A process journey occurs when the protection instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other gear to its trip state in response to an abnormal course of condition. In some circumstances, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is tested under real working situations, which offers a possibility to capture priceless valve diagnostic knowledge.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll discuss how DVCs might help decide the proof take a look at credit for an automatic valve after a process trip.
Process journey
A process journey occurs when the SIS detects an abnormal process situation by way of sensors corresponding to temperature and strain, executes the logic and locations the method in its safe state by tripping the final parts corresponding to closing an automated valve, stopping a pump, etc. The SIS might talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a process trip happens, the main objective is usually to restart the unit or equipment that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken units. Taking the opportunity to proof check an automatic valve won’t be a prime precedence or even an activity into consideration as a result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve may be thought-about carried out with diagnostic information captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help discover 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 course of journey meet most of the 12 necessities of a proof test.
Process trips versus proof checks
How can proof test credits be claimed for an automatic valve that has failed and brought on a process trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and ultimate components — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the ability of the SIS to take the method to its protected state when an irregular process condition is detected.
A proof check must be performed as per the proof test interval specified in the Safety Requirement Specifications. This interval is usually decided by way of an average probability of failure on demand calculation (PFDavg). Some customers could choose to proof check based on predetermined intervals, such as proof testing sensors every 24 months and final components each 48 months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams could be done offline or online. Offline proof checks are usually scheduled during a turnaround, when the process is totally shut down.
Proof testing an automated valve online normally requires a unit or equipment to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and costly.
But a proof check may also be achieved during a course of journey.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns as a outcome of precise demand on the SIS throughout operation could also be given credit as proof exams (fully or partial) under given conditions…the subsequent planned proof take a look at could also be skipped.”
These conditions are
The shutdown documents equal information as registered during corresponding proof check.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated must be examined separately.
The shutdown occurs within a predetermined most time window earlier than the subsequent deliberate proof check which can then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check can be thought of performed. A sample listing of activities performed throughout a proof test, along with those which may be carried out during a process journey, is shown in Figure 2. Even with out an automatic valve leak check, data captured by the DVC alone can doubtlessly account for a great quantity of proof take a look at coverage for an automated valve.
The exact protection is determined by the design of the automated valve, its DVC and its utility. The protection is estimated based mostly on the valve degradations, the likelihood of their occurrence and the percentage of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a process trip can often be enough to satisfy a significant a part of the proof test necessities.
If the method trip takes place within a predetermined maximum time window, the tip consumer could select to leverage the process journey as a proof test by completing steps one by way of 5 in Figure 2, that are normally not accomplished in a process journey. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the final half of the present proof check interval.
Figure 3. Data during a course of trip 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 most probably root causes of automated valve degradations — which may result in a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, velocity controller, and so on., as a result of moisture, particles or alignment issues. This causes a lack of practical margin and makes the valve sluggish to open or shut.
Binding, galling or different degradation of valve seats or associated flow management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, wear or looseness that reduces the strain obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball brought on by system circumstances, leakage or debris, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system parts such as solenoids, pilots, velocity controller, and so forth., due to moisture, particles or alignment. The valve will fail to open or shut.
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 drive. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously displays for inside faults as properly as its inputs such as provide stress. When ราคาเกจวัดแรงดัน sees the supply stress is too low, an alert is generated (Figure 4) so that the top consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide pressure, so it alerted the control system.
Other process journey benefits
Diagnostic knowledge captured throughout a process trip may reveal valve degradations that may not be detected during a proof test. For instance, diagnostic data captured during a course of journey may indicate an issue with the valve closing utterly against the total pressure of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is more accurate beneath actual working situations. This ends in a more accurate SIF response time (the time from detection of an irregular situation to ultimate factor reaching its journey state), which is in comparability with 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 process journey can provide priceless knowledge to prevent future failures. This data can assist with turnaround planning by ensuring the wanted components are available earlier than turnaround even begins to probably shorten the turnaround schedule.
Summary
A process journey can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process journey as a proof test. Even if the end person chooses to not take proof check credit for a process trip, the valve diagnostic knowledge offered by the DVC may help plant personnel make proactive valve maintenance choices..
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