Trip Setting Nomination and Process Safety Time
Harvey T.Dearden
BSc CEng FIET FInstMC FIMechE AFIChemE
Associate Consultant
HTS EngineeringGroup Ltd.
Any Safety Requirements Specification (SRS) worthy of the name will identify the trip setting and the
process safety time (PST). This latter is defined in the IEC 61508 standard to be the ‘period of time
between a failure, that has the potential to give rise to a hazardous event, occurring in the EUC or
EUC control systemand the time by which action has to be completed in the EUC to prevent the
hazardous event occurring’. (EUC stands for ‘Equipment Under Control’.) This is not quite right
however. Consider an example of a coolingwater failure; this will lead to a temperature rise , but
the initial rise beforethe trip point is reached does not constitute part of the PST. The PST is the
time between the trip setting being reached and the time bywhich the actionmust be complete if
the hazard is to be avoided. The process safety time is then useful as a component of the SRS in that
it identifies the maximumacceptabletotal execution timeof the Safety Instrumented Function (SIF).
Note that if the protection function is apermissive interlock (i.e. preventing an operation), there will
not be a process safety time. If the function is formitigation (e.g. fire detection), rather than
prevention, typically the response time will not be an issue. If the function is a trip derived from
detection of abinary status condition; drive on/off, valve open/closed, flame/no flame etc., then PST
is determined directly by considerations of the process and plant design. If the trip is derived froma
continuous process variable however e.g., pressure,temperature, level, then the PSTbecomes a
function of the trip point nominated. The farther fromthe hazard point the trip setting is, the
greater the PST. (Note that this is also true when a switch is used on a continuous process variable –
the trip point is implicit in the switch setting or level location.)
Often the trip setting will be nominated as a judgement based on past practice and experience
rather than any rigorous evaluation. Typically there will be ahandsome level of conservatism in the
specification of the true process limit e.g. equipment pressureor temperature rating, and a lack of
conservatism in the nomination of a trip setting would not be potentially hazardous. Formany
applications, conservatism in the specification of a trip setting would not be a critical concern, but
for some there may be profound implications for process performance and availability. Itmay be
that the closer the processmay approach a constraint the better the process yield or efficiency. This
is one reason for improving process control; itmay allow a set point closer to the trip setting through
reduced process variability. A critical examination of the trip setting nominationmay identify
opportunities to revise the trip setting itselfand allowoperationwith a reducedmargin to the
process limit.
The PST will also often also be nominated on the basis of established practice and judgement rather
than any formal evaluation, but although itmay not be recognised, implicit in a specification of PST
for a continuous variable trip is the approach speed of the variable to the true constraint, since:
= (
−
)
÷
ℎ