Many facilities have found value in performing diagnostics on valves before a turnaround or shutdown. This can help determine what valves will need work, what parts need to be ordered, and whether a valve can make it to the next turnaround. However, diagnostics are not as frequently used during commissioning and start up.
In a presentation during the 2016 Emerson Exchange, Gahan Mullen of BP and Sean Raymond of Emerson suggested, however, that diagnostics can actually make start-ups smoother and identify potential issues that may not be identified with traditional methods.
Traditionally, during a startup for a new process or coming out of a turnaround or shutdown, a valve is physically checked out by someone going to the valve to stroke it, either from the DCS or from a local current source. There may also be a calibration done on the positioner, and a functional check performed. The control system is also checked to verify that there are no alarms and that the valve strokes as required.
Generally, this is the end of testing for startup and commissioning, and the valve would be considered good. But diagnostics could be run at this point, especially if there has been any indication that a valve could have issues. Diagnostics can show faults not easily found with traditional methods.
The Value of Diagnostics
There is a wide variety of diagnostics software programs available from many manufacturers. ValveLink by Fisher, Valve Manager by Metso and ValVue by GE are just a few, but the function is basically the same. The software runs various tests to measure the performance of a control valve and enables graphical viewing of the results of those tests. It provides electronic documentation of configuration and calibration results and can help reduce and even eliminate manual diagnostic computations. It collects diagnostics information and makes the condition of the valve, actuator, positioner, control performance and even the operating environment visible to operators, whether they are on location or at a central monitoring depot.
During regular operations, this means that control valve performance can be maintained at optimum levels by continuously monitoring its condition and taking the necessary preventive actions.
According to Mullen and Raymond, however, these diagnostics can also be used going into a shutdown to help establish baseline data for later reference, and it can be used to make sure that the valve assembly is working as it should when commissioning and startup begin. Common diagnostics that could be run going into a shutdown are the valve signature, step response tests and a status monitor.
Dr.-Ing. Jörg Kiesbauer, in a paper for Samson Controls, also pointed out the value of running diagnostics at startup and noted that tests should be performed for early fault recognition with the objective to provide plant operators with a preventative, state-oriented maintenance. “Up-to-date diagnostic tools store all test results in data bases according to date and time. Consequently, trends of parameter changes can be recognized.” While Kiesbauer pointed out that careful sizing and selection of the control valve type is still the best guarantee for keeping costs of ownership low, efficient valve diagnostic programs run at startup allow operators to access comprehensive information about the control valve that can be used not only to determine problems at startup, but also to give insight into any communication problems between the positioner and control valve.
One example of the value of running diagnostics was with the case of a valve that showed no signs of issues when a visual inspection was done. Offline tests for the valve signature and performance all came back with no problems, and it would have passed traditional procedures for startup. However, by running a three point diagnostic test, it was found that the valve struggled to reach 100% performance. This resulted in replacement of the regulator. Without this diagnostic test, the valve would have performed poorly and could eventually have led to downtime.
In another example, a valve appeared to be functioning correctly and would have passed a traditional check out method but when diagnostics were run, the valve signature did not appear normal for the valve and actuator. By running diagnostic trouble-shooting, it was found that the setup wizard was done incorrectly, showing the wrong actuator. A new setup wizard and calibration was run incorporating the correct actuator model, so the problem was corrected. Mullen pointed out that, while this might not have led to downtime, the valve would not have performed optimally, and could have led to lost production.
Traditional startup and check out procedures may not always identify issues that can lead to problems at startup or once the process is up and running. It is possible to supplement traditional methods by using diagnostics software to identify issues at startup and commissioning that will ultimately make the process faster and smoother.