BS IEC 61888 pdf download – Nuclear power plants — Instrumentation important to safety — Determination and maintenance of trip setpoints
Analytical limits are derived from the safety limits through the study of accident cases. The margin between these two values allows to take into account the time dependency of the process itself and the response time of the whole instrument channel protection (sensor, signal process, actuator, etc.). Analytical limits (AL) represent a value that should not be exceeded prior to accomplishing the prescribed action. In establishing the analytical limit for a setpoint the overall function initiated by setpoint action shall be carefully considered. Generally, the analytical limit is provided through design documentation or other calculations. If an analytical limit is not available, the need to prepare a setpoint calculation should be carefully considered.
5.3 Limiting safety system setting (LSSS) The purpose of an LSSS is to assure that protective action is initiated before the process conditions reach the analytical limit, thereby limiting the consequences of a design-basis event to those predicted by the safety analyses. The LSSS is derived from the analytical limit in a manner determined by the setpoint calculation methodology. The LSSS is maintained by either the technical specifications or the plant-operating procedures. Figure 1 illustrates the relationships between an analytical limit and an LSSS. Detailed requirements for developing trip setpoints are given in subclauses 5.3.1 to 5.3.3.
5.3.1 Channel uncertainty and trip setpoint The trip setpoint shall be established for the channel. Data used to select the trip setpoint may be taken from any of the following sources: operating experience, equipment qualification tests, vendor design specifications, engineering analysis, laboratory tests, and engineering drawings. An allowance shall be provided between the trip setpoint and the analytical limit to ensure a trip or a safeguard action before the analytical limit is reached. The allowance used shall account for all applicable design-basis events and process instrument uncertainties which follow unless they were included in the determination of the analytical limit. Figure 1 illustrates the relationships between the trip setpoint and other parameters. Region A represents the uncertainties allowed between the analytical limit and the trip setpoint and is made up of a statistical combination of the uncertainties described in this clause and in 5.3.2. The approved setpoint methodology for the application identifies which uncertainties are to be included and how they are to be combined. This is called the channel uncertainty. Region B denotes the difference between the allowable value (see the definition in 5.3.4) and the trip setpoint. Region C denotes the difference between the analytical limit and the allowable limit and takes into account components not tested during periodic tests and accident condition effects.
Region D illustrates the difference between the expected value of the process variable during normal operation and the trip setpoint. Region E shows how the allowance around the trip setpoint is taken into account during the periodic checks. This acceptable allowance includes the calibration uncertainty of the part of the instrument channel being tested (also called adjustment error) and the uncertainty of the instruments themselves (electronic processing modules) in operating conditions (see item b) hereafter. Region F is the safety margin described by item i). Figure 2 shows an example where process related uncertainties should be considered as well as measurement uncertainties. Both are described below and apply to different portions of the channel.
a) Instrument calibration uncertainties caused by the
1 ) calibration standard;
2) calibration equipment; or
3) calibration method.
b) Instrument uncertainties during normal operation caused by
1 ) reference accuracy, including:
a) conformity (linearity);
b) hysteresis;
c) dead band;
d) repeatability;
e) foldover;
f) saturation;
2) power supply voltage changes;
3) power supply frequency changes;
4) temperature changes;
5) humidity changes;
6) pressure changes;
7) vibration (in service);
8) radiation exposure;
9) analog-to-digital (A-D) conversion;
1 0) digital-to-analog (D-A) conversion;
1 1 ) electromagnetic interference;
1 2) ageing effects;
BS IEC 61888 pdf download – Nuclear power plants — Instrumentation important to safety — Determination and maintenance of trip setpoints
