Valve proof check credit for a process trip

A process trip happens when the security instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other tools to its trip state in response to an irregular process situation. In some cases, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is tested beneath real working conditions, which supplies a possibility to capture valuable valve diagnostic data.
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 may help decide the proof check credit score for an automated valve after a process journey.
Process journey
A process journey happens when the SIS detects an abnormal course of situation via sensors corresponding to temperature and strain, executes the logic and locations the method in its secure state by tripping the final elements similar to closing an automated valve, stopping a pump, and so forth. The SIS might communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a process trip happens, the main objective is often 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 damaged units. Taking the opportunity to proof test an automated valve won’t be a prime precedence or even an activity under consideration as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof test of the valve may be considered carried out with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline might 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 journey meet a lot of the 12 requirements of a proof take a look at.
Process trips versus proof exams
How can proof check credit be claimed for an automatic valve that has failed and caused a process trip? A proof take a look at is a periodic test performed on SIS sensors, logic solvers and final parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or eliminated, may impair the ability of the SIS to take the process to its secure state when an abnormal course of condition is detected.
A proof take a look at should be performed as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is normally decided by way of a median probability of failure on demand calculation (PFDavg). Some users might choose to proof check based on predetermined intervals, such as proof testing sensors each 24 months and final parts each forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks may be accomplished offline or on-line. Offline proof exams are normally scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automatic valve online often requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and dear.
But a proof test may additionally be accomplished throughout a process journey.
According to ราคาเกจวัดแรงดันน้ำ -2 Section A.sixteen.three.1.three, “…shutdowns due to precise demand on the SIS throughout operation could also be given credit as proof tests (fully or partial) under given conditions…the subsequent planned proof test may be skipped.”
These circumstances are
The shutdown paperwork equal information as registered throughout corresponding proof check.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown occurs within a predetermined maximum time window before the next deliberate proof test which may then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test can be thought-about carried out. A sample record of actions performed throughout a proof check, together with these which may be performed during a course of journey, is proven in Figure 2. Even without an automated valve leak check, information captured by the DVC alone can potentially account for an excellent quantity of proof check protection for an automatic valve.
The precise protection depends on the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the probability of their incidence and the share of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a process journey can often be enough to fulfill a serious part of the proof take a look at necessities.
If the process journey takes place inside a predetermined maximum time window, the top person could choose to leverage the process journey as a proof test by finishing steps one by way of five in Figure 2, that are normally not accomplished in a process journey. The subsequent scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the current proof test interval.
Figure three. Data throughout a course of 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 more than likely root causes of automated valve degradations — which might lead to a process trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so forth., due to moisture, particles or alignment points. This causes a lack of practical margin and makes the valve sluggish to open or close.
Binding, galling or other degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the strain obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system components such as solenoids, pilots, velocity controller, and so on., as a end result of moisture, debris or alignment. The valve will fail to open or shut.
All of those circumstances may 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 assembly was new signifies potential valve leak. The Fisher DVC6200SIS also constantly monitors for inner faults as nicely as its inputs such as provide stress. When the DVC6200SIS sees the supply strain is simply too low, an alert is generated (Figure 4) so that the tip consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply strain, so it alerted the management system.
Other course of trip benefits
Diagnostic knowledge captured during a course of trip may reveal valve degradations that may not be detected throughout a proof take a look at. For instance, diagnostic information captured throughout a course of trip might point out an issue with the valve closing completely against the complete strain of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more correct beneath actual operating circumstances. This ends in a extra correct SIF response time (the time from detection of an irregular situation to ultimate element reaching its journey state), which is compared to the Process Safety Time to ensure the SIF is still meeting its design necessities.
Although automated valves are very dependable, failures recognized in a process journey can present priceless information to prevent future failures. This info can help with turnaround planning by making sure the wanted elements are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process trip can present the protection required to delay a valve’s next scheduled proof take a look at, thus saving time and money. Data captured by a DVC could be analyzed to leverage the process journey as a proof check. Even if the end consumer chooses not to take proof test credits for a course of trip, the valve diagnostic information supplied by the DVC can help plant personnel make proactive valve maintenance decisions..
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