A course of trip occurs when the security instrumented system (SIS) locations the process in its secure state by commanding an automatic valve (Figure 1) or different gear to its journey state in response to an irregular course of condition. In some circumstances, a spurious journey happens because of a valve malfunction. In that case, the valve meeting is tested underneath real operating situations, which supplies an opportunity to seize priceless valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs might help determine the proof test credit for an automatic valve after a course of trip.
Process journey
A course of trip occurs when the SIS detects an abnormal process situation by way of sensors corresponding to temperature and pressure, executes the logic and locations the process in its safe state by tripping the ultimate parts corresponding to closing an automated valve, stopping a pump, etc. ตัววัดแรงดันน้ำ may talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process journey happens, the primary objective is usually to restart the unit or tools that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof take a look at an automatic valve is not going to be a top 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 test of the valve may be thought of performed with diagnostic information captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline might help discover valve deficiencies and degradations that may not present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a process trip meet many of the 12 requirements of a proof check.
Process trips versus proof tests
How can proof take a look at credit be claimed for an automatic valve that has failed and triggered a process trip? A proof take a look at is a periodic check carried out on SIS sensors, logic solvers and last elements — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the ability of the SIS to take the method to its safe state when an irregular process situation is detected.
A proof test should be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is normally decided by way of a median likelihood of failure on demand calculation (PFDavg). Some users could select to proof take a look at based on predetermined intervals, corresponding to proof testing sensors every 24 months and ultimate parts every forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks can be accomplished offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and expensive.
But a proof check may additionally be completed during a process journey.
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 checks (fully or partial) underneath given conditions…the subsequent planned proof check may be skipped.”
These circumstances are
The shutdown documents equivalent info as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined most time window earlier than the next planned proof test which may then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about performed. A pattern listing of activities carried out during a proof check, along with these which are carried out during a course of trip, 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 coverage for an automatic valve.
The exact coverage is dependent upon the design of the automated valve, its DVC and its software. The protection is estimated primarily based on the valve degradations, the probability of their incidence and the share of these degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a process journey can typically be enough to fulfill a serious a part of the proof check necessities.
If the process trip takes place inside a predetermined maximum time window, the top person may select to leverage the method trip as a proof check by finishing steps one through five in Figure 2, which are often not accomplished in a process trip. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the final half of the present proof test interval.
Figure three. Data during a course of 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 ninety six.05.01, the more than likely root causes of automated valve degradations — which might lead to a course of journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, speed controller, and so on., due to moisture, debris or alignment issues. This causes a lack of useful margin and makes the valve sluggish to open or close.
Binding, galling or other degradation of valve seats or associated move control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the stress available to actuate the valve.
Minor harm to the valve obturator plug, disk or ball caused by system circumstances, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system components similar to solenoids, pilots, velocity controller, etc., due to moisture, particles 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 example, a valve that is 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 also repeatedly monitors for internal faults in addition to its inputs such as supply strain. When the DVC6200SIS sees the supply pressure is just too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the control system.
Other process trip advantages
Diagnostic information captured during a process journey might reveal valve degradations that may not be detected throughout a proof test. For instance, diagnostic information captured throughout a process trip might point out an issue with the valve closing fully against the complete stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more correct beneath real operating circumstances. This ends in a extra correct SIF response time (the time from detection of an irregular situation to last factor reaching its trip state), which is compared to 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 course of journey can provide useful data to prevent future failures. This information can assist with turnaround planning by making sure the needed elements are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
A process journey can provide the protection required to delay a valve’s subsequent scheduled proof take a look at, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process journey as a proof check. Even if the tip consumer chooses to not take proof take a look at credit for a course of journey, the valve diagnostic information provided by the DVC can help plant personnel make proactive valve upkeep selections..