Valve proof take a look at credit score for a process trip

A process journey occurs when the safety instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other equipment to its journey state in response to an irregular course of situation. In some instances, a spurious journey happens due to a valve malfunction. In that case, the valve assembly is examined beneath real working conditions, which supplies a chance to capture valuable valve diagnostic data.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs may help decide the proof test credit for an automated valve after a process journey.
Process trip
A process trip occurs when the SIS detects an irregular course of situation by way of sensors such as temperature and strain, executes the logic and locations the method in its protected state by tripping the final parts corresponding to closing an automated valve, stopping a pump, and so on. The SIS may 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 occurs, the primary aim is normally to restart the unit or gear that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. เกจแรงดัน are busy repairing damaged gadgets. Taking the chance to proof test an automated valve is not going to be a high priority or even an exercise under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof take a look at of the valve can be thought of performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline may help uncover valve deficiencies and degradations that received’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 during a course of trip meet most of the 12 necessities of a proof check.
Process journeys versus proof tests
How can proof take a look at credit be claimed for an automated valve that has failed and triggered a course of trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and last components — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the ability of the SIS to take the method to its safe state when an abnormal process situation is detected.
A proof test should be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually decided via a mean probability of failure on demand calculation (PFDavg). Some customers may select to proof test based on predetermined intervals, such as proof testing sensors each 24 months and last parts each 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests can be accomplished offline or on-line. Offline proof tests are often scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line usually requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and dear.
But a proof test can be achieved during a process trip.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns because of precise demand on the SIS during operation may be given credit as proof tests (fully or partial) underneath given conditions…the next planned proof take a look at may be skipped.”
These situations are
The shutdown documents equal info as registered during corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated must be tested separately.
The shutdown happens inside a predetermined most time window earlier than the following deliberate proof check which can then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at could be considered performed. A sample list of actions carried out during a proof take a look at, along with these that are performed during a course of trip, is shown in Figure 2. Even with out an automated valve leak check, knowledge captured by the DVC alone can probably account for an excellent quantity of proof test coverage for an automatic valve.
The precise coverage is determined by the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the probability of their prevalence and the share of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can usually be sufficient to fulfill a major a part of the proof take a look at necessities.
If the process trip takes place inside a predetermined most time window, the end user may choose to leverage the method journey as a proof test by completing steps one through 5 in Figure 2, which are usually not completed in a course of journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the last half of the current proof take a look at interval.
Figure 3. Data throughout a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for analysis.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most likely root causes of automated valve degradations — which can result in a course of trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, speed controller, etc., due to moisture, debris or alignment points. This causes a lack of practical margin and makes the valve slow to open or shut.
Binding, galling or different degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the strain out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management 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 these circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is caught open will exhibit itself as an increase in breakout pressure. Loss of seat load in comparability with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also constantly monitors for inside faults in addition to its inputs corresponding to provide pressure. When the DVC6200SIS sees the availability stress is too low, an alert is generated (Figure 4) in order that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero provide stress, so it alerted the control system.
Other process journey advantages
Diagnostic data captured throughout a process journey might reveal valve degradations that will not be detected during a proof test. For instance, diagnostic data captured throughout a process journey would possibly indicate an issue with the valve closing completely against the complete stress of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate beneath actual operating circumstances. This leads to a extra accurate SIF response time (the time from detection of an abnormal situation to final component reaching its trip state), which is in comparison with the Process Safety Time to ensure the SIF remains to be assembly its design requirements.
Although automated valves are very reliable, failures identified in a process trip can provide useful data to prevent future failures. This information can assist with turnaround planning by ensuring the wanted parts are available before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can present the protection required to delay a valve’s subsequent scheduled proof check, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method journey as a proof check. Even if the top user chooses to not take proof check credit for a course of trip, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve maintenance decisions..
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