Valve proof take a look at credit score for a process trip

A course of journey happens when the protection instrumented system (SIS) places the method in its safe state by commanding an automated valve (Figure 1) or different gear to its trip state in response to an irregular process condition. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is tested beneath real operating conditions, which provides an opportunity to capture priceless valve diagnostic information.
However, เกจวัดแรงดันออกซิเจนราคา can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs may help decide the proof take a look at credit for an automatic valve after a course of trip.
Process trip
A course of trip happens when the SIS detects an abnormal process condition by way of sensors such as temperature and stress, executes the logic and locations the method in its secure state by tripping the ultimate parts such as closing an automatic valve, stopping a pump, and so on. The SIS might talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of journey occurs, the principle aim is normally to restart the unit or tools that has been shut down as quickly as potential. เกจวัดแรงดัน4บาร์ are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged units. Taking the chance to proof check an automatic valve is not going to be a high precedence and 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 could be considered performed 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 present up in a proof check.
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 throughout a course of journey meet most of the 12 requirements of a proof test.
Process journeys versus proof exams
How can proof check credit be claimed for an automated valve that has failed and triggered a course of trip? A proof check is a periodic check performed on SIS sensors, logic solvers and final parts — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, might impair the flexibility of the SIS to take the method to its protected state when an irregular process situation is detected.
A proof check ought to be performed as per the proof take a look at interval specified within the Safety Requirement Specifications. This interval is often decided through a median probability of failure on demand calculation (PFDavg). Some users could select to proof take a look at based on predetermined intervals, similar to proof testing sensors every 24 months and ultimate parts each forty eight months as a substitute of counting on the PFDavg calculations.
Automated valve proof exams may be carried out offline or on-line. Offline proof tests are usually scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automatic valve on-line normally 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 test can additionally be accomplished during a course of journey.
According to IEC61511-2 Section A., “…shutdowns due to actual demand on the SIS throughout operation may be given credit as proof checks (fully or partial) under given conditions…the next deliberate proof take a look at could also be skipped.”
These conditions are
The shutdown documents equivalent data as registered during corresponding proof check.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated needs to be tested individually.
The shutdown happens within a predetermined maximum time window before the next deliberate proof test which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof check could be thought of carried out. A sample record of activities carried out throughout a proof test, along with those which might be carried out throughout a process journey, is shown in Figure 2. Even with out an automated valve leak check, data captured by the DVC alone can doubtlessly account for a great quantity of proof test protection for an automated valve.
The precise protection depends on the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the likelihood of their occurrence and the share of these degradations that could be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can often be enough to satisfy a significant part of the proof take a look at requirements.
If the process journey takes place within a predetermined maximum time window, the tip user might select to leverage the method journey as a proof check by completing steps one through five in Figure 2, that are normally not completed in a course of trip. The subsequent scheduled proof test 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 three. Data throughout a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made available 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 might result in a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, and so on., due to moisture, debris or alignment issues. This causes a loss of practical margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or related circulate control trim that restricts or resists valve motion.
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 attributable to system situations, leakage or debris, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components corresponding to solenoids, pilots, velocity controller, and so forth., due to moisture, debris or alignment. The valve will fail to open or shut.
All of these conditions 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 force. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly displays for inside faults in addition to its inputs such as supply pressure. When the DVC6200SIS sees the provision strain is simply too low, an alert is generated (Figure 4) in order that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply stress, so it alerted the management system.
Other course of trip benefits
Diagnostic knowledge captured throughout a course of trip could reveal valve degradations that is probably not detected during a proof check. For instance, diagnostic data captured throughout a process trip may indicate a problem with the valve closing fully in opposition to the total pressure of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is extra accurate underneath real working situations. This results in a extra correct SIF response time (the time from detection of an irregular condition to final factor reaching its journey state), which is compared to the Process Safety Time to make sure the SIF continues to be meeting its design necessities.
Although automated valves are very dependable, 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 elements can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
A course of trip can present the protection required to delay a valve’s subsequent scheduled proof take a look at, 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 tip person chooses to not take proof check credits for a course of journey, the valve diagnostic data offered by the DVC can help plant personnel make proactive valve maintenance choices..

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