Valve proof check credit for a process journey

A process journey occurs when the security instrumented system (SIS) locations the method in its protected state by commanding an automated valve (Figure 1) or other tools to its trip state in response to an irregular process situation. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined underneath actual operating situations, which supplies an opportunity to capture useful valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs may help determine the proof take a look at credit score for an automatic valve after a process trip.
Process trip
A process journey happens when the SIS detects an abnormal course of situation by way of sensors such as temperature and stress, executes the logic and locations the method in its secure state by tripping the final elements similar to closing an automatic valve, stopping a pump, and so on. The SIS might talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of journey happens, the primary objective is normally to restart the unit or gear that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the chance to proof check an automatic valve won’t be a prime precedence and even an activity into consideration 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 can be considered carried out with diagnostic information captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline might help uncover valve deficiencies and degradations that will not present up in a proof check.
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 process trip meet a lot of the 12 necessities of a proof take a look at.
Process journeys versus proof tests
How can proof check credits be claimed for an automatic valve that has failed and brought on a course of trip? A proof test is a periodic check performed on SIS sensors, logic solvers and last parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the ability of the SIS to take the method to its safe state when an abnormal process situation is detected.
A proof test must be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is normally determined via an average likelihood of failure on demand calculation (PFDavg). Some customers may select to proof check based mostly on predetermined intervals, such as proof testing sensors every 24 months and ultimate components each 48 months as an alternative of counting on the PFDavg calculations.
Automated valve proof exams could be done offline or on-line. Offline proof exams are usually scheduled during a turnaround, when the method is totally shut down.
Proof testing an automated valve on-line usually 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 check may also be achieved throughout a course of trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns as a end result of precise demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) beneath given conditions…the next deliberate proof test may be skipped.”
These circumstances are
The shutdown documents equivalent info as registered throughout corresponding proof check.
The shutdown covers all parts of the SIS, and if not, the gadget or SIS subsystem not activated needs to be tested separately.
The shutdown happens within a predetermined most time window earlier than the following planned proof test which can then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about carried out. A sample listing of activities performed throughout a proof take a look at, together with those which may be performed during a process trip, is proven in Figure 2. Even with out an automatic valve leak take a look at, information captured by the DVC alone can probably account for an excellent amount of proof test protection for an automated valve.
The precise protection is dependent upon the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the likelihood of their occurrence and the share of these degradations that may be detected by a DVC. Fortunately, data acquired by a DVC during a course of trip can typically be sufficient to satisfy a major a half of the proof test requirements.
If the process trip takes place inside a predetermined maximum time window, the tip person may select to leverage the method trip as a proof check by completing steps one via 5 in Figure 2, which are usually not completed in a process journey. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the last half of the current proof test interval.
Figure three. Data during a process trip 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 more than likely root causes of automated valve degradations — which may result in a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, speed controller, and so forth., because of moisture, debris or alignment points. This causes a loss of useful margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or associated flow control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the pressure available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system situations, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements such as solenoids, pilots, speed controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as a rise in breakout pressure. Loss of seat load in comparability with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously screens for internal faults in addition to its inputs similar to supply stress. When the DVC6200SIS sees the availability strain is too low, an alert is generated (Figure 4) in order that the end user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply strain, so it alerted the management system.
Other process trip advantages
Diagnostic data captured throughout a process journey could reveal valve degradations that will not be detected throughout a proof check. For example, diagnostic knowledge captured during a course of trip might point out a problem with the valve closing fully against the total pressure of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is more correct underneath actual working situations. This ends in a more correct SIF response time (the time from detection of an abnormal condition to final factor reaching its trip state), which is in comparability with the Process Safety Time to ensure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures recognized in a course of journey can provide priceless data to stop future failures. This data can help with turnaround planning by ensuring the needed elements can be found before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s subsequent scheduled proof test, thus saving time and money. เกจวัดแก๊สlpg captured by a DVC can be analyzed to leverage the process trip as a proof test. Even if the end consumer chooses to not take proof check credits for a process journey, the valve diagnostic data supplied by the DVC might help plant personnel make proactive valve upkeep choices..
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