API 521 7 th Edition Ballot Item 6.8 Work Item 54 Global Air Failure Background Work Item 54: # Source Section Comment Proposed Change Volunteer 54 Fall 2014 meeting Global instrument air failure scenario FC valves with instrument air actuators (local bottle with check valve to I/A supply) instead of spring loaded To be determined B. Otis / M. Porter / J. Golla / S. Verma / H. Attal / R. Wissbaum Background See attached presentation from API 521 Spring 2015 meeting by Brad Otis given after the proposed modifications.: Proposed Modification to API 521 6 th Ed 4.4.15.4 Loss of Instrument Air or Electric Instrument Power 4.4.15.4.1 General The loss of instrument air drives all air-operated valves to their specified fail position. This action of many valves can result in overpressure if the specified failure positions of the valves are not selected to prevent overpressure. Likewise, failure of electric instrument power can drive control systems and electrically operated valves to their specified failure positions. Consideration should be given to the effect on flare or vent system loading of valves failing open or closed due to instrument air failure or power failure. 4.4.15.4.2 Cautions for Double Actuated Valves The above guidance in 4.4.15.4.1 is based on decades of experience using air-operated control valves that rely on springs to move the valve to its specified failure position. Double actuated valves use instrument air to drive the valve to its specified failure. Typical designs have an instrument air pressure reservoir (air bottle) and all designs utilize pilot valves to re-route the instrument air to drive the valve to its failure position. Double actuated valves can be less likely to move to the specified failure position than spring valves (see latent failures 4.2.4). The user should consider the possibility that all double acting valves may not go the specified fail position during a loss of instrument air. Factors that may influence whether the user should consider this depend on the number
of double actuated control valves employed within the system, inspection and maintenance programs for double actuated control valves, and the owner s risk tolerance criteria.
API 521 POTENTIAL WORK ITEM: INSTRUMENT AIR FAILURE Author: B. A. Otis PTS/TTSM October 10, 2015
AGENDA Current Practices Issues Proposed Guidance Conceptual Scenario Assessment Spring Loaded Valve loss of instrument air Double Actuated Valve loss of instrument air Proposed API 521 Language
CURRENT PRACTICES Individual Instrument Air Failure Assess each control valve individually moving to its failure position Global Instrument Air Failure Assume all control valves simultaneously move to their failure position Usually this does not result in reliefs since control valve failure positions are normally specified to de-escalate the event
ISSUES Current API guidance for total instrument air failure assumes fairly high probability that valve moves to its failure position when air is lost Double actuator valves (spring with air assist or springless designs) may have an a PFD an order of magnitude higher than spring only valves Double actuator valves have a potential to be driven open during an instrument air failure
PROPOSED GUIDANCE - CONCEPTUAL Inform the user that double actuated valves may be less likely to move to their failure position upon loss of air than spring actuated valves. However, given that other overpressure scenarios are evaluated, it is unlikely that this potential will be the controlling case for relief device and/or flare system sizing. Inform the user that double actuated valves have a potential to go wide open upon total loss of instrument air. This is a possible but remote scenario. It is also conceivable that this mode of failure for some systems could be the governing case for relief device sizing.
SCENARIO ASSESSMENT Disclaimer Types of components and their PFD vary Stated PFD values are provided for illustration purposes ONLY.
SPRING ONLY ACTUATED VALVE: NORMAL CONDITIONS
SPRING ONLY ACTUATED VALVE: LOSS OF AIR
POTENTIAL FOR THE VALVE TO NOT FAIL CLOSE Failure modes that would result in the spring loaded failclosed valve not closing during an instrument air failure Stuck valve stem or actuator PFD (assuming clean instrument air):.00349 = (1/289 demands) if stroked every 4 years.00087 = (1/1100 demands) if stroked every year
DOUBLE ACTUATED VALVES Drive to reduce costs/weight results in using more double actuated valves instead of spring only actuated valves Some actuators have spring with air assist Some actuators have no springs (rely completely on air pressure)
DOUBLE ACTUATED VALVE RELIABILITY Designs can be complex (below) but we will discuss a simple design
SIMPLE DESIGN
LOSS OF INSTRUMENT AIR: INTENDED FUNCTION
LOSS OF INSTRUMENT AIR: INTENDED FUNCTION
POTENTIAL FOR THE VALVE TO NOT FAIL CLOSE Failure modes that would result in the fail-closed valve not closing during an instrument air failure Stuck valve stem or actuator Failure or partial failure of the pilot valves Factors that complicates the assessment are: Variations in the design of the pilots Both the regulator and positioner will consume air and slowly depressure that portion of the system The regulator will typically have no backflow prevention so the system will depressure back to the instrument air supply line Actuator pressures to close the valve must overcome friction and the forces caused by the process valve flow/pressure.
