Valves and actuators
crease plant uptime due to targeted safety integrity being maintained
for longer test intervals. The best diagnostic tools in safety valve
controllers could even see trends, such as load factors, predicting
future performance and warning users automatically without manual
work of failures, for example, valve stiction. While making life easier
with emergency valves, an intelligent partial-stroking device saves
labour and, regardless of fail open or close system, it improves the
safety of the plant by revealing failures in the complete valve assembly
that would otherwise remain hidden. Intelligent partial stroke devices
with modern asset management systems also give the end-user an
opportunity to plan his maintenance activities better and base them
on tested results in addition to general guidelines.
In the case where redundant main safety elements (ie a solenoid
or partial-stroke device) are used, the intelligent valve controller would
provide additional reduction of common cause failures since the intel-
ligent partial-stroke device is of a different type than normal solenoids
valves. Therefore, this triggers lower common cause failures than with
two solenoids. In some cases, one intelligent partial-stroke devicemay
replace two redundant solenoids and still meet the safety requirement
in single valve solutions. The best of safety valve controllers can test
their safety function automatically without the risk of spurious trip by
using an inbuilt testing procedure with self-diagnostics. This is done
in a controlled way without time delay. It is important that testing a
function of a main safety element is provided with an inbuilt system,
and not by utilising an external method such as a pulsing solenoid
with the signal coming from the SIS system. Such a method is com-
plex, would require additional software work, and increase the risk of
spurious trips, since uncontrolled time delays in external pulsing may
cause unwanted movement of the valve. In addition to increased risk
of a spurious trip, excessive rapid pulsing of the solenoid externally
may enhance the risk of coil burnout failures in the solenoid.
In the case of an emergency trip occurring, further studies may
lead to questions about the performance of the emergency valve
during the trip: for instance, what was the stroking time? Valve diag-
nostics in modern safety valve controllers such as Neles ValvGuard
are also available during the emergency trip. Some may have features
to provide full-stroke testing with inbuilt diagnostic information, which
can be used as a proof test for emergency valves during shutdown
periods. In addition to testing, a test report can easily be generated
with the help of asset management software. Hence, intelligent valve
controllers would provide added value, even without partial stroking.
Safety integrity
The high-end emergency valve applications discussed are safety
instrumented systems, which should be designed, installed and
maintained according to the principles of IEC61508/61511 [1,2]. The
standard has two fundamental concepts: the safety life cycle and
safety integrity level (SIL). Reliability (or SIL) calculations are typically
performed during the design evaluation phase of the safety life cycle
for each safety-instrumented function (SIF). This may contain some
preliminary base analysis cases with generic reliability data and then
re-evaluation by using data from selected vendors. The result of SIL
calculation includes, for example, an average probability of failure
on demand (PFD) for low demand mode applications. The SIL shall
be calculated for the complete safety loop, but PFD and suitability
for the SIL safety-related system can be evaluated individually also
for subsystems, such as the final element subsystem.
High-end final elements, such as the valves discussed in previous
chapters, may contain rather complex instrumentation to meet strict
reliability and availability requirements. The reliability verification of
the final elements can be done easily by using Metso’s SIL calculation
tool in an early phase of the project. The software is able to verify
the SIL capability and PFD value, even for relatively complex final
element subsystems, taking care of complete valve assembly and
including all necessary safety related accessories by using accurate
field proven reliability data.
Conclusion
Intelligence in emergency valves is combining correctly selected
and reliable valves with adequate safety factors and the added
value provided by intelligent valve controllers to achieve the full
benefit of valve diagnostics. In the case where redundant solenoids
are used, common cause failure can be reduced by replacing one of
the solenoids with an intelligent valve controller. Sometimes two
solenoids could be replaced by one intelligent partial-stroke device,
including an inbuilt solenoid, and still meet the safety requirements.
Partial stroking improves the final element PFD value, which brings
additional flexibility for proof test intervals and increases plant up-
time. While making life easier with emergency valves, an intelligent
partial-stroking device saves labour and, regardless of fail open or
close system, it improves the safety of the plant by revealing failures
in the complete valve assembly that would otherwise remain hidden.
Acknowledgement
This article was published in Hydrocarbon Engineering, November
2011 issue as ‘Thinking caps’.
References
[1] IEC 61508. 2010. Functional safety of electrical/electronic/program-
mable electronic safety-related systems
[2] IEC 61511-1. 2003. Functional safety. Safety instrumented sys-
tems for the process industry. Framework, definitions, system,
hardware and software requirements.
[3] EN ISO 23553-1. 2007. Safety and control devices for oil burners
and oil-burning appliances - Particular requirements - Part 1:
Shut-off devices for oil burners.
Bibliography
Kirmanen, J. Intelligent and safety. Hydrocarbon Engineering. No-
vember 2009.
API Standard 521. Pressure-relieving and Depressuring Systems.
Fifth edition.
Jari Kirmanen is the product manager for Neles, Metso’s Automation
Business Line. Enquiries: Email
A
bout the author
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July ‘13
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