Valve proof take a look at credit for a course of trip

A process journey occurs when the security 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 process situation. In some instances, a spurious trip occurs due to a valve malfunction. In that case, the valve assembly is examined underneath actual operating situations, which supplies an opportunity to seize priceless valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll discuss how DVCs may help decide the proof take a look at credit score for an automatic valve after a process journey.
Process journey

A course of journey occurs when the SIS detects an abnormal process situation through sensors corresponding to temperature and strain, executes the logic and places the method in its safe state by tripping the final components corresponding to closing an automatic valve, stopping a pump, and so on. 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 course of trip occurs, the main aim is normally to restart the unit or tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken units. Taking the chance to proof check an automatic valve won’t be a top priority and even an exercise into consideration as a end result of 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 carried out with diagnostic information captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline may help discover valve deficiencies and degradations that gained’t show up in a proof take a look at.
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 during a course of journey meet many of the 12 requirements of a proof check.
Process journeys versus proof tests

How can proof take a look at credit be claimed for an automated valve that has failed and triggered a course of trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and ultimate components — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or removed, could impair the ability of the SIS to take the process to its safe state when an abnormal process condition is detected.
A proof take a look at must be carried out as per the proof test interval specified in the Safety Requirement Specifications. This interval is normally determined via a mean chance of failure on demand calculation (PFDavg). digital pressure gauge could select to proof check based on predetermined intervals, similar to proof testing sensors every 24 months and last components each forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof exams may be done offline or online. Offline proof exams are usually scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automatic valve on-line often requires a unit or gear to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and dear.
But a proof test can additionally be achieved during a course of journey.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of precise demand on the SIS throughout operation may be given credit as proof exams (fully or partial) beneath given conditions…the subsequent planned proof check may be skipped.”

These conditions are

The shutdown paperwork equivalent data as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated needs to be examined separately.
The shutdown happens inside a predetermined maximum time window before the following planned proof test which can then be canceled

When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be thought of performed. A pattern listing of actions performed during a proof check, along with those that are performed during a course of journey, is shown in Figure 2. Even without an automatic valve leak take a look at, knowledge captured by the DVC alone can probably account for a good quantity of proof check protection for an automated valve.
The precise coverage depends on the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the chance of their occurrence and the proportion of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a process trip can usually be enough to satisfy a significant a half of the proof take a look at necessities.
If the process journey takes place inside a predetermined maximum time window, the top user might select to leverage the process trip as a proof check by completing steps one through five in Figure 2, which are normally not completed in a course of trip. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the current proof take a look at interval.
Figure three. Data throughout a process journey 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 most likely root causes of automated valve degradations — which may lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, velocity controller, etc., because of moisture, particles or alignment issues. This causes a lack of useful margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or associated flow management trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements similar to solenoids, pilots, velocity controller, and so on., because of moisture, debris or alignment. The valve will fail to open or close.
All of these circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is stuck open will exhibit itself as a rise in breakout pressure. Loss of seat load in comparison with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly screens for inside faults in addition to its inputs similar to provide stress. When the DVC6200SIS sees the availability strain is too low, an alert is generated (Figure 4) so that the end person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero supply pressure, so it alerted the control system.
Other process journey benefits

Diagnostic information captured throughout a course of trip may reveal valve degradations that is probably not detected during a proof check. For example, diagnostic knowledge captured during a process trip might indicate a problem with the valve closing utterly in opposition to the full pressure of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate beneath real working conditions. This ends in a extra accurate SIF response time (the time from detection of an irregular situation to last factor reaching its trip state), which is in comparability with the Process Safety Time to make sure the SIF continues to be assembly its design requirements.
Although automated valves are very reliable, failures recognized in a process journey can provide valuable information to forestall future failures. This info can help with turnaround planning by ensuring the wanted parts can be found before turnaround even begins to probably shorten the turnaround schedule.

A course of journey can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving money and time. Data captured by a DVC could be analyzed to leverage the process trip as a proof take a look at. Even if the tip person chooses not to take proof test credit for a course of journey, the valve diagnostic knowledge offered by the DVC might help plant personnel make proactive valve upkeep selections..


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