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A course of journey occurs when the security instrumented system (SIS) places the method in its secure state by commanding an automated valve (Figure 1) or other gear to its journey state in response to an irregular process condition. In some cases, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is examined beneath real working conditions, which supplies a chance to capture valuable valve diagnostic knowledge.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs may help decide the proof check credit for an automatic valve after a process journey.
Process trip
A course of journey occurs when the SIS detects an irregular process situation via sensors corresponding to temperature and strain, executes the logic and locations the method in its secure state by tripping the ultimate components corresponding to closing an automated valve, stopping a pump, etc. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of trip happens, the principle objective is often to restart the unit or tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof take a look at an automatic valve will not be a high precedence or even an exercise under consideration 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 data captured by the DVC. Also, a comparability of the diagnostic data 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 performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a process journey meet a lot of the 12 necessities of a proof take a look at.
Process trips versus proof tests
How can proof test credit be claimed for an automatic valve that has failed and caused a course of trip? A proof check 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 power of the SIS to take the process to its protected state when an irregular course of condition is detected.
A proof test should be performed as per the proof test interval specified in the Safety Requirement Specifications. This interval is usually decided via an average probability of failure on demand calculation (PFDavg). Some customers may choose to proof take a look at primarily based on predetermined intervals, such as proof testing sensors each 24 months and final parts each forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests may be accomplished offline or on-line. Offline proof checks are normally scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automated valve on-line normally requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and expensive.
But a proof test may additionally be completed throughout a course of journey.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns due to precise demand on the SIS throughout operation may be given credit score as proof exams (fully or partial) beneath given conditions…the subsequent planned proof test may be skipped.”
These circumstances are
The shutdown documents equal data as registered throughout corresponding proof check.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated must be tested separately.
The shutdown happens within a predetermined maximum time window before the subsequent planned proof take a look at which might then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof test could be thought-about performed. A pattern record of actions carried out throughout a proof test, together with those that are performed during a process trip, is shown in Figure 2. Even with out an automatic valve leak take a look at, data captured by the DVC alone can doubtlessly account for a great quantity of proof check protection for an automated valve.
The exact protection is dependent upon the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the chance of their occurrence and the share of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a process trip can typically be enough to satisfy a serious part of the proof test requirements.
If the process trip takes place within a predetermined maximum time window, the tip user might select to leverage the process trip as a proof take a look at by finishing steps one via five in Figure 2, which are usually not accomplished in a course of trip. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the last half of the present proof test interval.
Figure 3. 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 more than likely root causes of automated valve degradations — which may result in a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, speed controller, etc., due to moisture, particles or alignment points. This causes a lack of practical margin and makes the valve sluggish 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 attributable to compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system conditions, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements such as solenoids, pilots, pace controller, and so forth., due to moisture, debris or alignment. เกจวัดแก๊ส will fail to open or close.
All of those situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is caught open will exhibit itself as an increase in breakout force. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly screens for internal faults in addition to its inputs corresponding to supply pressure. When the DVC6200SIS sees the availability stress is 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 virtually zero provide strain, so it alerted the control system.
Other process journey advantages
Diagnostic data captured throughout a course of trip may reveal valve degradations that is most likely not detected during a proof check. For example, diagnostic data captured during a course of trip might indicate an issue with the valve closing completely against the full 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 accurate under real operating conditions. This leads to a extra accurate SIF response time (the time from detection of an abnormal situation to final element reaching its trip state), which is in comparison with the Process Safety Time to make sure the SIF remains to be assembly its design requirements.
Although automated valves are very reliable, failures recognized in a course of trip can present priceless information to prevent future failures. This data can assist with turnaround planning by ensuring the wanted parts are available before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of journey can present the protection required to delay a valve’s subsequent scheduled proof take a look at, thus saving money and time. Data captured by a DVC can be analyzed to leverage the process trip as a proof check. Even if the top person chooses not to take proof check credit for a process journey, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance choices..
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