Valve proof test credit score for a course of journey

A course of journey occurs when the protection 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 irregular course of situation. In some cases, a spurious journey occurs because of a valve malfunction. In that case, the valve meeting is tested under actual working situations, which provides a chance to seize priceless valve diagnostic information.
However, such diagnostics knowledge 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 check credit score for an automatic valve after a process trip.
Process journey

A course of journey happens when the SIS detects an abnormal course of situation via sensors corresponding to temperature and strain, executes the logic and locations the method in its protected state by tripping the final components similar to closing an automated valve, stopping a pump, and so on. The SIS may talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of journey occurs, the main objective is normally to restart the unit or equipment that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken units. Taking the chance to proof take a look at an automated valve is not going to be a high precedence and even an activity into account because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve could be considered performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline may help discover valve deficiencies and degradations that could 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 throughout a course of trip meet many of the 12 requirements of a proof take a look at.
Process journeys versus proof checks

How can proof check credit be claimed for an automatic valve that has failed and triggered a process trip? A proof take a look at is a periodic test carried out on SIS sensors, logic solvers and last parts — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its protected state when an irregular course of condition is detected.
A proof check ought to be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined via an average chance of failure on demand calculation (PFDavg). Some customers could choose to proof test based mostly on predetermined intervals, such as proof testing sensors each 24 months and last elements every forty eight months instead of counting on the PFDavg calculations.
Automated valve proof checks could be carried out offline or on-line. Offline proof tests are usually scheduled during a turnaround, when the process is totally shut down.
Proof testing an automatic valve on-line often requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and costly.
But a proof check can additionally be accomplished throughout a process journey.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns because of actual demand on the SIS during operation could also be given credit as proof exams (fully or partial) underneath given conditions…the subsequent planned proof take a look at could additionally be skipped.”

These circumstances are

The shutdown documents equivalent info as registered during corresponding proof check.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown happens inside a predetermined most time window earlier than the next deliberate proof test which may then be canceled

When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof check may be thought of performed. A sample listing of actions carried out throughout a proof test, together with these that are carried out during a process journey, is shown in Figure 2. Even without an automated valve leak take a look at, data captured by the DVC alone can potentially account for a great quantity of proof take a look at protection for an automated valve.
The actual protection is dependent upon the design of the automated valve, its DVC and its utility. The protection is estimated based on the valve degradations, the chance of their occurrence and the share of these degradations that might be detected by a DVC. Fortunately, knowledge acquired by a DVC during a course of trip can often be enough to satisfy a serious part of the proof test necessities.
If pressure gauge ไท วัสดุ takes place inside a predetermined maximum time window, the end user may choose to leverage the process trip as a proof take a look at by finishing steps one via 5 in Figure 2, which are normally not accomplished in a process journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the final half of the present proof take a look at interval.
Figure 3. Data during a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
Common causes of valve failure

According to ISA Technical Report ninety six.05.01, the most likely root causes of automated valve degradations — which can lead to a process journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, etc., due to moisture, debris or alignment issues. This causes a lack of useful margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or associated flow control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the strain available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to 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, pace controller, etc., due to moisture, particles or alignment. The valve will fail to open or shut.
All of these circumstances may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load in comparability with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly displays for inside faults as nicely as its inputs corresponding to provide strain. When the DVC6200SIS sees the availability strain is just too low, an alert is generated (Figure 4) so that the tip person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the management system.
Other course of trip advantages

Diagnostic data captured during a process trip might reveal valve degradations that may not be detected during a proof test. For instance, diagnostic knowledge captured during a process trip would possibly indicate an issue with the valve closing completely against the total strain 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 extra correct underneath actual working conditions. This leads to a more correct SIF response time (the time from detection of an irregular condition to ultimate factor reaching its journey state), which is in comparability with the Process Safety Time to make sure the SIF continues to be assembly its design necessities.
Although automated valves are very reliable, failures identified in a process journey can provide useful data to prevent future failures. This info can help with turnaround planning by ensuring the needed elements can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.

A process trip can present the coverage required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process journey as a proof check. Even if the top user chooses to not take proof take a look at credit for a process trip, the valve diagnostic knowledge offered by the DVC might help plant personnel make proactive valve upkeep selections..


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