Stiction webinar 2013

Improving Reliability & Safety Performance of Solenoid Valves

by Stroke Testing

exida Web Seminar

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Copyright © 2000-2013 exida

Improving Reliability & Safety Performance of Solenoid Valves

by Stroke Testing

Web Seminar April 24, 2013

Loren L. Stewart

exidaSellersville, PA USA

Audio is provided via internet.

Please enable your speaker (in

all places) and mute your

microphone.Copyright © 2000-2013 exida

Loren StewartBio

Loren Stewart graduated from Virginia Tech with a BSME.

She has 5 years of professional experience. She currently works for exida consulting as a safety engineer, focusing on the mechanical aspects of their customers. Along with assessing the safety of products and creating FMEDAs and reports, she researches stiction and is creating a database for the 2H initiative according to 61508.


4

William Goble has over 40 years of professional experience. His areas of expertise include safety and high availability automation systems, automation probabilistic analysis, new product development and market analysis. He developed many of the techniques used for probabilistic evaluation of safety and high availability automation systems. He was formerly Director, Critical Systems at a successful North American safety company where job duties included marketing, research and development including computer design, software design and development and engineering project management. He has written three books on topics of safety and reliability modeling. He is a fellow member of ISA. He has published many papers and magazine articles. Dr. Goble has a BSEE from Penn State, a MSEE from Villanova and a PhD from Eindhoven University of Technology in Eindhoven, Netherlands.


Dr. William Goble

network of excellence in dependable automation

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Asia Pacific

North America

Developmentformer Development Managers(Siemens Moore )

Instrumentation Engineers (UOP, BAYER, PDVSA, etc.)

Application Design + Operation

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former TÜV Managersformer Safety PLC Product Manager(Siemens)


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Engineering Tools SILAlarm - Alarm Management Tool PHAx – HAZOP Tool / Import SIL Selection Tool

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Cost effectively implement functional safety standards

SRS Tool SIL Verification Tool

Direct average calculation engine Instrumentation failure database built-in Variables include reality – test coverage, service

Proof Test Tool SILStat – Field Data Collection Tool


Reference Material exida authored most industry references for automation safety and reliabilityexida authored industry data handbook on equipment failure dataexida authored the most comprehensive book on functional safety in the market.

www.exida.comCopyright © 2000-2013 exida

Improving Reliability & Safety Performance of

Solenoid Valves by Stroke Testing


Overview• Mechanical Failure Rates• Cycle Testing – scaling cycle failure values• What is Stiction?• Evidence of Stiction Analyzed• Assessing Solenoid Valve Stroke Testing• Implications of Stroke Testing and Stiction

• Recommended Best Practices Copyright © 2000-2013 exida

Mechanical Device Failure Rates• Cycle Testing

• Accelerated motion testing until failure or a given number of cycles, used to simulate years

• For high or constant demand applications only

• FMEDA – Failure Modes Effects and Diagnostic Analysis

• Identifies the failure mode within the device which could relate those failures to the operation of the safety instrumented function

• For both high and low demand ‐ all applications


Cycle Testing• Cycle testing is used to estimate failure rates in dynamic (high demand) applications.

• Assumption is that premature wearout is the dominant failure mechanism and that no other failure mechanisms are significant.


Cycle Test Scaling


•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.

14Copyright © 2000-2011 exida

B10d Failure Data

•Table of example values from ISO 13849-1: 2006

Cycle Test Scaling


•The B10 number of cycles is converted to a time period by knowing the cycles per time period in any particular application.

•A failure rate is calculated by dividing the 10% failure count by the time period.

High Demand CertificationsSome certifications are based on failure data derived from “cycle testing” or other methods that require frequent movement of electro-mechanical products. This assessment is not valid for typical low demand process applications.


10 demands per year =

870 hours per cycle

What time period is valid?


•A failure rate is calculated by dividing the 10% failure count by the time period.•One year? 8760 hrs.•One month? 730 hrs.•One week? 168 hrs.

•How long can a device with a seal remain motionless before other failure mechanisms become significant such that premature wearout no longer dominates?

What is Stiction?

