Room Process Control

This section describes the devices used for control of cell-room operation and considers the effects of the magnetic field associated with the high currents. Some control devices are automatic instruments, some are manually operated, and some are built into the electrolyzers. 

Operation of the electrolyzers requires control of concentrations, temperatures, pressures, and amperage. These requirements place demands on other operating variables. Pressure control is in the gas headers, outside the cell installation proper. This is discussed along with instrumentation and control systems for the entire process in Chapter 11. 

The major types of cell all rely on the anode reaction 

To suppress the competitive formation of oxygen, it is necessary to hold some minimum chloride ion concentration in the anoly te. Feeding excess hrine to the cells maintains this concentration. The fact that the feed brine is relatively cool also limits the temperature inside the ceU. Concentration control is indirect, usually through feed rate control with the rate dependent on the operating amperage. Temperature control relies on adjustment of the temperature of the feed brine as well as its flow. 

Control of the flow(s) to each electrolyzer frequently is manual, with the aid of rotameters. This is especially true in monopolar installations with large numbers of electrolyzers. Monopolar diaphragm cells have essentially one chamber each for anolyte and catholyte and only one feed line (for brine). They rely on division of the feed brine 

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In the control room, process control encompasses the selection and installation of panel-mounted alarms, switches, recorders, and controllers, as well as Program Logic Controllers (PLC) and Distributed Control Systems (DCS). These include analog and digital input/output hardware, software to implement control strategies. 

EMEC 1974. Chemical Process Control and Control Rooms. Eoss Prevention Data Sheet No.7-45. Eactory Mutual Engineering Corporation, Norwood, MA. 

The location of the principal ancillary buildings should then be decided. They should be arranged so as to minimise the time spent by personnel in travelling between buildings. Administration offices and laboratories, in which a relatively large number of people will be working, should be located well away from potentially hazardous processes. Control rooms will normally be located adjacent to the processing units, but with potentially hazardous processes may have to be sited at a safer distance. 

At this point the location of the control panel(s) should be decided. Usually this should be a central location. This permits those watching the control panels to quickly investigate and determine the cause of any problems that might arise. As plants become more automated, it may be desirable to have two or more processes controlled from one location this could reduce the number of operators required. In this case the control room should be located in a relatively unexposed area near the edge of the processing area, but away from fired heaters.4 This is to protect both the employees and the equipment. 

The latest designs for onshore installations cater for a centralized control room, well distanced from the operating facility with sub control areas as part of a distributed control system (DCS). The sub-control areas are closer to the processes but contain fewer personnel and process control systems for the overall plant, so the overall risk level for the facility from a major incident is lowered. The outlying control buildings (sometimes referred to as PIBs or SIHs) still need to be sited against impacts from explosions and fires. 

The most utilized and reliable process control in the petrofeum and related industries is human observation and surveillance. Local pressure and level gages along with control room instrumentation are provided so that human observation and actions can occur to maintain the proper process conditions. First stage process alarms are provided to alert operators to conditions that they may not have already noticed. Typically when secondary alarm stages are reached, computer control systems employed to automatically implement remedial actions to the process. 

The distributed control system (DCS) hardware areas are often referred to as "process computer rooms." I/O Rooms contain the incoming and outgoing wiring, cables and data highway links, and often small transformers and other related electrical equipment. Often, additional space is needed for a master process engineering computer terminal/work station for process control system changes and for critical safety instrumented systems (SIS) for interlocks and emergency shutdowns. 

Computer rooms considered in this section are those installations of substantial size and importance providing primarily data processing and network support service for multiple large process units or for an entire plant site. These installations are typically much larger than ones for an individual processing facility s dedicated process control computer. 

At 1 14 p.m., the operator received a radio transmission from the PCC (Process Control Center) informing him that a "low" gas alarm had been received. Immediately, the operator was informed of two more gas alarms. Upon reaching the E l module, the operator noted the sound of gas escaping somewhere inside the production module. At approximately 1 22 p.m. he climbed the stairs to the E l control room and opened the door to check the fire and gas alarm panel which indicated a high gas atmosphere in the production modules. The operator then requested the Emergency Response Team be put on standby and went to inform three contract workers, in the area of the danger. 

Headblock—containing in-process control laboratories, offices, training, and meeting rooms, integrated into the main structure of the building. 

Neither one-point nor two-point calibrations have room to test the model or statistical assumptions, but as long as the model has been rigorously validated its use in the laboratory has been verified, these methods can work well. Typical use of this calibration is in process control in pharmaceutical companies, where the system is very well known and controlled and there are sufficient quality control samples to ensure on-going performance. 

The original GC control system took the form of a central room which monitors the flowllne6, oil, water, and utility sections, plus a smaller satellite control room monitoring the gas compression and gas conditioning section of the plant. Closed loop process control, such as separator liquid level, pressure, flow and temperature control were handled by local pneumatic analog controllers. The key process variables are displayed in the control room via electronic instrumentation. All the key process and equipment trouble alarms are annunciated m the control rooms, plus the on/off status of key machinery and open/close status of key valves are displayed. 

To safeguard future operations, additional reliability was built into the design of the permanent anti-foam injection unit. This included separate power supply sources ior the running and standby injection pumps with low-flow arid pump-failure alarms in the process control room... 

Permanent gamma ray scanning equipment was installed on each separator along with foam density and level recorders in the process control room. An emergency trip system that automatically shuts a valve in the appropriate separator gas line to isolate the separator from the compressor suction drums when a high foam level occurs was added also. A similar trip system was insulted in the gas feed line from the HP separator to the power station... 

It is assumed that chemical plant process workers will operate the production facilities as they will replace plants traditionally operated by them. Assuming that operators will not be expected to walk more than 200 yards from the control room, the control rooms will have to be spaced 400yards apart that is, one per 33 acres. Over 750 control rooms will be required for a million pounds per year operation. Building costs will therefore be much higher that on a conventional plant, and may well make the new process uneconomic, especially now that control rooms are being made stronger than in the past. 

On the basis of our own informal observations and discussions in process control rooms, e.g. Van dcr Schaaf (1989), and the classical literature on operator tasks (Grossman, 1974) the following set of elementary operator requirements in order to carry out. process control tasks correctly, has been identified ... 

The practical experiences in process control rooms mentioned earlier clearly point at biases in describing and classifying incidents the pilot studies mentioned in Chapter 2, the survey to be presented in Chapter 7, and also the Exxon case in Chapter 8 all show a tendency on the part of safety officers to concentrate on clearly visible S-B elements of task performance (e.g. pressing the wrong button) rather than on less obvious R-B errors (e.g. planning), let alone on the mainly cognitive, internal activities involved in K-B behaviour. 

A complete visual representation of the manufacturing process in flow chart format should be included. This flow chart should indicate the step in the process, the equipment and materials used, and the room where the operation is performed, and should provide a complete list of the in-process controls and tests performed on the product at each step. The diagram should also include information, including a descriptive narrative, on the methods used to transfer the product between steps (i.e., sterile, SIP connection, sanitary connection, open transfers under laminar flow units, etc.). 

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