Valve proof test credit for a course of journey

A process trip happens when the safety instrumented system (SIS) locations the process in its protected state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal process condition. In some instances, a spurious trip occurs due to a valve malfunction. In that case, the valve meeting is examined under real operating situations, which offers an opportunity to capture priceless valve diagnostic knowledge.
However, such diagnostics data can solely 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 take a look at credit for an automated valve after a process journey.
Process trip
A process journey happens when the SIS detects an abnormal process condition through sensors such as temperature and strain, executes the logic and places the process in its safe state by tripping the ultimate elements such as closing an automatic valve, stopping a pump, and so forth. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a course of trip occurs, the principle goal is usually to restart the unit or tools that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the chance to proof take a look at an automated valve won’t be a high precedence or even an activity into consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve could be thought-about carried out with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline may help discover valve deficiencies and degradations that gained’t 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 during a process trip meet a lot of the 12 necessities of a proof test.
Process journeys versus proof tests
How can proof test credits be claimed for an automatic valve that has failed and brought on a course of trip? A proof take a look at is a periodic check carried out on SIS sensors, logic solvers and final parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, could impair the power of the SIS to take the process to its protected state when an abnormal course of condition is detected.
A proof test should be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is often determined through a mean probability of failure on demand calculation (PFDavg). Some users could select to proof check primarily based on predetermined intervals, corresponding to proof testing sensors each 24 months and final parts every 48 months instead of counting on the PFDavg calculations.
Automated valve proof checks can be done offline or on-line. Offline proof exams are usually scheduled during a turnaround, when the method is totally shut down.
Proof testing an automatic valve online usually requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and costly.
But a proof check can additionally be achieved during a course of trip.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns as a result of actual demand on the SIS throughout operation could additionally be given credit as proof tests (fully or partial) underneath given conditions…the subsequent planned proof check may be skipped.”
These circumstances are
The shutdown paperwork equal data as registered during corresponding proof check.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown occurs inside a predetermined most time window earlier than the next deliberate proof take a look at which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof test could be thought of performed. A sample listing of actions performed throughout a proof check, along with these that are carried out during a process journey, is proven in Figure 2. Even without an automatic valve leak take a look at, knowledge captured by the DVC alone can probably account for an excellent quantity of proof test protection for an automatic valve.
The actual coverage 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 probability of their occurrence and the proportion of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a course of trip can usually be enough to satisfy a major a half of the proof take a look at necessities.
If the process trip takes place within a predetermined maximum time window, the top person could choose to leverage the method trip as a proof check by completing steps one via 5 in Figure 2, which are normally not accomplished in a course of trip. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the current proof check interval.
Figure 3. Data during 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 96.05.01, the more than likely root causes of automated valve degradations — which can lead to a process journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, velocity controller, and so on., due to moisture, debris or alignment points. This causes a lack of practical margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or associated circulate management trim that restricts or resists valve motion.
pressure gauge ราคา to compression, put on or looseness that reduces the pressure out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system circumstances, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, speed controller, and so forth., due to moisture, debris or alignment. The valve will fail to open or shut.
All of these circumstances 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 drive. Loss of seat load in comparison with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly screens for internal faults as properly as its inputs corresponding to provide stress. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) so that the end user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide pressure, so it alerted the control system.
Other process trip advantages
Diagnostic data captured throughout a course of trip might reveal valve degradations that may not be detected throughout a proof test. For example, diagnostic data captured during a process trip may indicate a problem with the valve closing utterly in opposition to the total pressure of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is more accurate under real working conditions. This results in a more accurate SIF response time (the time from detection of an irregular situation to ultimate factor reaching its journey state), which is compared to the Process Safety Time to make sure the SIF remains to be meeting its design necessities.
Although automated valves are very reliable, failures identified in a course of trip can present useful knowledge to forestall future failures. This data can assist with turnaround planning by ensuring the wanted components can be found before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can provide the protection required to delay a valve’s subsequent scheduled proof check, thus saving time and money. Data captured by a DVC can be analyzed to leverage the method trip as a proof take a look at. Even if the tip person chooses to not take proof take a look at credit for a course of trip, the valve diagnostic data provided by the DVC may help plant personnel make proactive valve maintenance choices..
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