End users and contractors alike strive to design and operate plants with maximum plant safety and efficient production, and safety instrumented system may be required to reduce risk to an acceptable level. At times, diagnostics beyond valve functional proof tests such as partial stroke tests are implemented to provide additional diagnostic coverage necessary to meet a target SIL or desired proof test intervals. Alerts and test trends can also provide predicative diagnostics. This article explores considerations when implementing diagnostics to maximize usage and benefits.
What are valve diagnostics and what are the benefits?
From a purely functional safety perspective, diagnostics are characterized by a method or test that detects dangerous faults. For valves, this includes partial stroke tests and other in-service tests of sub-assembly functionality, such as solenoid health testing and digital valve controller self-health assessment. This results in “yes/no” or “pass/fail” answers as to whether a dangerous fault was detected, but users are consistently driving for predicative diagnostic methods in addition to current pass/fail means. Predicative diagnostics include comparing a status or test result to a baseline and assessing for valve assembly degradation. This information will be used to take an action and/or make a maintenance decision.
Predicative diagnostics provide a significant benefit in the basic process control system side by reducing unplanned shutdowns. This benefit can extend to the safety valve, which can provide additional safety when the valve assembly begins trending toward a dangerous failure mode such as increased friction or breakout torque required for operation.
What to know when implementing diagnostics
Process designers must have the foresight to specify products that not only meet the required SIL target level and test intervals in accordance with the IEC61508 and 61511 safety lifecycle, but also need to consider the following when determining how diagnostic methods will be employed:
a) Internal functional safety specifications
In addition to functional safety standards such as IEC 61511, ISA 84.01 and other national regulatory requirements, corporations often have internal safety design and implementation specifications to extend a common safety culture over all owned facilities. These internal specifications outline points such as system architecture, test procedures, diagnostic management (does the partial stroke test need to be run from the logic solver, or can it be managed through the basic process control system, and/or asset management system?) and diagnostic alert management and alert response action plan as well as report automation and generation requirements to meet insurance, regulatory and/or audit needs. All decisions should be in accordance with this specification, unless a justifiable deviation is presented.
b) Project magnitude and scope
The nature of the project, brownfield or greenfield, will impact where diagnostics are implemented and how they are employed:
Brownfield or Retrofit Expansions
Retrofit or small brownfield expansion projects implementing valve diagnostics generally try to implement existing I/O without executing a major system upgrade. This means if a smart valve diagnostic device is installed, it should be able to communicate and operate over the same pair of wires as previously installed without adding to the I/O count.
Having an on-line asset management system (AMS) or HART enabled I/O card is ideal for an ongoing stream of diagnostic information, but this is not always an option when starting a predicative diagnostic program. Having a device that is flexible to support multiple infrastructures (HART I/O, AMS, multiplexer, laptop with diagnostic software, handheld, etc.) is useful in cases where an AMS is not available.
From a physical space constraint perspective, there may not be sufficient space at the valve to add a bypass or additional redundancy if online testing or additional safety integrity is required, making partial stroke testing a good option to safely extend time between proof tests.
System Architecture and I/O Selection
The I/O selection for greenfield facilities for valve assemblies comes down to a simple AO or DO discussion. Oftentimes intelligent PST devices are digital valve controller-based, which traditionally have been AO driven. A major benefit to utilizing a digital valve controller powered by 4-20mA AO is that the device is powered and able to collect diagnostic information even during a demand. HART-enabled AO cards provide direct integration from the device and its diagnostic information into the logic solver, which may be desired to implement PST and receive results directly using the safety system. Alternatively, traditional AO cards can be used, with the HART signal peeled off the wires and transferred into the AMS or diagnostic software.
For a greenfield project, system infrastructure can be designed to reflect plant culture and corporate work practices. Ideally, intelligent SIS valve devices are continuously connected to an AMS or diagnostic software that can provide live valve status and trends as well as automated SIS testing and diagnostics to ensure that tests necessary to the diagnostic coverage of the valve assembly are not delayed. This would especially be recommended when there is a large number of SIS valve assemblies installed requiring PST and predicative diagnostics, as this effort would quickly become insurmountable to perform one by one on hundreds of valves with point-to-point devices like handhelds.
At times, even with greenfield projects, bypasses cannot be built around a valve. This can be due to weight restrictions (as on FLNG platforms), or other physical constraints characteristic of the piping and instrumentation design. Like with the brownfield design, this is an opportunity to implement partial stroke testing if additional safety integrity is required or to extend the proof test interval.
c) Plant Culture
Last but not least, plant personnel and culture must be considered. Infrastructure required to execute diagnostic tasks such as PST initiation method, results interpretation and alert monitoring, should be designed to maximize accessibility to qualified personnel assigned to work with the SIS valve assemblies. Personnel comfort with performing tests that move the valve assembly (such as PST) should be gauged to determine whether the test is automated (scheduled or initiated from the control room), semi-automated or manually implemented at the valve. Utilizing equipment that is modular in design, easy to service, understand and use is important to ensuring implementation success within a plant. To further aid in establishing comfort with new test programs, thorough training (hands-on equipment as well as action plan training) and a transition plan, beginning with manually implemented tests and working toward automatic testing, should be considered. Gaining comfort with new diagnostic test procedures is critical to ensure tests are performed and to provide maximum diagnostic benefits.
There are many products on the market that provide some form of diagnostics for SIS valve applications. Understanding what diagnostics are desired and considering the specifications, project magnitude and scope, as well as plant culture in the design stages is key to specifying appropriate products, infrastructure and test methods to maximize plant safety and efficient production.