Alarm overload

Process alarms should be arranged to provide a major common alarm that is supplemented by an individual indicator, e g., a master alarm should be provided to indicate the status of the entire subsystem components. Multiple simultaneous alarms that can overload the senses of an operator should be avoided. 

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Alarm overload More alarm than manageable Alarm floods Nuisance alarms Chattering alarms Standing/stale alarms Redundant alarms Alarms which have no response Alarms with the wrong priority... 

Operators should respond to every alarm, regardless of priority, but alarm overload should be avoided. 

As we know all large control systems include a full range of alarm facilities and the problem is to avoid a proliferation of alarms that defeat the purpose when a real upset occurs. There have been several studies and reports on the subject of alarm overload and one of the best-known examples was the one at Milford Haven Refinery that was stated by a public enquiry to be a contributing factor to the huge explosion there in 1994. 

Torque indicator, overload alarm, motor cutout... 

Heat build-up Ensure frequent preventive maintenance checks due to worn or on bearings overloaded, Monitor and alarm bearing temperature bearings. Ensure proper belt tension CCPS G-12 CCPS G-23 CCPS G-29... 

During an overload condition, the relay may be set to give alarm if normal conditions do not restore by Ibe end of the cycle. 

Overeurrent alarm (Relay Code 51A) This may be provided to warn the operator of a likely overloading on the generator to prevent a trip and... 

When a process disturbance occurred, the plant computer printed a long list of alarms. The operator did not know what had caused the upset and he did nothing. After a few minutes an explosion occurred. Afterwards, the designer admitted that he had overloaded the user with too much information. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

It should be noted that the ability to focus not only emulates the way a human expert works, but also it avoids the problem associated with overloading the distributed process system with requests for information. While the expert system knows about all 20,000 measurement and alarm points in the process environment, only those of interest to the expert system need be accessed. 

The system has been used for a wide range of appHcations, the secret of its success being the attention to detail and a series of interlocking safety alarms. Several key parameters are continuously monitored and the built-in safety alarms will automatically shut the system down if there is a fault. As can be seen from Fig. 4.10, the balance the user has to make with these systems is between column overload and throughput. As the size of the injection increases, so the quahty of the chromatogram decreases and the purity of the fraction suffers. A compromise has to be reached in terms of the purity desired and the throughput that has to be maintained. 

Dry-type transformers are typically installed indoors and include ventilated, sealed, or gas filled, totally enclosed non-ventilated, and cast coil types. Cast coil transformers are the most expensive and they have the highest overload capability. Transformers should be sized based on their air to air (AA) rating. Provisions for future automatic fans that will provide a higher force air (FA) rating and an alarm that indicates when the fans are operating due to the temperature within the transformer should be provided. 

Motor overload protection, settings, status, pre-alarms and time to trip. 

Alarms are used to alert controllers to events or conditions in the process that they might not otherwise notice. They are particularly important for low-probability events. The overuse of alarms, however, can lead to management by exception, overload and the incredulity response. 

Keep spurious alarms to a minimum This guideline will reduce overload and the incredulity response. 

Prevent possibility of overloading of hoists by providing warning alarms and tripping of drive motor if there is excess weight. 

Vent absorbers are designed to receive gas, not liquid. The presence of liquid chlorine in a vent stream can disrupt flow in the scrubber, overload the system, and cause an early breakthrough. When appropriate, design should therefore include a gas/liquid separator to prevent carryover of liquid droplets. The separator should have the capacity to trap the maximum amount of liquid likely to be present and should include a temperature or level alarm to signify its presence. 

An alarm annunciation system that eliminates operator information overload during major plant transients. 

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