A course of journey occurs when the security instrumented system (SIS) locations the method in its protected state by commanding an automatic valve (Figure 1) or other tools to its trip state in response to an abnormal course of situation. In some circumstances, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is examined underneath real operating conditions, which offers an opportunity to seize useful valve diagnostic data.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help decide the proof check credit for an automatic valve after a course of journey.
Process journey
A process trip occurs when the SIS detects an irregular course of situation through sensors such as temperature and strain, executes the logic and locations the process in its safe state by tripping the ultimate components similar to closing an automatic valve, stopping a pump, and so forth. The SIS could 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 tools 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 opportunity to proof check an automatic valve will not be a top priority and even an activity into account because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof take a look at of the valve can be considered carried out with diagnostic knowledge captured by the DVC. Also, pressure gauge octa of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that will not show 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 throughout a process journey meet most of the 12 necessities of a proof check.
Process trips versus proof exams
How can proof take a look at credit be claimed for an automated valve that has failed and caused a process trip? A proof test is a periodic test performed on SIS sensors, logic solvers and final parts — such as automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the flexibility of the SIS to take the method to its safe state when an irregular course of situation is detected.
A proof test ought to be carried out as per the proof test interval specified in the Safety Requirement Specifications. This interval is normally decided by way of a mean chance of failure on demand calculation (PFDavg). Some customers might choose to proof take a look at based on predetermined intervals, such as proof testing sensors every 24 months and ultimate components every forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof exams could be accomplished offline or online. Offline proof exams are normally scheduled during a turnaround, when the method is totally shut down.
Proof testing an automatic valve online 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 test may also be accomplished throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns due to precise demand on the SIS during operation could additionally be given credit score as proof tests (fully or partial) underneath given conditions…the next planned proof test could additionally be skipped.”
These situations are
The shutdown documents equivalent info as registered during corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the gadget or SIS subsystem not activated must be tested individually.
The shutdown occurs inside a predetermined maximum time window earlier than the next deliberate proof check which can then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at could be thought-about carried out. A sample record of activities carried out during a proof check, together with these that are performed throughout a course of trip, is shown in Figure 2. Even without an automated valve leak test, information captured by the DVC alone can probably account for a great amount of proof test coverage for an automatic valve.
The exact protection depends on the design of the automated valve, its DVC and its utility. The coverage is estimated primarily based on the valve degradations, the chance of their incidence and the proportion of these degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a process journey can typically be enough to satisfy a major part of the proof check necessities.
If the method trip takes place within a predetermined maximum time window, the end consumer may select to leverage the method trip as a proof take a look at by completing steps one through 5 in Figure 2, that are normally not accomplished in a course of journey. The next scheduled proof take a look at 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 take a look at interval.
Figure 3. Data during a process trip 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 most probably root causes of automated valve degradations — which can result in a course of journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, pace controller, etc., due to moisture, debris or alignment points. This causes a loss 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 caused by compression, put on or looseness that reduces the pressure available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, 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 on., because of moisture, debris or alignment. The valve will fail to open or shut.
All of these circumstances 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 additionally constantly screens for inside faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the supply stress is just too low, an alert is generated (Figure 4) so that the tip person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the management system.
Other course of journey benefits
Diagnostic information captured during a course of journey could reveal valve degradations that will not be detected throughout a proof take a look at. For example, diagnostic information captured during a process journey would possibly indicate a problem with the valve closing fully in opposition to the full stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra correct beneath real working conditions. This results in a more correct SIF response time (the time from detection of an irregular situation to final component reaching its journey state), which is compared to the Process Safety Time to ensure the SIF is still assembly its design requirements.
Although automated valves are very dependable, failures recognized in a course of trip can present valuable data to forestall future failures. This info can assist with turnaround planning by ensuring the needed components can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can present the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC could be analyzed to leverage the process journey as a proof take a look at. Even if the tip consumer chooses to not take proof take a look at credits for a process journey, the valve diagnostic information offered by the DVC may help plant personnel make proactive valve upkeep selections..
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