Critical instruments testing

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. 

Delays to Safety Critical Instrument Test and Equipment Inspection Frequencies... 

There must be an approval mechanism to reduce unnecessary inspection and testing if repeated testing shows ideal performance. Many test frequencies are initially self-imposed to be annual, which could be too frequent. Those type of changes should have an approval trail which includes the unit manager, or second-level supervisor, the critical instrument testing supervisor and a process safety representative. 

Delays to safety critical instrument test and equipment inspection frequencies... 

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. 

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

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. 

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. 

Critical instruments assigned a Class 1 include those necessary to avoid a failure which may cause the perils listed above or instruments which fail to inform of upset conditions which may result in perils. Testing of these instrument systems may be mandated by regulatory agencies, in-house technical safety review committees, HAZOP studies, or designated as critical by operations supervisors. All of these shutdown systems and alarms must be prooftested in accordance with a proper schedule. [8]... 

Serious Consequences—Class 2. Equipment or the critical instruments serving equipment whose failure could possibly cause, or fail to warn of upset conditions, uncontrolled releases of dangerous materials, situations that could result in accidental fires and explosions. Furthermore these failures could result in serious conditions involving environmental releases, property or production losses, or other non-life-threatening situations. These particular pieces of equipment, the safety shutdown systems and the alarms that serve this equipment are given a slightly lower priority. However, they are also inspected, tested, or prooftested on a regular schedule, but may be allowed to have some leniency in compliance. 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

Maintenance records such as critical instrument checks, pressure relief vah e tests, pressure vessel inspections... 

The OQ protocols will test all critical parameters for the equipment. It will test all control devices, calibrate critical instruments, and test major vessels under operating conditions (pressure and vacuum). 

In the past, safety applications were called "critical instrument systems". Engineering rules, typical examples and best practices as well as test procedures were developed. 

The system suitability test is carried out to test critical instrument and method related parameters. In HPLC system suitability us determined ahead of running a method and checks that the parameters which can influence the outcome of the analysis are within specification. This normally includes reproducibUity, peak shape and tailing and resolution of the ingredients. 

Operating devices may be miscalibrated after a while, for example the temperature accuracy of a GC column oven or the wavelength accuracy the optical unit of a UV/visible detector. This can have an impact on the performance of an instrument. Therefore a calibration program should be in place to recalibrate critical instrument items. All calibrations should follow documented procedures and the results should be recorded in the instrument s logbook. The system components should be labeled with the date of the last and next calibration. The label on the instrument should include the initials of the test engineer, the form should include his/her printed name and the full signature. 

Figure 9.21 and Table 9.7 lists critical instrumentation for the inverted vertical LAD outflow tests. Ullage pressure was measured in order to deduce the pressure at the LAD screen. The stream pressure and temperature inside the LAD outflow line downstream of the channel but upstream of the Venturi flow meter was measured to ensure that there... 

Operators frequently have insight into which instruments are accurate and which are not. If those instruments subsequently prove critical, recalibration must be done prior to the unit test. Preliminary analysis of daily measurements and practice measurements will help to identify which are suspect and require instrument recalibration prior to me unit test. 

The assessor should also find out whether an effective testing program is in place to help ensure the serviceability of process measurement equipment. The successful toller should have an established calibration program to address the accuracy of critical measurement equipment. Safety critical process parameters should be monitored and critical process equipment should automatically interlock when monitoring instrumentation detects safety critical deviations. Interlocks should either facilitate a remedy to the critical deviation or bring the process to the zero energy state. These instruments and interlocking devices should be routinely tested to ensure operational reliability. 

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