Safety critical alarms

DEVIATIONS Alarms first-out safety-critical alarms many simultaneous or false alarms... 

Safety relief valves (SRVs) and safety critical alarms and critical shutdown systems can be modified easily by aging or tampering. The local environment surrounding the device including cycles of freezing and thawing, moisture, corrosive contact with equipment inter-... 

Occasionally, there may be business pressures or maintenance scheduling problems that would encourage the delay of prooftesting of safety critical alarms and shutdown systems. Such situations can also delay of vessel inspections and safety relief valve testing. Some type of variance procedure or review policy should be defined to handle this occasional need. Such a policy ought to require the review of all of the inspection and test records on the specific equipment involved as well as an approval of the superintendent of the area. 

ANSI/ISA-84.01-1996 excluded systems where the operator was the sole means of returning the process to a safe state. ANSI/ISA-84.00.01-2004-1 did not specifically exclude this type of system, but did not explicitly include it either. ANSI/ISA-84.00.01-2004-1 Clauses 11.3.1 through Clauses 11.3.3 provide requirements where the operator is required to take specific action in response to safety critical alarms and diagnostic alarms. When the Hazard and Risk Analysis (H RA) identifies a critical alarm as a protection layer, the detection and response may include many different components, such as sensor(s), logic solver, operator HMI, and final element(s). It is important that all elements, including the operator, be capable of achieving the required risk reduction. ISA-TR84.00.04-1 Annex B provides additional discussion on this subject. 

Provide critical alarms and safety systems independent of BPCS... 

First, safety critical systems must be reliable. These systems control releases in the event of accidents. It s necessary to have a critical analyzer, instrument and electrical system test program. This should consist of preventive maintenance and alarm and trip device testing for panel alarms, emergency isolation valves and other critical components. [7]... 

All of the effort expended in designing plant-safety systems is of little value unless accompanied by an adequate prooftest program and regular maintenance. These safety systems—consisting of such components as safety-relief valves, tank vents, critical alarms, and protective isolation and shutdown devices—do not operate on a continuous basis. Rather, they are only called into service periodically to warn of, or to prevent, conditions that could lead to plant accidents. [8]... 

A listing of critical alarms and safety interlocks essential to safe plant operations must be made. 

The Safety Instrumented Systems (SIS) and critical alarms assigned a Class 1 include those that have been mandated as such by state or federal agencies an in-house technical safety review committee HAZOP studies and specific alarms deemed critical by operations supervisors. All of these Safety Instrumented Systems and alarms are on a regular prooftesting schedule. 

Have Safety Critical process alarms and shutdown systems been modified to... 

Have the alarm listings and safety critical prooftest procedures been... 

Class 2 Safety Critical instruments include alarms or trips on refrigeration systems, rectifiers, cooling towers, kettles, and stills. [8]... 

A Safe Operating Procedure developed to create a uniform method to ensure that appropriate steps are taken prior to bypassing or removing an alarm, instrument, or shutdown system IWim service is described in the section that follows. This procedure can provide an effective way of communicating the status of an impaired instrument. The procedure has been in use for over five years. It assumes that all instrumentation has been classified into three safety critical systems. [7] (These classes have been defined in Chapter 9, but are repeated here.)... 

Class 2 Safety Critical instruments include alarms or trips on refrigeration systems, rectifiers, cooling towers, kettles, and stills. [71 Normal Consequences—Class 3. Instrument systems that are used to alert the chemical pracess operator of a nonhazardous abnormal condition that might otherwise be undetected. The failure to react to one of these alarms may create an off-spec ificalion product such as a low-temperature alarm on certain distillation columns. These systems are not included in the prooftest program. 

Decentralized decision making is, of course, required in some time-critical situations. But like all safety-critical decision making, the decentralized decisions must be made in the context of system-level information and from a total systems perspective in order to be effective in reducing accidents. One way to make distributed decision making safe is to decouple the system components in the overall system design, if possible, so that decisions do not have systemwide repercussions. Another common way to deal with the problem is to specify and train standard emergency responses. Operators may be told to sound the evacuation alarm any time an indicator reaches a certain level. In this way, safe procedures are determined at the system level and operators are socialized and trained to provide uniform and appropriate responses to crisis situations. 

Ensure all emergency equipment and safety devices are operable at all times during hazardous operations. Before safety-critical, nonroutine, potentially hazardous operations are started, inspect all safety equipment to ensure it is operational, including the testing of alarms. 

Define and communicate safe operating limits for all safety-critical equipment and alarm procedures. Ensure that operators are aware of these limits. Assure that operators are rewarded for following the limits and emergency procedures, even when it turns out no emergency existed. Provide for tuning the operating limits and alarm procedures over time as required. 

Alarms with defined operator response Critical alarms Safety instrumented systems Pressure relief devices Blast walls and dikes Deluge systems Flare systems... 

Mechanical integrity checks (critical alarms, equipment, monitoring safety system checks, etc.) ... 

The ES H Manual Supplement also considers applicable ANSI standards, including the basis for criticality requirements, record keeping, assessments for potential criticality events, criticality safety control parameters, conducting criticality safety analyses, preparation of plans and procedures, requirements for criticality alarms, personnel training, posting, and operational considerations. 

Controlled (CN). Has been countered by appropriate design, safety devices, alarm/caution and warning devices, or special automatic/manual procedures. Criticality categories include... 

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