Interrupts, alarm system

It generally is recommended, and often required, that gas dcicciiuii systems be installed in a fail-safe manner. That is, if power is disconnected or otherwise interrupted, alarm and/or process equipment shutdown (or other corrective action) should occur. All specific systems should be carefully reviewed, however, to ensure that non-anticipated equipment shutdowns would not result in a more hazardous condition tlian the lack of shutdown of the equipment. If a more hazardous situation would occur with shutdown, only a warning should be provided. As an example, a more hazardous situation might occur if blowout preventers were automatically actuated during drilling operations upon detection of low levels of gas concentrations than if drilling personnel were only warned. 

An alarm system should be provided to warn workers of power interruptions that have occurred during non-duty hours. Such evidence that engineering controls have been compromised alerts incoming personnel to the necessity for first entry monitoring of laboratory rooms. 

One application of photocells is in motion detectors for security alarm systems. A beam of light is emitted by a special device so that it strikes the photocell precisely, producing a tiny electric current. If something or someone breaks (interrupts) the beam, the current stops flowing and an alarm sounds. 

In many situations, the most likely scenario for failure of freezers is power interruption. In this case, the temperature monitoring and alarm systems need to function in the absence of power. In some settings, generator back-up is appropriate and required. Several types of temperature monitors are available, including those that record continuously on paper discs and recorders that electronically record, store, and alarm in case temperature parameters are exceeded. 

The dedicated environmental monitoring programmes and alarm systems that will respond to tmplanned radioactive releases, and the automatic devices to interrupt such releases. 

These emergencies include any interruption or loss of a utility service, power source, life-support system, information system, or equipment needed to keep the business in operation. Identify all critical operations, including electric power, gas, water, hydraulics, compressed air, municipal and internal sewer systems, and wastewater treatment services. Also consider security and alarm systems, elevators, lighting, heating, ventilation, air-conditioning systems, and electrical distribution systems. Evaluate transportation systems, including air, highways, railroads, and waterways. Determine the impact of service disruption. 

These are actions of the computer which are initiated by events, e.g. switching off a pump when a tank has been filled to a certain level. They form the basis of the response of computer-controlled plant to alarm conditions, e.g. if the temperature in the reactor described in Fig. 7.102 exceeds a specified value then action must be taken by the computer, such as rapidly increasing the flow of coolant or reducing the feed to the reactor. Generally, sensor-initiated operations must be performed within a specified maximum time interval and are often in the form of alarm interrupts (Section 7.18.3). Smaller systems will regularly scan a set of specific process variables via a multiplexer to see if any action is necessary. The latter arrangement is called polling. 

Should an emergency occur requiring immediate shutdown and over-ride of normal sequenhal shutdown, an EMERGENCY STOP pushbutton is provided for this purpose. This device interrupts power to all components in the oxidation unit control circuit, shuts down all motors, and closes the vacuum line valve and the chlorine pressure-reducing valve (PRV). The EMERGENCY STOP pushbutton is also used as a reset device to restore the system to normal operating status after an alarm situation has occurred. 

Modern communications are much more transaction based (a trend developed in the commercial and business sector), and these are less suited to traditional control system interfaces. Control systems have traditionally operated on a real-time basis, polling for data on a routine basis or being interrupt driven by alarms and events. These traditional systems are not well suited to transactional-based communications, and the use of object broker interfaces will overcome some of these problems. The new object-oriented interfaces will again be easier to validate since the object broker interfaces will be configured rather than programmed and will use standard software blocks to build the interfaces. 

The pipeline is assumed to be highly reliable. In certain intervals, however, the pipeline control system reports failures which are partly caused by misestimations of pressure but sometimes also due to micro-leakages. In any case, an inspection of the identified pipeline segment is necessary which forces an interruption of pipeline operations. It is assumed that the time span between two alarms (given in days) is Weibull distributed with... 

Figure 25 -1. Key components of a local exhaust system. If a local exhaust system is used to exhaust toxic contaminants, an alarm may be important to alert people when there is an interruption of air flow.

Similar systems are used where continuity of supply is important and it is undesirable for earth faults to lead to the interruption of the supply. In these systems, the limited fault current flowing causes an audible or visual alarm rather than tripping a circuit breaker. 

Similarly, the Revisions to UFC Article 80 required that a continuous gas-detection system shall be provided to detect the presence of gas at or below the permissible exposure limit or ceiling limit. The detection system shall initiate a local alarm and transmit a signal to a constantly attended control station. The alarm shall be both visual and audible and shall be designed to provide warning both inside and outside of the storage area. The audible alarm shall be distinct from all other alarms. It defined a continuous gas detection system as a gas-detection system where the analytical instrument is maintained in continuous operation and sampling is performed without interruption. Analysis may be performed on a cyclical basis at a frequency not to exceed 30 minutes. P1... 

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