Valve proof check credit for a process trip

A process journey occurs when the protection instrumented system (SIS) locations the method in its safe state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal process condition. In some instances, a spurious journey occurs because of a valve malfunction. In that case, the valve meeting is examined beneath actual operating circumstances, which supplies a possibility to seize useful valve diagnostic information.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In ราคาเกจวัดแรงดันน้ำ , we’ll discuss how DVCs may help decide the proof check credit for an automatic valve after a process journey.
Process trip
A course of trip happens when the SIS detects an irregular process situation via sensors such as temperature and pressure, executes the logic and locations the method in its protected state by tripping the ultimate parts corresponding to closing an automatic valve, stopping a pump, etc. The SIS could communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process journey happens, the primary goal is usually 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 gadgets. Taking the chance to proof take a look at an automatic valve won’t be a high 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 data captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline may help discover valve deficiencies and degradations that will not show up in a proof take a look at.
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 throughout a course of journey meet many of the 12 requirements of a proof test.
Process trips versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and triggered a process trip? A proof take a look at is a periodic test performed on SIS sensors, logic solvers and ultimate elements — such as automated valves — to discover hidden failures. These failures, if not corrected or removed, could impair the flexibility of the SIS to take the method to its secure state when an irregular course of condition is detected.
A proof test should be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often determined through an average chance of failure on demand calculation (PFDavg). Some users might choose to proof check based on predetermined intervals, similar to proof testing sensors each 24 months and final parts each 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof exams may be accomplished offline or online. Offline proof exams are often scheduled during a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line usually requires a unit or tools 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 be completed throughout a course of trip.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns due to actual demand on the SIS throughout operation could also be given credit score as proof tests (fully or partial) under given conditions…the next planned proof test may be skipped.”
These situations are
The shutdown documents equivalent data as registered during corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated must be examined individually.
The shutdown occurs within a predetermined most time window before the subsequent planned proof test which can then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought of performed. A pattern list of actions carried out during a proof take a look at, together with these which might be carried out during a course of trip, is shown in Figure 2. Even with out an automated valve leak take a look at, knowledge captured by the DVC alone can probably account for an excellent amount of proof check coverage for an automated valve.
The precise coverage is decided by the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the likelihood of their incidence and the percentage of those degradations that could be detected by a DVC. Fortunately, information acquired by a DVC during a process trip can typically be sufficient to satisfy a major a part of the proof take a look at requirements.
If the process journey takes place inside a predetermined maximum time window, the top person might choose to leverage the method trip as a proof take a look at by completing steps one by way of five in Figure 2, that are often not accomplished in a course of trip. The next 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 check interval.
Figure three. Data during a course of trip 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 96.05.01, the more than likely root causes of automated valve degradations — which might lead to a process journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment issues. This causes a loss of useful margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system situations, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system components corresponding to solenoids, pilots, speed controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of these situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously screens for inner faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the provision stress is simply too low, an alert is generated (Figure 4) in order that the end person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero supply stress, so it alerted the management system.
Other process journey benefits
Diagnostic knowledge captured during a process trip could reveal valve degradations that will not be detected during a proof test. For instance, diagnostic knowledge captured during a course of trip may indicate a problem with the valve closing completely towards the complete pressure of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is more correct beneath real operating conditions. This leads to a extra accurate SIF response time (the time from detection of an irregular situation to last element reaching its journey state), which is in comparability 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 process trip can present useful data to stop future failures. This information can help with turnaround planning by making sure the needed parts are available before turnaround even begins to probably shorten the turnaround schedule.
A process journey can provide the protection required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method journey as a proof test. Even if the top person chooses not to take proof check credit for a course of trip, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance decisions..

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