Valve proof check credit for a course of journey

Concealed of trip occurs when the protection instrumented system (SIS) locations the process 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 instances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is tested beneath actual working conditions, which provides a chance to capture priceless valve diagnostic data.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help decide the proof test credit score for an automated valve after a process journey.
Process journey
A process trip happens when the SIS detects an irregular process condition by way of sensors similar to temperature and strain, executes the logic and locations the process in its secure state by tripping the final parts similar to closing an automatic valve, stopping a pump, etc. The SIS may talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a course of trip occurs, the main objective is usually to restart the unit or equipment that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the chance to proof take a look at an automated valve won’t be a prime priority or even an activity into account as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof check of the valve could be considered performed with diagnostic information captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline may help uncover valve deficiencies and degradations that might not present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a process trip meet a lot of the 12 requirements of a proof take a look at.
Process trips versus proof checks
How can proof test credits be claimed for an automatic valve that has failed and triggered a course of trip? A proof check is a periodic check carried out on SIS sensors, logic solvers and ultimate parts — such as automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the method to its safe state when an abnormal process situation is detected.
A proof take a look at ought to be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined via a median chance of failure on demand calculation (PFDavg). Some users could select to proof check primarily based on predetermined intervals, such as proof testing sensors each 24 months and last components every forty eight months as a substitute of counting on the PFDavg calculations.
Automated valve proof tests can be done offline or online. Offline proof exams are usually scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line often requires a unit or gear to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and expensive.
But a proof check can also be accomplished during a course of trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns as a result of actual demand on the SIS throughout operation could also be given credit as proof exams (fully or partial) underneath given conditions…the next planned proof take a look at may be skipped.”
These situations are
The shutdown paperwork equal info as registered throughout corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown happens within a predetermined maximum time window before the next deliberate proof check which may then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be considered performed. A pattern record of activities carried out throughout a proof check, together with these which are carried out during a process trip, is proven in Figure 2. Even without an automatic valve leak check, knowledge captured by the DVC alone can doubtlessly account for an excellent quantity of proof take a look at coverage for an automated valve.
The exact coverage is determined by the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the chance of their occurrence and the share of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a course of trip can typically be enough to satisfy a serious a half of the proof test requirements.
If the process trip takes place inside a predetermined most time window, the top user could select to leverage the method journey as a proof take a look at by finishing steps one via five in Figure 2, which are normally not completed in a course of journey. The subsequent scheduled proof test can then be canceled. According to Simple as ABC , the maximum time window ought to be the final half of the current proof test interval.
Figure 3. Data throughout a course of journey 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 96.05.01, the more than likely root causes of automated valve degradations — which can result in a process journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, etc., due to moisture, particles or alignment issues. This causes a loss of functional margin and makes the valve sluggish to open or close.
Binding, galling or other degradation of valve seats or associated move control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements such as solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment. The valve will fail to open or shut.
All of these situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s caught open will exhibit itself as a rise in breakout pressure. Loss of seat load in comparability with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also continuously displays for inside faults as well as its inputs similar to supply pressure. When the DVC6200SIS sees the supply pressure is simply too low, an alert is generated (Figure 4) so that the end consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero provide pressure, so it alerted the management system.
Other process journey advantages
Diagnostic data captured during a process journey might reveal valve degradations that may not be detected during a proof check. For instance, diagnostic knowledge captured during a process trip might indicate an issue with the valve closing fully against the total pressure of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is extra accurate under real operating conditions. This results in a more accurate SIF response time (the time from detection of an irregular condition to last factor reaching its trip state), which is compared to the Process Safety Time to make sure the SIF continues to be meeting its design requirements.
Although automated valves are very reliable, failures recognized in a course of trip can present priceless knowledge to prevent future failures. This information can assist with turnaround planning by making sure the needed components can be found before turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process trip as a proof check. Even if the end person chooses to not take proof test credits for a process journey, the valve diagnostic information supplied by the DVC might help plant personnel make proactive valve upkeep selections..
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