API 521 7th 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 6th 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. OtisPTS/TTSMOctober 10, 2015

AGENDA

Current Practices Issues Proposed Guidance – Conceptual Scenario Assessment Spring Loaded Valve – loss of instrument airDouble Actuated Valve – loss of instrument air

Proposed API 521 Language

CURRENT PRACTICES

Individual Instrument Air FailureAssess each control valve individually moving to its failure

position

Global Instrument Air FailureAssume all control valves simultaneously move to their

failure positionUsually 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 lostDouble 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 fail-closed 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 pilotsBoth 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 #4Pilot

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

Compare this to PFD for spring loaded fail-closed valve PFD (assuming clean instrument air): .00349 = (1/289 demands) if stroked every 4 years .00087 = (1/1100 demands) if stroked every year

About an order of

magnitude different

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 airPotential 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 REVISIONS4.4.15.4 Loss of Instrument Air or Electric Instrument PowerThe 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 air-operated 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.