Valve proof check credit score for a course of trip

A process journey occurs when the protection instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or different gear to its journey state in response to an irregular course of situation. In some instances, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is tested beneath real operating conditions, which provides an opportunity to capture valuable valve diagnostic knowledge.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help determine the proof test credit score for an automated valve after a course of trip.
Process trip
A course of journey happens when the SIS detects an irregular process condition through sensors similar to temperature and strain, executes the logic and locations the process in its safe state by tripping the ultimate elements similar to closing an automated valve, stopping a pump, and so forth. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process trip occurs, the primary objective is often to restart the unit or tools that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof take a look at an automatic valve is not going to be a high precedence or even an activity into account as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve can be thought-about performed with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline can help discover valve deficiencies and degradations that won’t show up in a proof take a look at.
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 course of trip meet a lot of the 12 requirements of a proof test.
Process journeys versus proof tests
How can proof test credit be claimed for an automated valve that has failed and brought on a course of trip? A proof check is a periodic test performed on SIS sensors, logic solvers and ultimate parts — such as automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its secure state when an abnormal process situation is detected.
A proof take a look at should be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined via a mean chance of failure on demand calculation (PFDavg). Some users might select to proof test based mostly on predetermined intervals, similar to proof testing sensors every 24 months and final parts every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks may be accomplished offline or on-line. Offline proof checks are normally scheduled during a turnaround, when the method is completely shut down.
Proof testing an automated 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 test can also be completed during a course of journey.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns as a end result of precise demand on the SIS during operation could additionally be given credit score as proof exams (fully or partial) under given conditions…the subsequent planned proof take a look at could also be skipped.”
These circumstances are
The shutdown paperwork equal data as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be tested separately.
The shutdown occurs inside a predetermined maximum time window before the following planned proof take a look at which may then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof test can be thought-about performed. A sample listing of actions carried out during a proof check, along with these which may be performed throughout a process trip, is proven in Figure 2. Even with out an automated valve leak test, information captured by the DVC alone can potentially account for a good quantity of proof take a look at protection for an automatic valve.
The actual protection is dependent upon the design of the automated valve, its DVC and its application. The protection is estimated based on the valve degradations, the likelihood of their occurrence and the percentage of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of trip can typically be enough to satisfy a significant part of the proof test requirements.
If the process journey takes place inside a predetermined most time window, the top consumer may choose to leverage the process trip as a proof take a look at by completing steps one via five in Figure 2, that are usually not completed 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 current proof check interval.
Figure 3. Data during a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most likely root causes of automated valve degradations — which might lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, etc., because of moisture, debris or alignment points. This causes a lack of useful margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or related flow management trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components similar to solenoids, pilots, speed controller, and so forth., as a result of moisture, particles or alignment. The valve will fail to open or close.
All of these circumstances may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is stuck open will exhibit itself as an increase in breakout force. Closet of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also continuously screens for internal faults in addition to its inputs corresponding to supply strain. When the DVC6200SIS sees the availability strain is simply too low, an alert is generated (Figure 4) in order that the tip consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply stress, so it alerted the control system.
Other course of trip benefits
Diagnostic knowledge captured during a process trip could reveal valve degradations that is most likely not detected throughout a proof test. For instance, diagnostic data captured throughout a course of trip may point out an issue with the valve closing completely towards the full strain of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more correct under actual working circumstances. This ends in a extra accurate SIF response time (the time from detection of an abnormal situation to final component reaching its journey state), which is compared to the Process Safety Time to make sure the SIF continues to be meeting its design necessities.
Although automated valves are very dependable, failures identified in a course of journey can present useful knowledge to stop future failures. This data can assist with turnaround planning by making sure the wanted parts are available before turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of journey can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof take a look at. Even if the end consumer chooses not to take proof check credits for a process journey, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve upkeep selections..
Share

Leave a Comment