Hazardous situations

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

Alarms should act as early warning devices to anticipate a potentially hazardous situation. Alarms that are essential to safety should be identified and classified separately from process alarms. Redundancy may be required. 

Hazardous situations can develop with changes in volume witli temperature. Tlie effect of temperature on gases and liquids is mentioned on pages 45 and 65, respectively. 

Generally the substances in this class are unstable when heated or exposed to light they should be stored cool and in the dark. However, for liquids with added stabilizer cooling may cause separation of the material from the stabilizer. Similarly, precipitation of a potentially explosive compound from a diluent may occur on cooling. In both cases this can represent a hazardous situation. 

Enrichment of a room atmosphere >25% creates a hazardous situation enrichment in a confined space is particularly dangerous. 

Reactants can accumulate when the chosen reaction temperature is too low, and as such the reaction continues even after the end of the addition. In such a case, a hazardous situation could occur if cooling were lost as exemplified by Barton and Rogers [5]. 

Inherently safe A system is inherently safe if it remains in a non-hazardous situation after the occurrence of non-acceptable deviations from normal operating conditions. 

Can equipment be designed such that it is difficult or impossible to create a potential hazardous situation due to an operating error (for example, by opening an improper combination of valves) ... 

A vvhat-if analysis identifies hazards, hazardous situations, and accident events with undesirable consequences (CCPS, 1992). What-if analysis considers deviations from the design, construction, modification, or operating intent of a process or facility. It is applicable at any litc stage of a process. 

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. 

The final element in management s communication of a desire to reduce human error is the identification and elimination of error-likely situations. Every task is an opportunity for a human error, but some situahons represent greater risks than others. Identifying these high-risk situations is not easy and an expertise in applying human factors principles to the workplace is an essential prerequisite for this identification. Eliminating these hazardous situations is often relatively simple once they have been identified. In some cases it may be appropriate to provide error-tolerant systems, which are those that facilitate identification of and recovery from the errors. 

Rasmussen, J. (1990). Human Error and the Problem of Causality in Analysis of Accidents. In D. E. Broadbent, J. Reason, A. Baddeley (Eds.). Human Factors in Hazardous Situations. Oxford, U.K. Clarendon Press. 

Pneumatic controls are used widely in hazardous situations such as chemical plants and oil refineries. The same risk of chafing applies as with capillary tubes. Pneumatic tubing is more usually in copper and is correctly secured. 

Before demolishing the old chemistry building on its c pus, a university called in a disposal service to get rid of old chemicals stored here and there. While this was not cheap, it was felt to be the safest and most efficient way to handle what could have been a potentially hazardous situation. 

Because personal protection is limited to the user and the equipment must be worn for the duration of the exposure to the hazard, it should generally be considered as a last line of defence. Respiratory protection in particular should be restricted to hazardous situations of short duration (e.g. emergencies, maintenance, or temporary arrangements while engineering control measures are being introduced). Occasionally, personal protection may be the only practicable measure and indeed even a legal requirement. If it is to be effective, its selection, correct use and condition are of paramount importance. 

For low-hazard situations, blast-resistant design is not required because a fire is more likely than an explosion in case of a flammable vapor release. Where little or no explosion hazard (low hazard) exists, it is only necessary to meet conventional building code requirements, including those for fire protection. 

Instruments are provided to monitor the key process variables during plant operation. They may be incorporated in automatic control loops, or used for the manual monitoring of the process operation. They may also be part of an automatic computer data logging system. Instruments monitoring critical process variables will be fitted with automatic alarms to alert the operators to critical and hazardous situations. 

Alarms are used to alert operators of serious, and potentially hazardous, deviations in process conditions. Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. Where delay, or lack of response, by the operator is likely to lead to the rapid development of a hazardous situation, the instrument would be fitted with a trip system to take action automatically to avert the hazard such as shutting down pumps, closing valves, operating emergency systems. 

A safety trip can be incorporated in a control loop as shown in Figure 5.24a. In this system the high-temperature alarm operates a solenoid valve, releasing the air on the pneumatic activator, closing the valve on high temperature. However, the safe operation of such a system will be dependent on the reliability of the control equipment, and for potentially hazardous situations it is better practice to specify a separate trip system such as that shown in Figure 5.24b. Provision must be made for the periodic checking of the trip system to ensure that the system operates when needed. 

A formal operability study is the systematic study of the design, vessel by vessel, and line by line, using guide words to help generate thought about the way deviations from the intended operating conditions can cause hazardous situations. 

Fires starting in a plenum communicated to the outside are unlikely to cause concern in habitable areas. If the plenum is isolated from the outside, a fire starting in it is more likely to cause a hazardous situation in the room below if the plenum is communicated with other plenums. 

The purpose of a what-if analysis is to identify hazards, hazardous situations, or specific accident events that could produce an undesirable consequence. The what-if analysis is described in detail in Guidelines for Hazard Evaluation Procedures (CCPS, 1992). 

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