So What’s an Impact Analysis?

Wednesday, March 06, 2013

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It doesn’t take much to remember a time you wish you had “looked before you leaped.” The time you bought furniture that would not fit through the doorway, or the small tree you planted whose roots are now cracking through the sidewalk. Don’t you wish you had given more thought to these changes and their effects BEFORE they were implemented? Well, an impact analysis can help.

An impact analysis is a systematic approach for evaluating changes to a system. It looks at the new feature, enhancement, or problem to be fixed, the underlying reason for change or root cause, and the proposed solution in terms of the existing system and its constraints and requirements. It is part of an overall…

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Welcome to the 2H Initiative

Friday, March 01, 2013

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exida has been gathering field failure data for over a decade.  At this point we have a total of over 60 billion unit operating hours of field data for process industry devices.  This data has been used to create a 2H database for components.  exida is using this component database in the FMEDA process to predict Route 2H compliant failure rates for derivative devices used within the defined environmental profiles. 

Route 2H is an alternative to the 1H method which uses minimum hardware fault tolerance tables based on safe failure fraction concepts.  Route 2H has simplified minimum hardware fault tolerance requirements that are identical to the IEC 61511 proven in use requirements.  exida will apply the 2H approach on…

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The Real PFDavg Variables

Wednesday, February 27, 2013

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I was recently asked how the PFDavg value in an exida FMEDA report was calculated. Those PFDavg values are calculated using the exSILentia^TM program. The person who posed the question was using the equations from Part 6 of IEC 61508 and got a different value. Why?

exSILentia does a detailed calculation using discrete time Markov models and considers all realistic variables including:

• All dangerous failure rates
• Proof test intervals
• Bypass time
• Proof test coverage
• Maintenance capability
• Other application parameters

This is all done semi-automatically making it very easy to do professional verification calculation.

The equations in Part 6 of IEC 61508 are just simplified equations that consider only a subset of the important variables.…

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Why Personnel Functional Safety Certification Programs are Needed

Friday, February 22, 2013

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The publication of the international functional safety standards IEC 61508 (1999), IEC 61511 / ISA-84 (2004) and IEC 62061 (2005) were significant milestones in the effort to increase equipment and process safety. ^1,2,3  Unfortunately, the release of safety standards does not by itself make the world a safer place.

Analysis of past process safety incidents underscores the role that people can play in preventing these accidents. A study by the UK’s Health and Safety Executive (HSE) showed that most safety incidents could have been prevented.  It found that the number one cause of accidents was incorrect and incomplete design specification (44%). Another 15% was from improper design & implementation. The study also found that a significant percentage of…

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It’s Just a Little Change…

Tuesday, February 19, 2013

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Once there was a laundry room with old, worn-out linoleum floor.  It was functional but did not have the look and feel of newer homes.  The lady of the house decided to replace the flooring with new ceramic tile.  She engaged her husband with the idea, outlining the benefits of this enhancement, and coaxed him to agree.  While tedious and time consuming, he was willing to save some money and do the work himself.  The material was selected and ordered.  When installation time arrived, the washer and dryer had to be moved out temporarily, which required some effort as the appliances just barely fit through the narrow doorway.  The husband removed the old floor, placed the alignment marks down, and…

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Stiction: Fact or Fiction & Start Time Prediction

Thursday, February 14, 2013

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Click here to read the blog “What is Stiction?”

How Does Stiction Affect Me?

Stiction is a phenomenon that is beginning to become an accepted theory among engineers.  It is no longer a question of is this happening, but how does it happen and when does it start to affect me?  Different experts find this in varied ways, but are all honing in on the same time frame.

Mechanical engineers and technicians who routinely work with solenoid valves are familiar with the experience of trying to “stroke”, i.e., move through its range of motion, a valve that has…

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How Does Stiction Affect Me?

Tuesday, February 12, 2013

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Click here to read the blog “What is Stiction?”

In normal operation of a solenoid valve, the O-rings in the valve create a smooth transition while the plunger or spool is in motion. Because the O-rings are in direct contact with the valve chamber walls, in order for the plunger/spool to begin to move, it must first overcome the stiction between the O-rings and the chamber walls.  Once in motion, the plunger/spool must overcome the sliding or dynamic friction between the O-rings and walls, however since the dynamic friction is usually significantly less than the nominal level of stiction, it is generally not of concern.  By design the magnetic and…

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What is Stiction?

