exida explains Blog

Entries tagged with: Loren Stewart

Back to Basics 21 – The B10 Method

Back to Basics 21 – The B10 Method

The B10 method uses cycle test data to predict failure rates. 

A cycle test is done on a set of products (>20) until 10% of the units under test fail. The number of cycles until failure is called the B10 point.

The B10 number of cycles is converted to a…

Read More...

Back to Basics 22 – Cycle Testing

Back to Basics 22 – Cycle Testing

A cycle test is done on a set of products (>20) until 10% of the units under test fail. 

The number of cycles is converted to a time period by knowing the cycles per hour in any particular application. 

A failure rate is calculated by dividing the 10% failure…

Read More...

Back to Basics 23 – Stiction

Back to Basics 23 – Stiction

What is Stiction? 

stiction

Stiction is the resistance to the start of motion usually measured as the difference between the external force being applied in order to overcome the static friction and the force to maintain movement between the two contacting or working surfaces.

It can…

Read More...

Back to Basics: Failure Rates

Back to Basics: Failure Rates

Failure rates are the number of failures per unit time for a piece of equipment which are usually assumed to be a constant value. They can be broken down into several categories, such as safe and dangerous, detected and undetected, and independent/normal and common cause. Failure rates are often…

Read More...

Back to Basics: Failure Rates - FITS

Back to Basics: Failure Rates - FITS

Failures ITime or Failure UnIT

FITs is the number of failures per billion hours for a piece of equipment. 

It is mentioned in both IEC 61508 and IEC 61511 standards as a preferred unit of measurement expressed by 10-9 hours.

Example: 5 FITs is expressed as 5 failures every 10-9 hours (5x10-9). 

When you…

Read More...

Back to Basics: Failure Rates - λ

Back to Basics: Failure Rates - λ

The Greek symbol lambda, λ, represents failure rates in functional safety, usually expressed in the unit of measurement of FITS.

λ can be expressed as a total failure rate for a device (λT), or it can be broken down into more specific groupings:

  • Safe detected (λSD)
  • Safe undetected (λSU)
  • Read More...

Back to Basics: Failure Rates - λD

Back to Basics: Failure Rates - λD

The Greek symbol  λD represents dangerous failure rates in functional safety, usually expressed in the unit of measurement of FITs, and can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λD is the number of dangerous failures per…

Read More...

Cycle Testing for Static Applications? NO!

Cycle Testing for Static Applications? NO!

The failure rates of certain mechanical components used in solenoid valves, actuators, and valves vary substantially depending on operation. Seals such as O-rings, for example have fundamentally different failure modes when used in applications with frequent movement (dynamic) versus applications with infrequent movement (static). 

Static is generally…

Read More...

Dare to Be Different

Dare to Be Different

exida’s Safety Reliability Analysis (SRA) and How It May Help You

exida’s Safety Reliability Analysis (SRA) and How It May Help You

We all know that an FMEDA is only as good as the assumptions made regarding typical design engineering practices and the database used for the failure rates.  Here is the catch, not all products are designed and manufactured per the same rules!  Many manufacturers boast that their…

Read More...

FMEDA Predictions
  • by Loren Stewart, CFSE
  • Thursday, September 01, 2016
  • Certification

FMEDA Predictions

The FMEDA technique is performed on a specific device (e.g., ball valve, pressure transmitter, temperature sensor, electronic module, etc.) specified down to the manufacturer and series/model. Based on the specifics of the design, the parts used to execute the design, the design margins, any automatic diagnostics, the…

Read More...

How can I improve my SIL?

How can I improve my SIL?

Click here to read the first entry in this blog series (What is SIL compliance?)

Click here to read the second entry in this blog series (How is SIL Used?)

As we now know, a Safety Integrity Level (SIL) can only be given to a…

Read More...

How Does Stiction Affect Me?

How is SIL Used?
  • by Loren Stewart, CFSE
  • Tuesday, September 22, 2015
  • Certification

How is SIL Used?

Click here to read the first entry in this blog series 

Once the Safety Integrety Level (SIL) of a product is found, it will be used FOUR ways:

  1. To establish risk reduction requirements
  2. Probabilistic limits for hardware random failure
  3. Architectural constraints 
  4. Read More...

Outer Banks: How Could This Massive Power Outage Happen?

Outer Banks: How Could This Massive Power Outage Happen?

I, like many others, was stunned to hear that there was a major power outage covering the Outer Banks. Being an engineer, my instincts were to immediately investigate what happened and what steps occurred to lead to that point. Even before I did research, my mind was already asking…

Read More...

Setting Goals for 2019: Are You Thinking About Functional Safety?

Setting Goals for 2019: Are You Thinking About Functional Safety?

When I went to set my goals for 2019, I set both personal and professional goals. One of my professional goals is to clean off my desk, but another is to get more people to think about functional safety before an accident occurs, instead of when they are normally…

Read More...

SILSafe Data Website

SILSafe Data Website

exida has launched the web site www.silsafedata.com. SILSafe Data is a web site listing a number of product categories used in process control and the expected range of failure rates for process industry applications. The use of realistic and application appropriate failure rate data has a significant…

Read More...

Stiction: Fact or Fiction & Start Time Prediction

The exida Scheme

Today I was asked: “What is the exida Scheme?”  If one person had this question, it is likely that others may be interested as well.  Therefore, I decided to take a moment and explain the exida structure.  Any Certification Body operates each product certification per a “scheme,” which references…

Read More...

The Undetected Detective; PVST

The Undetected Detective; PVST

Have you ever wondered why some dangerous failures are classified as undetected and others are classified as detective?  Aren’t dangerous failures all dangerous? Why do we classify them differently?

Let's say we have a plant that is running in normal operation mode and the system needs to trip. The…

Read More...