• Stiction – Static + Friction

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

• Can result from: corrosion, cold welding, break down of lubrication, build‐up of deposits, chemical reactions, breakdown of the sealing components…


Who cares about Stiction?• Started studying stiction to determine the maximum time period before stiction impacts failure rates.

• This is the maximum time period for scaling cycle test results.


Evidence of Stiction Analyzed• Expert Knowledge

• Technicians and engineers routinely work on valves• Stiction occurring after a month or more

• O‐ring Manufacturer's Guides• Stationary between 1 week and 1 month• “Delay between cycles” plateaus at approximately 300 hours

• Experimental Study • Studies conducted on lubrication thickness in magnetic thin‐filmed disks. • As equilibrium rest time increases, stiction increases to a plateau or

around 275 hours, depending on lubrication type

• ISO 13849• “Valve must be operated at least once per week or once per shift to insure

the intended function”


Impact of Stiction • We discovered that stiction becomes significant after one week therefore never scale cycle test results beyond 275 hours.

• We also realized that improvements in safety and reliability can be obtained with stroke testing.


Solenoid Background• Solenoid Valves

Open Solenoid Valve Closed Solenoid Valve

Cross section of an open and closed solenoid valve, showing how the plunger movement directly controls the process fluid flow.


Impact of Stiction • We also discovered that once stiction is overcome via movement, bonds creating the binding are destroyed and must start reforming.

• Even with a small movement.


Assessing Solenoid Valve Stroke Testing

Values of failure rates from the solenoid valve (manufacturer X, model Y) used in computing PFDavg

( ) p g g

Parameter

failures/109 hrs operation λDstiction 103.7

λDnon-stictionDetectable 84.3 λDnon-stictionUndetectable 1.9


Assessing Solenoid Valve Stroke Testing• Case 1: No Valve Stroke Testing

• λD = λDStiction + λDnon-StictionDetectable + λDnon-StictionUndetectable

• λD = 103.7 + 84.3 + 1.9 = 189.9 fits

• Case 2: Infrequent Valve Stroke Testing• λD = λDStiction + λDnon-StictionDetectable + λDnon-StictionUndetectable

• λD = 103.7 + 84.3 + 1.9 = 189.9 fits

• Case 3: Frequent Valve Stroke Testing• λD = λDnon-StictionDetectable + λDnon-StictionUndetectable

• λD = 84.3 + 1.9 = 86.2 fits


Implications of Stoke Testing and Stiction

Case 1: proof testing every 2 yearsCase 2: valve stoke testing performed once every 6 monthsCase 3: valve stoke testing performed once every week


PFD

PFDavg

Implications of Stoke Testing and Stiction

Case 3: valve stoke testing performed once every week

0.00E+00

2.00E‐04

4.00E‐04

6.00E‐04

8.00E‐04

1.00E‐03

1.20E‐03

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

PFD avg (t)

Time in years

Case 3

Upper Limit SIL 3


Implications of Stroke Testing and Stiction


Recommended Best Practices

• Implement automatic or semi‐automatic stroke testing in final element designs.

• Make certain that the movement in actuator‐valve assemblies is small so as to not cause process disturbances.

• Do not let actuator pressure drop too far as stiction bonds might let loose can cause a false trip.


Recommended Best Practices

• Begin valve stroke testing on a weekly basis unless maximum cycle ratings are below 52 * useful life (years)

• After a few months of testing, if the number of times the valve is found stuck, cut the testing period in half

• Repeat the testing procedures until an optimum test interval is identified.


Recommended Best Practices• Valve stroke testing can have significant beneficial impacts on safety

• Most beneficial practice would be a frequent valve stroke testing of once per week or more.

• We believe there is a reduction in false trip rate as well but need more research.


Future Web Seminar: Design Options for Partial Stroke Testing of Final Element

Assemblies, May 2013

Questions ? Comments?More Information:1. Free Web Seminars – see www.exida.com 2. White Papers3. Safety Automation Equipment List – www.sael-

online.com4. Books: www.isa.org, www.exida.com

Email me at: [email protected]


Stiction webinar 2013

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Transcript of Stiction webinar 2013