A process journey occurs when the protection instrumented system (SIS) locations the process in its secure state by commanding an automated valve (Figure 1) or other gear to its trip state in response to an abnormal course of condition. In some instances, a spurious journey occurs because of a valve malfunction. In that case, the valve assembly is examined underneath real operating situations, which offers a chance to seize valuable valve diagnostic information.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs may help decide the proof take a look at credit for an automatic valve after a course of journey.
Process trip
A process trip happens when the SIS detects an abnormal course of condition via sensors corresponding to temperature and stress, executes the logic and locations the method in its protected state by tripping the ultimate elements corresponding to closing an automated valve, stopping a pump, etc. The SIS may communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process journey happens, the main aim is usually 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 broken gadgets. Taking the opportunity to proof check an automated valve won’t be a high priority or even an activity into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve can be thought of performed with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline can 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 knowledge captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet many of the 12 requirements of a proof take a look at.
Process journeys versus proof exams
How can proof check credits be claimed for an automated valve that has failed and brought on a process trip? A proof take a look at is a periodic test carried out on SIS sensors, logic solvers and last components — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the process to its secure state when an irregular course of situation is detected.
A proof take a look at should be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is normally determined through a mean chance of failure on demand calculation (PFDavg). Some customers could choose to proof test based on predetermined intervals, similar to proof testing sensors every 24 months and ultimate components every 48 months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams can be carried out offline or online. Offline proof exams are usually scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve online usually requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious trip, making it inconvenient and dear.
But a proof take a look at may additionally be accomplished throughout a process trip.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns because of precise demand on the SIS during operation could also be given credit as proof tests (fully or partial) under given conditions…the next planned proof test could additionally be skipped.”
These circumstances are
The shutdown documents equivalent data as registered throughout corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the gadget or SIS subsystem not activated needs to be tested individually.
The shutdown occurs within a predetermined maximum time window before the following deliberate proof take a look at which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of carried out. A sample list of actions carried out throughout a proof check, together with those that are performed throughout a process trip, is shown in Figure 2. Even with out an automated valve leak test, information captured by the DVC alone can doubtlessly account for a good amount of proof test coverage for an automatic valve.
The actual protection depends on the design of the automated valve, its DVC and its utility. The protection is estimated based mostly on the valve degradations, the likelihood of their incidence and the proportion of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can typically be sufficient to fulfill a serious a half of the proof take a look at necessities.
If the method journey takes place inside a predetermined maximum time window, the tip person could select to leverage the process trip as a proof check by finishing steps one through 5 in Figure 2, which are normally not completed in a process journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the last half of the current proof test interval.
Figure three. Data throughout 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 may result in a process journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so forth., as a end result of moisture, debris or alignment points. This causes a loss of useful margin and makes the valve gradual to open or close.
Binding, galling or other degradation of valve seats or related flow management trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, put on or looseness that reduces the pressure out there to actuate the valve.
เกจวัดแรงดัน4นิ้วราคา to the valve obturator plug, disk or ball attributable to system conditions, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components corresponding to solenoids, pilots, speed controller, and so forth., because of moisture, debris or alignment. The valve will fail to open or shut.
All of these conditions 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 force. Loss of seat load in comparability with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly monitors for internal faults as well as its inputs corresponding to supply pressure. When the DVC6200SIS sees the availability strain is too low, an alert is generated (Figure 4) in order that the end consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply strain, so it alerted the control system.
Other process journey benefits
Diagnostic knowledge captured throughout a course of trip could reveal valve degradations that is most likely not detected throughout a proof check. For example, diagnostic information captured throughout a process trip may point out an issue with the valve closing fully in opposition to the full stress of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is more accurate underneath real working situations. This ends in a more accurate SIF response time (the time from detection of an irregular situation to final factor reaching its journey state), which is compared to 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 course of trip can provide useful data to forestall future failures. This information can assist with turnaround planning by ensuring the wanted components are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process journey as a proof take a look at. Even if the top user chooses not to take proof take a look at credits for a course of trip, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve maintenance decisions..
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