Abnormal condition

The invahd state is an abnormal condition that is generally handled in a manner similar to process alarms. The transition state is not considered to be an abnormal state but may be implemented in either of the following ways ... 

Failure logic. This logic provides for responding to abnormal conditions, such as equipment failures. 

Of these, the failure logic can easily be the most demanding. The simplest approach is to stop or hold on any abnormal condition, and let the process operator sort things out. However, this is not always acceptable. Some failures lead to hazardous conditions that require immediate action waiting for the operator to decide what to do is not acceptable. The appropriate response to such situations is best determined in conjunc tion with the process hazards analysis. 

The purpose of an alarm is to alert the process operator to a process condition that requires immediate attention. An alarm is said to occur whenever the abnormal condition is detected and the alert is issued. An alarm is said to return to normal when the abnormal condition no longer exists. 

The arm logic must consistently detect abnormal conditions within the process. 

The alarm logic must not issue an alert to an abnormal condition when in fact none exists. 

The controller action can be disabled by triggering a binary deactivate input signal, during process startup, shutdown, or when some abnormal conditions exist. 

By attempting to maintain process conditions at or near their design values, the process controls so attempt to prevent abnormal conditions from developing within the process. Although process controls can be viewed as a protective layer, this is really a by-product and not the primaiy func tion. Where the objective of a function is specifically to reduce risk, the implementation is normally not within the process controls. Instead, the implementation is within a separate system specifically provided to reduce risk. This system is generally referred to as the safety interlock system. 

If proper instruments are provided, the tank and pump can be interlocked, so the pump will stop when abnormal conditions are detected. This may help keep the tank from collapsing, but the gravity-discharge case should also be checked to ensure that failure will not occur after the interlock stops the pump. In all cases where instrumentation is used, the consequences of potential failure should be considered. 

Emergency Relief Device A device that is designed to open during emergency or abnormal conditions to prevent rise of internal fluid pressure in excess of a specified value. The device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief valve, a nonreclosing pressure relief device, or a vacuum relief valve. 

Interlock System A system that detects out-of-limits or abnormal conditions or improper sequences and either halts further action or starts corrective action. 

Intrinsically Safe Equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture or hazardous layer. 

This is a location safer than Zone I with a likelihood of concentration of explosive gases, chemical vapour or volatile liquids during processing, storage or handling. This would become a fire hazard only under abnormal conditions, such as a leakage or a burst of joints or pipelines etc. Such a condition may exist only for a short period. A standard motor with additional features, as di.scussed below, may also be safe for such locations. A non-sparking type. Ex. n , or an increased safety motor, type Ex. e , may also be chosen for such locations. 

In Table 11.6 the values of colurrm 2 relate to normal operating conditions. Abnormal conditions may prevail when the machine is exposed to overhead lines, the... 

If a system disturbance requires extra generating capacity, the generator may drop speed, reducing the frequency proportionately. The under-frequency relay 81-G will detect such a condition and operate a systematic load shedding in a programmed manner in order to meet the load demand. If the abnormal condition persists, the generator is taken off the main supply system. 

It is generally agreed that a shelter with a roof having ridge ventilation and with curtain walls not extending lower than 8 feet above the operating platform would be freely ventilated. Because a gas compressor would not be a source of hazard, except under abnormal conditions such as an equipment failure, this type of compressor shelter is usually classified as a Division 2 area. 

There are no pyrophoric materials such as peroxides, acetylides or sulfides, or other internal ignition possibilities such as static present, either normally or through abnormal conditions. 

Use materials that are applicable over the full range of operating conditions such as normal, startup, routine shutdown, emergency shutdown, and draining the system. For example, carbon steel may be acceptable for normal operating conditions but may be subject to brittle fracture at low temperatures under abnormal conditions (as in the case of a liquefied gas). Cold water, of less than 60°F, during hydrotest may cause brittle fracture of carbon steel. 

Avoid incompatible materials that could come into contact in abnormal conditions. 

From a broader perspective, the Abnormal Situation Management Consortium is working to apply human factors theory and expert system technology to improve personnel and equipment performance during abnormal conditions. In addition to reduced risk, economic improvements in equipment reliability and capacity are expected (Rothenberg and Nimmo, 1996). 

For critical, high consequence systems, simulators are useful to practice diagnosis and correction of errors and abnormal conditions in emergency conditions (CCPS, 1994a). 

The user and facilitiy engineer must determine the critical nature of each installation and determine the type of construction desired. He or she must consider such things as intended service, compressor location, the consequences of downtime, and frequency of up-set or abnormal conditions. 

A nonincendive device is one which will not release sufficient energy under normal operating conditions to ignite a specific substance. Under abnormal conditions, such as a malfunction of the device, it may release enough energy to cause ignition. Because of this, such devices are suitable for use only in Division 2 and unclassified areas. 

This incident shows the importance, during hazard and operability studies (see Chapter 18), of considering abnormal conditions, such as failure of utilities, as well as normal operation. 

Before a permit to weld or carry out other hot work is issued, it is normal practice to make sure there are no leaks of flammable gas or liquid nearby and no abnormal conditions that make a leak likely. The meaning of nearby depends on the nature of the material that might leak, the slope of the ground, and so on. For highly flammable liquids, 15 m is often used. 

Embrey, D. E., Humphreys, P. (1985). Support for Decision-Making and Problem-Solving in Abnormal Conditions in Nuclear Power Plants. In L. B. Methlie R. H. Sprague (Eds.), Knowledge Representation for Decision Support Systems. Amsterdam North Holland. 

This is the simplest of all relay operating principles. Any current above, or voltage below, a set level may be taken to mean that a fault or some other abnormal condition exists inside the zone of protection. Figure 2 shows a definite time and an inverse time overcurrent relay. 

The types and numbers of vehicles moving around the site and vehicles arriving from outside the plant must be considered. Identify interdepartmental vehicle movements. It is useful to start with the records from the gatehouse, which will indicate the number of vehicles arriving at the plant each day. This will need to be confirmed with the plant manager who will know if abnormal conditions existed during the time for which the records are valid. Aim to establish average and maximum numbers. 

Sympathomimetics (drugs that mimic the sympathetic nervous system) are used primarily to treat reversible airway obstruction caused by bronchospasm associated with acute and chronic bronchial asthma, exercise-induced bronchospasm, bronchitis, emphysema, bronchiectasis (abnormal condition of the bronchial tree), or other obstructive pulmonary diseases. 

Mlcrochromatographlc Methods During the past two years rapid. Inexpensive, miniaturized column chromatographic methods for the separation of hemoglobins have been developed These methods are designed for the qualitative detection and quantitative determination of hemoglobins In normal and abnormal conditions and cover the quantitation of Hb-A2 the detection of Hb-S, Hb-C other abnormal Hbs differentiation of various conditions In adults and the detection of hemoglobinopathies especially sickle cell anemia at birth (27, 28, 29, 30) ... 

---