Deviation alarms

DEVIATIONS Alarms first-out safety-critical alarms many simultaneous or false alarms... 

High deviation alarm, 20 672 High dust plant, 10 101 High early strength cements, 5 498t, 500-501... 

Remember that the BPCS operates with signals that are relatively dynamic. This makes BPCS failures generally detectable by plant personnel. Example diagnostic methods include flat line outputs, quality indicators, pre-alarms, deviation alarms, and out of range signals. While a BPCS operates under relatively dynamic conditions, safety instrumented system signals are static Boolean variables. Since the SIS only takes action when a potentially dangerous condition is detected, it can be very hard for operations and maintenance persormel to detect certain failure modes of a SIS. 

Do current maintenance practices support the SIL of the SIFs - is the assumed Mean Time to Repair (MTTR) on fault detection met This should include analog input maintenance based on deviation alarms. 

An operator-initiated SIF is often associated with a never exoeed never deviate alarm, where the operator is expected to mitigate risk in much the same manner as an automated SIF. Operator-Initiated SIFs are generally used when it is not possible to completely automate the function. The manually initiated action is typically comprised of the sensor detecting the hazardous condition, the logic solver that determines that the safety condition exists, alarm presentation, human response, and the equipment used by the operator to bring the process to a safe state. When risk reduction is taken for an operator-initiated SIF, the PFDavg should be determined for the instrumented system. This is discussed further in B.6. 

In the event that the operator receives a deviation alarm and determines that one of the transmitters has failed, the operator is trained to contact the maintenance department to repair the transmitter within a specified allowable time (e.g., the MTTR). Since the SIF is designed to tolerate a single hardware fault, the pressure transmitter may be repaired, tested, and returned to service within the specified MTTR without bringing the process to the safe state, as defined in the operating procedure. 

Normal Operation. The designer of a chemical plant must provide an adequate interface between the process and the operating employees. This is usually accompHshed by providing instmments to sense pressures, temperatures, flows, etc, and automatic or remote-operated valves to control the process and utility streams. Alarms and interlock systems provide warnings of process upsets and automatic shutdown for excessive deviations from the desired ranges of control, respectively. Periodic intermption of operations is necessary to ensure that instmments are properly caUbrated and that emergency devices would operate if needed (see Flow measurement Temperaturemeasurement). 

With respect to their response, the discussion should emphasize why these are important anci why they adjust certain control settings. Among the deviations on which analysts should focus the discussion are the high and low alarm settings. Some alarms will require rapid response. Alarms may give insight into equipment-operation boundaries as well as process constraints. 

Do look for potential causes of deviation - don t wait until the alarm bells ring. 

Supports the Early Detection of Abnormal Process States Although some diagnostic information is available in the activated alarms, the workers still need to know the size of any deviations from the target states and require a concise picture of those critical parameters which are in alarm and those which are nearing it. This information is available on the overview display and facilitates the early detection of process deviations. 

The system should be able to establish the norm based on trends developed over time. However, the analyst must establish the level of deviation that triggers the alarm. The level of deviation in rate depends on the mechanical condition of each machine-train. For a new machine in good operating condition, the limit is typically set at two... 

The added capability of calculating unknown values based on measured inputs will greatly enhance the system capabilities. For example, the neither fouling factor nor efficiency of a heat exchanger can be directly measured. A predictive maintenance system that can automatically calculate these values based on the measured flow, pressure and temperature data would enable the program to automatically trend, log and alarm deviations in these unknown, critical parameters. 

The second method uses dynamic limits that monitor the rate of change in the measurement parameters. This type of monitoring can detect minor deviations in the rate that a machine or system is degrading and anticipate when an alarm will be reached. The use of dynamic limits will greatly enhance the automatic diagnostic capabilities of a predictive maintenance system and reduce the manual effort required to gain maximum benefits. 

Alarms are used to alert operators of serious, and potentially hazardous, deviations in process conditions. Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. Where delay, or lack of response, by the operator is likely to lead to the rapid development of a hazardous situation, the instrument would be fitted with a trip system to take action automatically to avert the hazard such as shutting down pumps, closing valves, operating emergency systems. 

Control of the process. Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms, trips together with good operating practices and management. 

Storage chambers should be validated with respect to their ability to maintain the desired conditions, and, if so equipped, the ability to sound an alarm if a mechanical or electrical failure causes the temperature to deviate from preestabilished limits. They should also be equipped with recording devices, which will provide a continuous and permanent history of their operation. Logbooks should be maintained and frequent readings or mercury-in-glass, National Institute of Science and Technology traceable thermometers recorded. 

In general, hazard identification criterion represents the deviation of one or more measured variables from specified values. This is the basis upon which a significant percentage of risk analyses are done. For a chemical process, a number of measurable variables, physical properties, and states or positions of various parts of the overall equipment, e.g., pumps, valves, and motors, can be specified for every time or phase of the process. Certain deviations from the "standard" recipe or settings can then be defined in advance as hazardous, and thus can be used for initiation of an alarm at the early stage of a runaway or upset condition. 

Along the same line, UV-vis based scanning for deviations from a blank reference in industrial wastewaters can indicate problems on an upstream level that remain undetected using conventional parameters. The so-called alarm parameters need not even be based on detailed interpretation of all absorption bands. In a coarse manner, absorption in certain spectral regions may correlate with lump parameters such as total organic carbon (TOC), dissolved organic carbon (DOC) and the like. ... 

The basic results from the individual units are processed and then combined to form the final result which is produced on the report printer. Results that deviate from an expected value by more than a preset tolerance may be marked or commented on. Additional information, such as sample identification and origin, is also made available. To ensure complete control by the analyst, the basic raw results may also be recorded in analogue form. Sample identification is provided so that the data can be re-analysed. Fully automatic systems require careful monitoring of the supply of reagents and the disposal of waste chemicals. To achieve this, fluid levels are monitored, and if they are low, an alarm signal is issued to the operator. 

The major problem for control based on material states, however, is the quality control culture that requires that parts be accepted based on adherence to a preset cycle within specified limits. Because state-based inferential control systems could theoretically come up with a new cure cycle every time, this sort of specification cannot be used with such systems. Specifications instead have to be in terms of the process plan used for the cure. The satisfactory completion of a certain cure history without alarm states would be assumed to produce an acceptable part. Once the culture was able to accept that difference for autoclave curing, production costs at the U.S. air force s McClellan AFB Logistic Center were substantially reduced [32], This type of specification could also give material review boards a head start on investigations because they would know that a part did not meet specification as well as what sorts of flaws might result from the deviation. The experience at McClellan is that there are fewer parts to review. It is even conceivable that, with improvements to sensors, much of the current postcure nondestructive evaluation used to verily the quality of parts could be incorporated into the process, building quality in rather than inspecting it in after the fact. 

D3 To what extent should plant be shut down for any deviation beyond the operating limits Does this require the installation of alarm and/or trip Should the plant be partitioned differently How is plant restarted ( 9.6)... 

Hydrogen should be monitored at die economizer inlet and the superheater outlet. A hydrogen analyzer-recorder can actuate an alarm when the hydrogen concentration of the feedwater or steam deviates from the safe value, which is specified for the plant. Deviation from the normal hydrogen concentration can indicate that corrosion is taking place within the water-steam system. 

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