A process trip happens when the security instrumented system (SIS) places 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 process condition. In some cases, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is tested under real working situations, which supplies an opportunity to capture valuable valve diagnostic information.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs might help determine the proof take a look at credit score for an automated valve after a course of trip.
Process journey
A process trip occurs when the SIS detects an abnormal course of situation via sensors corresponding to temperature and stress, executes the logic and places the method in its protected state by tripping the final elements corresponding to closing an automated valve, stopping a pump, etc. The SIS could talk 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 occurs, the principle goal is normally 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 devices. Taking the opportunity to proof check an automated valve will not be a prime precedence or even an activity into account due to 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 thought of performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help discover valve deficiencies and degradations that may not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller throughout a course of journey meet most of the 12 requirements of a proof check.
Process journeys versus proof checks
How can proof test credit be claimed for an automated valve that has failed and brought on a process trip? Reduced take a look at is a periodic check performed on SIS sensors, logic solvers and final elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the flexibility of the SIS to take the process to its protected state when an irregular course of situation is detected.
A proof take a look at should be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is normally determined through a mean likelihood of failure on demand calculation (PFDavg). Some users may select to proof take a look at based on predetermined intervals, such as proof testing sensors every 24 months and final parts every 48 months as a substitute of counting on the PFDavg calculations.
Automated valve proof tests may be done offline or on-line. Offline proof checks are normally scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic 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 costly.
But a proof test can be accomplished during a process trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns due to actual demand on the SIS throughout operation may be given credit as proof tests (fully or partial) under given conditions…the next deliberate proof take a look at may be skipped.”
These conditions are
The shutdown paperwork equal data as registered throughout corresponding proof test.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated needs to be tested separately.
The shutdown happens inside a predetermined maximum time window earlier than the following planned proof test which may then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about performed. A pattern listing of actions carried out throughout a proof take a look at, together with those which are performed throughout a course of trip, is proven in Figure 2. Even without an automated valve leak take a look at, knowledge captured by the DVC alone can doubtlessly account for a good amount of proof check coverage for an automated valve.
The exact coverage depends on the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the probability of their incidence and the share of those degradations that might be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can usually be sufficient to satisfy a serious part of the proof take a look at necessities.
If the process journey takes place within a predetermined maximum time window, the end person may choose to leverage the process journey as a proof test by completing steps one via 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 utmost time window should be the last half of the current proof take a look at interval.
Figure 3. Data throughout a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made available 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 might result in a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, speed controller, etc., as a result of moisture, debris or alignment issues. This causes a lack of functional margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or associated flow control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball brought on by system situations, leakage or debris, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components corresponding to solenoids, pilots, pace controller, and so on., due to moisture, debris or alignment. The valve will fail to open or close.
All of these situations may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is caught open will exhibit itself as an increase in breakout force. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly monitors for internal faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the supply pressure is just too low, an alert is generated (Figure 4) so that the end person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the management system.
Other process journey benefits
Diagnostic information captured throughout a course of trip may reveal valve degradations that may not be detected during a proof take a look at. For example, diagnostic information captured during a process journey may point out an issue with the valve closing utterly in opposition to the full stress of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is more correct under actual operating conditions. This results in a extra correct SIF response time (the time from detection of an abnormal situation to ultimate factor reaching its trip state), which is in comparison with the Process Safety Time to make sure the SIF continues to be assembly its design necessities.
Although automated valves are very reliable, failures identified in a process journey can present valuable data to prevent future failures. This info can assist with turnaround planning by ensuring the needed elements are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s subsequent scheduled proof test, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process journey as a proof test. Even if the tip user chooses to not take proof test credits for a process journey, the valve diagnostic knowledge supplied by the DVC might help plant personnel make proactive valve maintenance decisions..
Shareg