Valve proof check credit for a process journey

A process journey occurs when the protection instrumented system (SIS) locations the process in its secure state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal process situation. In some instances, a spurious journey occurs because of a valve malfunction. In that case, the valve meeting is tested beneath real operating situations, which offers a possibility to seize valuable valve diagnostic data.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs can help decide the proof take a look at credit for an automatic valve after a process trip.
Process trip

A course of journey happens when the SIS detects an abnormal process situation through sensors similar to temperature and stress, executes the logic and locations the process in its protected state by tripping the ultimate components similar to closing an automatic valve, stopping a pump, and so forth. ไดอะแฟรม ซีล could 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 happens, the primary aim is usually to restart the unit or gear that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the opportunity to proof test an automated valve will not be a high priority and even an activity into account because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof check of the valve can be considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline may help uncover valve deficiencies and degradations that will not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of journey meet many of the 12 necessities of a proof test.
Process trips versus proof checks

How can proof take a look at credit be claimed for an automatic valve that has failed and caused a process trip? A proof take a look at is a periodic check carried out on SIS sensors, logic solvers and ultimate parts — such as automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the flexibility of the SIS to take the method to its safe state when an irregular course of situation is detected.
A proof check ought to be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is normally decided by way of an average likelihood of failure on demand calculation (PFDavg). Some customers could choose to proof take a look at based mostly on predetermined intervals, similar to proof testing sensors each 24 months and ultimate components every 48 months instead of counting on the PFDavg calculations.
Automated valve proof exams may be done offline or on-line. Offline proof checks are usually scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automatic valve online often requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and dear.
But a proof check may also be accomplished throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns because of actual demand on the SIS throughout operation may be given credit as proof checks (fully or partial) under given conditions…the next planned proof test may be skipped.”

These situations are

The shutdown paperwork equal info as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown occurs inside a predetermined most time window before the following planned proof test which can then be canceled

When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check could be thought of performed. A sample list of actions carried out throughout a proof test, together with these which are performed throughout a course of trip, is proven in Figure 2. Even with out an automated valve leak test, data captured by the DVC alone can probably account for an excellent amount of proof take a look at coverage for an automatic valve.
The exact coverage depends on the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the likelihood of their prevalence and the percentage of these degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a process trip can usually be enough to satisfy a significant part of the proof check requirements.
If the method trip takes place within a predetermined maximum time window, the end person may choose to leverage the method journey as a proof check by completing steps one through 5 in Figure 2, which are normally not accomplished in a process journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the final half of the current proof take a look at interval.
Figure three. Data during a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there 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 may lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, velocity controller, etc., as a result of moisture, debris or alignment issues. This causes a loss of useful margin and makes the valve sluggish to open or close.
Binding, galling or different degradation of valve seats or associated flow control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball caused by system conditions, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system parts similar to solenoids, pilots, pace controller, and so on., as a end result of moisture, debris or alignment. The valve will fail to open or shut.
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 an increase in breakout drive. Loss of seat load in comparability with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously monitors for inner faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the supply strain is just too low, an alert is generated (Figure 4) so that the end user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero supply strain, so it alerted the control system.
Other course of journey advantages

Diagnostic knowledge captured during a course of journey might reveal valve degradations that may not be detected throughout a proof take a look at. For instance, diagnostic data captured during a process journey may indicate an issue with the valve closing fully in opposition to the total strain of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more correct underneath actual operating situations. This results in a extra correct SIF response time (the time from detection of an irregular situation to ultimate factor reaching its trip state), which is compared to the Process Safety Time to ensure the SIF is still meeting its design requirements.
Although automated valves are very dependable, failures identified in a process trip can present useful information to forestall future failures. This info can help with turnaround planning by making sure the wanted elements are available earlier than turnaround even begins to doubtlessly 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 could be analyzed to leverage the method trip as a proof test. Even if the top consumer chooses to not take proof take a look at credits for a course of journey, the valve diagnostic information provided by the DVC can help plant personnel make proactive valve maintenance choices..


Scroll to Top