ISA S84.01Application of Safety

Instrumented Systems for the Process Industry

Testing of SIS Valves

ISA SP84.01 Origins / Direction

• Grew out of OSHA (1910.119) and EPA (40CFR Part 68) mandates

• ISA specification will be superseded by IEC 61511, Functional safety of Safety Instrumented Systems for the Process Industry Sector

• IEC 61511 imposes additional redundancy requirements to achieve high SIL rating; these can be mitigated where diagnostics are shown to be used to provide predictive maintenance.

ISA SP84-01 Requirements• 1.1.2 The SIS includes all elements from sensor to the final

element, including inputs, outputs, power supply and logic solvers. SIS user interface may be in the SIS.

• 1.1.3 Other interfaces to the SIS are considered part of the SIS if they have potential impact on its safety function.

• 7.9.3 a) SIS shall be designed in accordance with the maintenance and testing requirements defined in the Safety Requirement Specifications.

• 7.9.1 Where the interval between scheduled process downtime is greater than the functional test interval, then on-line testing facilities are required.

ISA SP84-01 Requirements• 9.7 Functional testing - Not all system faults are self

revealing. Covert faults that may inhibit SIS action, this can only be detected by testing the entire system

• 9.7.1 Periodic functional tests shall be conducted using a documented procedure to detect covert faults that prevent the SIS from operating as per the Safety Requirement Specifications.

• 9.7.2 The entire SIS shall be tested including the sensor(s), the logic solver, and the final element(s) (e.g. shutdown valves, motors)

• Integral (entire system) testing not required except for pre-startup acceptance

ISA SP84-01 Requirements (cont’d)

• 7.9.3 b) The operator shall be alerted to the bypass of any portion of the SIS via an alarm and/or operating procedure. (ISA S84.01)

• 7.9.3 c) Bypassing of any portion of the SIS shall not result in the loss of detection and/or annunciation of the condition being monitored. (ISA S84.01)

ISA SP84-01 Requirements (cont’d)

• The PFD for the entire SIS is the sum of the PFD for each element.

• Low complexity Field devices contribute most to total PFD.

• “85% of the PFD is allocated to the field devices and the remaining 15% to the Programmable Electronic System. Any safety system design that does not fully comprehend the effect of the field devices (sensor and final control elements) is woefully incomplete and consequently inadequate.”

(ISA S84.01)

Probability of Failure (PFD)PFD = (λD) * TI/2

Where

λD = component dangerous failure rate (1)

TI = testing interval

PFD = DCpt(λD) * TIpt/2 + (1- DCpt)(λD) * TIft/2

Where

DCpt = Diagnostic Coverage Factor

TIpt = testing interval, partial stroke

TIft = testing interval, full stroke(1) See OREDA, 1997

Safety Integrity Level (SIL)

• SIL 1, 10-1 < PFD < 10-2

• SIL 2, 10-2 < PFD < 10-3

• SIL 3, 10-3 < PFD < 10-4

Graph of PFD against time

10%10%

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1010--33

1010--22

1010--11

t = 0t = 0

90%90%50%50%

Test interval timeTest interval time

Graph of PFD against timeto achieve SIL level

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1010--33

1010--22

1010--11

t = 0t = 0

SIL 1SIL 1

SIL 2SIL 2

SIL 3SIL 3TestTest

TestTest

TestTest

Graph of PFD against time to achieve SIL 2O

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1010--33

1010--22

1010--11

t = 0t = 0

SIL 1SIL 1

SIL 2SIL 2

SIL 3SIL 3TestTest

TestTest

TestTest

TestTest

TestTest

TestTest

Factors Effecting Testing Frequency

• SIL Level• Failure rate of valve in intended service• Valve manufacturer’s recommendation• Operational constraints• Level of redundancy• Good engineering practice

On-Line SIS Valve Testing Alternatives

• Bypass Valves• Partial Stroke Testing

Bypass Valves

• Pros• “Complete” diagnostic

coverage factor• Can allow for ESD

Valve removal / repair with unit running

• Cons• Expensive• May limit process

throughput unless full size bypass used

Partial Stroke Testing

• Allows more flexible testing intervals• Diagnostic coverage credit varies from

50% - 70% credit (vs. full stroke test) for detecting valves dangerous failure modes; DCpt a function of the specific partial stroke mechanism used

Partial Stroke Testing

Mechanical / Jammers

Pros• High Diagnostic

Coverage• Simple• Generally inexpensive

Cons• Labor intensive• Require tight

administrative procedures• Added risk of spurious

trip• No diagnostics• Valve unavailable during

test

Partial Stroke Testing (Cont’d)

SIS Logic SolverPros

• High Diagnostic Coverage• Can provide diagnostics• Testing can be automated• Valve available during test

Cons

• Generally expensive• Added software in logic

solver• Added risk of spurious

trip• Violates principle of using

separate technology / hardware between DCS, SIS and testing

Partial Stroke Testing (Cont’d)

Proportional Control

Pros• Can provide

diagnostics• Testing can be

automated• Valve available during

test

Cons• Requires addition of

digital valve positioner (adds to system PFD)

• Higher wiring costs• May not have high

diagnostic coverage• Doesn’t test valve at

actual speed of operation• Increased spurious trip

rate

Partial Stroke Testing (Cont’d)Manufacturer “D”

Pros• Provides diagnostics• High diagnostic coverage• Testing can be automated• Valve available during test• Tests valve in real time

operating speed• Does not increase PFD of

system• Available feature to allow

data capture during trip

Cons• Higher wiring costs• Slightly increased

spurious trip rate

Conclusion

• Testing of SIS valves represents a significant challenge, involving safety and operational constraints

• Partial stroke testing can:– Provide improvement in PFD over full stroke

testing alone– Provide diagnostic capability about current

valve operation and future potential failures

The The DrallimDrallim LMT SystemLMT System

And International/Company And International/Company Standards on Safety Related Standards on Safety Related

Systems.Systems.

D.M.EssamD.M.Essam DrallimDrallim Industries LtdIndustries Ltd