CONTROL VALVE FAILS TO CLOSE: CAUSE #1 Actuator stuck
CONTROL VALVE FAILS TO CLOSE: CAUSE #2 Pilot valve partially stuck?
CONTROL VALVE FAILS TO CLOSE: CAUSE #3 Pilot valve stuck Regulator and positioner air consumption and backflow may eventually depressure the piston and allow it to close
CONTROL VALVE FAILS TO CLOSE: CAUSE #4 Pilot valve stuck Valve might close if normal top piston pressure is high enough to offset friction and the upward forces caused by process pressure. Top piston pressure will slowly depressure.
POTENTIAL FOR THE VALVE TO NOT FAIL CLOSE Failure modes that would result in the fail-closed double actuator valve not closing during an instrument air failure Stuck valve stem or actuator PFD (assuming clean instrument air):.00492 = (1/203 demands) if stroked every 4 years.00123 = (1/813 demands) if stroked every year Failure of a pilot valve Approximate PFD (assuming clean instrument air):.00526 = (1/190 demands) if stroked every 4 years.00131 = (1/760 demands) if stroked every year Combined PFD.0207 = (1/48 demands) if stroked every 4 years.00517 = (1/193 demands) if stroked every year About an order of Compare this to PFD for spring loaded fail-closed valve magnitud PFD (assuming clean instrument air): e different.00349 = (1/289 demands) if stroked every 4 years.00087 = (1/1100 demands) if stroked every year
POTENTIAL FOR THE VALVE TO FAIL OPEN Is it possible for a the fail-closed actuator to be driven open during an instrument air failure?
LOSS OF INSTRUMENT AIR: VALVE FAIL OPEN? Leak: large enough to depressure bottle within 15 minutes but small enough to operate with normal air supply
LOSS OF INSTRUMENT AIR: VALVE FAIL OPEN? Leak: large enough to depressure bottle within 15 minutes but small enough to operate with normal air supply
LOSS OF INSTRUMENT AIR: VALVE FAIL OPEN? Leak: large enough to depressure bottle within 15 minutes but small enough to operate with normal air supply
LOSS OF INSTRUMENT AIR: VALVE FAIL OPEN? Leak: large enough to depressure bottle within 15 minutes but small enough to operate with normal air supply Valve might go wide open after air bottle pressure is lost if process pressure is sufficient to overcome control valve stem and actuator friction
POTENTIAL FOR THE VALVE TO FAIL OPEN Probability of Failure upon Demand for fail-closed double actuated valve to go wide open during instrument air failure. Expect 1-10% of air bottles would depressure within 24 hours after loss of instrument air Potential for air bottles to have a leak small enough to not be noticed but large enough to depressure bottle within 15-30 minutes is on the order of 0.1% to 1% So, IF process pressure is sufficient to overcome control valve stem and actuator friction, the probability of the valve going wide open within 15-30 minutes during an instrument air failure is on the order of.001 to.01 Note: if this occurs, the valve would likely open slowly since air pressure on the top piston is lost slowly
RECOMMENDED API-521 REVISIONS 4.4.15.4 Loss of Instrument Air or Electric Instrument Power The loss of instrument air drives all air-operated valves to their specified fail position. This action of many valves can result in overpressure if the specified failure positions of the valves are not selected to prevent overpressure. Likewise, failure of electric instrument power can drive control systems and electrically operated valves to their specified failure positions. Consideration should be given to the effect on flare or vent system loading of valves failing open or closed due to instrument air failure or power failure. The above guidance is based on decades of experience using airoperated control valves that have springs to move the valve to its specified failure position. Double actuator valve designs can also be used. These actuators use instrument air to drive the valve to its specified failure position. Some designs have a spring and some designs don t have a spring. All designs have a pressure reservoir and all designs utilize pilot valves to re-route the instrument air to drive the valve to its failure position. Because double actuated valves have more components, these valves may have about an order of magnitude higher probability of not moving to their specified failure position during instrument air failure than spring-only actuators.
RECOMMENDED API-521 REVISIONS There may be a potential for a double actuated valve to go wide open upon total loss of instrument air. This is a possible but remote scenario. It is also conceivable that this mode of failure for some systems could be the governing case for relief device sizing.