and discharge piping of each unit as well as in the station piping, route gas through the one suction header and two discharge headers via various configurations.
The control system at the station is comprised of compressor unit controls and an overall station controller. Each compressor unit has a self-contained controller, while a separate controller provides supervisory control of the station.
The unit controllers contain control logic responsible for safe operation of each unit. The unit controllers monitor operating parameters and initiate unit alarms and shutdowns as appropriate. The station programmable logic controller (PLC) contains the logic for coordinating the operation of each compressor unit according to the pipeline conditions.
Significant events log-in
Control set points and station operating parameters reside in the station PLC. Station control set points may be set at the station through a human-machine interface (HMI) or remotely via PG&E's SCADA.
The control system allows unattended operation of the station from PG&E's Brentwood terminal 200 miles south of the station. The station PLC receives and carries out the commands from Brentwood sent via the SCADA system.
The station PLC fulfills the SCADA commands by issuing start/stop commands to the appropriate unit control system and by increasing or decreasing the speed set points to the appropriate unit controllers in response to changing pipeline conditions.
In addition to implementing commands from the Brentwood terminal, the station control system continuously monitors selected pipeline, station, and unit conditions. Significant events and abnormal conditions log into and alarm the station and Brentwood controllers.
Vent gas to atmosphere
The station and the unit controllers have a hierarchical system of responses to specific abnormal conditions. The PLC has the decision-making logic and controls the responses to various atypical operating conditions.
The first order or most severe response is an emergency shutdown, also referred to as a station shutdown lockout. This response isolates the station from the two main pipelines and vents all of the station piping out to the atmosphere. This level of response requires local manual intervention before operation can resume.
The next most severe condition response is a shutdown nonlockout. A station nonlockout initiates a shutdown of all the units in operation or those units connected to a specific pipeline.
If the condition is due to an improper station valve configuration, all operating units are shut down. If the condition is because of an abnormal discharge pressure or temperature condition in one header only, the units compressing into that affected pipeline stop.
This response level does not blow down the station piping and does not require local manual intervention to restore the system to operation. Should the initiating condition return to normal within a predetermined time period, the station resumes remote automatic operation.
The lowest level occurrence invokes a warning alarm.
Algorithm controls speed
Each unit controller receives separate speed control set points from the station PLC via 4-20 mA signal.
When the unit configuration switch is on auto, the station PLC determines the unit speed set points based on the process conditions. There are separate speed set point control algorithms for L400 and L401.
The difference between the control set points and the actual discharge pressures and temperatures determines the speed set point signals from the station PLC to the unit controllers.
The station PLC applies a proportional-plus-integral control algorithm to these four error signals (discharge pressure and temperature for both L400 and L401) and then compares the output of each of the two L400 control algorithms.
It then selects the smallest signal as the controlling parameter for all units compressing into L400. The station PLC also compares the output of each of the two L401 control algorithms and selects the lowest signal for any units compressing into L401.
In common discharge header mode, the L400 control algorithms determine the unit speed set point for all units in operation because actual line pressures and pressure set points are within 10 pounds per square inch, gauge, (psig) of each other.
Unmanned station wanting
Owing to the importance of the Delevan station in PG&E's delivery system and also to the fact that it operates as an unmanned station, the need for a highly reliable control system is crucial. The compressor units need to be ready to start, stop, or change speed at a moment's notice as dictated by the ever-changing pipeline conditions.
Any impediments to station reliability that reduce available horsepower on the pipeline can be costly. Recall that a station shutdown represents up to $10,000 per day of lost revenue.
Also, whenever the station blows down to the atmosphere as a result of planned or unplannedshutdown, the cost of the lost gas is about $8,000. In this case, redundancy justified its cost, as the redundant aspects of the system themselves did not reduce overall system reliability.
The most critical control point in the control system architecture at Delevan was the station PLC. Failure of the station PLC to function properly would have an immediate detrimental effect on the station operation.
Furthermore, the station PLC needs to shut down occasionally for maintenance, troubleshooting, and logic changes. By designing the station PLC as a redundant system, these planned shutdowns can happen without affecting pipeline throughput.
PG&E used an I/O system that has the embedded capability of recognizing two central processor units (CPUs) on a single bus. When the two CPUs configure as a redundant system, the I/O also recognizes which CPU is the master and which is the backup.
This system implements redundancy by simply installing duplicate PLC racks and then programming a continuously running communication routine, which keeps the PLCs synchronized.
As long as the master CPU detects no failures within itself, it stays in control and sends all data (i.e., set points from SCADA and the HMI, timer and counter values, and latches) required for synchronous operation to the backup CPU.
Thus the backup is always functioning with the most the current operating data (no more than one PLC-scan old), and seamlessly takes control of the station should the master fail.
If the master fails or detects a failure of one of its modules, it takes itself out of control.
Putting it all together
The critical and complex nature of the application required personnel from different departments, each with different concerns and objectives, to become involved in the design process and to determine the specific requirements and operating parameters.
Requirements from technicians, foremen, engineers, and managers melded into a design philosophy from which a methodical and structured approach to PLC programming emerged.
In order to account for and document the many operating parameters, the PLC program design team used carefully structured and written logic commands that in some instances were more basic than might otherwise have been used.
For example, differentiating bits to determine status conditions was accomplished with simple relay-style programming instead of more efficient data manipulation techniques, such as word level masking.
The resulting sacrifice in system performance is more than offset by the cost reductions realized by shorter implementation and start-up time, more thorough testing, and reduced troubleshooting time resulting from simpler program documentation.
Ultimately, the PLC program structure, even with less than optimal data processing performance, effectively increases station availability and still has a maximum scan rate of an acceptable 50 milliseconds per scan.
The experience at the Delevan compressor station shows that a critical, complex application can be accomplished using small, commercially available PLCs as long as a methodical, structured programming technique is used.
The benefits of using this technique are that the program logic is more explicit and easier for local technicians and engineers to follow than if higher-level programming techniques are used.
The result is a reliable integrated control system conducting multiple complex tasks that is easy to support and maintain.
Figures and Graphics
- Control system architecture
- Gas regulation ways and means
- Station control panel
- Two compressing units
George Gaebler, P.E., is a senior gas engineer with the California Gas Transmission division of PG&E.
Mark Kazimirsky is a senior gas engineer with the California Gas Transmission division of PG&E.