Valve proof test credit for a course of journey

A process journey occurs when the safety instrumented system (SIS) locations the process in its secure state by commanding an automated valve (Figure 1) or other tools to its trip state in response to an abnormal course of situation. In some instances, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is examined underneath actual working conditions, which provides a chance to seize useful valve diagnostic information.
However, such diagnostics knowledge can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs can help decide the proof take a look at credit for an automated valve after a course of journey.
Process journey
A process journey occurs when the SIS detects an irregular process condition by way of sensors corresponding to temperature and strain, executes the logic and places the method in its secure state by tripping the ultimate elements such as closing an automated valve, stopping a pump, and so on. The SIS might talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process trip happens, the main goal is often to restart the unit or equipment that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken units. Taking the chance to proof check an automatic valve will not be a top priority or even an exercise under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve can be thought of carried out with diagnostic information captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline might help discover valve deficiencies and degradations that will not present up in a proof take a look at.
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 during a process trip meet most of the 12 necessities of a proof take a look at.
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 carried out on SIS sensors, logic solvers and last components — such as automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the ability of the SIS to take the process to its safe state when an abnormal course of condition is detected.
A proof check must be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is often determined by way of a median probability of failure on demand calculation (PFDavg). Some users may select to proof test primarily based on predetermined intervals, similar to proof testing sensors each 24 months and last elements every forty eight months instead of counting on the PFDavg calculations.
Automated valve proof tests could be carried out offline or online. Offline proof tests are normally scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automatic valve online often requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and costly.
But a proof take a look at can also be achieved throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to precise demand on the SIS during operation may be given credit score as proof checks (fully or partial) under given conditions…the next deliberate proof test could also be skipped.”
These conditions are
The shutdown paperwork equal data as registered throughout corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated must be tested individually.
The shutdown occurs inside a predetermined most time window earlier than the next planned proof check which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof check may be thought of carried out. A pattern listing of activities carried out during a proof test, together with those which are carried out throughout a course of trip, is proven in Figure 2. Even with out an automatic valve leak check, data captured by the DVC alone can doubtlessly account for an excellent quantity of proof check coverage for an automatic valve.
The exact coverage is dependent upon the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the probability of their prevalence and the share of those degradations that might be detected by a DVC. Fortunately, knowledge acquired by a DVC during a process journey can typically be sufficient to fulfill a major part of the proof take a look at requirements.
If the method journey takes place within a predetermined maximum time window, the tip person may choose to leverage the process trip as a proof check by finishing steps one through five in Figure 2, that are often not completed in a course of trip. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the present proof check interval.
Figure 3. เพชเชอร์เกจ throughout a process journey 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 probably root causes of automated valve degradations — which may lead to a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, pace controller, and so forth., as a result of 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 move control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, put on 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 conditions, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components such as solenoids, pilots, speed controller, etc., because of moisture, debris or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also constantly screens for internal faults in addition to its inputs similar to supply pressure. When the DVC6200SIS sees the supply pressure is just 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 nearly zero supply stress, so it alerted the management system.
Other process trip benefits
Diagnostic data captured throughout a process trip could reveal valve degradations that may not be detected throughout a proof test. For example, diagnostic information captured throughout a course of trip would possibly indicate a problem with the valve closing fully in opposition to the complete stress of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of journey is more accurate under actual working situations. This results in a extra accurate SIF response time (the time from detection of an irregular condition to final component 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 journey can present priceless information to forestall future failures. This information can help with turnaround planning by making sure the wanted elements are available before turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can provide the coverage required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC could be analyzed to leverage the method journey as a proof test. Even if the end user chooses to not take proof check credit for a course of trip, the valve diagnostic information provided by the DVC can help plant personnel make proactive valve upkeep selections..
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