Valve proof take a look at credit for a course of journey

A course of journey occurs when the protection instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or different gear to its trip state in response to an irregular process situation. In some cases, a spurious journey happens because of a valve malfunction. In that case, the valve meeting is examined underneath real operating situations, which provides a possibility to capture 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 talk about how DVCs can help decide the proof check credit score for an automatic valve after a course of journey.
Process journey
A process trip occurs when the SIS detects an irregular process condition via sensors corresponding to temperature and strain, executes the logic and places the process in its secure state by tripping the final 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 shut a valve downstream of the automated valve as a secondary action.
When a process journey happens, the principle aim is normally to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged gadgets. Taking the opportunity to proof test an automatic valve won’t be a prime priority and even an exercise into consideration as a outcome of 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 could be thought of carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help discover valve deficiencies and degradations that may not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet most of the 12 necessities of a proof test.
Process journeys versus proof tests
How can proof check credit be claimed for an automated valve that has failed and brought on a process trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and final parts — 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 process to its secure state when an abnormal course of condition is detected.
A proof check ought to be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually decided by way of a mean probability of failure on demand calculation (PFDavg). Some users could choose to proof test primarily based on predetermined intervals, such as proof testing sensors each 24 months and last components every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks could be accomplished offline or online. Offline proof exams are often scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automated valve online often requires a unit or equipment to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and expensive.
But a proof test can also be accomplished during a process trip.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of actual demand on the SIS during operation may be given credit as proof tests (fully or partial) beneath given conditions…the subsequent planned proof take a look at may be skipped.”
These circumstances are
The shutdown documents equal data as registered throughout corresponding proof check.
pressure gauge 4 นิ้ว covers all components of the SIS, and if not, the device or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined maximum time window before the subsequent planned proof check which may then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought of performed. A sample listing of activities carried out during a proof test, together with those which are performed throughout a course of journey, is shown in Figure 2. Even with out an automatic valve leak take a look at, information captured by the DVC alone can potentially account for a good amount of proof check protection for an automatic valve.
The actual protection is decided by the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the likelihood of their prevalence and the percentage of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of trip can typically be enough to fulfill a serious part of the proof check necessities.
If the method journey takes place within a predetermined most time window, the end user may select to leverage the process journey as a proof take a look at by completing steps one by way of five in Figure 2, that are often 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 ought to be the final half of the present proof check interval.
Figure 3. Data during a process journey 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 most probably root causes of automated valve degradations — which might result in a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements similar to solenoids, pilots, speed controller, etc., because of moisture, debris or alignment points. This causes a lack of useful margin and makes the valve gradual to open or close.
Binding, galling or other degradation of valve seats or associated flow management trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the strain obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system conditions, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements such as solenoids, pilots, pace controller, and so on., due to moisture, debris or alignment. The valve will fail to open or close.
All of these conditions could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s caught open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also continuously screens for inside faults as properly as its inputs corresponding to provide pressure. When the DVC6200SIS sees the supply stress is simply too low, an alert is generated (Figure 4) in order that the end person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide pressure, so it alerted the management system.
Other process trip benefits
Diagnostic knowledge captured during a course of journey might reveal valve degradations that will not be detected throughout a proof take a look at. For example, diagnostic information captured throughout a process journey might 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 throughout a process trip is more correct underneath real working situations. This ends in a more accurate SIF response time (the time from detection of an abnormal situation to ultimate element reaching its journey state), which is in comparability with the Process Safety Time to ensure the SIF continues to be assembly its design necessities.
Although automated valves are very reliable, failures identified in a process trip can present valuable knowledge to prevent future failures. This info can help with turnaround planning by making sure the wanted components are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
A course of journey can present the protection required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC could be analyzed to leverage the process trip as a proof take a look at. Even if the top user chooses to not take proof test credit for a course of journey, the valve diagnostic information offered by the DVC may help plant personnel make proactive valve maintenance choices..

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