Valve proof check credit score for a process trip

A course of trip happens when the protection instrumented system (SIS) places the process in its protected state by commanding an automatic valve (Figure 1) or different equipment 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 meeting is tested under real operating conditions, which provides a possibility to seize useful valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs might help decide the proof check credit for an automatic valve after a process journey.
Process journey
A course of journey happens when the SIS detects an abnormal course of condition via sensors such as temperature and stress, executes the logic and locations the process in its secure state by tripping the final parts such as closing an automated valve, stopping a pump, and so on. The SIS might 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 trip happens, the principle objective is normally to restart the unit or gear that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the chance to proof test an automated valve is not going to be a high priority or even an exercise into account because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve may be thought-about performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline might help uncover valve deficiencies and degradations that may not present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet many of the 12 requirements of a proof check.
Process trips versus proof exams
How can proof take a look at credit be claimed for an automatic valve that has failed and caused a course of trip? A proof check is a periodic test carried out on SIS sensors, logic solvers and final parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, might impair the flexibility of the SIS to take the process to its protected state when an irregular process situation is detected.
A proof check must be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is usually decided via a mean chance of failure on demand calculation (PFDavg). Some customers may select to proof check based on predetermined intervals, such as proof testing sensors each 24 months and ultimate elements every forty eight months instead of counting on the PFDavg calculations.
Automated valve proof tests could be done offline or online. Offline proof exams are often scheduled during a turnaround, when the method is completely shut down.
Proof testing an automatic valve online normally requires a unit or equipment 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 completed throughout a course of journey.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns because of precise demand on the SIS throughout operation could also be given credit score as proof checks (fully or partial) underneath given conditions…the subsequent planned proof check could also be skipped.”
These situations are
The shutdown paperwork equivalent data as registered throughout corresponding proof check.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be tested separately.
The shutdown happens within a predetermined maximum time window before the subsequent planned proof check which can then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof test may be thought of performed. A sample record of activities performed throughout a proof check, together with these which are carried out throughout a process trip, is shown in Figure 2. Even with out an automatic valve leak take a look at, information captured by the DVC alone can potentially account for a great quantity of proof take a look at coverage for an automatic valve.
The exact coverage is determined by the design of the automated valve, its DVC and its utility. The protection is estimated primarily based on the valve degradations, the chance of their incidence and the proportion of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a process journey can typically be sufficient to satisfy a serious a part of the proof take a look at requirements.
If the process journey takes place inside a predetermined most time window, the end consumer might choose to leverage the method trip as a proof take a look at by finishing steps one through 5 in Figure 2, that are often not completed in a process journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the present proof check interval.
Figure three. Data during a course of 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 might result in a course of trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, velocity controller, and so on., as a result of moisture, debris or alignment issues. This causes a loss of useful margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or related move control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the strain obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system conditions, leakage or debris, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system elements similar to solenoids, pilots, speed controller, etc., due to moisture, particles or alignment. The valve will fail to open or close.
All of those situations may 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 force. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly screens for inner faults as properly as its inputs such as provide stress. When the DVC6200SIS sees the provision stress is simply too low, an alert is generated (Figure 4) so that the end person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero provide stress, so it alerted the management system.
Other course of journey benefits
Diagnostic information captured during a process trip may reveal valve degradations that is probably not detected during a proof take a look at. For Dependable , diagnostic information captured throughout a process trip would possibly point out an issue with the valve closing completely in opposition to the full strain 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 correct beneath real working situations. This leads to a extra accurate SIF response time (the time from detection of an irregular condition to last element reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF remains to be meeting its design necessities.
Although automated valves are very reliable, failures identified in a course of journey can present priceless data to stop future failures. This info can help with turnaround planning by ensuring the wanted components can be found earlier than turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of trip can provide the coverage required to delay a valve’s subsequent scheduled proof check, thus saving money and time. Data captured by a DVC could be analyzed to leverage the method trip as a proof take a look at. Even if the top person chooses not to take proof test credits for a process journey, the valve diagnostic data provided by the DVC might help plant personnel make proactive valve upkeep choices..
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