Valve proof test credit for a course of trip

A course of journey happens when the security instrumented system (SIS) places the process in its safe state by commanding an automatic valve (Figure 1) or other gear to its journey state in response to an irregular course of situation. In some cases, a spurious trip occurs due to a valve malfunction. In that case, the valve meeting is examined underneath actual operating situations, which offers a chance to seize priceless valve diagnostic data.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs can help determine the proof take a look at credit for an automatic valve after a process journey.
Process journey
A course of journey occurs when the SIS detects an irregular course of situation via sensors such as temperature and stress, executes the logic and places the method in its safe state by tripping the ultimate components corresponding to closing an automatic valve, stopping a pump, and so on. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a process trip occurs, the primary goal is normally to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the chance to proof check an automated valve is not going to be a prime precedence or even an activity into consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve may be thought of performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline can help uncover valve deficiencies and degradations that could not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet most of the 12 necessities of a proof take a look at.
Process journeys versus proof tests
How can proof take a look at credits be claimed for an automatic valve that has failed and caused a process trip? A proof check is a periodic test carried out on SIS sensors, logic solvers and final components — such as automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the power of the SIS to take the process to its safe state when an irregular process condition is detected.
A proof check should be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is normally determined by way of an average likelihood of failure on demand calculation (PFDavg). Some users could select to proof take a look at based mostly on predetermined intervals, such as proof testing sensors every 24 months and final components every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks may be carried out offline or on-line. Offline proof exams are usually scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automatic valve online often requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and costly.
But a proof test may additionally be achieved throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns as a outcome of actual demand on the SIS during operation may be given credit as proof exams (fully or partial) under given conditions…the next planned proof check may be skipped.”
These circumstances are
The shutdown paperwork equal data as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the gadget or SIS subsystem not activated must be examined separately.
The shutdown happens within a predetermined most time window earlier than the next deliberate proof test which may then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be thought-about carried out. A sample list of activities carried out throughout a proof test, together with those that are carried out throughout a process trip, is shown in Figure 2. Even without an automatic valve leak test, data captured by the DVC alone can probably account for a great quantity of proof take a look at protection for an automated valve.
The actual protection depends on the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the likelihood of their prevalence and the proportion of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a course of trip can usually be enough to satisfy a major a half of the proof check requirements.
If the process trip takes place inside a predetermined maximum time window, the end user might choose to leverage the method trip as a proof take a look at by completing steps one via five in Figure 2, which are often not completed in a course of journey. The subsequent scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the final half of the current proof test interval.
Figure three. Private during a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made available for evaluation.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the more than likely root causes of automated valve degradations — which may result in a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, velocity controller, and so forth., as a end result of moisture, debris or alignment points. This causes a loss of practical margin and makes the valve sluggish to open or close.
Binding, galling or different degradation of valve seats or related circulate control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system parts such as solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment. The valve will fail to open or shut.
All of those conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as a rise in breakout pressure. Loss of seat load in comparison with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also constantly displays for internal faults in addition to its inputs corresponding to supply strain. When the DVC6200SIS sees the supply stress is just too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero supply stress, so it alerted the control system.
Other course of journey benefits
Diagnostic knowledge captured during a course of journey may reveal valve degradations that will not be detected during a proof check. For instance, diagnostic knowledge captured throughout a course of trip might point out an issue with the valve closing utterly towards the complete pressure of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is extra accurate beneath real operating circumstances. This leads to a more accurate SIF response time (the time from detection of an irregular situation to ultimate element reaching its journey state), which is in comparability with the Process Safety Time to make sure the SIF is still assembly its design requirements.
Although automated valves are very reliable, failures recognized in a course of trip can provide priceless data to stop future failures. This data can help with turnaround planning by ensuring the needed elements are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
A process trip can present the coverage required to delay a valve’s subsequent scheduled proof test, thus saving money and time. Data captured by a DVC can be analyzed to leverage the process journey as a proof take a look at. Even if the tip user chooses to not take proof test credit for a process trip, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance choices..

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