Valve proof take a look at credit for a course of trip

A process journey occurs when the safety instrumented system (SIS) places the method in its safe state by commanding an automated valve (Figure 1) or different gear to its journey state in response to an irregular process situation. In some circumstances, a spurious journey happens due to a valve malfunction. In that case, the valve assembly is examined under actual operating circumstances, which offers a possibility to capture valuable valve diagnostic knowledge.
However, เกจวัดแรงดันถังแก๊ส can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs may help decide the proof check credit for an automatic valve after a process journey.
Process journey
A course of trip happens when the SIS detects an irregular course of condition through sensors corresponding to temperature and strain, executes the logic and places the method in its secure state by tripping the ultimate components such as 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 action.
When a course of journey happens, the main objective is often to restart the unit or tools that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof check an automated valve will not be a prime precedence or even an exercise into account because of 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 could be thought-about 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 won’t present up in a proof take a look at.
Figure 2. เกจอาร์กอน vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet many of the 12 requirements of a proof test.
Process journeys versus proof checks
How can proof test credit be claimed for an automated valve that has failed and caused a course of trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and last elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may 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 ought to be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is often determined through a median chance of failure on demand calculation (PFDavg). Some users could choose to proof take a look at primarily based on predetermined intervals, similar to proof testing sensors every 24 months and final parts every forty eight months instead of counting on the PFDavg calculations.
Automated valve proof tests can be accomplished offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line usually requires a unit or equipment to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and expensive.
But a proof check can additionally be accomplished during a process trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to actual demand on the SIS during operation may be given credit score as proof tests (fully or partial) under given conditions…the subsequent planned proof check may be skipped.”
These situations are
The shutdown paperwork equal info as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the gadget or SIS subsystem not activated needs to be examined separately.
The shutdown occurs inside a predetermined most time window before the following planned proof test which may then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof test can be considered performed. A pattern list of actions carried out during a proof check, together with those which might be performed throughout a process journey, is shown in Figure 2. Even with out an automatic valve leak test, knowledge captured by the DVC alone can probably account for an excellent quantity of proof test coverage for an automated valve.
The exact protection is dependent upon the design of the automated valve, its DVC and its application. The protection is estimated based on the valve degradations, the chance of their incidence and the share of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can usually be enough to fulfill a serious part of the proof check requirements.
If the method trip takes place within a predetermined most time window, the end consumer may select to leverage the method journey as a proof check by finishing steps one through five in Figure 2, that are often not completed in a course of journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the current proof test interval.
Figure three. Data throughout a process 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 ninety six.05.01, the most likely root causes of automated valve degradations — which might lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, pace controller, etc., as a result of moisture, particles or alignment issues. This causes a loss of practical margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or related circulate management trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system conditions, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements such as solenoids, pilots, velocity controller, and so forth., due to moisture, debris or alignment. The valve will fail to open or close.
All of these circumstances may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load in comparability with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly screens for internal faults in addition to its inputs such as supply stress. When the DVC6200SIS sees the supply stress is merely too low, an alert is generated (Figure 4) so that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero supply strain, so it alerted the control system.
Other course of trip advantages
Diagnostic knowledge captured throughout a process trip may reveal valve degradations that is in all probability not detected throughout a proof test. For instance, diagnostic information captured throughout a course of journey would possibly indicate a problem with the valve closing utterly in opposition to the full stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is extra correct underneath real operating situations. This leads to a extra correct SIF response time (the time from detection of an irregular condition to last factor reaching its trip state), which is in comparability with the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very dependable, failures identified in a process trip can present priceless knowledge to forestall future failures. This information can assist with turnaround planning by ensuring the wanted parts are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process journey can present the coverage required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method journey as a proof take a look at. Even if the tip user chooses not to take proof check credits for a process trip, the valve diagnostic information provided by the DVC might help plant personnel make proactive valve upkeep selections..
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