likelihood or the impact severity of an identified emergency
event. In a modern pipeline, the SIS is designed to prevent or
reduce hazardous events by initiating a process to ensure a safe
state when predetermined conditions are violated. It is commonly
known as an emergency shutdown (ESD) system and in a pipeline,
it can include a variety of shutdown valves.
ESD valves play a key role in various safety instrumented
functions (SIF). For example, they may be installed to protect
upstream pipelines from over-pressure, oil leaks and other
emergency conditions. As final control elements, ESD valves do
not continually move like typical control valves, but are normally
expected to remain static in one position and then operate
only when an emergency situation arises. However, it is widely
recognised that final control elements contribute 50% to SIS
failure.
Innovations in testing techniques
There are four safety integrity levels (SIL), with SIL4 having the
highest availability for a given safety function and SIL1 having the
lowest level. The risk reduction factor (RRF) is determined by a
hazard and operability analysis in which dangerous events are
identified and quantified, factoring the potential for serious injury,
equipment damage, lost production and environmental threat. If
the probability of these events is high, or the ramifications of such
incidents would be intolerable, an RRF is calculated to bring the
risk to an acceptable level. Operating companies can substantially
increase their SIL loop rating if they adopt a rigorous maintenance
and testing programme on their safety valves. By combining partial
stroke testing (PST) of valves with more frequent inspection,
companies can achieve a higher SIL rating without spending
increasing costs on additional hardware.
Pipeline operation tests to ensure valves meet safety
requirements can become burdensome and expensive as well as
pose concerns when ESD valves are rendered non-operational.
In addition, traditional methods of testing safety valves do not
provide any internal component diagnostics, which can include
hidden damage to the equipment.
In recent years, online PST techniques have been developed
to test a percentage of the possible valve failure modes without
the need to physically close the valve, otherwise known as full
stroke testing (FST). A programme of periodic PSTs can help ensure
that a SIS functions as designed in an emergency by addressing
the single biggest cause of SIF faults – the final control element
– while reducing the frequency of full stroke tests. The PST can
be started both locally on the device in a time-controlled manner
or from a control room. The position control device evacuates
the energising port until the predefined position change occurs
(typically 15 - 20˚). If this happens within the set torque/thrust
vs time limit, an alarm can be output. Additionally, monitoring is
performed to determine whether the valve has moved out of its
end position within a defined break torque/thrust period. If it
does not, the test is classed as ‘failed’ and an alarm is displayed.
This behaviour prevents a blocked valve from suddenly freeing
itself from the end position and thereby disrupting the process. At
the end of the PST, the position control device moves the valve
to the last valid position and reverts to the most recently active
control mode.
The typical advantages of PST include:
)
Improvement to the SIL rating.
)
Access to predictive maintenance data.
)
Extension of FST intervals.
)
Reduced need for valve bypasses.
)
Availability of valves to respond to process demands during
test periods.
The main benefit of PST is providing a measure of confidence
that a valve can move freely, and it does so at short intervals. This
has both a preventive and corrective aspect. The valve movement
can dislodge any dirt build-up to help prevent sticking. If it is
stuck, the test will detect that and corrective measures can be
applied. The system can either be brought to an orderly shutdown
to perform repairs, or, if it can be completed quicker, the valve
may be temporarily bypassed. By conducting a PST, pipeline
operators can significantly reduce maintenance costs associated
with complex and disruptive full stroke testing. Additionally, by
checking the valves more frequently, they can achieve increased
reliability and reduce service interruptions.
Latest technology advances
Partial stroke testing has been performed in different applications
for many years. The latest developments enable PST to be carried
out with limited human intervention in a semi-automatic way.
Figure 2.
Typical automated ball valve with stainless steel
controls.
102
World Pipelines
/
JUNE 2015
