New Wave Gas Detection

Saturday, June 23, 2012 @ 12:06 PM gHale


Traditional Gas-Detection Design Codes Can’t Cover Fire, Flammable Gas, and Toxic Gas Behavior on Tank Farms and Sprawling Petrochemical Facilities.
By Edward Marszal and Srinivasan Ganesan
The design of hydrocarbon-gas-detection systems using risk analysis methods is drawing more attention these days.

Industry experts decided design codes used in traditional gas-detection-system design work are not sufficient for open door process areas having serious hazards like fire, flammable gas, and toxic gas.

The ISA Technical Report TR 84.00.07 provides guidelines for the design of fire and gas systems in unenclosed process areas in accordance with the principles given in IEC 61511 standards.

Here are risk assessment methods that are in the ISA technical report and how they apply to gas detection systems.

We also include an overview of the performance based safety life cycle of gas detection systems from conceptual design stage to operations and maintenance.

Safety at Core
Risk assessment techniques are becoming vogue in the design of engineered safeguards, such as Fire and Gas Detection and Suppression Systems (FGS), Safety Instrumented Systems (SIS), and Alarm Systems.

The principles of risk assessment used in the design of SIS can also work in the design of Gas Detection Systems. A Gas Detection System is a type of instrumented safeguard intended to reduce risks posed by process plants, such as safety risk, environmental risk, and asset risk (commercial/business) to tolerable levels.

However, gas detection systems are only capable of mitigating the consequence of a loss of containment, whereas safety instrumented systems are capable of preventing the consequence from occurring altogether.

All automated safety systems such as FGS, SIS, and High Integrity Pressure Protection Systems (HIPPS) need a “basis of safety” for the selection and design of its functional elements (sensor, logic solver, and final control elements).

In the design of Gas Detection Systems, it is important to select detectors of the appropriate technology and to diligently position the right number of detectors at the correct location for the system to respond on demand.

In addition, the basis of safety specifies the mechanical integrity requirements for the equipment with respect to the type and frequency of preventive maintenance tasks required. In short, the basis of safety is at the core of decisions that one makes with reference to selection and maintenance of instruments.

Unaddressed Standards
The two options for choosing basis of safety are – prescriptive and performance-based.

Prescriptive basis of safety (such as NFPA 72 and EN 54 for fire alarming equipment) specifies the type of equipment, its location for installation, and addresses the requirements to maintain them.
Not only do the prescriptive standards for FGS design provide a very comprehensive set of rules for designing equipment, but they are also so well-established for the design of fire alarm systems, that we often employ them for the signaling portion of gas detection systems too.

These standards have evolved to be very effective for the fire alarms in occupied buildings, such as office buildings, hospitals, and schools, but often fall short for gas detection and even for fire detection in open process areas.

Prescriptive standards provide detailed requirements for many gas and fire system applications. However, they do not provide detailed requirements for gas detection in open-door areas, such as chemical process units and hydrocarbon storage tank farms. These prescriptive standards do not adequately cover some of the gas-detection system elements (sensor, logic solver, final control element) that we typically find in chemical process facilities
In addition, they do not provide an optimal solution to deal with the hazards associated with process facilities, such as oil refineries and petrochemical plants. Specifically, their design is not necessarily suitable for hazards such as combustible hydrocarbon gases and toxic gases.

As a matter of fact, toxic gases are completely unaddressed by these prescriptive standards, and combustible gases only slightly.

Jumpstarting Risk Assessment
It is worthwhile to point out the institutions that developed these prescriptive standards are cognizant of their shortcomings and therefore allow the use of performance-based basis of safety in areas where the users of the standards believe that the prescriptive guidance is ineffective.

Performance-based standards use risk assessment techniques for decisions involving selection, design, and maintenance of gas detection systems. The intent of the performance-based approach is not to replace the prescriptive method, but to supplement it where prescriptive methods are ineffective.

Industry practitioners recognized the need for more guidance for performance-based design for gas detection systems and came to a consensus that this guidance has to come from a standards organization like the International Society of Automation (ISA). ISA Standards Panel 84 created a special working group called “working group 7” specifically to address performance-based design of fire and gas systems.

