Instrument critical

Control valve fails to respond 3. Same 3. Place valve on critical instrumentation JFL 1/93... 

Facilities Reviews There are many lands of facilities reviews that are useful in detec ting and preventing process safety problems. They include pre-start-up reviews (before the plant operates), new-plant reviews (the plant has started, but is stiU new), reviews of existing plants (safety, technology, and operations audits and reviews), management reviews, critical instrument reviews, and hazardous materials transportation reviews. 

Review of planned operation of process, especially the possibility of upsets, modes of failure, unexpec ted delays, redundancy of equipment and instrumentation, critical instruments and controls, and worst-credible-case scenarios... 

Another critical instrument specification is the total extra-column dispersion. The subject of extra-column dispersion has already been discussed in chapter 9. It has been shown that the extra-column dispersion determines the minimum column radius and, thus, both the solvent consumption per analysis and the mass sensitivity of the overall chromatographic system. The overall extra-column variance, therefore, must be known and quantitatively specified. 

Instrument Power - Instrument power failures are evaluated on a basis similar to that described for a power failure. Included in the normal considerations for PR sizing should be the failure of power supply to all instruments in and controlled from a single bus bar. Reliability feahires should include secondary selective power supply to control rooms, with emergency generator or battery backup for critical instruments and control computers. Critical controls should be able to continue operation independently of control computers. 

Experience with inherent safety reviews indicates that project investment costs are frequently reduced as a result of this exercise. Eliminating equipment and reducing the need for safety critical instrumentation are typically the main contributors to investment reduction. Capturing these potential savings depends greatly on the timing of the reviews. 

Last critical instrument check for No. 3 ketde (Maintenance records )... 

Equipment data materials, including specifications, maintenance and lubrication schedules andd records, critical instrument records, and so on... 

Centers usually adapt company-wide guidelines to activities within their scope. For example, the S/B (styrene/butadiene) Latex Technology Center has developed a Critical Instruments Program specifically for Dow s latex plants around the world. In identifies typical latex production equipment likely to be controlled by critical instruments. The program then draws upon Dow (and non-Dow) latex and other plant and research knowledge and experience to propose test procedures and test intervals for such instruments. 

The Technology Center audit team also addresses safety, reactive chemicals and critical instruments programs, and industrial hygiene and environmental quality efforts and performance which are specific to the technology. 

A Critical instrument is a device and its accompanying system whose proper functioning is necessary to prevent a serious incident. The system that reads the signal, interprets it, and displays it is as critical as the device that generates the signal. 

A serious incident is one in which serious personal injury or serious health, environmental, or economic damage could occur. Some examples of critical instruments and instrument systems are ... 

Dow s Critical Instruments Program describes identification of critical instruments, and provides guidelines for their design, installation, testing, maintenance, and documentation. 

Identification of critical instruments is the responsibility of the owner of the process and usually is a part of process research and design. In addition to recognizing critical instruments, the operation must be reviewed to make certain that instrumentation is adequate to prevent a serious incident. 

Once a critical instrument loop is identified, a procedure for testing the entire loop must be written. The test procedure will influence the design of the new system, since, if possible, the test should be an actual performance test. For example, if a high temperature should close a valve, the ideal test would consist of raising the temperature to see if the value closes. Efforts should be made to avoid test procedures which require temporary wiring disconnects, valve closures, and so on, which might not be returned properly to operating condition. 

A minimum interval of three months between tests of each critical instrument loop is recommended in no case should the interval exceed 12 months. However, frequency of testing should be based on plant judgment and experience, as well as on Technology Center information. 

Figure 10 was prepared by a Dow technology center as a logic diagram to illustrate an approach to be taken to determine how often a critical instrument loop should be tested. 

Fig. 10. Determining test interval for critical instruments. Example of tanker off loading which is operator supervised. The tank has a local level indicator (LI) and separate level alarm (LA) and level switch (LSW). Offloading is performed 10 times per year, (x) is the number of operations per year. Probability of overfill equals l(T4(x). Proposed target is less than or equal to 3 x 10 5. Testing interval 10-4(x) < 3 xl0 5(x) 3 x 10 1 = 4 months. Note that if only one tanker is offloaded per year, it is better to check LSW prior to offloading.

As described above, performance testing is an important consideration in the design of a critical instrument loop. Components of the system must be selected for ease of testing, as well as for their ruggedness and reliability. 

In computer systems, program logic is an important component of the critical instrument loop. Security precautions must be taken to prevent inadvertent changes in programming. In addition, manual overrides are discouraged. For example, if it is critical to shut down a pump during an emergency situation, a manual on switch may not be desirable. 

Dow s Critical Instruments Program requires that written records of critical instrument tests, with the name of the tester and information on any unsatisfactory performance, be kept for the current and preceding year. In addition, the Critical Instruments Program is included in the plant s program of safety audit and is audited on an annual basis. 

Monitoring programs for critical quality attributes and operating conditions, including calibration of critical instruments... 

Product critical instrument—where failure may have a direct effect on product quality (normally aligning with the defined quality-related critical parameters)... 

Safety/environmental critical instrument—where failure may have direct effect on safety/environment... 

In addition to inspection and calibration of instrumentation carried out as part of an SAT, the need for recalibration of critical instruments prior to IQ, OQ, and PQ should be reviewed and the decision documented in the respective qualification report. All site calibration activity should be conducted in accordance with quality standards and the respective engineering procedures. Any remedial work should be undertaken under document control, and where necessary, evaluated under change control. 

Calibration of critical instruments and system components must be controlled by a calibration schedule in order for call-off dates to be determined. The calibration periodicity should be determined by the process owner, its quality representative, and the maintenance engineer, taking into account the manufac-... 

The calibration test sheets form the evidence necessary to demonstrate the accuracy of data gathered during product manufacture and as such are key inspection documents. Critical instruments must be provided with a calibration test sheet/certificate that details both the test results and their limits of uncertainty. Calibration test sheets must be checked and approved by an authorized person. 

Deviations from approved calibration standards on critical instruments must be reported immediately and investigated to determine if this could have adversely affected earlier testing or product quality since the last calibration. 

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