RELIABILITY & AVAILABILITY
Within The Facility
presented by Geoff Cope, PE
Reliability & Availability for Mission Critical Design
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Reliability & Availability Basics
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Reliability & Availability Modeling
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Practical Examples
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Conclusions
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Questions & Answers
Reliability & Availability for
Mission Critical Design
Reliability/Availability study is used for making comparisons.
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Industry standards
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Infrastructure architecture (making design decisions)
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Infrastructure improvements (Before & After)
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Identify most important component (Weakest Link)
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Helping with financial decisions (reliability Vs cost)
Reliability & Availability for
Mission Critical Design
Industry Standard “Uptime Institute”
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Developed Tier Classifications
 Tier I & Tier II may have single points of failure (99% available)
 Tier III may have some single points of failure (99.9% available)
 Tier IV has no single points of failure (99.99% available)
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Recently modified white paper to include electrical distribution concepts
 Tier III & Tier IV are provided with dual corded mechanical systems and
redundant pathways.
Reliability & Availability for
Mission Critical Design
Infrastructure Architecture
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System Architecture
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UPS Systems
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Stored Energy Systems
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Mechanical Systems
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Single Point of Failure (SPOF)
Reliability & Availability for
Mission Critical Design
System Architecture
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Single System (N)
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Multi-unit systems
 Parallel System (N)
 Parallel Redundant (N+1, N+2, etc…)
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2N or System + System
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2(N+1) two parallel redundant systems
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Isolated redundant system
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Catcher System
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Distributed Redundant System
These configurations greatly affect the reliability of a system
Reliability & Availability for
Mission Critical Design
UPS Systems
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Double Conversion
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Rotary
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line interactive
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Off-Line
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Low Voltage
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Medium Voltage
UPS types do not generally affect the reliability analysis for Tier III & Tier IV Systems.
Reliability does not equal power quality.
Reliability & Availability for
Mission Critical Design
Electrical System Stored Energy
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Chemical Batteries (wet cell, dry cell)
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Fly Wheels
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Super Capacitors
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Fuel Cells
Mechanical System Stored Energy
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Ice Storage
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Chilled Water Storage
Stored Energy Approaches may have a significant impact.
Reliability & Availability for
Mission Critical Design
Mechanical Systems
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Reliability modeling is not typically used
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Data is not as readily available for mechanical components.
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Useful models can be made
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Reliability level on mechanical system is significantly lower than electrical system
Reliability & Availability for
Mission Critical Design
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A single point of failure (SPOF) is a point within a system that will bring the system’s process to a
halt.
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Common SPOFs include:
 EPO systems
 Single module systems
 Parallel redundant systems
 Automatic transfer switches
 Shared infrastructure space
Reliability & Availability for
Mission Critical Design
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A facilities reliability is dependant on the number of single points of failure within a system.
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If your facility can pass the following tests, then you have a highly reliable facility.
 Shotgun
 Chainsaw
 Fire bomb
 Hand grenade
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Eliminate Single Points of Failure to Improve reliability
Reliability & Availability for
Mission Critical Design
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The only way to survive a catastrophic facility failure is to have:
 Redundant Staff
 Data Mirroring Capabilities
 Geo-Redundancy (A Redundant Facility)
Reliability & Availability Basics
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Reliability Vs Availability
 757 Example:
 A Boeing 757 is a very reliable aircraft in which the jet may fly halfway around the world two or
three times and is then placed into a maintenance shop for one to two weeks for maintenance.
This aircraft is very reliable, but due to the amount of required maintenance, it has a low
availability.
Reliability & Availability Basics
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Bathtub curve shows failure over various region of life. It is a function describing the variation
of hazard rate with time ( life of a unit)
Reliability & Availability Basics
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Reliability: probability that an item will perform its function under given condition for a stated
time interval . . . It is not a guarantee!!
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Availability: Probability that an item will perform its required function under given conditions at
a stated instant in time
Steady-State Availability
Or :
Reliability & Availability Basics
MTTR & MTBF
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Mean Time To Repair = MTTR (1/μ)
It is the mean time to restore the system to an operating condition
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MTBF = Mean Time Between Failures (1/)
 λ = Failure rate (probability that an item will
fail in the time (t0, t0+Δt) giving the item is
operable at time t0
Reliability & Availability Basics
MTBF
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MTBF is also the Statistical point at which 63% of a large homogenous population of
items will fail
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Example: a homogenous population of UPS's have an MTBF of 250,000h. This means
that in about 28 years 63% of the UPS’s have failed to perform their function
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MTBF has nothing to do with Lifetime. MTBF significantly exceeds the lifetime.
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Example: Current word Average Life Expectancy is 66 years. The U.S. has a failure
(death) rate of 9.6 people per 1000 persons/year, therefore an MTBF of 104 years!!
Reliability & Availability Basics
Reliability Vs Availability
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Reliability is time dependant
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The longer the time, the lower the reliability, regardless of the system design.
