Functional Safety 2014

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