Alarm setting

To achieve early and reliable warnings of leakages, the sensitivity of detectors should be at the highest level commensurate with the level of false alarm rates. 

From a safety viewpoint the lower the alarm levels are set the better. However the lower the level of alarm the greater the possibility of false alarms and disruption to operations. On the other hand 

In general practice, the gas detection alarm set points are the settings recommended by the manufacturer of the equipment or by requirements of an operating company to effect an acceptable compromise on any given field of operation. The lower the set points the higher the sensitivity to possible leakage emissions. 

Operation of detectors with their associated alarm panels should be checked and calibrated after installation. Detector performance can be impaired in a hostile environment by blockages to the detector (i.e., ice, salt crystals, wind blown particles, water or even fire fighting foam, or by inhibition of the catalysts by airborne contaminants such as compounds of silicon, phosphorus, chlorine or lead. It is essential that detectors and alarm panels be checked and re-calibrated on a routine basis. 

It is also possible for detectors to be calibrated using one gas (e.g., methane) for use thereafter in detecting a second gas (e.g., propane or butane) provided the relative sensitivity of the detector to each of the gases is known. A procedure for calibration of the detector for a different gas than that which is being used is normally available from the manufacturer of the detector. 

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Displacer units are tolerant of specific gravity shifts within their range, require no power to operate, are intrinsically safe, and can be easily field cahbrated. A single displacer unit can control up to three alarm set points or up to three pumps with different spans. They can be caHbrated for set points or pump spans from 0.15—30 m from the top of the vessel mounting. 

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. 

A board-mounted low pressure alarm, set at a positive pressure, and a remote control valve operator, so that repressuring gas can be admitted from the control center, should the automatic system fail to function. 

When a failure or a malfunction occurs, first it needs to be detected. The detection time is influenced by the choice of appropriate alarm settings or by the use of more sophisticated alarm systems, as described in Section 10.4.6. Most important, is choosing the appropriate parameter which must be monitored to detect a malfunction. The design of alarms, interlocks, and control strategies is an important part of process design and should always follow the principles of simplicity in the concept of inherently safer processes (see Section 10.3). 

This has essential consequences for the design of emergency measures. A technical measure to prevent a runaway could be a temperature alarm set at, for example, 10 K above the process temperature. This works well with nth-order reactions, where the alarm is activated at approximately half of the TMRld. However, autocatalytic reactions are not only accelerated by temperature, but also by time. This can lead to a sharp temperature increase. In the case shown in Figure 12.2, a temperature alarm is not effective, because there is no time left to take measures in the example given, only a few minutes are left before runaway. Therefore, it is important to know if a decomposition reaction is of autocatalytic nature or not that is, the safety measures must be adapted to this type of reaction. 

Predictive alarming Yes, alarm on 10, 20, or 30 min before it thinks that you will hit that number based on the current trend. It estimates a future number by using algorithms and vector technology. (One parent has her child s alarm set at 20 min notice for highs and 30 min notice for lows)... 

NDIR-type C02 monitors are used in HVAC systems and for industrial applications. The portable units are usually battery operated, and their ambient sample is received by a combination of diffusion and convection effects, without using any pumps or filters. These units are provided with digital displays, one or two alarm settings, and digital output signals. 

An operator interface is used to record data gathered by the PLC.1 The operator interface is usually another computer (sometime called the human-machine interface or HMI). The HMI uses process displays with real-time sensor readings so that the operator can quickly assess the status of the system (see Figure 6.17). The operator uses the control panel to adjust alarm settings and to turn on and off process equipment. Once running, however, the PLC controls and runs the system automatically, without further input from the operator. Common HMI status indicators are listed below ... 

Provision of independent functions for alarms (including management and calibration as appropriate of alarm set points and management of alarm log records)... 

Is there a hard copy of valid alarm settings ... 

Each temperature controller includes an alarm output to avoid gas overheating above alarm set value. 

The essential method to obtain sterile air, whether packed-bed or cartridge filters are used, is to reduce the humidity of the air after compression so that the filter material always remains dry. The unsterilized compressed air must never reach 100% relative humidity. Larger plants install instrumentation with alarms set at about 85% relative humidity. Careful selection of the cartridge design or the design of packed-bed filters will result in units that can operate in excess of three years without replacement of filter media. If a fiber material is used in a packed-bed type filter, the finer the fiber diameter the shallower the bed depth needs to be for efficient filtration. Other filter media are less common and tend to have special problems and/or shorter life. The bed depth of filters is only 10 to 18 inches for fibers of less than 10 microns. These filters run clean for 2 weeks or longer before being resterilized. 

Display Unit. The Radiacmeter Control Units have digital readout, automatically recognize the attached probes, and show rate information in the appropriate units. One can program alarm set points and background subtraction. The power supply is three 9-volt batteries, BA3090, or vehicular power systems. 

When a main alarm has been given, it is no longer permitted to work without respiratory protection. As long as the concentration has not been fallen below the main alarm setting, only those workers needed to solve the problem should stay within the danger zone. All others must leave this area. 

Even with simple objects the lack of standards for object models has proven to be a problem in the past. The object used to describe a PID function in one control system will be different from the object used in a different system because of variations in the number of alarm settings, in the way the derivative action is calculated and so on. This variation in object design is one of the reasons why true integration between modern systems is such a problem. 

It may be a large dial micrometer with alarm setting for an axial movement of 0.4 millimeter and shutdown at 0.8 millimeter, or An oil pressure gauge connected to an oil leak-off device may also be used as an axial position indicator. The oil is supplied at say 500 kPa, flows through an orifice and leaks off through a nozzle. The pressure between the orifice and nozzle depends on the distance between the nozzle and shaft thrust collar the larger the distance the lower the... 

In all the events irrespective of a leak size no increase in pressure in the expansion tank up to the alarm setting has been observed and the operational power plant safety has remained within the limits. 

Data verification includes confirmation that data used within the application software is correct and where appropriate unique (for example that TAG names are uniquely assigned, that data is not misused by subsequent functions and that constants such as alarm set points are valid and correct). 

Instrument loops, including alarm settings and interlock logic Operating procedures Operator training records Preventive maintenance procedures Isolation valve list Electrical one-line diagrams Blind lists Inventory levels Relief valve information. 

Often the design based on separation (or settling) ehar-aeteristies is overridden by the necessity of certain fill-up times between the different alarm settings. These are based on the operation of the outlet valves versus shutdown eriteria for the proeess. Standard shutdown eriteria range from 30 s to 1 min between the different alarm levels. Following the applicable API specifieation (1), the vessel size is seleeted and possibly increased until the 200-pm cut-size eriterion is met. Proprietary faetors and relations may modify the design shghtly. 

Any design and implementation changes in safety alarm settings, interlocks and process trips... 

Stage (5) was started and the temperature increased in a normal profile up to 140°C. Again, evaporation was severely constrained by the reactor pressure which rose to 2.6 bar g. No further alarm had been given, because the pressure had not fallen below the alarm setting since the initial excursion. 

Verify the alarm set-points given in Table 4.5 have been entered in the PLC. 

Check the low-level alarm setting in the PLC Check that the tank inlet water valves are open... 

Check that the high alarm set-point corresponds to the high-high tank level Check that the tank inlet valves are closed, and the divert valves are open Check that the tank feed water supply pumps/systems are not operating (should turn off automatically due to a process interlock)... 

Check the alarm set-point of the high pressure switch... 

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