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The availability of a system is always calculated the same regardless of past or future events
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An availability of 99.9% could mean that a facility was down for 8.76 hours per year or suffer
525 one minute outages per year.
Reliability & Availability Basics
Reliability
Not a Commonly used Metric for Data Center Discussions
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People are used to hearing about 5-9s of availability.
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Reliability numbers are much lower (2-9s).
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Reliability numbers are directly proportional to a period of time.
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Reliability numbers will indicate how often a facility’s infrastructure requires
maintenance.
Reliability & Availability Basics
Availability
Common Misuses of the Availability Metric
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Extracting the mean downtime (MDT) from Inherent Availability
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Equating the hours of downtime per year. If a facility has 99.999% availability, it implies
that the facilities downtime may be 5.26 minutes per year, but a better assumption
would be that it may suffer a 63 minute outage over a 12 year life expectancy of the
facility infrastructure.
Reliability & Availability Basics
System Reliability
 A system is a collection of components arranged according to a certain structure.
 Subsystem: unit or device that is part of a larger system
 Reliability modeling:
- Analyzing components and their relationships.
- Relating components states (working or failed) to system state.
 System reliability analysis tools:
- Reliability Block Diagram (RBD)
- Fault Tree Analysis (FTA)
Reliability & Availability Basics
Other Factors to Consider
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Maintainability: Probability that an item can be repaired in the interval (0, t). If all single
points of failure are eliminated, in most cases the system will be maintainable.
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Scale-ability: A system should be design such that it can be increased in capacity so that the
infrastructure can grow with the load.
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Flexibility: Design your facility such that the electrical and mechanical systems can be easily reconfigured to adapt to changing technologies.
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Simplicity: Keep it Simple. The more complex a system is, the more unreliable the system will
become.
Reliability & Availability Basics
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Inherent Availability is used in lieu of Operational Availability
 Considers only down time for repair of failures
 Excludes unscheduled and scheduled maintenance
 Excludes Logistics Time: Part & Technician Availability
 Excludes delays related to schedule
 Is useful as a metric for analyzing the system design as uncontrolled parameters need to
be kept the same.
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By eliminating all of the variations, it is possible to calculate the inherent availability of a system
regardless of all logistic matters.
Reliability & Availability Modeling
Assumptions: Binary Reliability Analysis
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The system has n components;
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A component may only be in two possible states, working or failed. Xi is used to indicate the
state of component i.
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The system may only be in two possible states, working or failed; Φ(X) is used to denote the
state of the system (Structure function). The state of the system is completely determined by
the states
of the components.
Reliability & Availability Modeling
Typical System Reliability Models
 Series systems: System fails if one component fails.
 Parallel systems: System works if there is one component working. Paralleled systems always
share a single point of failure.
Reliability & Availability Modeling
Typical System Reliability Models
 Series-parallel systems:
 Parallel-series systems:
Reliability & Availability Modeling
Typical System Reliability Models
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K-out-of-N systems
(NODES)
 Example: UPS Set
Reliability & Availability Modeling
Modeling Examples
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Existing infrastructure
 2N UPS
 2N Generator Plant
 2N UPS Distribution
 N+2 Mechanical System
 Mechanical equipment single corded
 System Availability = 99.99%
Modeling Examples
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Identified single points of failure
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Identified most important component
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Made Recommendations
Modeling Examples
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Proposed infrastructure
 Eliminate single points of failure
 Eliminate most important component “ATS 4”
 Dual cord the Mechanical equipment
 System Availability = 99.995%
Modeling Examples
Model Redundant Utilities
 Small improvement on availability
 System Availability = 99.997%
 Expensive to implement
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Recommended not to implement
Modeling Examples
Program Requirements
 2N Utility
 2(N+1) UPS
 N+2 Generator Plant
 2N UPS Distribution
 N+2 Mechanical System
 Dual Corded Mechanical Equipment
 Modeled System Availability = 99.9995%
 Cost: $20,000,000
Modeling Examples
Distributed Redundant Option
 2N Utility
 N+1 UPS
 N+2 Generator Plant
 2N UPS Distribution
 N+2 Mechanical System
 Dual Corded Mechanical Equipment
 Modeled System Availability = 99.9992%
 Cost: $16,000,000
Modeling Examples
Medium Voltage Design
 2N Utility
 2N UPS
 N+2 Generator Plant
 2N UPS Distribution
 N+2 Mechanical System
 Dual Corded Mechanical Equipment
 Modeled System Availability = 99.9994%
 Cost: $18,000,000
Modeling Examples
Mechanical system
 2N Mechanical system
 Modeled System Availability = 99.97%
Modeling Examples
Mech. & Elec. System
 2N Mechanical system with 2N Electrical system
 Modeled System Availability = 99.92%
RELIABILITY VS COST
Conclusion
Questions & Answers