Alarm systems process safe time

Now with the help of Fig. IX/4.2.2-1, an attempt will be made to see how process safe time is determined. When an untoward incident occurs after a small gap of time, alarm appears with the help of sensor, alarm logic, and HMI. The small gap told here is sensor detection time and reaction time of alarm logic and HMI. After appearance of alarm in HMI there will be some time for operator s detection, diagnosis, and response as shown in Fig. IX/4.2.2-1. Now after operator responds actual thing to happen there will be reaction time, which comprises of process dead band and system reaction time. Sum total of operator action time and reaction time will be minimum necessary for any corrective action to happen. So, if hazardous event occurs after these time from T = 0, then system will be safe. Thus, process safe time wiU be as shown in Fig. IX/4.2.2-1. 

In this case, the process engineer determined that upon detection of a dangerous fault, the SIF should be initiated. A diagnostic alarm is displayed on the BPCS HMI to alarm when the SIF level signal falls below -5%, which indicates a failure of the SIF level transmitter. When the operator receives the alarm, the operator manually activates the SIF per the operating procedures to bring the system to a safe state. The hazard and risk analysis indicated that there was sufficient process safety time for the operator to respond effectively (refer to Annex B for more information on operator alarm with response). Alternatively, the SIF could be configured such that on detection of transmitter failure, the SIF is automatically initiated. 

Failure rates of complex equipment can often be greatly reduced through redundant arrangements. Redundancy provides multiple means of warning and shut down systems to ensure personnel and equipment are safe from failures that could result in a hazardous situation. Redundancy is based upon how critical an event is to worker safety or process continuation. Two separate switches or two separate alarms provide redundancy. In addition, there is standby redundancy. In this case, a piece of equipment operates until a failure is indicated, at which time another duplicate piece of equipment is turned on either automatically or manually. An excellent example of redundancy is fire water pumps. There are always at least two or more. A unit does not want to experience a fire, the site has only one pump, and that is the day the pump fails. 

An alarm (usually in the form of a buzzer and/or flashing signal on the control panel) tells the operators that the system is either in an unsafe condition or is moving in that direction. Safe limits, as defined by the process safety program, provide potential values for alarm values. One is hi and the other hi-hi. There are also corresponding lo and lo-lo values. The hi alarm warns that an unsafe condition is developing but that the operator has time to think through as to what actions to take. A hi-hi alarm means that the situation has become serious, and immediate action is needed. 

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