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A new installation must be fully tested to ensure that the equipment is in full working order prior to its being started. This testing is typically divided into three phases:
This is the testing of each instrument for correct calibration and operation prior to its being installed in the field. Such testing is typically performed in a workshop which is fully equipped for that purpose and should contain a means of generating the measured variable signals and also a method of accurately measuring the instrument input and output (where applicable). The test instruments should have a standard of accuracy better than the manufacturer’s stated accuracy for the instruments being tested and should be regularly certified.
Normally, instruments are calibration checked at 5 points (0, 25, 50, 75, and 100 percent) for both rising and falling signals, ensuring that the readings are within the manufacturer’s stated tolerance.
After testing, instruments should be drained of any testing fluids that may have been used and if necessary, blown through with dry air. Electronic instruments should be energized for a 24-hour warm-up period prior to the calibration test being made. Control valves should be tested in situ after the pipework fabrication has been finished and flushing operations completed. Control valves should be checked for correct stroking 0, 50, and 100 percent open, and at the same time the valves should be checked for correct closure action.
This is an important operation done before loop testing.
All air lines should be blown through with clean, dry air prior to final connection to instruments, and they should also be pressure tested for a timed interval to ensure that they are leak free. This should be in the form of a continuity test from the field end to its destination for example the control room.
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Impulse lines should also be flushed through and hydrostatically tested prior to connection of the instruments. All isolation valves or manifold valves should be checked for tight shutoff. On completion of hydrostatic tests, all piping should be drained and thoroughly dried out prior to reconnecting to any instruments.
All instrument cables should be checked for continuity and insulation resistance before connection to any instrument or equipment. The resistance should be checked core to core and core to ground.
Cable screens must also be checked for continuity and insulation. Cable tests should comply with the requirements of the relevant regulations for electrical installations or the rules and regulations with which the installation has to comply. Where cables are installed below ground, testing should be carried out before the trenches are back filled. Coaxial cables should be tested using sine-wave reflective testing techniques. As a prerequisite to cable testing, it should be ensured that all cables and cable ends are properly identified.
The objective of loop testing is to ensure that all instrumentation components in a loop are in full operational order when interconnected and are in a state ready for plant commissioning.
Before loop testing is done, inspection of the whole installation, including piping, wiring, mounting, and so forth, should be carried out to ensure that the installation is complete and that the work has been done in a professional way. The control room panels or display stations must also be in a fully functional state.
Loop testing is generally a two-person operation, one in the field and one in the control room. These persons are equipped with some form of communication for instance field telephones or radio transceivers. Simulation signals should be injected at the field end equivalent to 0, 50 and 100 percent of the instrument range, and the loop function should be checked for correct operation in both rising and falling modes. All the results should be properly documented on calibration or loop check sheets. All auxiliary components in the loop should be checked at the same time.
Alarm and shutdown systems must also be systematically tested and all systems should be checked for “fail-safe” operation, including the checking of “burn-out” features on thermocouple installations. At the loop-checking stage all the auxiliary work should be completed, such as setting zeros, filling liquid seals, and fitting of accessories such as charts, ink, fuses, etc.
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Commissioning is the bringing “on-stream” of a process plant and the tuning of all instruments and controls to suit the process operational requirements. A plant or section thereof is considered to be ready for commissioning when all instrument installations are mechanically complete and all testing, including loop testing has been accomplished.
Prior to commissioning, it should be ensured that all air supplies are available and that all power supplies are fully functional, including any emergency standby supplies. It should be also be ensured that all auxiliary devices are operational, such as protective heating systems, air conditional, etc. All control valve lubricators (when fitted) should be charged with the correct lubricant.
Commissioning is usually accomplished by first commissioning the measuring system with any controller mode overridden. When a satisfactory measured variable is obtained, the responsiveness of a control system can be checked by varying the control valve position using the “manual” control function. Once the system is observed to respond correctly and the required process variable reading is obtained, it is then possible to switch to “auto” in order to bring the controller function into action. The controller responses should be then adjusted to obtain optimum settings to suit the automatic operation of plant.
Alarm and shutdown systems should also be systematically brought into operation, but it is necessary to obtain the strict agreement of the plant operation supervisor before any overriding of trip systems is attempted or shutdown features are operated.
Lastly, all instrumentation and control systems would require to be demonstrated to work satisfactorily before formal acceptance by the plant owner.
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