Valve proof take a look at credit score for a process journey

A process trip occurs when the safety instrumented system (SIS) places the process in its protected state by commanding an automatic valve (Figure 1) or different gear to its trip state in response to an abnormal course of situation. In some cases, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined beneath actual working situations, which offers an opportunity to capture priceless valve diagnostic information.
However, such diagnostics knowledge can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs can help determine the proof check credit for an automated valve after a process journey.
Process journey
A process trip happens when the SIS detects an irregular course of situation via sensors such as temperature and stress, executes the logic and locations the method in its protected state by tripping the final parts similar to closing an automatic valve, stopping a pump, etc. The SIS may talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a course of trip occurs, the principle objective is often to restart the unit or equipment that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken units. Taking the opportunity to proof test an automatic valve is not going to be a high priority and even an activity under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve could be thought of carried out with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help uncover valve deficiencies and degradations that may not 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 throughout a course of trip meet most of the 12 necessities of a proof check.
Process trips versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and triggered a course of trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and final elements — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the ability of the SIS to take the method to its safe state when an irregular process situation is detected.
A proof test must be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often determined via a mean chance of failure on demand calculation (PFDavg). Some users may choose to proof check based mostly on predetermined intervals, corresponding to proof testing sensors each 24 months and final elements every forty eight months instead of relying on the PFDavg calculations.
Automated valve proof exams may be done offline or online. Offline proof exams are usually scheduled throughout 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 stop a spurious trip, making it inconvenient and expensive.
But a proof check can be accomplished during a process trip.
According to เกจวัดแรงดันต่ำ -2 Section A.sixteen.three.1.3, “…shutdowns because of precise demand on the SIS during operation may be given credit score as proof tests (fully or partial) under given conditions…the subsequent planned proof take a look at may be skipped.”
These conditions are
The shutdown documents equal data as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be examined separately.
The shutdown occurs within a predetermined most time window earlier than the following planned proof test which can then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof test may be thought of carried out. A sample record of activities performed during a proof check, together with these which would possibly be performed during a process trip, is proven in Figure 2. Even with out an automated valve leak test, information captured by the DVC alone can doubtlessly account for an excellent amount of proof test protection for an automatic valve.
The precise coverage is dependent upon 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 incidence and the share of those degradations that may be detected by a DVC. Fortunately, information acquired by a DVC during a process trip can typically be enough to fulfill a major part of the proof check necessities.
If the process journey takes place within a predetermined maximum time window, the end consumer could select to leverage the method trip as a proof check by finishing steps one by way of five in Figure 2, which are normally not completed in a process journey. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the last half of the current proof take a look at interval.
Figure three. Data during a course of journey 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 probably root causes of automated valve degradations — which might lead to a course of journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, velocity controller, and so forth., as a outcome of moisture, debris or alignment issues. This causes a lack of functional margin and makes the valve gradual to open or close.
Binding, galling or different degradation of valve seats or associated circulate management trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system conditions, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components similar to solenoids, pilots, pace controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances can 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 pressure. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly monitors for internal faults as properly as its inputs similar to provide strain. When the DVC6200SIS sees the supply strain is too low, an alert is generated (Figure 4) so that the top consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the management system.
Other process trip benefits
Diagnostic data captured during a process journey may reveal valve degradations that may not be detected during a proof test. For instance, diagnostic information captured during a course of journey might indicate an issue with the valve closing fully towards the total stress of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is more accurate underneath actual working conditions. This ends in a more accurate SIF response time (the time from detection of an irregular situation to last element reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF continues to be assembly its design necessities.
Although automated valves are very dependable, failures recognized in a process trip can present priceless knowledge to forestall future failures. This info can help with turnaround planning by making sure the needed components can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A process journey can provide 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 top consumer chooses to not take proof check credit for a process trip, the valve diagnostic data provided by the DVC might help plant personnel make proactive valve maintenance decisions..
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