Valve proof test credit for a process trip

A course of trip occurs when the safety instrumented system (SIS) locations the process in its safe state by commanding an automatic valve (Figure 1) or different equipment to its trip state in response to an abnormal course of situation. In some circumstances, a spurious journey occurs because of a valve malfunction. In that case, the valve assembly is tested under real working circumstances, which supplies an opportunity to capture valuable 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 may help decide the proof take a look at credit score for an automatic valve after a course of trip.
Process trip
A process journey happens when the SIS detects an abnormal process situation via sensors corresponding to temperature and pressure, executes the logic and locations the process in its safe state by tripping the final components corresponding to closing an automated valve, stopping a pump, and so on. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a course of trip happens, the principle objective is often to restart the unit or equipment that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged gadgets. Taking the opportunity to proof check an automated valve won’t be a prime precedence and even an exercise into account 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 may be considered performed with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help uncover valve deficiencies and degradations that will not show 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 course of trip meet most of the 12 necessities of a proof check.
Process journeys versus proof tests
How can proof check credits be claimed for an automatic valve that has failed and triggered a course of trip? A proof test is a periodic test performed on SIS sensors, logic solvers and final components — corresponding to automated valves — to find hidden failures. These failures, if not corrected or eliminated, might impair the flexibility of the SIS to take the process to its protected state when an abnormal course of situation is detected.
A proof take a look at must be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is often determined through a median likelihood of failure on demand calculation (PFDavg). Some customers might select to proof take a look at primarily based on predetermined intervals, corresponding to proof testing sensors each 24 months and ultimate elements each 48 months as an alternative of counting on the PFDavg calculations.
Automated valve proof exams may be accomplished offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automated valve on-line normally requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and costly.
But a proof take a look at can be achieved during a course of journey.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns because of precise demand on the SIS throughout operation could additionally be given credit score as proof exams (fully or partial) under given conditions…the subsequent deliberate proof check may be skipped.”
These circumstances are
The shutdown paperwork equivalent data as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated needs to be examined individually.
The shutdown occurs within a predetermined maximum time window earlier than the subsequent deliberate proof test which can then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof check may be considered carried out. A sample listing of activities performed during a proof test, along with these which would possibly be performed throughout a course of trip, is proven in Figure 2. Even without an automated valve leak test, data captured by the DVC alone can doubtlessly account for a good amount of proof take a look at coverage for an automatic valve.
The precise protection is decided by 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 prevalence and the proportion of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a course of trip can typically be sufficient to fulfill a major a half of the proof take a look at requirements.
If the method journey takes place within a predetermined maximum time window, the end consumer might choose to leverage the process journey as a proof take a look at by completing steps one via 5 in Figure 2, which are often not accomplished in a course of trip. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the current proof check interval.
Figure three. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
เกจ์วัดแรงดัน of valve failure
According to ISA Technical Report 96.05.01, the most probably root causes of automated valve degradations — which might result in a course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, velocity controller, etc., because of moisture, particles or alignment points. This causes a loss of practical margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or related circulate control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, wear or looseness that reduces the strain obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball caused by system conditions, leakage or debris, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system components corresponding to solenoids, pilots, speed controller, and so forth., as a result of moisture, particles or alignment. The valve will fail to open or shut.
All of those situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as an increase in breakout drive. Loss of seat load in comparability with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also continuously monitors for inner faults in addition to its inputs corresponding to supply strain. When the DVC6200SIS sees the availability strain 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 supply strain, so it alerted the control system.
Other course of journey benefits
Diagnostic data captured during a process trip may reveal valve degradations that may not be detected during a proof take a look at. For instance, diagnostic knowledge captured during a course of trip might indicate a problem with the valve closing fully in opposition to the complete strain of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is more accurate underneath actual operating circumstances. This leads to a extra accurate SIF response time (the time from detection of an irregular condition to ultimate factor reaching its journey state), which is compared to the Process Safety Time to ensure the SIF continues to be meeting its design requirements.
Although automated valves are very reliable, failures identified in a process journey can present valuable data to prevent future failures. This info can assist with turnaround planning by ensuring the wanted components can be found before turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can provide the protection required to delay a valve’s subsequent scheduled proof take a look at, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method trip as a proof check. Even if the tip person chooses not to take proof check credit for a course of journey, the valve diagnostic data supplied by the DVC may help plant personnel make proactive valve maintenance choices..
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