Thursday, February 07, 2013

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Many studies and organizations have defined stiction, or static friction, in different ways. A few examples of the various definitions are as follows:

• According to Entech [1], “stiction is a tendency to stick-slip due to high static friction. The phenomenon causes a limited resolution of the resulting control valve motion. ISA terminology has not settled on a suitable term yet. Stick-slip is the tendency of a control valve to stick while at rest and to suddenly slip after force has been applied.”
• The Instrument Society of America (ISA)(ANSI/ISA-S51.1-1979) [2], described “stiction as the resistance to the start of motion, usually measured as the difference between the driving values required to overcome static friction upscale and downscale.” The…

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How Do You Handle Interposing Relays?

Tuesday, February 05, 2013

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Many times in the design of a safety instrumented system it is necessary to utilize an interposing relay. Typically there is not a certified relay available. Since no certified product is being used it is important to follow the requirements to justify prior use and also to design the circuit in such a way as to minimize the dangerous failure rate. Users of the exSILentia software tool can find a proven in use template in the software and an example is shown below (Figure 1). Those not utilizing exSILentia should develop a standard template to document the justification of non-certified equipment.

If the Proven in Use justification is sufficient then…

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New Working Group (WG7) Formed for Alarm Management

Wednesday, January 30, 2013

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The ISA-18.2 committee on alarm management has launched a new working group (WG7) that is focused on developing a standard, recommended practice, or technical report on the application of alarm management to process plants utilizing multiple packaged equipment systems. The work will be based on and complement the existing ANSI/ISA-18.2 standard “Management of Alarm Systems for the Process Industries.” (2009)

The objective of this group is to define alarm management practices for plants that utilize packaged process equipment systems. In such a plant it is often very challenging to achieve consistency in alarm system design and operation. (Think of how difficult it is to bring alarm data from disparate systems into a common operator HMI in a way…

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Understanding Our Customers – One Step Closer to Excellence

Monday, January 28, 2013

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Customer service isn’t about slashing prices or offering great rates.  Sure these types of strategies can promote new customers and generate business, but how often do the “new customers” return if the service or product didn’t meet their expectations?  It’s always said that a great sales person can sell anything to anyone once.  However, it’s not until a customer returns for additional service(s) when you know expectations have been met.

So how do you achieve more than great customer service?  The answer is to “Understand” the customer.  A good company can achieve good customer service, but a great company KNOWS their customer’s needs.  Asking a few key questions and the willingness to hear the honest truth will help promote…

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2012 - Good Progress for Cybersecurity and Functional Safety

Thursday, December 20, 2012

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I think it is wise for individuals to periodically review things. I like to do my professional review at the end of the year. 2012 was a good year.

Product Certification

Over 60 new products received functional safety or cybersecurity certification this year. Those products and more can be found on our Safety Automation Element List. Most significant to me are the new product categories including:

• Microprocessors
• Integrated circuits
• Middleware packages
• Complete safety systems

Many new manufacturers are using exida as their functional safety Certification Body. Even manufacturers who have an older certificate from another agency have come to exida for FMEDA analysis or new certificates.

exida Certification has expanded its scope…

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Replace Before Failure

Thursday, December 13, 2012

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The solenoid valve manufacturer provides an FMEDA report that lists “useful life = 12 years.”  Useful life is a reliability engineering term that indicates a point in time when the failure rate of a device will begin increasing rather rapidly.  Why do we care about this time interval?  That means that any safety function equipment operated past that point in time will not achieve the designed risk reduction factor. The IEC 61511 standard states that our maintenance procedures must require that all equipment be replaced before the end of its useful life rating.  OK, there is a good reason for this.

Then we ask the maintenance supervisor to sign the purchase requisition for the new device.  First response is a…

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Spaces and Braces and Comments – Oh My!

Thursday, December 06, 2012

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What constitutes a good coding guideline?

Serious software development for functional safety requires a coding standard or coding guideline. Most organizations have one, but how do you know if it’s good?

You need to look beyond the boilerplate and physical code layout that are often defined in a coding guideline. These are needed and useful, but they are mostly aesthetic. You need to look to the “meat” of the guideline.

A good coding standard should:

• Be specific for the selected language
• Identify good practices to be used and unsafe practices to avoid
• Be accepted and used by the developers and enforced by code reviews
• Cite module size and complexity metrics to be met
• Specify defensive programming practices to…

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Tool Qualification and Safety Standards

Thursday, November 29, 2012

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When building a product such as an anti-lock braking system for an automobile, or a railroad, or process control safety system, making sure that the product works as specified is a big part of functional safety. When it is time for the system to engage to prevent an accident, you need to know that the system is going to work almost all of the time. One area that is often overlooked in the effort to ensure that the system works properly is the effects of software tools used in the development process. Examples of such tools include:

• Compilers
• Static checkers
• Verification tools
• Test automation tools

Since these tools are usually purchased from a third party, they are normally…

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