A process journey happens when the security instrumented system (SIS) places the method in its secure state by commanding an automatic valve (Figure 1) or different equipment to its trip state in response to an irregular course of condition. In some instances, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is examined under real working situations, which provides a possibility to capture useful 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 focus on how DVCs can help determine the proof take a look at credit score for an automatic valve after a course of trip.
Process journey
A course of trip happens when the SIS detects an abnormal course of condition via sensors similar to temperature and pressure, executes the logic and locations the process in its protected state by tripping the ultimate elements similar to closing an automated valve, stopping a pump, etc. The SIS may communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process journey occurs, the primary goal is usually to restart the unit or gear that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof take a look at an automatic valve will not 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 equipped with a DVC, the proof test of the valve could be thought of carried out with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline may 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 carried out (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of journey meet most of the 12 necessities of a proof take a look at.
Process trips versus proof checks
How can proof test credits be claimed for an automatic valve that has failed and brought on a course of trip? A proof check is a periodic check performed on SIS sensors, logic solvers and ultimate elements — corresponding to automated valves — to find hidden failures. These failures, if not corrected or eliminated, may impair the power of the SIS to take the method to its protected state when an irregular course of situation is detected.
A proof check should be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is usually determined via a mean chance of failure on demand calculation (PFDavg). Some users could choose to proof check primarily based on predetermined intervals, corresponding to proof testing sensors each 24 months and last components each 48 months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams can be carried out offline or online. Offline proof checks are often scheduled during a turnaround, when the process is totally shut down.
Proof testing an automatic valve online often requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and expensive.
But a proof take a look at can be accomplished during a course of trip.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns as a result of actual demand on the SIS during operation may be given credit as proof checks (fully or partial) under given conditions…the subsequent planned proof take a look at could additionally be skipped.”
These circumstances are
The shutdown documents equal info as registered during corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the gadget or SIS subsystem not activated must be tested separately.
The shutdown occurs within a predetermined most time window earlier than the subsequent deliberate proof test which can then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof test can be considered carried out. A pattern listing of activities carried out during a proof test, along with those which are carried out during a process trip, is shown in Figure 2. Even without an automated valve leak check, information captured by the DVC alone can potentially account for a good amount of proof check protection for an automatic valve.
The precise 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 likelihood of their incidence and the proportion of these degradations that may be detected by a DVC. Fortunately, data acquired by a DVC during a course of journey can usually be enough to satisfy a serious a part of the proof test requirements.
If the process trip takes place inside a predetermined most time window, the top person might choose to leverage the process trip as a proof check by finishing steps one via five in Figure 2, which are usually not accomplished in a process journey. The subsequent 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 take a look at interval.
Figure three. Data during a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for evaluation.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most likely root causes of automated valve degradations — which may lead to a process trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, speed controller, etc., because of moisture, debris or alignment issues. This causes a loss of functional margin and makes the valve sluggish to open or close.
Binding, galling or different degradation of valve seats or related circulate management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, wear or looseness that reduces the strain available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system situations, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, speed controller, etc., as a result of moisture, debris or alignment. The valve will fail to open or shut.
All of those situations may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For เกจวัดแก๊สlpg , a valve that’s stuck open will exhibit itself as a rise in breakout pressure. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously screens for inner faults in addition to its inputs corresponding to provide pressure. When the DVC6200SIS sees the availability stress is merely too low, an alert is generated (Figure 4) so that the end user 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 advantages
Diagnostic data captured during a process journey might reveal valve degradations that is probably not detected during a proof check. For example, diagnostic knowledge captured throughout a course of journey may point out a problem with the valve closing fully against the full stress of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra accurate under real working situations. This leads to a extra accurate SIF response time (the time from detection of an irregular situation to ultimate element reaching its trip state), which is compared to the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very dependable, failures identified in a process trip can provide valuable knowledge to forestall future failures. This information can assist with turnaround planning by making sure the needed components are available earlier than turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of journey can present the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method journey as a proof check. Even if the tip consumer chooses to not take proof take a look at credit for a process trip, the valve diagnostic knowledge offered by the DVC might help plant personnel make proactive valve maintenance choices..
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