A course of trip occurs when the protection instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or different equipment to its journey state in response to an irregular process condition. In some cases, a spurious journey occurs because of a valve malfunction. In that case, the valve meeting is tested underneath real working conditions, which provides a possibility to seize priceless valve diagnostic information.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs might help determine the proof take a look at credit score for an automatic valve after a course of trip.
Process journey
A process journey happens when the SIS detects an abnormal course of situation via sensors similar to temperature and stress, executes the logic and locations the method in its safe state by tripping the ultimate components similar to closing an automated valve, stopping a pump, and so on. The SIS could talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of trip happens, the main aim is often to restart the unit or tools that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the chance to proof check an automatic valve is not going to be a prime precedence and even an exercise into account 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 could be thought of performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help uncover valve deficiencies and degradations that gained’t present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller throughout a course of trip meet many of the 12 requirements of a proof test.
Process trips versus proof checks
How can proof check credits be claimed for an automatic valve that has failed and brought on a course of trip? A proof take a look at is a periodic take a look at carried out on SIS sensors, logic solvers and final parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the flexibility of the SIS to take the process to its secure state when an irregular course of situation is detected.
A proof test should be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is normally decided by way of an average likelihood of failure on demand calculation (PFDavg). Some users could select to proof test based on predetermined intervals, similar to proof testing sensors every 24 months and final parts each 48 months instead of counting on the PFDavg calculations.
Automated valve proof exams may be accomplished offline or on-line. Offline proof tests are usually scheduled during a turnaround, when the process is totally shut down.
Proof testing an automatic valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and expensive.
But a proof check may additionally be accomplished throughout a course of trip.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns due to precise demand on the SIS during operation may be given credit score as proof checks (fully or partial) beneath given conditions…the next deliberate proof check could also be skipped.”
These conditions are
The shutdown paperwork equal info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the gadget or SIS subsystem not activated must be tested individually.
The shutdown happens within a predetermined maximum time window earlier than the subsequent planned proof test which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof test may be considered carried out. A sample record of activities performed during a proof take a look at, along with those that are performed throughout a course of journey, is shown in Figure 2. Even without an automatic valve leak check, knowledge captured by the DVC alone can doubtlessly account for a great amount of proof test coverage for an automatic valve.
The exact coverage is dependent upon the design of the automated valve, its DVC and its application. The protection is estimated based on the valve degradations, the probability of their prevalence and the percentage of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a process journey can often be sufficient to satisfy a serious a half of the proof check requirements.
If the process journey takes place inside a predetermined most time window, the top consumer may select to leverage the method journey as a proof take a look at by completing steps one through five in Figure 2, that are often not completed in a process journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the final half of the present proof take a look at interval.
Figure three. Data throughout 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 96.05.01, the most likely root causes of automated valve degradations — which might result in a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, speed controller, etc., because of moisture, debris or alignment points. This causes a loss of practical margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or associated flow control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, wear or looseness that reduces the pressure obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system parts such as solenoids, pilots, speed controller, and so forth., due to moisture, particles or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as an increase in breakout drive. Loss of seat load in comparability with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously monitors for inner faults as properly as its inputs corresponding to provide pressure. When the DVC6200SIS sees the provision pressure is just too low, an alert is generated (Figure 4) in order that the top user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the management system.
เกจวัดแรงดันดิจิตอล captured during a course of journey could reveal valve degradations that may not be detected during a proof take a look at. For example, diagnostic information captured during a course of journey might point out an issue with the valve closing fully in opposition to the full stress of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra correct underneath actual working situations. This results in a extra correct SIF response time (the time from detection of an irregular condition to ultimate element reaching its journey state), which is compared to the Process Safety Time to ensure the SIF is still meeting its design requirements.
Although automated valves are very reliable, failures recognized in a course of trip can provide useful data to prevent future failures. This information can assist with turnaround planning by ensuring the wanted parts can be found before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process trip can provide the coverage required to delay a valve’s subsequent scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process trip as a proof check. Even if the tip user chooses to not take proof check credits for a course of trip, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve maintenance choices..
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