End of Life Expectancy for Final Control Elements in Safety and Control Systems

by | Nov 20, 2020 | Safety, Valves, Actuators & Regulators

Riyaz Ali

Senior Director, Valve Instruments

Bathtub curveMechanical items, such as shut down valves, isolation valves, control valves, actuators and other pneumatic accessories, if selected and sized properly during the design stage to meet process parameters/conditions, fluid comparability, environmental conditions, etc., will typically follow a bathtub curve aging lifecycle.

The bathtub curve consists of three periods: an infant mortality period with a decreasing failure rate followed by a normal life period (also known as “useful life”) with a low, relatively constant failure rate and concluding with a wear-out period that exhibits an increasing failure rate.

Failures during infant mortality are highly undesirable and are always caused by issues such as material defects, design blunders, errors in assembly, etc. Normal life failures are normally considered to be random cases of “stress exceeding strength.” However, many failures often considered normal life failures are infant mortality failures. Wear out is caused by fatigue or depletion of materials, such as lubrication depletion in bearings. A product’s useful life is limited by its shortest-lived component.

Emerson’s Fisher valve and actuation technology products have been in the field for many decades. Design improvements made over time as products mature, increase ruggedness and reliability to provide extended life in the field. In fact, I just got a request from customer for a spare part (linkage between positioner and actuator) for a Fisher control valve supplied in 1974. This customer still insists that this valve is working fine but that the external linkages need replacement due to corrosion. You can imagine the reliability of these mechanical items.

Manufacturers and producers are typically focused on reliability centered maintenance (RCM) to increase performance in plant availability. Reliability engineering concepts has been in these industries for a long time. However, reliability has become more prominent in the past two decades due to the use of electrical/electronic/programmable logic devices. These are used for field assets and control system, which have been migrating towards smart microprocessor-based technology for better accuracy, more precise control, easier operation and maintenance, diagnostics, and more.

Unlike mechanical failures, electrical, electronic or microprocessor-based failures are categorized to ensure failure data can be organized in a consistent way.

ISA Technical report ISA-TR84.00.02-2002 –Part 1 talks about two failure modes:

  • Physical (random) failures
  • Functional (systematic) failures

For these failure modes:

  • Physical or random failures result from the degradation of one or more hardware mechanisms. These are often permanent and attributable to some component or module. For example, when a control valve is at the end of travel and not moving with the change in the control signal due to a broken shaft, the failure has occurred because of a physical failure of the component in the valve.
  • Functional or systematic failures are failures related in a deterministic way to a certain cause, which can be eliminated by a modification of the design or manufacturing process, operational procedures, or other relevant factors. For example, a computer program has crashed and there is no physical damage, but the system has failed. The result is that the program is not working, and a failure has occurred due to a systematic error in programming code.

Mechanical components’ life is a function of applications, process fluid, fluid physical properties, operational history, maintenance records, environmental conditions, test philosophy, etc. However, preventive maintenance practices play an important role in extending the life of mechanical items. Especially if they are re-built correctly each time during a shutdown/outage/turnaround back to original manufactures specifications.

The same is applicable for electrical, electronic or microprocessor-based field devices, where preventive maintenance, specifically of software parts, should be done at each interval recommend by the manufacturer, to extend their useful lives.

During failure modes, effects, and diagnostic analysis (FMEDA) for EUC (Equipment under control), the failure rates are calculated and rough benchmarks can be established for the life expectancy as per below formula.

Mean time to failure (MTTF) = Integration of Reliability function with respect to time from 0 to infinity limit = ∫ R(t) dt

or simply

1/(sum of all the part failure rates) = 1 /λ {Assumption: Single or series of components with constant failure rates}

To summarize, mechanical equipment such as shutdown valves, control valves, actuators, and accessories such as positioners are designed keeping in mind a plant’s life of around 25 years. Though you may not find any written or stated document from any supplier or manufacturers since mechanical deterioration depends on many parameters like internal process-related conditions or external environmental conditions, operating media (air, hydraulic fluid, etc.), proper care exercised during application selection & sizing, and more.

Visit the Valves, Actuators & Regulators section on Emerson.com for more on these technologies and their designs for long life. You can also connect with other final element experts in the Valves, Actuators & Regulators group in the Emerson Exchange 365 community.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.