Limits, action alarm

Most DCSs provide a pair of alarm bits associated with the instrument limits. For an instrument output signal, the limits prevent transmitting a value that is outside of the specified ranges. If an input value is outside the limits, an alarm action is taken (see Chapter 10). 

Finding 4 Although the rapid detection of a chemical/ biological attack and identification of the agent used are worthwhile objectives, a defensive strategy that depends exclusively on a detection-system alarm before action is taken (i.e., employment of a detect and react strategy) has several serious limitations. 

In a modem laboratory, automatic sensors are often used to detect unwanted changes in laboratory conditions and warn laboratory staff. Basic laboratory conditions, such as temperature, humidity and particulates, can all be monitored continuously using sensors. The results can either be fed to chart recorders, or into computer-controlled laboratory management systems, which can take corrective action or sound alarms in the event of the limit for a particular condition being exceeded. 

Flammable gas detection systems are typically used to initiate an alarm at a concentration level below the lower flammable limit (LFL). Two gas alarm levels (low and high) are often utilized to allow early warning prior to taking automatic actions. Detection systems may also be used to stop electrical power and initiate process shutdown. The low alarm setpoint should be —20% LFL and the high alarm level set point should be between 40%-60% LFL. Where these devices are used to initiate process shutdown or activate fire protection systems, it is common practice to use some form of voting, typically 2 out of 2, such that the frequency of spurious shutdowns or system activation is minimized. 

Although the temperatures that are monitored vary slightly depending on the details of facility design, temperature detectors are used to provide three basic functions indication, alarm, and control. The temperatures monitored may normally be displayed in a central location, such as a control room, and may have audible and visual alarms associated with them when specified preset limits are exceeded. These temperatures may have control functions associated with them so that equipment is started or stopped to support a given temperature condition or so that a protective action occurs. 

The data are plotted on a control chart in time sequence. This enables the analyst to readily observe changes in the measured value. The analyst can define warning and action limits on the chart to act as alarm bells when the system is going out of control. In Figure 4, it shows that all the results of the analysis of the QC samples are within the warning limits except for one result which is between the upper warning and upper action limit. 

Alternatively, a two-point control is enacted when the compression force is outside an acceptable band outlined by the upper and lower tolerance limits. Thus, there are separate control limits, rejection limits, alarm limits, and shutdown limits, and no respective action is taken when the signal is inside these limits. 

Apr-94 JACADS (Unusual Occurrence) LIC/MPF airlock MDB filter unit taken off-line to perform maintenance. Air dampers on spare filter did not operate properly causing a fluctuation in the MDB. Agent maintained under engineering controls. Agent alarm in LIC/MPF airlock. Cause limit switch failure. Corrective actions taken. 

Name/Location Description Never Exceed Interlocks Alarms Operating Range Remarks R—Reason for Limit or Effect of Exceeding Limit C—Correction and/or Control Action T—Troubleshooting Guide... 

Distracted in its early years by the focus on tariffs, and hobbled by the limitations of its committee structure, the association at first did little about pollution. It intervened briefly in the oil pollution debate of 1924, but concerted action came only when the minds of top executives were concentrated by an imminent threat of government regulation. A water pollution committee was established in the spring of 1936 its immediate purpose was to formulate an industry response to Senator Lonergan s clean water bill. The air pollution committee followed only in 1949, a consequence of the alarm created by the Donora disaster. 

Comparison of the feedback from the final element (such as limit switches or position transmitters) to the requested state may be performed to verify that the expected actions have been taken. Sufficient time delays should be used to filter the alarm for valves in transition (for example, from fully opened to fully closed). This comparison of the feedback from the final element to the requested state can only be considered to be a diagnostic if the valve periodically changes to the safe state as part of normal operation (for example, batching operation). 

This standard is frequently titled the safety manual of the component or device. It may, however, be comprised of the suppliers standard Installation, Maintenance and User s Manuals with an additional document specifying those aspects relating to its use in SIF applications, the limitations of use in these applications, the actions that should be taken on diagnostic alarms and the known failure modes. It should also define those features, configurations and/or program statement types that should not be used when the component or device is used in a SIF application. 

Outward motion can be a more significant threat to equipment and people. The consequence of control system failures which cause rod withdrawal are limited by several system features. The first is the limit on rod withdrawal speed due to the maximum amount of power that can be delivered to the CRD motor. Second is the control circuitry which limits the number of control rods that may be withdrawn at any one time. Third is the alarm system which will inform the operator of the improper rod motion so that he may take corrective action. Finally, a reactor trip signal will cause rods to be inserted. 

Process interlocks make up an automatic system that detects an abnormal condition and either halts process action or takes corrective action to return the process to normal. Process interlock systems may be part of the Basic Process Control System (BPCS). The BPCS is a system of measurements and controls including alarms and interlocks that function to keep the process within acceptable operating limits. The BPCS is usually associated with producing good quality or in-spec finished product. 

The provisions made against hydrogen risk consist to clean under inert atmosphere (Na), to limit the Ha maximal theoretical concentration to 1% in the effluent gas by limiting the nominal water concentration in the first steps of the process and to measure continuously the H2 concentration in the effluent. Alarms and automatic actions are triggered if the Ha... 

Volumetric feeders offer no verification of the feed rate and permit no simple means to record their historic performance. Apart from an ongoing certification of performance, registered measurable values allow alarms to be actuated and failsafe or corrective action to be automatically engaged, should the feed rate fall outside acceptable limits. 

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