The ISA Technical Report TR 84.00.07 that came out of the working group 7 provides guidelines for fire and gas systems in accordance with the principles provided in IEC 61511 standards. The Technical Report TR84.00.07 generated considerable interest among oil & gas operating companies and EPC companies and it jumpstarted the application of risk assessment techniques to design fire and gas detection and suppression systems.

Performance Metric
The basis of the IEC 61511 standard is to specify targets for performance metrics for each safety instrumented function that is protecting the plant from process-related risks. The target is selected based on the risk associated with the hazard that the safety instrumented function is intended to prevent.

Gas detection systems pose challenges when trying to use risk analysis techniques that are compliant with ISA84/IEC 61511 standards for safety-instrumented systems. The hazards associated with gas detection systems (especially as applied in the chemical process industries) are general in nature and it is difficult to characterize them in the context of layer of protection analysis (LOPA).

Initiating events caused by leaks due to corrosion, erosion, and other physicochemical forces are not included in LOPA. Although the concept of probability of failure on demand is applicable to gas-detection-system functions, component equipment failures are not the only consideration and usually not even the most important.

The inability of a gas-detection-system function to detect a gas leak because of lack of coverage can also lead to failure on demand. Recent data from the UK North Sea area indicate automated systems did not detect more than 30% of major gas releases.

The ISA 84 working group 7 determined a gas detection system could be similar to a SIS if we consider detector coverage as an additional performance metric. In addition to assigning targets for safety availability (equivalent to SIL), targets for detector coverage need to be assigned for gas detection systems so that the verification and validation of detector coverage is included in the gas detection system design.

Design Life Cycle
The safety life cycle defined in the ISA Technical Report TR84.00.07 for fire and gas systems is very similar to the one defined for safety instrumented systems in the IEC61511 standard. We must identify risk scenarios before the selection of fire and gas systems for a particular application.

The analysis of hazards and consequences associated with each scenario must occur, taking into account the impact on human lives and assets. It is also important to consider the frequency of occurrence of the consequence while making decisions on the fire and gas system. If the decision is that the consequence will occur quite frequently, then we need to consider a more rugged risk mitigation system.

A risk assessment takes place before making a decision on the need for a fire and gas system. If the unmitigated risk were tolerable, there would be no design for a fire and gas system. If the unmitigated risk were not tolerable, recommendations to design a fire and gas system to reduce the overall risk to tolerable levels happen.

If the decision is to install a fire and gas system, the initial design is typically uses heuristics (rules of thumb). The ISA Technical Report TR 84.00.07 proposes that the procurement and installation of fire and gas systems should not immediately follow the initial design.

Instead, the technical report suggests the coverage provided by the detector layout in the initial design be calculated and verified to check if it meets its target. In addition to the coverage, the safety availability (equivalent to SIL) for each function should be calculated and verified in a way identical to verifying the SIL of a safety instrumented system in accordance with the IEC 61511 standard.

The typical workflow in the safety life cycle of performance-based fire and gas system design is below.


Editor’s note: Ganesan and Marszal discuss each of the steps in this performance-based FGS life cycle in their paper “Performance-Based Gas Detection System Design for Hydrocarbon Storage Tank Systems.” – Nicholas Sheble (nsheble@isssource.com)

Edward Marszal, PE (edward.marszal@kenexis.com) is president of Kenexis and is responsible for instrumented safeguard design basis development and verification/validation projects. He is an ISA Fellow, a teacher of ISA safety courses, a member of the ISA SP 84 and SP 18 standards committees, and author of the book Safety Integrity Level Selection. Srinivasan Ganesan, PE, is a senior engineer at Kenexis DMCC and handles Kenexis’ operations in the Middle East. He is a chemical engineer with 15+ years experience in the design, engineering, operation, and maintenance of chemical process units.



2 Responses to “New Wave Gas Detection”

  1. chenzhongsheng says:

    I am a student ,and I am very interested in FGS. Recently,I am read the article “Performance‐Based Gas Detection System Design for Hydrocarbon Storage Tank Systems”.When I reference the article in my thesis ,I don’t find the time when it published.So,I need you help .Would you told me the time when the article was published?

  2. gHale says:

    The article published on ISSSource June 23, 2012…


Leave a Reply

You must be logged in to post a comment.