3.3 A sample description of an alarm control strategy.
If the control system included human–machine interface color graphic screens on personal computer-based operator workstations, the alarms will be recorded (with a time stamp) in the computer’s hard drive. If the hard drive is full, then new alarms will overwrite old ones unless provisions are made to store alarm logs on removable media (e.g., compact discs, digital versatile discs, backup tape drives, flash drives, or other external hard drives). In addition, alarms for personal computer-based systems must be acknowledged at the operator workstation (an industrial-grade computer mounted on a control panel or a desktop personal computer in a control room, or a wireless workstation or smartphone device). Some systems may also include an alarm printer in the control room that prints out all alarms as they occur. Printouts include an alarm description, the time it happened, priority level (e.g., critical, warning, or equipment status) and whether and when the alarm was acknowledged by an operator. Similar information is also available at the alarm summary screen of the personal computer-based operator workstations. Alarms can also be stored in a dedicated Historian Server for long-term storage and retrieval (see the “Suggested Readings” section at end of this chapter for more information).
If a system must be inspected after a malfunction or alarm before it can be restarted, then the system will need a reset function to clear it after the problem is corrected. Designers have several reset options. For example, they can provide a reset button or combine a reset button with the control system’s stop button. The subsystem or equipment could be momentarily disengaged from the automatic control system and then reconnected. It is important to note, however, that a motor overload can only be reset at the motor control center [MCC] starter, typically via a reset pushbutton on the MCC starter compartment’s front door that pushes the spring-loaded overload relay contact block into position. This feature forces the facility to send qualified personnel such as electricians to properly diagnose the nature of the electrical trip.
When writing the PCN for system resets, the designer should describe when resets are necessary and how each one functions, especially for complicated systems with multiple subsystems. Typically, reset functions release all latched alarms and associated permissives so the automatic control system can function normally. Associated permissives refers to all conditions that must be satisfied before the equipment can be started; they are monitored by sensors, the signals of which are wired to the control system.
Process control narratives should include a list of all remote indicators (e.g., alarms, monitored setpoints, status lights, etc.). A remote indicator is any indicator at a site other than the equipment’s local control panel or station.
The PCN should include local pushbutton control stations for manual operations, maintenance, and testing. Designers should specify the type of controls and indicators needed such as start and stop, local or remote, speed control, and status lights (run, stop, fault, etc.).
The PCN should specify which equipment must have UPS power in case the utility’s main power source fails. Designers should not only note this information, but also which equipment can withstand a short power interruption (minutes) until on-site generators are fully online; designers should also calculate the power load and duration (in minutes) required. This information should be shared with the project’s electrical engineer. Particular attention should be paid to associated network devices (switches, routers, and media converters) to prevent a situation in which equipment does not transition properly during outages because of lack of communication.
The PCN should provide simple descriptions of vendor-supplied package controls, their functions, and which parameters must be monitored or controlled remotely. Designers also should provide all related vendor information (e.g., functional descriptions, control panel drawings, wiring schematics, and equipment catalog cuts) to I&C and electrical engineers as soon as possible. In addition, designers should let I&C engineers know what instruments the packaged equipment will need that are not provided by the vendor so that these instruments can be included in I&C specifications. Finally, to ensure that the design is well coordinated, I&C and electrical engineers should be asked to review packaged-equipment specifications. Every effort should be made to match existing packaged control system (PCS) hardware whenever possible to alleviate long-term O&M requirements for facilities. The specifications should require that all licenses and copies of programs for all PCSs be provided to owners for maintenance if the owner desires.
2.1.1.12 Programming Standards
The following standards, in particular, should be considered when specifying programming standards (see Chapters 13 and 14 for more details):
• Programmable logic controller programming standards,
• Graphics standards, and
• Alarming standards.
2.1.1.13 Field Instruments
The following items should be considered when specifying field instruments (see Chapters 8 and 9 for more details):
• Online analyzers and
• Physical parameter instruments.
2.1.2 Drawings
2.1.2.1 Process Flow Diagrams
A PFD is a simplified facility process diagram that shows key process equipment, key process variable measurements, and other elements and their interconnections. Typically, PFDs are prepared by process engineers early in the project cycle and represent the precursor documents from which detailed P&IDs are developed. For additional information on PFDs, see Instrumentation and Control Systems Documentation (Meier and Meier, 2011).
2.1.2.2 Process and Instrument Diagrams
Process and instrument diagrams typically are the most important design documents for a control system because they define the process, physical facilities, interconnections between units, measurement types and points, control elements, and control loops. These documents reflect the consensus of the designer and owner, and help design teams coordinate their efforts.
A typical wastewater project requires between 40 and 50 P&IDs, all of which meet the International Society of Automation (ISA) S5.1, S5.3, S5.4, and S5.5 standards. Smaller facility upgrades or retrofit projects require fewer drawings depending on the extent of process modifications mandated by the project. The documents are typically printed on 28-cm by 43-cm (11-in. by 17-in.) or 56-cm by 86-cm (22-in. by 34-in.) or larger sheets of paper so the lettering is easy to read (see Chapter 4 for more details).
2.1.2.3 Process Control System Architecture Diagram
The process control system architecture diagram, or system configuration diagram, is a basic drawing of control system components and their locations. The system configuration and communication schematics show all of the process control system’s significant components, including workstations, printers, modems, RTUs, PLCs, interfaces between units, networks, communication media, and communication protocols (see Chapter 4 for a detailed example of a process control system architecture diagram).
