The completed instrument installation must be fully tested to ensure that the equipment is in full operating order before starting a new installation. This test normally falls into three phases, i.e. pre-installation testing; piping and cable testing; pre-commissioning or loop testing.
This test is done to each instrument before it is installed in the field for correct calibration and operation. Such testing is normally performed in a workshop that is fully equipped for the purpose and should include a means to generate the measured variable signals as well as a method to accurately measure the input and output of the instrument.
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. Instruments are normally checked for both rising and falling signals at five points (i.e., 0, 25, 50, 75, and 100 percent).
Piping and Cable testing:
Before the final connection to the instruments, all air lines should be blown through with clean, dry air and pressure tested for a timed interval to ensure that they are free of leakage. This should be in the form of a continuity test from the field end to its destination.
Impulse lines should also be flushed through before the instruments are connected and hydrostatically tested. Tight shut-off should be checked for all isolation valves or manifold valves. Upon completion of hydrostatic tests, all pipes should be drained and dried thoroughly before reconnecting to any device.
Before connecting to any instrument or device, all instrument cables should be checked for continuity and insulation resistance. Core to core and core to ground should be checked for resistance.
If cables are installed below ground, testing should take place before the trenches are filled back. Sine - wave reflective testing techniques should be used to test coaxial cables.
Loop testing shall ensure that all instrumentation components in a loop are fully operational when interconnected and ready for commissioning at the plant.
In order to ensure that the installation is complete and that the work has been carried out in a professional manner, inspection of the entire installation, including piping, wiring, mounting, etc., should be carried out before loop testing. The panels of the control room or display stations must also be fully functional.