Applying ISA recommended practice 67.04 to setpoint calculations

The ISA issued draft 7 of Recommended Practice (RP) 67.04 Methodologies of the Determination of Setpoints of Nuclear Safety-Related Instrumentation in September of 1990. The draft RP describes methods for calculating and combining the uncertainties associated with instrument loop setpoints (e.g., temperature, pressure, and radiation effects, etc.). However, in some cases, the guidance provided on how to apply these uncertainties and other factors to setpoint calculations is not completely developed. In addition the RP does not address how these uncertainties might affect the setpoints of other instrument loops with a system. This paper addresses these issues. The method for applying the uncertainties to instrument loop setpoints presented in this paper were developed while performing setpoint calculations for instruments located in harsh environments at WNP-2. Considerations for applying uncertainties when determining setpoints include: the effects of cascading setpoints, the effects of setting tolerances, switch reset, and two sided analytical limits