Best Practices in SIS Documentation

Ed Marszal, President, Kenexis

Presenters

Ed Marszal

Gary Hawkins

Introduction

Safety Instrumented System Design per “ISA S84”* is becoming a common practice

Poor documentation is being generated due to “safety case” mentality

Current practices ignore audience of documents and “good practices” for specifications in general

* ANSI /ISA 84.00.01-2004 (IEC 61511-Mod)

FEED Phase SIS Documents

List of Safety Instrumented Functions (SIF) Grouping of Instrumented Protective Functions (IPF)

– Group by equipment or process• Compressors• Reactors

• Fired Heaters

P&ID representation of SIF Logic description

– cause and effect tables– Boolean logic diagrams– Narrative (“plain English” description of operation)

Testing procedures (with documentation of results)

Preliminary Design Steps

SIF List should be precursor to SIL selection HAZOP/LOPA without knowledge of typical SIF leads

to errors HAZOP is a final check on a good design, not a

design task Typical SIF based on experience, standards, codes,

and judgment

Instrumented Protective Function Groups

Group instruments together that are functionally related

Typically based around major equipment– Compressors– Fired Heaters– Reactors

Typically contains multiple SIF Also can contain non-SIF instruments and logic

Typical Plant Groupings

IPF Grouping for Separator

V-101

PT101B

PV101B

PT101D

LT101A

LT

101B

PI101C

PIC101B

H

L

A

D

PT101C

PV101A

PIC101A

PT

101A

USC101

USC101

LG101A

LG101B

H

H

L

Detail “A”

Detail “A”

Advantages of IPF Grouping

Compact information with minimal duplication Facilitates programming – programmer shielded for

single instruments in multiple SIF Facilitates design and I/O counting Facilitates test plan development and testing

P&ID representation

Symbology for SIS, specifically tag naming in inconsistent (I, X, UC, USC)

Use of “S” is technically correct, but leads to more confusion (PSV is always a relief valve??)

Use of typicals to minimize clutter

Typical SIS I/O Details

Detail “A” - SIS Inputs

XIXXX

XTXXX

HAXXX

XAHHLLXXX

XAHLXXX

HSXXX

XDAXXX

USCXXX Indicator

Bypass Switch

Trip Alarm

Pre-Alarm

Bypass Alarm

Deviation Alarm

Safety Requirements Specs

Specifications (emphasis on ‘s’) Limit information to what is required for audience (SIL

not required on C&E or P&ID) Use “general requirements” statements for common

features such as bypassing Refer to other documents for non-critical information

Typical Bypass Note

1.1 Bypass / Override SIS Logic Solver

Each of the functional groups that are described in this Safety Requirements Specification shall require a shutdown bypass function for maintenance and testing. The bypass functionality described in this note shall not be used for normal operations. I f a bypass is required for normal operations such as start-up, a dedicated hard-wired bypass facility shall be provided. The SIS shall be configured so that bypasses are implemented using a two-step process that includes activation of a unit-specific “bypass enable” switch and activation of an input-specific BPCS bypass soft switch. Only when both of these items are activated shall the input be bypassed. When an input is placed in “bypass”, the SIS logic solver shall hold the input in the non-trip state, regardless of the status of the bypassed input.

Reference External Documents

Conclusions

Room for improvement in SIS documentation practices

Consider the audience for the documents Use good engineering practice Minimize data duplication Leads to shorter preparation time and fewer errors

Business Results Achieved

Decreased implementation time and cost– Compact documentation is easier to prepare and more

accurate– Use of standard modules instead of custom development– Minimal clarification and rework

Decreased ongoing maintenance effort and cost– Updates only occur in one document– Likelihood of inconsistent data in multiple documents

decreased

Safer processes– Lower probability of systematic errors in system resulting

from poor documentation

Summary

SIS design can be made safer and more cost effective through documentation method improvements

Specification preparation time can be reduced by as much as 50%*

Please fill out comment cards and e-mail any feed back you have to the authors

Questions?!?

Where To Get More Information

ISA Bookstore – Safety Integrity Level Selection Kenexis Web Site

– HTTP://www.kenexis.com/resources

Emerson SIS Lifecycle Workbook– At Delta V SIS Booth during EGUE– Contact Emerson After EGUE