A process trip happens when the security instrumented system (SIS) places the process in its safe state by commanding an automatic valve (Figure 1) or other tools to its journey state in response to an abnormal process situation. In some circumstances, a spurious journey happens due to a valve malfunction. In that case, the valve meeting is examined underneath real operating circumstances, which provides a possibility to seize priceless valve diagnostic information.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs might help determine the proof take a look at credit for an automated valve after a course of journey.
Process journey
A process trip happens when the SIS detects an irregular process situation through sensors such as temperature and stress, executes the logic and places the method in its safe state by tripping the final components corresponding to closing an automatic valve, stopping a pump, and so on. The SIS may communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of journey happens, the main aim is usually to restart the unit or gear that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged gadgets. Taking the chance to proof check an automated valve won’t 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 test of the valve can be thought of performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline can help uncover valve deficiencies and degradations that may not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of journey meet many of the 12 necessities of a proof take a look at.
Process trips versus proof tests
How can proof check credits be claimed for an automated valve that has failed and triggered a process trip? A proof check is a periodic check carried out on SIS sensors, logic solvers and ultimate parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the power of the SIS to take the process to its safe state when an irregular course of situation is detected.
A proof test should be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is often decided via a median chance of failure on demand calculation (PFDavg). Some users might select to proof check based mostly on predetermined intervals, similar to proof testing sensors every 24 months and last parts every forty eight months as a substitute of counting on the PFDavg calculations.
Automated valve proof checks could be carried out offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automated valve on-line normally 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 take a look at may additionally be achieved throughout a process trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to actual demand on the SIS throughout operation could additionally be given credit score as proof tests (fully or partial) beneath given conditions…the subsequent deliberate proof test could also be skipped.”
These conditions are
The shutdown paperwork equivalent info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be tested separately.
The shutdown occurs inside a predetermined maximum time window before the subsequent planned proof check which can then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof check can be thought-about performed. A pattern list of actions performed during a proof test, together with those which might be performed throughout a course of trip, is shown in Figure 2. Even without an automated valve leak take a look at, data captured by the DVC alone can doubtlessly account for a great amount of proof check coverage for an automatic valve.
The actual coverage is decided by the design of the automated valve, its DVC and its application. The protection is estimated primarily based on the valve degradations, the likelihood of their occurrence and the proportion of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a course of journey can typically be enough to satisfy a major part of the proof test requirements.
If the process trip takes place within a predetermined maximum time window, the tip user might choose to leverage the process trip as a proof check by finishing steps one through five in Figure 2, which are normally not completed in a process trip. The subsequent scheduled proof test can then be canceled. According to เกจวัดแรงดันน้ำประปา , the maximum time window ought to 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 out there for evaluation.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most probably 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 useful margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or related move management trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the strain obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components similar to solenoids, pilots, speed controller, etc., because of moisture, debris or alignment. The valve will fail to open or close.
All of these situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as an increase in breakout drive. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly displays for inside faults in addition to its inputs corresponding to supply pressure. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) so that the top user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the control system.
Other course of trip benefits
Diagnostic information captured throughout a course of journey may reveal valve degradations that may not be detected during a proof check. For instance, diagnostic information captured during a course of journey may indicate an issue with the valve closing utterly towards the total stress of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more correct under actual operating conditions. This ends in a extra accurate SIF response time (the time from detection of an irregular situation to ultimate factor reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF continues to be assembly its design requirements.
Although automated valves are very dependable, failures identified in a process trip can present useful information to forestall future failures. This data can help with turnaround planning by ensuring the needed components are available before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process journey can present the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method trip as a proof take a look at. Even if the end user chooses to not take proof take a look at credits for a process trip, the valve diagnostic knowledge supplied by the DVC might help plant personnel make proactive valve maintenance selections..
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