Valve proof test credit for a course of journey

A course of trip happens when the security instrumented system (SIS) locations the method in its protected state by commanding an automatic valve (Figure 1) or other equipment to its trip state in response to an abnormal course of condition. In some cases, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is examined underneath real operating situations, which offers a possibility to seize priceless valve diagnostic information.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs can help decide the proof take a look at credit for an automatic valve after a course of journey.
Process trip
A course of journey occurs when the SIS detects an irregular process condition via sensors corresponding to temperature and stress, executes the logic and places the method in its secure state by tripping the ultimate parts corresponding to closing an automatic 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 action.
When a process trip occurs, the principle objective is normally to restart the unit or gear that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the chance to proof test an automatic valve is not going to be a top precedence or even an activity under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof check of the valve may be considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help discover valve deficiencies and degradations that received’t present up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a process journey meet a lot of the 12 requirements of a proof check.
Process journeys versus proof exams
How can proof take a look at credits be claimed for an automatic valve that has failed and triggered a process trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and last elements — such as automated valves — to discover hidden failures. These failures, if not corrected or eliminated, could impair the power of the SIS to take the method to its safe state when an abnormal course of condition is detected.
A proof check ought to be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is normally decided by way of a median chance of failure on demand calculation (PFDavg). Some users may select to proof check based mostly on predetermined intervals, such as proof testing sensors each 24 months and last elements every 48 months instead of counting on the PFDavg calculations.
Automated valve proof tests can be done offline or on-line. Offline proof exams are often scheduled during a turnaround, when the method is totally 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 prevent a spurious trip, making it inconvenient and dear.
But a proof take a look at may also be completed throughout a process journey.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns due to precise demand on the SIS during operation could also be given credit as proof tests (fully or partial) under given conditions…the subsequent deliberate proof check could also be skipped.”
These circumstances are
The shutdown paperwork equivalent data as registered throughout corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown occurs inside a predetermined maximum time window before the following deliberate proof test which can then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof check could be thought-about performed. A sample listing of activities carried out throughout a proof take a look at, along with these which may be carried out during a process trip, is proven in Figure 2. Even without an automated valve leak test, data captured by the DVC alone can probably account for a good amount of proof check coverage for an automatic valve.
The actual coverage depends on the design of the automated valve, its DVC and its utility. The protection is estimated primarily based on the valve degradations, the probability of their occurrence and the proportion of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of trip can usually be sufficient to satisfy a significant a half of the proof test necessities.
If the process trip takes place inside a predetermined maximum time window, the top person may select to leverage the process journey as a proof test by finishing steps one via five in Figure 2, which are normally not completed in a process trip. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the last half of the present proof test interval.
Figure 3. Data during a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for analysis.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most probably root causes of automated valve degradations — which may lead to a course of journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, pace controller, and so on., due to moisture, debris or alignment points. This causes a loss of functional margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or associated circulate control trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, wear or looseness that reduces the pressure out there to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system circumstances, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements corresponding to solenoids, pilots, velocity controller, etc., as a result of moisture, debris or alignment. The valve will fail to open or close.
All of these situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is caught open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS also continuously displays for internal faults in addition to its inputs corresponding to provide stress. When the DVC6200SIS sees the provision strain is simply too low, an alert is generated (Figure 4) so that the top consumer can take corrective actions.
pressure gauge trerice ราคา . The DVC6200SIS digital valve controller detected almost zero supply stress, so it alerted the control system.
Other course of trip advantages
Diagnostic data captured throughout a course of trip could reveal valve degradations that may not be detected throughout a proof take a look at. For example, diagnostic data captured throughout a course of journey would possibly point out an issue with the valve closing completely against the full pressure of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is extra accurate under actual working situations. This leads to a more accurate SIF response time (the time from detection of an abnormal situation to final element reaching its journey state), which is in comparability with the Process Safety Time to ensure the SIF remains to be meeting its design necessities.
Although automated valves are very dependable, failures identified in a course of journey can provide useful information to prevent future failures. This information can assist with turnaround planning by ensuring the needed components are available before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can present the coverage 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 method journey as a proof take a look at. Even if the tip consumer chooses not to take proof take a look at credit for a process journey, the valve diagnostic knowledge provided by the DVC may help plant personnel make proactive valve maintenance selections..
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