Statistical management of an instrument setpoint drift based on plant specific as-found/as-left data

Instrument setpoints are affected by many sources of uncertainty. ANSI/ISA-S67.04.01-2000 (ISA, 2000a) and ISA-RP67.04.02-2000 (ISA, 2000b) provide a basis for establishing safety-related instrument setpoints under several uncertainty terms including a random drift. However, the current methodologies and the recommended practices for a setpoint drift analysis need to be modified in order to control plant specific setpoint drift of an instrumentation device, especially when there are many ties in the measurements. In this paper, we propose an extended procedure supplemented with non-parametric and graphic tools for managing the setpoint drift, based on the plant-specific as-found/as-left data. The proposed procedure has three major advantages. First, supplementing non-parametric statistical methods can handle non-standard measurement data even when they have many ties. Second, adopting statistical graphic tools facilitates identifying data characteristics. Third, applying statistical process control (SPC) techniques may provide plant staff with an intuitive way of managing the instrumentation setpoint drift for a long term.