Valve proof check credit for a process journey

A process journey happens when the security instrumented system (SIS) places the process in its safe state by commanding an automated valve (Figure 1) or other gear to its journey state in response to an abnormal course of condition. In some instances, a spurious trip happens due to a valve malfunction. In that case, the valve meeting is examined under actual working circumstances, which provides a possibility to capture valuable valve diagnostic knowledge.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs can help decide the proof test credit score for an automated valve after a process journey.
Process trip
A course of journey happens when the SIS detects an abnormal process situation through sensors similar to temperature and strain, executes the logic and places the process in its secure state by tripping the ultimate components such as closing an automatic valve, stopping a pump, and so forth. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process trip happens, the primary aim 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 units. Taking the chance to proof test an automatic valve will not be a prime precedence and even an exercise into account as a end result of 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 can be thought of carried out with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline can help uncover valve deficiencies and degradations that received’t show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a process journey meet many of the 12 necessities of a proof check.
Process journeys versus proof tests
How can proof check credit be claimed for an automatic valve that has failed and brought on a course of trip? A proof test is a periodic test performed on SIS sensors, logic solvers and final components — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its protected state when an irregular process condition is detected.
A proof check ought to be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is usually determined through an average probability of failure on demand calculation (PFDavg). Some users could choose to proof take a look at based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate parts every forty eight months as a substitute of counting on the PFDavg calculations.
Automated valve proof exams could be carried out offline or on-line. Offline proof exams are usually scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or gear to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and dear.
But a proof check can additionally be achieved during a course of trip.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns due to precise demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) under given conditions…the next deliberate proof take a look at could additionally be skipped.”
These situations are
The shutdown documents equivalent information as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated needs to be examined separately.
The shutdown happens inside a predetermined most time window before the following deliberate proof test which may then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at could be thought of carried out. A pattern list of actions performed throughout a proof check, along with those which are performed throughout a course of trip, is proven in Figure 2. Even with out an automatic valve leak take a look at, data captured by the DVC alone can doubtlessly account for a great quantity of proof take a look at coverage for an automated valve.
The actual protection is decided by the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the probability of their occurrence and the percentage of these degradations that might be detected by a DVC. Fortunately, data acquired by a DVC during a process journey can typically be enough to satisfy a significant a half of the proof test necessities.
If the method trip takes place inside a predetermined most time window, the end consumer may select to leverage the method trip as a proof test by completing steps one by way of five in Figure 2, which are usually not completed in a course of journey. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the last half of the present proof test interval.
Figure 3. Data during a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most probably root causes of automated valve degradations — which might result in a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts corresponding to solenoids, pilots, speed controller, and so on., because of moisture, debris or alignment issues. This causes a loss of functional margin and makes the valve sluggish to open or shut.
Binding, galling or different degradation of valve seats or associated flow management trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the pressure out there to actuate the valve.
เกจแรงดัน to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components 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 these conditions may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also constantly monitors for inside faults as well as its inputs corresponding to supply pressure. When the DVC6200SIS sees the availability stress is too low, an alert is generated (Figure 4) in order that the end person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the management system.
Other course of journey advantages
Diagnostic data captured throughout a process journey might reveal valve degradations that is most likely not detected during a proof test. For example, diagnostic knowledge captured during a course of trip might indicate an issue with the valve closing utterly against the full pressure of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is extra correct beneath real working circumstances. This leads to a extra correct SIF response time (the time from detection of an abnormal condition to last factor reaching its trip state), which is in comparison with the Process Safety Time to make sure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a process journey can present valuable information to stop future failures. This info can assist with turnaround planning by ensuring the needed parts can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
A process journey can present the coverage required to delay a valve’s subsequent scheduled proof check, thus saving time and money. Data captured by a DVC could be analyzed to leverage the method journey as a proof check. Even if the tip user chooses to not take proof check credit for a course of journey, the valve diagnostic information offered by the DVC may help plant personnel make proactive valve upkeep choices..

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